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Implant Biomechanics, Screw Mechanics, and Occlusal Concepts for Implant Patients

An important goal of prosthodontic rehabilitation is the long-term stability of restorations, supporting structures (eg, tooth, implants, bone, soft tissues), muscles of mastication, temporomandibular joints (TMJs), and the neuromuscular control system during normal function and anticipated occlusal changes. These natural tissues and artificial materials can change over time due to biologic (eg, aging, disease), biochemical (eg, drugs, lifestyle, oral habits), and biomechanical factors. This chapter focuses on the role of biomechanics by considering the action (occlusion, parafunction) and reaction (design parameters) to forces and summarizes relevant clinical considerations that promote long-term success.

Occlusal Forces: Actions and Reactions

Functional and parafunctional occlusal forces and moments (moment = force × distance) result in cyclic stress (stress = force / area) and strain (strain = deformed length / original length) within the prosthetic materials, implant components, and surrounding natural tissues. For natural biomaterials such as bone and soft tissues, the strain magnitudes, frequency, and duty cycle act cooperatively with the local biochemical signals to mediate biologic processes ranging from atrophy, necrosis, resorption, maintenance, remodeling, regeneration, and fracture depending on the load conditions. For artificial biomaterials such as polymers, metals, ceramics, and composites, the stress magnitudes will influence the material deformation, wear, creep, stress relaxation, and fatigue failure. Because stress is induced by occlusal forces, the magnitude of forces will be discussed first.

Magnitude: Maximum and nominal functional occlusal forces

The maximum biting forces that human masticatory muscles can generate depends on the force detection method, muscle mass, vertical dimension, distance from the TMJ fulcrum, periodontal support, health status of the TMJs, occlusal arrangement, age, gender, restoration type, and loading symmetry (bilateral vs unilateral). Given the wide range of variables, it is not surprising that the maximum biting forces for naturally dentate patients have been reported to range from 100 N to 1,700 N over many studies. In general, fully dentate patients generate greater maximum forces than partially dentate patients. Higher occlusal forces are typically observed more in the posterior and medial directions than in the anterior and lateral directions. For partially dentate patients, dental implants generally increase bite force, improve masticatory efficiency, and lead to overall higher patient satisfaction when compared to the partially edentulous state. For completely edentulous patients, implant-assisted overdentures exert greater maximum forces (~170 N) than those with root- supported overdentures (130 N) and conventional dentures (100 N).1

Force measurement depends on the technology used for force recording. Numerous devices such as Black’s gnathodynamometers, strain gauges, semiconductors, strain-sensitive films (piezoelectric and polymer), pressure-sensitive foils, and microprocessor devices have been used with varying degrees of measurement errors and measurement ranges (sensitivity). Also, some of these devices (eg, mechanical dynamometers) require higher vertical jaw separation than others (eg, films), thus the measurements are made with different condylar orientation, skeletal muscle length, contraction velocity, and other factors that can alter force generation.2 These differences are not easily discerned given the large variability in human bite force, and needs have yet to be met for an accurate force recorder that can measure forces at a single-tooth level under a variety of controlled loading conditions and for a system to reproducibly generate the clinical loads.

While maximum voluntary biting force may serve as a potential indicator of masticatory system health, such maximum forces do not predict masticatory efficiency. In fact, maximum forces are typically not the culprit for the common clinically observed material failures. Besides occlusal overload during parafunction, most components fail due to time- dependent processes that occur at lower masticatory loads over millions of chewing cycles. Most healthy vertebrates can masticate involuntarily. Similar to the muscles involved in crucial cyclic movements such as breathing, walking, and gastric motility, the muscles of mastication are controlled by a central pattern generator (CPG) in the brain stem.3 Each CPG contains a group of specialized neurons that receives both sensory and reflex feedback to modulate its basal rhythmic, oscillatory electrical patterns. This phase, amplitude, and frequency of the CPG basal pattern modulates the contraction and relaxation of the masticatory muscles. The net biting force is summed by adding the corresponding resolved force vectors from all muscles of mastication. CPGs operate effectively in the absence of voluntary muscle movement and can be further influenced by external stimuli and voluntary muscle commands. Hence, more emphasis is needed on material selection and design that can prevent the cumulative damage that occurs over millions of functional loading cycles.

The CPG-generated cyclic functional forces are typically 25% to 40% of maximum loads for each individual, with greater forces at the molars and less in the incisors due to the mechanical lever system. Fully dentate patients with complete dentition or implants generate higher cyclic functional forces (~130–150 N) than edentulous patients with implant- supported overdentures (~70 N), root-supported overdentures (60 N), and conventional dentures (~40 N).1 However, the implant overdenture group produced the lowest chewing efficiency, and this difference has been attributed to the implant-related attenuation of sensory feedback to the CPG. Note that the actual forces depend on distance from the muscle and TMJ fulcrum. The human mandible acts as a third-class lever when the food is anterior to the anterior border of the masseter muscles (premolars to incisors) and behaves as a second-class lever when food is posterior to the center of mass of the masseters (molars). Therefore, under the same applied biting force, the mechanical load at the molar can easily double the apparent load at the incisor.

Direction: Axial and nonaxial loading

Forces are vectors. Moments due to a force acting on a point from a distance are axial vectors. Forces and moments acting on implants result in stress and strain throughout the supporting structures. Individual implants osseointegrated in bone can deform or move along directions that are combinations of six basic motions (or six degrees of freedom) around the implant-abutment complex’s center of mass (Fig 3-1): three translational (linear up/down, left/right, forward/backward) and three rotational (yaw, pitch, roll). In the geometric context, pure translational movements occur when every infinitesimal point within an object moves the same distance. By that definition, translation can only occur when an object moves without rotation, tipping, bending, changing shape, or changing size. Conversely, pure rotations occur when all infinitesimal points within an object rotate around an axis of rotation, except those points that lie along the axis. In practice, most intraoral movements are comprised of components of translation and rotation.

Fig 3-1 Six degrees of freedom that an implant restoration can undergo in response to intraoral forces. The center of mass is the point of lowest resistance to motion at the intersection of the three principal axes.

The three translational forces that act directly on a point tend to produce movement and deformation. Movement can be linear if the force vector acts on the object’s center of mass, resulting in rotation-free bodily movement parallel to the force vector. These forces also produce pure compression/tension on the objects. All other force vectors produce nonaxial movements that involve rotation along with translation. Pure movement happens only in the rare occurrence where an axial force vector is oriented concentrically with the normal axis that lies through the center of mass. By definition, the normal axis is the orthogonal axis that lies perpendicular to the surface. For implants supported by healthy bone, a combination of both translation and compression results. When a vertical force acts on the central axis of an integrated implant, the apical movement (translation) is accompanied by deformation (compression) of the implant and the bone apical to the implant.

Most intraoral forces produce a combination of axial and nonaxial strain components that produce bending, tipping, and rotation around a point of minimum moment of inertia that offers the lowest resistance to each specific motion. By definition, the moment of inertia—or more precisely, the second moment of inertia, also termed the area moment of inertia—is a geometric factor that determines how the 3D shape of an object affects the resistance to deformation such as bending. For maximal resistance (ie, the maximum second moment of inertia), it is desirable to maximize the fractional area to be as far away from the center of mass (more on centroidal axis later in the chapter) as possible. During mastication and oral function, teeth contact and slide over each other by 1.4 to 2.6 mm depending on the type of food and occlusal arrangement.4 The net effect of the applied biting force on each point depends uniquely on the magnitude and direction of the force as well as the distance between the vector and the point. That same force also exerts influence on points that are not aligned along the force axis. The resultant nonaxial force produces rotation and translation of movable objects, bending and shearing of immovable objects, and a combination of both during mastication (Fig 3-2). This is the reason narrow occlusal tables (see section entitled “Occlusal design for partially edentulous patients”) are beneficial by decreasing the length of the moment arm. As another example, occlusal contact on a cantilever pontic causes axial compression on the pontic material directly below the contact point and causes nonaxial bending at the connector between the pontic and abutment. Because multiple points contact during mastication, the sum of all resolved vectors determines the orientation and net magnitude of deformation or movement.

Fig 3-2 Purely axial forces produce pure translation when the contact planes are perpendicular to the load vector. In all other loading scenarios, nonaxial forces are introduced, adding bending and rotation to the deformation.

Nonaxial forces are usually not aligned to take advantage of the resistance offered by the supporting bone, and most materials are weaker in shear than in compression. Therefore, the reduction of excessive nonaxial forces should be a key design rule if long-term stability is a goal of prosthodontic therapy. At the single-tooth level, wide occlusal tables with steep cuspal inclines pose a greater risk of inducing moments throughout the 3D Posselt envelope of motion and should be avoided when the implant support is suboptimal and whenever the expected loads are high (eg, short or narrow implant in compromised bone).

During function, the implant will experience axial and nonaxial loads simultaneously. Several observations have confirmed the favorable effect of axial forces in distributing the stresses evenly within the peri-implant bone.5–7 Axial loading occurs during closure of the mouth at maximal intercuspation. This is further enhanced if the opposing cusp is occluding against a flat surface, as the applied loads will be parallel to the long axis of the implant. Eventually, the implant and the surrounding bone will be under compression. On the other hand, nonaxial forces will subject the implant to bending moments. These forces can be caused by lateral or oblique application of loads on an implant prosthesis that may occur in situations of occlusal discrepancies, excursive occlusal contacts, bruxism and parafunctional activities, cantilever loading, or occlusal contact on inclined surfaces.8,9 As a result, the nonaxial forces tend to accentuate the stresses within the bone, prosthesis, and implant components along directions that offer the weakest resistance. This was historically referred to as load magnification and can result in detrimental biologic and mechanical complications.5,6,10

Osseointegrated implants are considerably different from natural teeth biomechanically. Natural teeth are attached to the alveolar bone via the periodontal ligament (PDL), which absorbs and efficiently distributes forces applied on the tooth to the surrounding alveolar bone. In addition, the PDL provides proprioception abilities and maintains some degree of physiologic mobility of the natural teeth.7,11,12 As a result, the natural tooth is less vulnerable to suffering from occlusal overloading and can withstand excursive contacts. In comparison, a dental implant is unique in being rigidly connected to bone, so any applied force on the implant prosthesis is directly transferred to the bone. This may lead to excessive stresses and deformation of the surrounding bone, and because the implant is not surrounded by a PDL, there is a reduction of proprioception and mobility. Instead, proprioception and movement of the implant can occur by deformation of bone. In contrast, physiologic mastication results in compression of the natural teeth within their sockets. Therefore, when implants are restored adjacent to natural teeth, the occlusion should allow the natural teeth to compress slightly to allow the implant and natural teeth to distribute the loads evenly. Consequently, the implant and its prosthesis can be subjected to excessive loading, and this overloading can manifest clinically in the form of alveolar bone loss10 and mechanical complications, such as ceramic chipping, screw loosening, material fatigue, creep, and component fracture.5,6,12

The following sections describe the biomechanics of each critical component within the implant-prosthesis system, starting from the dental occlusal design at the most coronal level, down to the prosthetic substructure, the abutment, the implant, and then finally the bone. The important design parameters for each component that influence long-term clinical success are highlighted.

Biomechanics of occlusal design

Numerous case reports, articles, and textbooks have described the importance of dental occlusion when restoring implants. With digital planning using implant component libraries, ideal occlusal schemes can now be designed and executed much more predictably. Planning the appropriate occlusion is important for controlling the load on implant prosthesis-bone systems. Otherwise, occlusal discrepancies predispose to loosening and premature fracture of abutment and prosthetic screws. Besides patient comfort, occlusal discrepancies affect the stability and mobility of implant-assisted overdentures, accelerating wear of attachments and implant connecting bars. Excessive bending moments may also cause micro- fractures within the bone-anchoring implants, triggering a resorptive remodeling response of the anchoring bone and eventual bone loss and implant loss. Occlusal overloading may also have a profound impact on the success or failure of immediately loaded provisional prostheses. Various occlusal schemes and philosophies, whether they were originally developed for complete dentures or fixed prosthodontics, are currently being employed for dental implants depending on the system design.

To overcome the biomechanical limitations of implant prostheses, special occlusal considerations have been proposed in the literature for reducing the likelihood of implant overloading.8,10,12,13 These considerations are centered on increasing the proportion of axial forces and reducing the proportion of nonaxial forces. This involves the number of implants, orientation of the implants, prosthesis design, and application of protective devices.12 To ensure that the implants and the surrounding bone are not overloaded, a sufficient number of well-distributed implants should be placed to support the prosthesis. The implants should be placed in a way that will allow loading in a direction that is parallel to the implant. The occlusion for the prosthesis should be designed for the purpose of reducing lateral forces applied to the implant. The occlusal design is largely dependent on the prosthesis design and location as well as the condition of the adjacent teeth and opposing dentition. In general, the occlusal design should be optimized by reducing prosthesis cantilever, reducing cuspal inclination, and narrowing the occlusal table.12,14–17 Heavy maximal intercuspation and lateral occlusal contacts can be further controlled by splinting multiple implants and reducing the lateral contacts on the implant prosthesis by relying on the remaining natural dentition. In addition, the damaging effect of heavy occlusal forces can be controlled by providing an occlusal splint.

Biomechanics of prosthesis design

If occlusal design and muscles of mastication dictate the magnitude and direction of occlusal forces, then prosthesis design governs how these occlusal forces are transmitted to the underlying abutments, implants, and ultimately, bone.

Individual vs splinted configuration

In a well-osseointegrated implant, the supporting bone resists against translation, and the motion of the implant becomes tipping, bending, and rotation. For example, a premature working-side interference on a posterior implant can result in disclusion of other teeth, reduction of contact surface area (fewer teeth touching), and translation of the load point toward the fulcrum TMJ. All these combine to increase stress that tips, bends, and rotates the implant around its center of mass. Even with the most ideally designed prostheses, normal mastication and routine function can produce nonaxial stresses and can result in moments. The design goal is to minimize—not eliminate—nonaxial stresses. In practice, time-tested procedures such as clinical remounts will reduce stress concentration and overall nonaxial stresses (see later in the chapter).

Splinting can distribute the dynamic loads and strains during function and parafunction more uniformly to the supporting implants, thereby reducing the probability of localized overloading. While some finite element analysis studies show that splinting does not reduce peak strains significantly when there is optimal crown-to-implant ratio, especially when using implants with internal connections, it is critical to understand that although episodic peak loads can be dangerous, greater uniformity reduces the cyclic loads and directly increases the number of fatigue cycles before failure. Furthermore, dentists cannot adequately predict which implants will suffer from future peri-implant bone loss. As bone resorbs around an implant, the center of mass moves apically, increasing the distance between the occlusal table and the center of mass. This ultimately results in greater moments, producing more nonaxial deformation and motion on nonsplinted implants.

Perhaps more important than load distribution, splinting reduces the amount of rotation around the implant axis (yaw), thereby minimizing shear stress on implant-bone interface around the vertical long axis of the implant. A good example that illustrates the significant antirotation effect of splinting is the fact that nonengaging abutments can be used successfully to splint two implants together, but nonengaging features will not resist rotation as single-implant abutments. Compared to single-unit implant restorations, splinting stabilizes the supporting implants by reducing the degree of freedom against implant rotation and mesiodistal tipping. Besides reducing stress on implants and bone, splinting also reduces rotational deformation on abutment screws, incidence of screw failure, and other complications.18

With the widespread availability of digital planning and milling as well as the ability to achieve passive fit and emergence profile to facilitate oral hygiene, the authors recommend splinting implants in most clinical situations, especially in posterior regions. Systematic review of 36 finite element analyses conclude that splinting may reduce crestal marginal bone loss, especially in short implants.19 Based on clinical experience, splinting is recommended when the applied stresses are high and when the supporting abutments (eg, external hex, rotating ball joints, nonengaging abutments) or implants (eg, short, narrow, compromised bone) may cause excessive stresses that can expedite fatigue failure, wear, plastic deformation, rupture, creep, fretting fatigue, and failure of implant-prosthesis components. The clinical justification for splinting will be detailed further in subsequent chapters (see chapter 11, section entitled “Individual implants versus splinted implant designs”).

For multi-unit prostheses, nonaxial forces at connectors can be countered by shortening the span of the unsupported segment or increasing the cross-sectional area of the connectors to maximize the angular mass or rotational inertia. Otherwise, the pontic connectors are at risk of mechanical failure. For posterior teeth in Class I occlusion, this resistance against vertical forces is dominated by the span length (L) and connector apical-occlusal height (h) (Fig 3-3), as deflection is related to the third power of both L and h. For maxillary anterior teeth with Class II occlusion and steep vertical overlap, resistance against horizontal forces is related to the third power of the connector width. The dimensions should not be excessive; they must be optimized for esthetics and allow access for oral hygiene. In many cases, the span length is beyond the control of the prosthodontist or restorative dentist, hence the primary control defaults to connector dimension—more specifically, the dimension that parallels the load axis governs the inertial resistance against deformation (occlusal gingival height for posterior teeth, buccolingual width for anterior teeth).

Fig 3-3 Prosthesis design parameters that control bending deformation (D_max), span length (L), and connector height (h) are the most important for posterior teeth under a vertical loading force (F). The moment of inertia for elliptical cross section is shown. Other cross-section geometries seen in dentistry would alter the equation, but the term wh3 remains unchanged.

Linear vs curvilinear configurations

The previous section suggests that antirotation and load uniformity are the primary benefits of splinting. That is certainly true with implants that are splinted linearly, that is, the centroid of all connecting implants are roughly aligned along a straight line (this forms the so-called “centroidal axis”). In the resting, unloaded state, there is no stress or strain anywhere within the prosthesis. When this linear configuration is loaded, stress and strain are distributed throughout the prosthesis except the path of a second axis that runs perpendicular to the load axis throughout the cross section of the prosthesis—the neutral axis, which is defined as the line along which stress and strain (hence “neutral”) are zero when the prosthesis is subjected to linear elastic bending moments. In linear configurations, the centroidal axis and the neutral axis are usually collinear for isotropic, symmetric prosthetic structures prior to visible bending or plastic deformation. Linear configuration is the common design when multiple implants are connected to restore four or fewer teeth.

In contrast, curvilinear (curved line) designs where the implant prostheses span across the arch are more common to treat fully edentulous patients. Mechanically, free-floating curvilinear configurations differ from linear configurations in that they are more prone to bending and torsion and feature noncollinearity between the centroidal axis and the neutral axis20 (Fig 3-4). However, when connected rigidly to osseo- integrated implants, cross-arch stabilization of the implant prosthesis results in reduced tipping motion in all directions. A good clinical illustration of the cross-arch stabilization effect is that even nonretentive abutments (eg, multi-unit abutments) that lack antitipping and antirotation features can be used to rigidly secure curvilinear full-arch prostheses. Individually, these nonretentive abutments cannot be used to retain single-tooth implant restorations because they do not resist rotation and tipping. However, these nonretentive attachments are used frequently to secure cross-arch, full-arch prostheses splinting where the curvilinear design reduces tipping and rotation forces.

Fig 3-4 The centroidal axis is the line that passes through the geometric center of a bar made of a homogeneous material. The neutral axis is the line along which the materials experience no stress or strain. In a straight beam (upper) with symmetric cross section, the centroidal axis and neutral axis are collinear. In a curved beam (lower) with symmetric cross section, the centroidal axis and neutral axis do not align, and there is a shift of the neutral axis from the geometric centroidal axis.20 By definition, no stress (hence no strain) is experienced along the neutral axis during bending, while the materials above and below the neutral axis planes experience compression and tension depending on load orientation. The shift in neutral axis, difference in stress distribution, and cross-arch connection combine to provide antirotation and antitipping for full-arch curvilinear prostheses.

As shown by the Skalak models,21 the force directed on each implant supporting the curvilinear prosthesis is linearly correlated to the vertical load and inversely correlated to the number of implants. The original Skalak models provided an estimate for maximum force (F_max) on any implant to be F_max = 2P / N, where P is the applied load and N is the number of implants. Various modifications by Brunski22 and others to incorporate more jaw flection and prosthesis deformation have been introduced. In general, the use of more implants (eg, increase N from 4 to 5) will yield biomechanical benefits only when the additional implant is positioned to increase the arc that is spanned by the implants (anteroposterior [AP] spread; Fig 3-5). By the same principle, the use of distally tilted, longer implants further lengthens the arc, increases the AP spread, and shortens the distal cantilever. It should be emphasized that most implants and implant components are engineered to withstand these stresses; therefore, the main goal of stress reduction is to minimize the stresses on the freshly created implant-bone interface, especially for early and immediate loading cases in compromised bone.

Fig 3-5 Implant stress can be reduced by increasing the arc length by spreading out the implants and tilting the terminal implant distally.

This biomechanical approach of treatment planning differs from the original biologic/anatomical approach favored by Brånemark, which involved placing all implants vertically in the anterior segments of the edentulous maxilla and mandible to take advantage of the denser bone structures in those regions. Although the success rate was high, this arrangement produced a linear configuration that precludes second premolar occlusion in order to minimize bending moments associated with long posterior cantilevers. Spreading out the implants farther into posterior sectors and tilting them (eg, all-on-four, maxillary V-4) represent optimization of the biomechanical and biologic/anatomical approaches.23

Biomechanics of Prosthesis- Abutment Connection

Implant screw mechanics

Screws are commonly used to connect implant components together to maintain a union between implant fixtures, abutments, and prostheses. The design of implant screws must meet the opposing demands of being a reliable clamping force, on one hand, and yet being the first-to-fail in deference to other components. It should be emphasized that screws and similar retention mechanisms must be engineered to be part of a failsafe strategy to avoid catastrophic failure of components (eg, implant fixtures) that are not easily replaced.

When a torque is applied to a screw, tension or “preload” is generated within the screw, and the corresponding compressive force clamps components together to create a screw joint. Longer screws with wider-diameter threads generally offer greater resistance for screw joint displacement than short, narrow screws. Preload is affected by the amount of torque applied and the type of alloy of the screw and jointed components. Controlling factors include the surface features of the screw and components, the screw head design, the shape of component fitting surfaces, and the presence of lubricants.

The actual preload force F_preload is simply F_preload = δ σ_y A, where δ is percent of yield strength, σ_y is the screw material yield strength, and A is the cross-sectional area of the screw under tension. For example, δ of 75% yield strength is cited most frequently as the ideal preload for abutment and prosthetic screws.24 One study showed that during the final stages of screw tightening, each degree of clockwise rotation lengthens the screw by ~1 micron and increases the preload by ~50 N.25 For example, for a typical implant system, when the torque value of 32 Ncm is reached, the abutment screw would lengthen by ~13 microns, and a total preload of > 500 N is produced on the implant-abutment interface.

As most dental implant screws are ductile materials, 75% yield strength is approximately 65% of the ultimate torque to failure. Because it is not practical in the clinic to determine exactly when 75% yield is reached during clinical crown delivery, another popular approach is to estimate the preload force from applied torque (T) from the torque wrench: T = F_preload D _Ktorque, where D is the nominal screw diameter and K_torque is the “torque coefficient” that summarizes the effects of friction and thread geometry in the joint. Most K_torque values range from 0.1 to 0.3, and 0.2 is considered a reasonable value when details of the joint are unknown. It should be noted that although implant manufacturers market various torque drivers to apply exact torque levels to abutment and prosthetic screws, the actual preloads are highly dependent on the actual fit of the screw joint components, the quality of the screw-thread interfaces, and variability in the coefficient of friction that comprises K_torque as well as the accuracy of the torquing device. Ideally, the compressive clamping forces are adequate to withstand the joint-separating forces that may occur between components. Joint-separating forces can arise during function as a result of excursive contacts, off-axis centric contacts (where angled abutments are used or where a wide occlusal table exists), interproximal contacts, cantilever contacts, and where frameworks are not fitting passively. While it is tempting to increase preload to overcome potential joint-separating forces, it is important to note that higher preloads will accomplish higher clamping force by increasing percent yield but will also directly shorten fatigue life because excessive preloads can increase mean stress that promotes microcrack propagation. It must be emphasized that the very existence of a preload equates to stress concentration in the screw thread system, and choosing the proper preload is a compromise between screw loosening and screw fracture.

After initial tightening, loss of preload can occur due to material deformation such as creep, wear, corrosion, and other mechanisms that alter the fit between the screw and the threads. Preload relaxation can result in screw loosening and fracture and is more likely when the initial surface rough spots on the screw-thread interface initiate contact and subsequently undergo wear and/or deformation over time. This is expected in most bolt joint assembly and is consistent with studies reporting loosening torque values that are usually 80% to 90% of tightening torque.26 These studies are based on delivering a normal range of recommended torque values, and when excessive tightening torques are used, phenomena such as cold welding and galling occur that may increase the removal torque. Excessive tightening torque can occur if the torque drivers are not calibrated or are used improperly. In these simple mechanical joint configurations, the screw compresses the separate components together and by itself offers minimal resistance to shear, tensile, torsion, and bending deformations. Without additional joint design features to add resistance, micromotion leads to component wear. Galling is a common adhesive wear and transfer of material between contacting metallic surfaces, causing them to seize together, but can be reduced by application of lubricants. In order to better appreciate screw mechanics, it is useful to understand some physical concepts relating to contact between surfaces. When contact is achieved between two metallic surfaces, the initial interaction is between high points found on the surfaces, such as the screw thread tips, known as the asperities. When movement is applied, the surface irregularities penetrate the opposing surface, causing friction and/or plastic deformation, and pressure is induced, which increases heat and adhesion. Deformation of the contacting surfaces is known as settling, and when components are pressed together, microsurface irregularities on the component surfaces, screw head, and screw threads are flattened out. The magnitude of settling that occurs is a direct function of the initial surface roughness, surface hardness, and the magnitude of loading force applied. Rougher surfaces and larger loads are associated with greater settling. This “settling” effect also occurs when the strength and stability of the implant-abutment connections are inadequate, and more stresses are transmitted to the screw. This is one of several reasons that nonsplinted external hex connections tend to have more screw loosening than internal connections (hex, octagon, conical, Morse taper, etc). Fortunately, preload relaxation can be reversed by retightening the screws over time. To counter the effects of settling clinically, it is recommended that screws are initially torqued down, with further torque applied after waiting 10 minutes once settling has occurred.27 Many textbooks still teach the practice of retightening 1 week after initial tightening because as more settling occurs, less friction is available because of cyclic loading and wear. New screws are recommended after multiple tightening and loosening cycles.

To stabilize the joints, antitipping and antirotational features are incorporated into both internal and external implant connection designs to provide resistance to joint-separating forces. The original Brånemark implants have an external hexagonal connection, but the minimal height of the hexagon limits the extent of stability achievable. This has been addressed with the Brånemark Mk III and Mk IV implants (Nobel Biocare) by increasing the hexagonal wall height to between 0.7 mm (narrow and regular platform) and 1.0 mm (wide platform). Other implant systems with an external hexagon connection include Biomet 3i (Osseotite and Naontite External Hex Implant System), Southern Implants (External Hex Implants), and Straumann (Narrow Neck). Alternatively, a variety of internal connections have been developed with tapered or conical designs are available from Straumann, Bicon, Astra Tech, Southern Implants, and ITS Implants, and internal connections are available in hexagon (Certain 6-point, Zimmer Biomet), double hexagon (Certain 12-point, Zimmer Biomet; Astra Tech Implant System, Dentsply), or trilobe (Nobel Biocare Tri-Lobe; Southern Implants Tri-Lobe). Antirotational features are particularly important for single-implant restorations where engaging abutments with interlocking features are used, as opposed to nonengaging abutments for multi-unit applications. The internal indexing design with seven-point configuration would engage the implant and ensure specific orientation of the individual custom abutment. While this is beneficial for single-tooth restorations, these features may complicate multiple-unit implant restorations with divergent implant angulations.

The properties of screw materials determine both the optimal preload as well as the ability to maintain preload under cyclic loading. Materials used for implant screws include gold alloy, titanium, gold-plated gold-palladium titanium alloy (Gold-Tite, Zimmer Biomet), and polytetrafluorethylene (PTFE)-coated titanium (TorqTite, Nobel Biocare). Titanium is prone to galling and settling. The friction between titanium and other metals is initially low, but with repeated tightening following loosening, it increases to about that of titanium against titanium. However, all screw materials are susceptible to screw loosening. Coating screw surfaces with pure gold creates a very smooth surface, reducing settling effects. Gold alloy screws are capable of greater preload than titanium screws. Dry lubricant coatings reduce friction, allowing greater preloads to be achieved. A recent study reported that saliva contamination reduces reverse torque on implant screws, but blood and fluoride contamination had no effects.28 Interestingly, the same study showed that chlorhexidine contamination actually increased the reverse torque values above statistical significance. It should be noted that previous studies did not report a saliva effect.26

Gold-coated screws (Gold-Tite) have very smooth surfaces due to a thin coating of pure gold, while in contrast, PTFE-coated screws (TorqTite) have rougher surfaces. With repeated loosening and retightening, delamination of the PTFE-coated surface has been observed, so it is recommended that TorqTite PTFE-coated screws should be replaced rather than reused and retightened repeatedly. Historically, to ensure screws remained tightly clamped, some implant manufacturers recommended wrapping PTFE tape around the threads (Integral System, Calcitek) or applying a bonding agent to the threads (Ceka Bond, Preat). With current screw materials and surfaces, these precautions are not generally needed. Generally speaking, due to galling and settling effects affecting thread shape, repeatedly loosened screws should be replaced to ensure optimal preloads are attained and to avoid the risk of screw fracture associated with increased friction when torque is applied.

The most effective way to prevent screw loosening is to ensure that they are tightened sufficiently at the time of placement. The maximum torque generated by hand is approximately 15 to 25 Ncm,29–31 and torque wrenches are necessary to obtain optimal tightening torques beyond this. It is debatable whether to tighten screws to the point where they are difficult to loosen; this depends upon the type of prosthesis used to restore the patient. For example, implant-assisted implant connecting bars retaining overdentures are subject to wear and may need to be removed and remade 15 to 20 years following initial placement. Individual attachments retaining overdentures also wear and need periodic replacement; therefore, the screws retaining these devices must be removable. Screw loosening is less of an issue to manage for screw- retained prostheses, whereas for cement-retained prostheses where retrieveability is difficult, it is much more important to prevent screw loosening. Use of torque wrenches is therefore more critical for cement-retained prostheses. However, torque wrenches are consistently inaccurate32–34 and must be recalibrated on a regular basis. Even with calibrated torque wrenches, parameters such as friction that determine K_torque would affect the actual preload achieved.

Torque wrench designs available for implant application are categorized into the following five types: (1) friction grip wrench; (2) spring action toggle (ratchet) wrench (metal or ceramic bearings); (3) spring action beam wrench with graduated- force scale; (4) break-open torque-limited wrench; and (5) electronic motor–controlled torque wrench (needs to be recalibrated every 6 months because of the wear associated with the bearings).

It is important to understand that different screw designs have correspondingly different maximum torque preload affecting their application. Prosthetic screws tend to be shorter and of smaller diameter and have finer thread pitch and smaller thread surface area than abutment screws. Abutment screws are generally larger in dimension to resist the forces between abutments and the implant fixture. Abutments are designed to support the overlying prostheses, so forces acting at the screw interface between the abutment and prosthesis are reduced to allow smaller screws to be used.

Screws used to retain prostheses on custom abutments or custom substructures by cross-pinning (also known as set screws) are designed for placement transverse to the long axis of the implant prosthesis (Fig 3-6). Joint-separating forces acting in the transverse direction are relatively minimal compared to those acting down the long axis of the implant in response to occlusal forces, so screws used for cross-pinning can be relatively small and only require finger tightening to remain effectively clamped. The ability to use smaller screws for cross-pinning is a useful feature. Such an approach may be dictated by limited restorative space, resulting in insufficient height of the axial walls of the custom abutment for retention of the prosthesis with cement.

Fig 3-6 (a and b) Cross-pinning screws used to secure the prosthesis to customized milled abutments. (c) Prosthesis in position.

Laboratory screws are designed for fabrication of implant components in the dental laboratory. These screws should not be used in the clinic because of potential damage secondary to repeated application of torque. Laboratory screws often have thread sections that are shorter (so fewer turns are required to tighten them), and they may not have the same thread coatings as the screws designed for clinical use. Consequently, the preloads attainable with laboratory screws are not as high. Therefore, using old laboratory screws for clinical crowns will compromise the ability to achieve and maintain the desired preload and risk premature loosening or screw fracture.

Biomechanics of implant design

Implant manufacturers are constantly introducing new implant design features, but many of these innovations have not produced clinical evidence of higher implant success. Among the numerous design parameters, implant surface topography and surface chemistry have received tremendous attention (see chapter 2). This section address the more macroscopic implant body design features that have been reported to influence implant success rate. It should be noted that all of these design parameters are highly dependent on one another. This interdependence can complicate data analysis from clinical studies that compare implant systems that typically differ in more than one parameter.

Abutment connections

Most implants connect to their abutments by either an external or internal interface. The external hexagonal interface used in the initial Brånemark system has a long track record of clinical success. However, the short hex height provided suboptimal antitipping and antirotation during millions of masticatory cycles. The small interfacial contact area subjects the interface to concentrated stress that accelerates micromotion and wear, resulting in loosening and fracture of the abutment screw. While these problems are reduced by splinting and cross-arch stabilization as discussed earlier, they present challenges for long-term maintenance of single-tooth implant restorations.

In contrast, internal connection allows the apical end of the abutment to engage the internal features of the implant body. Besides reducing screw failures by redirecting stresses away from the abutment retention screw to the implant, the diameter of the components that comprise the internal connection design are more able to resist bending forces than the screws and small hex used in external connections. However, this size increase at the abutment level is achieved by reducing the implant fixture’s collar wall thickness. The thinning of the collar wall effectively compromises implant fixture fracture strength, and the problem is worsened with smaller-diameter implants and with smaller internal taper angles that further thin down the implant walls (Fig 3-7).

Fig 3-7 (a) Worn external hex. (b and c) Fractured implants with internal connections and thin walls. (Part c courtesy of Dr N. AbouJaoude.)

One-piece implants incorporate the abutment features into the implant body. This simple design eliminates the thin collar wall problems, and the absence of a retention screw eliminates screw loosening and failures. Furthermore, the absence of a bone-level abutment-implant interface eliminates the microgap that is believed to favor biofilm formation. However, the absence of the abutment screw and abutment-implant connection interface results in a highly stiff joint that tends to distribute more stress to the surrounding bone. Clinically, the one-piece implants have been associated with more extensive marginal bone loss than size-matched two-piece implants.35,36 It appears that of the two parameters (inflammation and mechanical overload) that are most associated with crestal bone loss around implants, mechanical overload may play a larger role in the failure of one-piece small-diameter implants. Although the bone stress can be modulated by other implant design parameters at the thread level, one-piece implants illustrate the importance of a failsafe mechanism to avoid catastrophic damage.

Short vs long implants

Implant length. Under mechanical loading, most of the occlusal stresses are concentrated in the coronal third of each implant. This information, along with the reasonable implant success rate, has been used to justify the selection of short implants over bone grafting in order to facilitate the placement of longer implants. While short implants are worthy options when bone volume is inadequate and bone grafting and nerve repositioning are not indicated, they have the disadvantage of reduced bone-to-implant contact (BIC) surface area. Under the same occlusal force, higher and more concentrated stresses are expected even though most of the stress is confined to the most coronal 3 to 4 mm of crestal bone.19 Although computer modeling shows that longer implants do not reduce stress around the crestal bone and clinical evidence does not show higher success, the role of biology must be respected. Bone can resorb over the lifetime in service due to many reasons (eg, peri-implantitis), and short implants have less “runway” for bone loss. Therefore, grafting for longer implants is recommended unless the risk-to-benefit ratio is unfavorable for bone graft surgery.

Implant diameter. In general, wider-diameter implants offer more contact surface area of bone-implant interface. Wider implants also have a higher probability of engaging the crestal cortical bone. Therefore, most finite element analysis would report overall reduction of stress concentration (and strain) in wider-diameter implants for axial and nonaxial loads.19 However, when space is inadequate (thin alveolar width, short mesiodistal interproximal spaces) and grafting options are limited, smaller implants can be considered as long as the following precautions are accepted by the dentist and the patient. Besides having thinner walls (two-piece implants) that compromise the fatigue resistance of the implant, small- diameter implants also have less BIC surface area. Under the same occlusal force, these translate into higher and more concentrated stresses that reduce fatigue life expectancy and risk bone overloading.

Thread design and pitch

Thread geometry plays an important role in implant placement, primary stability, and stress transfer at the bone-implant interface. Thread design must also facilitate cutting into bone in both clockwise and counterclockwise directions. Threads also play a critical role in primary stability by increasing the surface area of bone-implant interface and converting shear stresses into normal stresses that are better resisted by the bone.

In general, implant threads with sharp edges (eg, V-shape, buttress) can cut bone faster than square-shaped threads, which means they are also more likely to transmit shear forces to the bone, which is weakest against shear. Sharp threads are typically seen in implants designed for type I and type II bone, where initial fixation is easily achieved. The sharp cutting features that cut bone cleanly in both forward and reverse directions without excessive compression are favored. In contrast, threads with square edges transmit less shear stress than sharp threads, which are converted into compressive and tensile stress. When all other parameters are held constant, buttress designs are more resistant against pull-out, and square threads are more resistant against push-in. Square threads are used mostly in type III and type IV bone, where initial fixation is not often achieved due to the low bone density. As sharp threads are not necessary for the softer bone, square threads with aggressive pitch, large surface area, and thread angles can better resist the intraoral forces.

The selection of implant design features based on bone density is likely more important for immediate loading applications, as implant positioning and clocking are crucial for the surgical phase, and the initial fixation is critical to support the immediate mechanical loads.

Numerous studies on implant design suggest that thread design (nonthreaded, V-shaped, buttress, square) did not produce significant differences in clinical success when the loading of these implants is delayed conventionally. At this time, many implant manufacturers are introducing universal implant designs with combinations of sharp apical threads that cut through bone and square coronal threads to provide surface area and initial stability. Long-term studies will be needed to evaluate the performance in type III and type IV bone between the conventional and universal thread designs.

Biomechanics of Bone Along the Bone-Implant Interface

The bone-implant interface, its relation to bone-implant interfacial shear strength, and bone biomechanics have been described in chapter 2. This section focuses on the reaction to mechanical loading by biologic tissues, specifically bone around dental implants.

Occlusal forces that are transferred to the bone-implant interface produce dynamic response of bone tissue. The biologic response depends on the health status of the local tissue, loading parameters (ie, force vector direction, magnitude, frequency, and duty cycle), and greatly on the individual’s biologic environment.

Julius Wolff was the first to make observations related to the functional adaptation of bone architecture and local trabeculae pattern to the amount of mechanical loading.37 His concepts were refined over the next century to Frost’s “mechanostat” hypothesis, which states that bone cells respond to the amount of mechanical strain in their local environment such that with mild increases in strain, the slightly overloaded bone cells are stimulated to deposit more bone.38 Besides the hypothetical “minimal effective strain” for adaptive bone remodeling to occur, bone resorption dominates when the cells lack the minimum stimulation. Subsequent variations of this general approach hypothesize the existence of homeostatic set points.39 Below a minimal disuse threshold, bone resorption dominates. Above the remodeling thresholds, bone deposition dominates. Between these two set points, resorption and deposition both occur at similar rates to produce no net gain or loss. In practice, the magnitude of actual strains are so small that they are measured in units of microstrain (με; where 1 με = 10–6ε; 10 με = 1% strain). Frost proposed ranges of microstrain that are associated with disuse-related bone weakening (< 400 με), homeostatic bone remodeling (400–1,000 με), bone growth/repair (1,500–3,000/με, pathologic overload/microdamage (> 3,000 με), and fracture (> 25,000 με).

The exact strain thresholds depend on biologic status, load frequency, and duty cycle, and several minor modifications of the mechanostat hypothesis have been suggested. It should be noted that peri-implant bone, especially the mandible, experiences stresses that are unrelated to the presence of the implant. The attachment positions of the muscles of mastication exert significant stress and strain within the mandible, enough to cause bending and torsion. These strains, and smaller ones during speech, swallowing, and other jaw movements, cumulate and enhance the total microstrain history on the bone, in addition to the strain transmitted by the implant.

While precise mechanisms remain elusive, the roles of mechanical stimulation on numerous natural tissues have been elaborately studied and mathematically modeled in fundamental biologic processes. Various bone remodeling theories have been proposed. The most common approach relates local bone density changes and internal bone remodeling, while others relate mechanical stimulus to periosteal bone remodeling. The internal remodeling approach may be more applicable to dental implants. Mechanical forces have been shown to play a major role in organogenesis, growth and development, remodeling, disuse, cancer metastasis, and tissue regeneration. For dental implants in human jaw bones, these strain windows provide guidelines into bone resorption at low stimulation, bone remodeling and deposition at intermediate strain (Fig 3-8),40 bone microfracture at higher strain, and resorption and fracture during heavy loading.

Fig 3-8 There is anecdotal evidence that when implants are placed in the anterior mandible of a patient with a severely resorbed mandibular body and an implant-supported prosthesis (either a fixed prosthesis or an implant-supported overdenture) is fabricated, the bone mass of the body of the mandible will increase.40 Cortical layers thicken, and the vertical height of the body of the mandible can increase as much as 3.0 mm in some patients. (a) Mandibular body before delivery of the prosthesis. (b) Mandibular body several years after delivery of the prosthesis. (Courtesy of Dr H. Davis.)

The disuse window is reached when insufficient dynamic loading is delivered to produce the necessary microstrain stimulation, and bone resorption dominates. Examples include prolonged microgravity, disuse, stress shielding, paralysis, and prolonged bed rest. The bone remodeling window applies to normal daily activities. Bone growth/repair occurs with realignment of internal bone microstructure via controlled osteoblastic bone deposition and osteoclastic bone resorption to produce bone architecture optimized for the applied load (see chapter 7 section entitled “Treatment of the Severely Resorbed Mandible”).

When excessive loading is delivered to exceed microstrain tolerance, mechanical overload can lead to pathologic bone remodeling, microdamage, and resorption. Excessive stresses that accumulate damage faster than bone repair may result in fatigue and fracture. Excessive overload can be due to high stresses (eg, parafunctional habits, small localized contact area, long cantilevers, improper occlusal scheme) or poor resistance (eg, poor-quality bone, inadequate implant number, inappropriate implant body and thread design, inadequate implant dimensions). Crestal bone loss, screw loosening, and material failures are warning signs of mechanical overload. It should be emphasized that the strain window also depends on the stress state. Bone is generally weaker in shear, moderately weak against tension, and stronger in compression. Implant designs such as threads, collar, and any nonlinear protrusions and recesses along the bone-implant interface essentially convert significant portions of shear strain into tensile and compressive strain.

It should be noted that these microstrain windows vary with animal species and frequency. For example, low amplitude (< 500 με) at hyperphysiologic frequency (eg, ~30 Hz, or cycles per second) can be actually osteogenic even though the same microstrain magnitudes at normal frequency (< 1 Hz) would be catabolic, resulting in disuse atrophy. Further decrease in microstrain (~10 με) can be compensated by high frequency (~50 Hz) to maintain osteogenicity.41 Based on these observations, micro-piezoelectric implant stimulators have been proposed to deliver high-frequency, low-amplitude stimulation to promote osseointegration. However, these microstrain frequency effects are nonlinear or somewhat unpredictable due to the complex relationship between mechanoreceptors, mechano-signal transduction, biochemical signals, and osteoblast-osteoclast coupling.

When bone is inflamed or compromised (eg, smoking, bone metabolic diseases, medications), the pro-inflammatory cascade of biochemical signals may significantly influence the cell responses over time. This alteration of the mechanostat is analogous to adjusting the sensitivity of the thermostat controller. Although notable exceptions exist, many in vitro and in vivo studies have shown that inflammation tends to promote bone resorption and reduction of inflammation tends to promote osteogenesis. Besides inflammatory cytokines, the local biology (ie, vasculature, stem cell availability, biochemical signals) may be responsible for the observations that the microstrain windows may differ from person to person, from region to region within the same individual (eg, loaded bone vs non–weight-bearing bone), and even within the same site over time (eg, hence the concepts of immediate, early, conventional, and delayed loading).

Even in mature bone, both osteoblasts and osteoclasts possess the machinery to sense their mechanical environment, and some cells are more reactive than others. Although the microstrain magnitude may differ between long bones and jaw bones, the mechanostat concept seems to apply to most cells, as physical forces play an important role in cell folding, migration, patterning, and differentiation during embryogenic growth and development. Two main mechanotransduction mechanisms have been investigated: (1) stretch-sensitive ion channels and (2) surface-bound adhesion receptors that bind to specific extracellular matrix molecules and other ligands. Once these mechanoreceptors are activated, cells interpret the complex mixture of dynamic mechanical signals and biochemical signals to orchestrate the remodeling of the local bone matrix. It is believed that as cells lay down bone matrix in response to the stimulation, the newly formed bone mass will increase local resistance to the mechanical loading, thereby lowering the microstrain window. Exactly how cells regulate and process the vast input data remains largely unknown, and it is possible that mechanical strain plays a critical role only in the instances of extreme highs (pathologic overload) and lows (disuse) by altering the net effects of local biochemical signals.

Clinical Applications of Biomechanics

Occlusal design for partially edentulous patients

Single posterior implant

Because a single posterior implant prosthesis should not influence the occlusion of the whole arch, the occlusion should conform to the existing static and dynamic lateral occlusal relationship. When designing and fabricating the single- implant crown, it is recommended that the occlusal contacts be located against flat surfaces as close to the implant screw channel as possible to direct the forces along the long axes of the implant (Fig 3-9). Tripodized contacts are not recommended because they may accentuate the lateral forces to a degree that may become clinically significant, thereby increasing the risk of mechanical or biologic complications. For mandibular crowns, their mandibular central fossae should be engaged by the maxillary lingual cusps, hence a “lingualized occlusion” (Fig 3-10). Conversely, an implant restoration in the maxillary arch should be designed with contact by the buccal cusps of the mandibular teeth in the central fossa or a “buccalized occlusion.”

Fig 3-9 (a and b) When a posterior implant crown in the mandible opposes natural dentition in the maxilla, the occlusal contacts are lingualized where the maxillary palatal cusps occlude against mandibular central fossae, as shown.

Fig 3-10 (a) Note the shallow cusp angles of the posterior implant crowns. (b and c) This is an example of the lingualized occlusal concept. There are no buccal cusp contacts either in centric occlusion or during lateral excursions.

The biomechanics of an implant prosthesis restoring posterior quadrants are generally more favorable if the occlusal table is narrow and shallow cusp angles are used (Fig 3-11). This will reduce the cantilever forces applied on the implant from functional and parafunctional activities. Further, exaggerated cusp inclinations may cause unwanted interferences during function and may expose the implants to potentially destructive lateral forces. In terms of occlusal guidance, it is more favorable if the posterior implant crown does not exhibit lateral contacts. This means relying on the remaining natural healthy teeth to control occlusal guidance, as they have a PDL and better sensory feedback to control forces. As a result, the occlusion can be maintained as mutually protected occlusion or group function occlusion with no lateral contacts on the implant crown.

Fig 3-11 The occlusal table in the molar region should be no wider than a premolar when 4- or 5-mm-diameter implants are used. (a) The first and second molars restored with implants. (b) The first molar restored with implants.

Multiple posterior implants

In general, the occlusal considerations for a posterior multiple- implant prosthesis depend on extension of the prosthesis and the existing occlusal scheme. As for the posterior single-implant prosthesis, a “lingualized” or “buccalized” type of centric contact is preferred. Combined with anterior guidance, this will centralize the posterior occlusal forces along the long axis of the implants and minimize lateral forces.

When performing an occlusal analysis for a prospective implant patient who has lost dentition in the posterior quadrants, it is essential to evaluate the existing occlusal guidance and determine whether it is desirable to change the occlusal scheme. The existence of canine guidance or anterior tooth guidance will simplify the treatment. Whenever possible, it is desirable to relieve the implant prosthesis from lateral occlusal contacts; however, this may not always be possible, especially in situations where the implant prosthesis is of large span. Likewise, patients who present with group function and show no discernible signs or symptoms of a pathologic occlusion may be best left functioning with group function, so long as the wear to the remaining dentition is minimal. The implant prosthesis should not introduce nonworking interferences, as this may stimulate parafunctional activities in some individuals.42,43 If the posterior implants are contributing to occlusal guidance, it is advised to splint the implants together44 and fit the patient with an occlusal bite plane, thereby reducing the potential detrimental effects from lateral or shear-type forces during parafunctional activities. However, every effort should be made to restore the anterior guidance.

In some clinical presentations, some authors have advocated for the use of short cantilever when restoring a posterior quadrant with multiple implants. While this option does not appear to influence the peri-implant bone level, based on relatively short-term clinical outcome studies, it is associated with a greater incidence of mechanical complications.45–48

If wear to the dentition is moderate to severe and the edentulous space constitutes the entire posterior quadrant, then restoration of the anterior guidance is desirable to reduce the lateral forces on the posterior implants. Similarly, if patients have lost occlusal vertical dimension (OVD), restoration of the OVD with implant-supported prostheses can be considered. A mutually protected occlusal scheme using anterior guidance is preferable and should be approached after a thorough evaluation and in adherence to sound prosthodontic principles49,50 (Fig 3-12). Indiscriminately opening the OVD should be performed with caution, because implants are ankylotic in nature and will not intrude into bone as do the natural teeth. Patients that exhibit severe parafunctional activities such as central nervous system bruxism, those presenting with a history of fracturing multiple teeth (especially teeth not previously restored) and/or severe wear, or those who are brachycephalic (Fig 3-13) should be approached with caution. The risk-to-benefit ratio of treatment with dental implants in these patients should be carefully considered. A diagnostic wax-up should be completed prior to initiating treatment to assess the type of restorations necessary and to determine what occlusal adjustments need to be made to the remaining posterior teeth. An occlusal splint can be used prior to treatment to establish a treatment position that is compatible with the patient’s envelope of function.

Fig 3-12 The mandibular first molar and second premolar are restored with implants. Note the anterior guidance during laterotrusion.

Fig 3-13 An example of a patient with a brachycephalic profile.

Central occlusal contact is defined as follows. The implant crowns are adjusted so that two thicknesses of shim stock (see Fig 3-20b) will pull through the occlusal surface of the implant restoration when the patient is in the closed position and while the remaining teeth hold one layer of shim stock in position. When the patient clenches, two thicknesses of shim stock should be required to hold onto the implant crown. This will allow for compression of the PDL during function and parafunctional activity and help protect the opposing teeth from trauma.

Single anterior implant

Solitary implants should not be the sole means of guidance in the esthetic zone. Guidance should be provided primarily by adjacent natural teeth (Figs 3-14 and 3-15). This is generally not a concern when restoring an individual lateral incisor or a central incisor defect, as implant crowns restoring these spaces can generally be made to restore the full esthetic contours without interfering with lateral occlusion. However, it can be problematic when restoring the canine, where the clinician may be forced to compromise the length of the implant crown in order to conform to this principle. Nevertheless, situations may arise that necessitate using the canine implant crown for guidance. In these scenarios, while the canine implant restoration can be designed to full anatomical contour, the guidance should be kept as minimal as possible and within the limits of the condylar inclination of the patient. This can be accomplished by reducing the guidance steepness of the implant crown or by adjusting the opposing guiding tooth. In addition, the authors strongly recommend the use of a conical connection implant with a placement that maintains sufficient buccal bone (at least 2.0 mm). The conical connection implant has been reported to more evenly distribute the stresses within the bone and possibly reduce the lateral forces on the retaining screw.51,52 When placing implants to restore a solitary canine, every effort should be made to angulate the implant so that the occlusal forces can be directed axially.53

Fig 3-14 An illustration of the suggested occlusal contact between an implant crown and a natural tooth in the esthetic zone: (a) Centric contact position exists on the cingulum region. (b) During the pathway of the anterior guidance, there should not be any contact. (c) Edge-to-edge contact can exist after maximal protrusion and excursion.

Fig 3-15 (a) The left canine has been restored with an implant. Note that it is shorter than the natural canine on the right. These altered contours are dictated by the occlusion. (b) Note the lack of horizontal and vertical overlap. This occlusal relationship will mitigate occlusal forces during translational movements. Guidance is provided by the adjacent teeth.

Immediate loading—Provisional single crowns in the esthetic zone

Some implants in the esthetic zone can be restored by an immediate provisional restoration when the implant is inserted within healed or partially healed alveolar bone, or as part of immediate implant placement that involves tooth extraction and implant placement. A key indication of immediate provisionalization is adequate primary stability. Studies have shown that when an implant becomes mobilized during the healing period beyond certain limits (100–150 µm), the implant fails to osseointegrate.54,55 Thus, the occlusal contacts should be controlled in a way that will not illicit implant movement. As a result, many clinicians have recommended that if an immediate provisional restoration is deemed necessary to preserve dental esthetics and peri-implant soft tissue contours, it should be adjusted short of occlusal contact in centric occlusion and during lateral excursions56 (Fig 3-16). The challenge with this technique is that the implant is still loaded if the patient is not careful and can lead to osseointegration failure; therefore, this approach is usually only used in a fully compliant patient.

Fig 3-16 A full-contour provisional restoration secured to an implant replacing the maxillary right central incisor. Notice that the prosthesis is designed to be short of occlusal contact during the healing period.

Alternatively, natural soft tissue contour can be established by customizing the healing abutment, and the edentulous space is restored by an alternative means such as bonding of a denture tooth to adjacent dentition, a partial denture, or a thermoplastic appliance. An example of such a provisional restoration is shown in Fig 3-17 (see also chapter 13). The subgingival contours of the customized abutment are developed by molding provisional material attached to a provisional abutment cylinder. This apparatus extends slightly above the gingival margin. A denture tooth of the desired shape and shade is adapted to fill the edentulous space. Care is taken to avoid contact with the peri-implant soft tissues and the customized abutment. The denture tooth is then bonded to the adjacent teeth with composite resin. This kind of provisional restoration has the advantage of preserving the gingival contour with no occlusal load transfer to the implant fixture, which makes it suitable for a patient who has no or limited compliance during the healing period.

Fig 3-17 (a) Customized healing abutment. (b) The denture tooth was shaped and bonded to the adjacent natural dentition. This design restores full esthetic contours without exposing the implants to occlusal loads during the healing period.

Multiple anterior implants

As per the single anterior implant situation, centric contacts (as defined earlier) can be achieved with a flattened cingulum region to facilitate axial loading of the implant and reduce the lateral forces. The design of the occlusal guidance is dependent upon the number of implants placed, their position in the arch, and the AP spread. In general, and consistent with our philosophy of overengineering when implants are arranged in a linear configuration or when only two implants have been placed, guidance during excursions is provided by the remaining natural dentition57 (Fig 3-18). This will reduce the lateral and oblique forces applied on the implant prosthesis.

Fig 3-18 (a) Two implants have been placed to restore the canine and the lateral incisor. (b) Guidance is provided by the natural premolars and the central incisor. (Courtesy of Dr M. Hamada.)

When three or more implants with suitable AP spread have been placed, the implants can be employed to provide anterior guidance aided by the remaining natural dentition in some instances, or unaided if appropriate57 (Fig 3-19). A mutually protected occlusal scheme utilizing canine guidance is preferable. The guidance established should be compatible with the condylar guidance of the patient. In addition, if the implant prosthesis is guiding the occlusion, splinting adjacent implants is recommended. Conical connection implants should be considered as noted earlier.51,52 A diagnostic wax-up or setup should be completed prior to initiating treatment to assess the type of restorations necessary and to determine what occlusal adjustments need to be made to the remaining anterior and/or posterior teeth.

Fig 3-19 (a and b) A sufficient number of implants were placed with appropriate AP spread to permit the implant prosthesis to be in contact during excursions.

Fitting the implant crown and occlusal adjustments

When an implant crown is fitted, the interproximal contacts should be evaluated to ensure that they do not interfere with crown seating on the implant. The quality of the interproximal contacts is evaluated with shim stock so that a double layer of shim stock should be able to pass through the contact area without tearing (Fig 3-20a). This allows for vertical movement of the adjacent natural tooth during function without being impacted by the implant restoration, thereby reducing the risk of mesiodistal movement of the natural teeth and the development of open interproximal contacts. Dental floss can also be utilized to check the interproximal contact. Some clinicians maintain that the ideal contact between the natural tooth and the implant restoration should allow passing of dental floss with some resistance. In addition, the clinician can use the dental floss through the adjacent proximal contacts of natural teeth before and after the implant crown is placed. If the proximal contact of an implant restoration is accurate, dental floss should pass through the adjacent proximal contact with the same resistance. Mesial drift may still occur, so it may be prudent to use materials in the implant prosthesis that can be easily modified (see chapter 20).

Fig 3-20 (a) Interproximal contact evaluation. The shim stock should pull through the proximal contact area after full seating of the implant crown. (b) Evaluation of occlusal contacts on the implant crowns. This thickness of shim stock pulls through the implant-crown contact areas with light resistance in a closed position and is held in the clenched position.

Next, the occlusal contacts are checked and adjusted as needed. Given the compressibility of the PDL of the adjacent natural dentition, the following sequence is recommended for occlusal adjustment (Fig 3-20b). The purpose is to ensure the implant prosthesis is not in contact with the opposing dentition during light occlusion, while occlusal contacts can exist with the opposing dentition during clenching. Preferably, there should be no contact on the implant prosthesis during excursion. Before seating the implant restoration, the occlusal contacts of the natural teeth are checked with single shim stock (8–12 µm thickness). The patient is directed to gently close on the shim stock without clenching to verify the occlusal contacts that exist between the articulating natural teeth. After seating of the implant restoration, the pattern of initial occlusal contacts on the natural teeth should be maintained. This can be ensured by adjusting the implant restoration so that two layers of shim stock can pull through the occlusal surface of the implant restoration when the patient is in the closed position and with the remaining teeth holding one layer of shim stock. When the patient clenches, two thicknesses of shim stock should be required to hold onto the implant restoration. This will allow for PDL compression during functional and parafunctional activities and help protect the opposing teeth and implants from trauma. However, if the implant restoration is restoring an extension area with no natural posterior tooth, then they must be in contact during closure to provide support for the TMJs. This will prevent overloading the anterior tooth segments and distribute occlusal contact through the dental arches. The lateral occlusal contacts are evaluated and adjusted as per earlier recommendation. Once the occlusal adjustments are complete, the prosthesis is returned to the laboratory for appropriate finishing.

Bite planes

Following the delivery of an implant prosthesis, nocturnal clenching and tooth grinding are likely to continue in patients with parafunctional activities, especially those who present with central nervous system bruxism. Protection from the excessive occlusal forces generated during sleep is best addressed with a maxillary occlusal guard13 (Fig 3-21). Impressions are made, and the casts are mounted on an articulator with a centric relation record. The occlusal guard should be configured to ensure even occlusal contact in centric occlusion with shallow anterior guidance. Some authors have recommended that the nightguard be relieved in the areas overlying the implant restorations to minimize or prevent occlusal forces from being applied to the implant and its superstructure.58

Fig 3-21 (a) Bite plane nightguard equilibrated in centric occlusion. (b) Adjusted in excursions.

Occlusal design for completely edentulous patients

Implant-assisted overdentures

The selection of the occlusal scheme for implant-assisted overdentures depends on the number and distribution of implants as well as the type of attachment. The most common form of implant-assisted prosthesis is the mandibular overdenture retained by two implants placed in the anterior portion of the mandible, so the occlusion should be controlled to reduce the lateral interferences that induce loss of retention, patient discomfort, and wear of retentive components. In addition, implant-assisted overdentures are often used to restore the maxilla and should be designed according to similar occlusal principles.15 In general, similar principles to conventional complete denture occlusion are followed. This involves establishing an occlusion according to centric relation, ensuring all teeth are in contact in centric occlusion, maintaining esthetics, allowing occlusal adjustments, and avoiding interferences in any direction.59

Bilateral balanced occlusion has long been the standard of care when designing and fabricating complete dentures.60,61 Otherwise, constant tipping of the dentures is thought to predispose to tissue destruction and ultimately bone resorption of the edentulous denture-bearing surfaces, particularly in individuals who present with significant parafunctional activities (chronic clenching and bruxing). The same principles of occlusion apply when fabricating implant-assisted overdentures. With these designs, support is shared between the implants and the denture-bearing surfaces. Although retention and stability are immensely improved, these prostheses are not entirely implant-supported and are compressed into the tissues during a forceful closure. Moreover, the additional movement triggered by overdentures that are not properly balanced will accelerate attachment wear and bone resorption of the denture-bearing surfaces.

A form of balanced articulation can be achieved with a variety of tooth forms. This is best achieved by using a semiadjustable articulator with the master casts mounted with facebow transfer and centric relation records. Once the occlusal plane and the anterior tooth relationship are determined, a protrusive record is useful to establish the condylar guidance. Alternatively, an average condylar angle value can be applied.62,63 Eventually, anatomical denture teeth are arranged consistent with the curve of Spee, the curve of Wilson, and the condylar guidance. If nonanatomical teeth are used, balancing ramps can be added to permit balanced articulation.

In general, for an implant-assisted overdenture opposing a conventional complete denture, the selection of posterior denture teeth depends on the coordination of mandibular movements of the patient, bone contours of the opposing maxilla, denture history, and jaw relation. Implant-assisted mandibular overdentures are relatively stable, as the attachments retain the denture and resist lateral displacement during function. Therefore, anatomical teeth can be successfully used in most patients with a Class I jaw relationship, especially if the maxillary bearing surfaces retaining the conventional maxillary denture are favorable.64 In these situations, the posterior teeth relationship can be achieved with the functional cusp occluding against the central fossae of the opposing teeth. Alternatively, the balanced occlusion can be modified to lingualized occlusion, where only the functional maxillary cusps occlude against the mandibular central fossae. This scheme is considered easier to achieve, is less likely to cause lateral interferences, centralizes the forces on the posterior teeth of the mandibular denture, and provides acceptable denture esthetics.59 In patients with Class II or Class III jaw relationships or patients presenting with a moderately resorbed maxilla predisposing to an unstable maxillary denture, lingualized maxillary teeth opposing nonanatomical mandibular teeth with posterior balancing ramps to maintain bilateral balanced occlusion (balanced articulation) are favored14,65 (Fig 3-22). At delivery, a new centric relation record should be made for a clinical remount, and the occlusion should be refined as necessary (Fig 3-23).

Fig 3-22 (a and b) Denture setup: Lingualized maxillary teeth opposing nonanatomical mandibular teeth with posterior balancing ramps added to permit balanced articulation. (Part a reprinted with permission from Chang et al.65)

Fig 3-23 These are anatomical posterior teeth. The dentures have been remounted using remount casts and are equilibrated in (a) centric occlusion, (b) working occlusion, and (c) balancing and protrusive movements.

Implant-supported fixed prostheses

With these prostheses, all occlusal forces are supported by the implants. The occlusal scheme of implant-supported fixed prostheses depends on the number, location, and orientation of implants; opposing dentition; prosthesis material; cantilever dimension; and presence of parafunctional activities. Because these prostheses restore the occlusion of the whole dentition, centric relation should be followed during the establishment of the occlusion. It is preferred for the opposing contacts to occur on flattened occlusal surfaces instead of inclined surfaces.

If the opposing arch is restored with a complete denture, balanced articulation is recommended.16 As per implant overdenture treatment, during the try-in appointment, the centric relation record should be verified, followed by recording the protrusive record to establish the condylar inclinations, which can then be transferred to the articulator (Fig 3-24). During the delivery appointment, a remount record is made and the occlusion is refined, first in centric and then in all lateral excursions (Fig 3-25).

Fig 3-24 (a) The protrusive record is made 4 to 6 mm anterior to centric relation. (b) The record is transferred to the articulator, and the condylar inclination (c) is adjusted accordingly.

Fig 3-25 The occlusion is refined based on a clinical remount record. This is a lingualized occlusal scheme. It is refined in (a and b) centric occlusion and in (c) excursions. In this patient, balanced articulation was used because the opposing arch was restored with an implant-assisted overdenture.

However, if the opposing arch is dentate or is restored with an implant-supported prosthesis, a mutually protected scheme of occlusion is advised (Fig 3-26). Care should be used to establish the anterior guidance as shallow as possible. Tooth contacts during excursion on the working side should not extend posteriorly into the cantilevered portion of the implant-supported prosthesis.

Fig 3-26 Fixed maxillary and mandibular prostheses. A mutually protected scheme of occlusion was used. (a) Centric occlusion. (b) Right working position. (c) Left working position. (d) Protrusive position.

Immediate load prosthesis

The authors define immediate loading as the delivery of the prosthesis onto the implants within 24 hours of implant placement. These full-arch immediate prostheses are in occlusal contact with the dentition or a prosthesis of the opposing arch. When the immediate prosthesis opposes a dentate arch or an implant-supported prosthesis during a healing period, there is no consensus as to whether the occlusion should be a mutually protected occlusion, group function occlusion, or balanced occlusion. However, clinical remounts at delivery are strongly recommended in order to harmonize the centric occlusal relationships and establish the guidance selected for the patient (Fig 3-27). Patients that present with a natural dentition in the opposing arch may be restored with a mutually protected occlusal scheme. If the immediate prosthesis is opposed by a conventional denture or an implant-assisted overdenture, the occlusion, which is based on the needs of the weakest arch, should be balanced articulation (bilateral balanced occlusion). It is recommended to reduce or even eliminate the posterior cantilever.66 Eventually, the occlusal scheme can be changed to group function or a mutually protected occlusion at a later date when the definitive prosthesis is fabricated based upon the weakest arch.

Fig 3-27 (a) The immediate prosthesis is secured in position with a stent while it is being luted to the implants. (b) The clinical remount record. The occlusion is refined both in (c) centric occlusion and in lateral excursions. (d) Completed immediate prosthesis.

Summary

Due to the significant biomechanical differences between natural teeth and implants, alternative occlusal considerations should be applied for implant prostheses. By controlling implant prosthesis occlusion, occlusal discrepancies can be avoided, and the implants are protected from excessive bending moments, which in some patients can lead to both biologic and restorative complications, failure of the restoration, or even implant failure. When restoring posterior quadrants with implant-supported fixed dental prostheses, anterior guidance is preferred. When restoring single teeth in the esthetic zone, the existing adjacent dentition should be used for guidance. When multiple implants are used to restore extended edentulous areas in the esthetic zone, guidance is dependent upon the size of the defect, the number of implants used, and their distribution pattern. Single-tooth provisional restorations placed immediately upon implant placement should minimize occlusal contacts in centric occlusion as well as excursions during the healing period. When designing occlusal schemes for implant-assisted overdentures for edentulous arches, balanced articulation is advised. When fixed restorations are fabricated, the occlusal scheme is selected based upon the nature of the opposing arch. If it is fully dentate or restored with an implant-supported prosthesis, a mutually protected occlusal scheme is recommended. If the opposing arch is restored with a conventional denture or an implant- assisted overdenture, balanced articulation is advised. When an immediate prosthesis opposes a dentate arch or an implant- supported prosthesis, there is no consensus as to whether the occlusion should be balanced articulation, group function, or mutually protected occlusion. However, clinical remounts at delivery are strongly recommended in order to harmonize the occlusal relationships.

This chapter highlighted the basic principles of implant biomechanics and their clinical applications under some common scenarios. The chapter also summarized prevalent theories behind actions (occlusion, parafunction) and reactions (design parameters) to forces, as well as time-tested clinical practices that promote long-term success. Although the mechanostat hypothesis and Skalak models have assumptions that constrain their predictive power in many clinical situations, the fundamental biomechanics are always in effect, even if biologic factors may introduce additional dominant variables. The basic biomechanics will be applied in the following clinical chapters to explore their importance in our goal to maximize long-term stability of implant-supported restorations, natural tissues and artificial materials, muscles of mastication, TMJs, and the neuromuscular control system.

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