Читать книгу Fundamentals of Fixed Prosthodontics - James C. Kessler - Страница 44

An articulator is a mechanical device that simulates the movements of the mandible (Fig 3-1). The principle employed in the use of articulators is the mechanical replication of the paths of movement of the posterior determinants, the temporomandibular joints (TMJs), and, in some cases, the anterior guidance. The instrument is then used in the fabrication of fixed and removable dental restorations that are in harmony with those movements. The articulator is a tool, and, as with all tools, its value to the dentist is determined by its appropriate use.

The outer limits of all excursive movements made by the mandible are referred to as border movements. All functional movements of the mandible are confined to the three-dimensional envelope of movement contained within these borders.¹ The border movements are of significance in discussing articulation because they are limited by ligaments. As such, they are highly repeatable and useful in setting the various adjustments on the mechanical fossae of an articulator. The more nearly the articulator duplicates the border movements, the more nearly it will simulate the posterior determinants of occlusion. As a result, the harmony between the fabricated restoration and the posterior determinants (ie, the TMJs) will be improved.

Articulators vary widely in the accuracy with which they reproduce the movements of the mandible. At the lower end of the scale is the nonadjustable articulator. It is usually a small instrument that is capable of only a hinge opening. The distance between the teeth and the axis of rotation on the small instrument is considerably shorter than it is in the skull, with a resultant loss of accuracy.

As the mandible moves up and down in the retruded position, the cusp tip of a mandibular tooth moves along an arc in a sagittal plane, with the center for that rotation located at the transverse horizontal axis (THA), which passes through the condyles (Fig 3-2). If the location of the axis of rotation relative to the cusp tip differs markedly from the patient to the articulator, the radius of the arc of closure of the cusp tip may be different, producing an error. Drastic differences between the radius of closure on the articulator and in the patient’s mouth can affect the placement of morphologic features such as cusps, ridges, and grooves on the occlusal surface.

The casts mounted on a smaller articulator will have a much shorter radius of movement, and a tooth will travel a steeper arc during closure of the small articulator (Fig 3-3). If the casts are mounted at an increased dimension of occlusion (ie, with a thick interocclusal record), the teeth will occlude in a different intercuspal position on the articulator than in the mouth.² A slight positive error resulting in a deflective occlusal contact could develop between the mesial incline of the maxillary teeth and the distal incline of the mandibular teeth.³

The mediolateral location of the centers of rotation (ie, the intercondylar distance) will change the radius of tooth movement, which in turn will affect the arc traveled by a tooth cusp in the horizontal plane during a lateral excursion of the mandible. On a small hinge articulator, the discrepancy between the arcs traveled by a cusp on the instrument and in the mouth can be sizable, particularly on the nonworking side (Fig 3-4). The result is an increased possibility of incorporating a nonworking occlusal interference into the restoration.

A semi-adjustable articulator is an instrument whose larger size allows a close approximation of the anatomical distance between the axis of rotation and the teeth. If casts are mounted with a facebow transfer using no more than an approximate THA, the radius of movement produced on the articulator will reproduce the tooth closure arc with relative accuracy, and any resulting error will be slight (Fig 3-5). Placing the casts a small distance closer to or farther from the condyles through the use of an approximate THA will produce an error of only a small magnitude during lateral excursions⁴ (Fig 3-6).

The semi-adjustable articulator reproduces the direction and endpoint but not the intermediate track of some condylar movements. As an example, the inclination of the condylar path is reproduced as a straight line on many articulators, when in fact it usually traverses a curved path. On many instruments, the lateral translation, or Bennett movement, is reproduced as a gradually deviating straight line, although several recently introduced semi-adjustable articulators do accommodate the immediate lateral translation.

Intercondylar distances are not totally adjustable on semi-adjustable articulators. They can be adjusted to small, medium, and large configurations, if at all. Restorations will require some intraoral adjustment, but it should be inconsequential if the restoration is fabricated carefully on accurately mounted casts. This type of articulator can be used for the fabrication of most single units and fixed partial dentures.

Fig 3-1 The articulator should simulate the movements of the mandible.

Fig 3-2 As the mandible closes around the hinge axis (mha), the cusp tip of each mandibular tooth moves along an arc. (Reprinted from Hobo et al² with permission.)

Fig 3-3 The large dissimilarity between the hinge axis of the small articulator (aha) and the hinge axis of the mandible (mha) will produce a large discrepancy between the arcs of closure of the articulator (dotted line) and of the mandible (solid line). (Reprinted from Hobo et al² with permission.)

Fig 3-4 A major discrepancy exists between the nonworking cusp path on the small articulator (a) and that in the mouth (m). (Reprinted from Hobo et al² with permission.)

The most accurate instrument is the fully adjustable articulator. It is designed to reproduce the entire character of border movements, including immediate and progressive lateral translation, and the curvature and direction of the condylar inclination. Intercondylar distance is completely adjustable. When a kinematically located hinge axis and an accurate recording of mandibular movement are employed, a highly accurate reproduction of the mandibular movement can be achieved.

Fig 3-5 The dissimilarity between the hinge axis of the full-size semi-adjustable articulator (aha) and the mandibular hinge axis (mha) will cause a slight discrepancy between the arcs of closure of the articulator (dotted line) and of the mandible (solid line). (Reprinted from Hobo et al² with permission.)

Fig 3-6 There is only a slight difference between cusp paths on a full-size articulator (a) and those in the mouth (m), even though the cast mounting exhibits a slight discrepancy. (Reprinted from Hobo et al² with permission.)

This type of instrument is expensive. The techniques required for its use demand a high degree of skill and are time-consuming to accomplish. For this reason, fully adjustable articulators are used primarily for extensive treatment requiring the reconstruction of an entire occlusion.

Fig 3-7 The angle between the condylar inclination and the occlusal plane of the maxillary teeth remains the same in an open (a) and a closed (b) arcon articulator (a₁ = a₂). However, the angle changes in an open (c) and a closed (d) nonarcon instrument (a₃ ≠ a₄). For the amount of opening illustrated, there would be a difference of 8 degrees between the condylar inclination at an open position (where the articulator settings are adjusted) and the closed position at which the articulator is used.