Screw fixation is a technique that is at the heart of internal fixation using AO techniques. Screws can be used in a variety of ways to stabilize bone fragments and to secure plates to bone.

2.1.1 Drilling and tapping holes in bone

Screws are inserted into bone through drill holes. Drilling is an important process since it helps determine the nature of the bone-screw interface. Use of a hand brace or drill may cause the operator to drill oval holes due to drill bit wobble. Power drilling is recommended and combined with the use of appropriately sized drill guides, will help prevent the drilling of oval holes.

Drill hole quality is also dependent on the drill bit itself. A good sharp drill bit that is designed for use in bone should drill at a rate of 1 mm/s. Sharp drill bits are necessary for drilling in bone. Drilling rates slower than 1 mm/s may be related to a dull bit or the accumulation of swath material in the drill bit's flutes that prevent further penetration and increase heat production. Drill bits may become dull instantly when they strike a metal surface as can occur when drilling into a previously placed screw. A sharp bit can drill through more than 1 meter of bone before it becomes dull. Dull bits should be discarded. Resharpening is not recommended for large animal surgeries because the drill bit accumulates cyclic fatigue in rotational bending as it is being used. Older drill bits are therefore more easily broken than new ones.

Older drill bits break more easily due to fatigue accumulated in rotational bending.

Increased pressure on the drill bit improves efficiency, but can cause bending.

Power drilling results in a round, symmetric hole.

Use a sharp drill bit, and drill at a rate of 1mm/s.

Periodically cleaning the drill bit is important in preventing heat generation.

When a drill bit is used to drill a hole in bone the tip of the bit creates heat due to friction. High temperatures in bone (>54°C) may occur causing protein coagulation and bone necrosis. Temperature generation is inversely related to drilling rate when sharp drill bits are used. Increased pressure on the drill bit will increase the cutting rate and reduce bone heat generation but the introduction of bending to the drill bit by a surgeon pressing on the drilling machine may lead to drill bit breakage. Cooling of the drill bit is impractical since it has been shown that more than 500 ml/min of saline are necessary to adequately cool the bone [1]. Temperature control is possible however by using saline as a lubricant in the drill hole to decrease friction at the point of drilling. Much of the frictional heat generated during the drilling process is incorporated into the swath material. Periodic removal of this bony material from the flutes of the drill bit during the drilling cycle will decrease heat buildup and allow for further swath material accumulation. Impaction of swath material in the drill bit's flutes will decrease cutting rates of the drill bit since there will be nowhere for the cut bone to go. Saline should be supplied as a lubricant to the drill point at the time of drilling and can be placed into the drill hole when the bit is removed periodically for cleaning. Drill bits are designed to circulate fluid for lubrication. The fluid descends via the lands and ascends with the swath material (Fig. F2A).

Pretapping insures a good interface between bone and screw.

Pretapping of drill holes prior to screw insertion insures a good interface between the bone and screw. It also permits the screw to be inserted with less torque. Using saline for lubrication increases the ease of hole tapping and screw insertion as well. Screws without tapping flutes on their tip can easily be removed and reinserted during surgery without danger of cross threading the hole in the bone. Special care must be taken when using self-tapping screws in this regard. In equine bone self-tapping screws may not work well because the flutes of the tap may fill up before the entire cortex is penetrated, leading to imperfectly threaded holes and heat generation.

Fig. F2A: Lubricating fluid circulates along a drill bit by descending behind the lands and ascending with the swath material.

Avoid the use of self-tapping screws in thick cortices.

2.2 Screw types

There are two basic types of screws. The cortex screw is fully threaded, has relatively fine threads and is used in cortical bone and dense cancellous bone. It is the screw that is used most commonly in equine orthopedics. The cancellous bone screw is partially threaded and has a larger coarser thread. It is used in soft cancellous bone and may be used as a substitute for a cortex screw if the cortex screw has stripped the threads in the bone during insertion. Both screws are available in a wide variety of sizes and special large 5.5 mm cortex screws have been developed with the horse in mind.

2.2.1 Cortex screws

Fig. F2B: A fully threaded cortex screw is made to act in lag fashion by overdrilling the cis cortex and cutting threads only in the trans cortex.

A cortex screw is fully threaded and can be used as a fixation position screw with threads holding in both cortices to attach a plate to a bone. It can also serve as an interfragmentary screw that compresses two fragments together by drilling a glide hole in the near cortex (cis cortex) while providing a threaded hole in the far cortex (trans cortex) (Fig. F2B). Cortex screws are available in a large range of sizes with the 3.5 mm, 4.5 mm, and 5.5 mm diameters used most commonly in the horse. Testing of the various screw sizes has been carried out and the results show that in general screw strength is related to its core diameter [2]. Therefore, increased strength comes with larger screw diameters. When soft bone is encountered, the large-diameter screw threads hold better. In general, equine bone is so dense that cancellous bone screws are rarely needed. Therefore when dealing with a stripped screw hole it would be better to replace the stripped screw with a cortex screw of a larger diameter rather than substituting it with a cancellous bone screw.

Cortex screws used as lag screws require a glide hole.

Larger-diameter screws provide better holding power in soft bone.

To use a cortex screw as a lag screw it is necessary to use a large drill bit, equivalent to the outside diameter of the screw thread, to drill the glide hole through the cis cortex while a smaller drill bit, equivalent to the approximate core diameter of the screw, is needed to drill the smaller threaded hole into the trans cortex. For each cortex screw size, drill bits of the proper diameter are available to drill both the glide and threaded holes; see table (Fig. T2A).

Fig. T2A

The techniques for insertion of lag screws are shown in Video P1LAGSCR and in the animation Video DBASICS. After reduction of the fragments using bone clamps, K-wires, or some other device, a large glide hole is drilled through the cis cortex using a drill guide to prevent drill bit wobble and to protect the overlying soft tissues. This hole must be drilled across the fracture plane which may include cancellous bone as well as cortical bone, especially near the metaphyses. A drill insert is then placed into the glide hole and pushed across the fracture plane. This guide will center the thread hole precisely. The thread hole is drilled through the trans cortex using this drill guide along with the appropriate diameter drill bit. The drill bit and drill guide are then removed and the hole is countersunk to provide a seat for the head of the screw. It is important to turn the countersink tool a full 360° in a clockwise direction to avoid any ridges that would be created by simply oscillating the instrument back and forth. The countersink depression should only be deep enough to support the head of the screw and prevent its bending during tightening. Next the hole is measured with the depth gauge to determine the proper screw length; then it is tapped. Tapping of the thread hole is accomplished by inserting the tap through the tap sleeve into the glide hole and cutting threads in the trans cortex. The tap should be advanced into the bone by using two half turns forward and then one quarter turn back to keep the bone cuttings clear of the cutting edges. If the tap starts to bind, cutting should stop so that the tap can be removed and cleaned before continuing. The tap must be maintained free of all swath material in its longitudinal grooves in order to cut satisfactorily. Finally, a screw of the proper diameter and length is chosen and inserted using the screwdriver. As mentioned above, saline used as a lubricant will decrease the torque occasioned by tapping and screw insertion.

The glide hole should extend just beyond the fracture plane.

Rotate the countersink through a full 360°.

Power tapping is convenient, but can quickly lead to breakage or stripping.

Video P1LAGSCR: Techniques for insertion of lag screws.

Video DBASICS: Introduction to drill basics.

Although power drilling is recommended for drilling holes in bone, power tapping is reserved for applications where many screws are to be used in an internal fixation, as with a long plate or a double plating procedure. Special attention should be paid to directing the tap in the same plane as the drill hole. If the tap misses the hole in the trans cortex it may jam or break or the cis cortex threads may be stripped. When inexperienced operators begin to use power equipment for tapping bone, the torque of the machine may be lowered by decreasing the air pressure. This will help preserve instruments as well as the newly cut threads in the cis cortex. Power tapping should not be used when only a few screws are to be used since each hole is so important for fixation that stripping one cortex may be a reason for fixation failure. Insertion of screws using power equipment will decrease the time of a surgical procedure as well as the fatigue of the operator. All screws should be tightened by hand following power insertion.

2.2.2 Cancellous bone screws

Cancellous bone screws are available in 6.5 mm and 4.0 mm diameters. The 6.5 mm screw has three different thread lengths—16 mm, 32 mm and fully threaded (Fig. F2D).

F2D: Cancellous bone screws are available as fully threaded or in two different thread lengths: 16 mm & 32 mm.

Only the partially threaded cancellous bone screws will produce interfragmentary compression at the fracture site using a single-sized drill bit. If interfragmentary compression is desired then it is important to choose the proper length thread combination so that the threads are located only within the trans cortex/ cancellous fragment (Fig. F2E). Cancellous bone screws should only be used in soft cancellous bone since it may be impossible to remove them from hard cortical bone without breaking them.

Fig. F2E: When inserting a cancellous bone screw, the threads must only engage the trans cortex if it is to act as a lag screw.

Fig. F2F: In this figure, two cancellous bone screws inserted in lag fashion are illustrated.

Only the partially threaded cancellous bone screws will produce interfragmentary compression at the fracture site using a single-sized drill bit.

Cancellous bone screws should only be used in soft cancellous bone .

To insert a large cancellous bone screw a 3.2 mm or 3.6 mm hole (hard cancellous bone) is drilled across the entire bone. A 6.5 mm tap is then used to tap the thread into the bone and measure the length of the screw to be used. The tap does not cut the entire 6.5 mm thread, leaving some uncut bone to be cut by the screw itself at the time of insertion; therefore, the torque necessary to insert a cancellous bone screw will be greater than that of a cortex screw and much greater in hard bone than in soft bone (Fig. F2F).

Screws should be placed perpendicular to the long axis of the bone if weight bearing loads are to be expected.

2.3 Screw position

Screws are designed to provide purchase in bone that will be advantageous in fracture fixation. They are designed to be loaded in tension and not in bending or shear. Interfragmentary compression is greatest when the forces on the surfaces of the fragments are normal (perpendicular to the fracture plane). To accomplish this, the screw must be placed perpendicular to the fracture planes in all directions (Fig. F2G). This means that when a fracture spirals, the screws used to fix this fracture must spiral as well. The loads experienced by the reduced and stabilized fracture will be those imposed by the screws and by the loads associated with use (i.e., postoperative weight bearing). Vector analyses will show that the loads of weight bearing change the resultant forces (loads) through the bone so that simple fixations become far more complex systems when subjected to weight bearing (Fig. F2H). As a rule of thumb, screw placement should be perpendicular to the long axis of the bone if weight bearing loads are to be expected. Screws placed perpendicular to the fracture plane will be subjected to shear forces during weight bearing. A decision on screw placement may represent a compromise if the screw itself will induce large shear forces independent of weight bearing forces. In these cases, the screw should be placed at an angle between perpendiculars drawn to the fracture plane and to the along axis of the bone. In all instances, the screw placement should spiral in the longitudinal plane in concert with the fracture (Fig. F2G).

Normal forces on the fragment surfaces create the greatest compression.

Fig. F2G: To efficiently create interfragmentary compression, the screws must be placed perpendicular to the fracture plane in all three dimensions.

Fig. F2H: Weight bearing loads can result in shear forces at the fracture site and displacement of the fragments if the screws are inserted perpendicular to the fracture plane.

2.4 Plate fixation

A variety of plates are available for use in internal fixation. Recent introductions of new plate designs with new materials have been utilized in many animal species including humans; the most commonly used plate in equine orthopedics, however, continues to be the dynamic compression plate (DCP) fabricated in stainless steel. This is available in two cross-sectional dimensions, the narrow and broad configuration. The narrow DCP has the screws placed in a straight line while the broad plate has the screws placed in a staggered configuration. The sectional properties of the plates, especially around the screw holes, determine their strength and fatigue resistance. The larger cross-sectional dimensions of the broad plate make it the choice for most applications. The dynamic condylar screw (DCS)- and corresponding dynamic hip screw (DHS) plates have correspondingly larger cross-sectional dimensions and would therefore be stronger than the broad DCP. They are only manufactured, however, with the large sliding screw at one end which limits their application in the horse. Choosing the proper length plate for use in any specific situation may represent a dilemma based on soft tissue viability, surgical approach, and configuration of the fracture. While four cortex screws are recommended as a minimum on each side of the fracture with each screw threaded into both cortices (eight cortices) it is best to plate most equine long bone fractures from end to end. Two or more plates are often used in the repair of fractures in the horse. These plates should be placed at right angles to each other to optimize the inertial properties of the fixation. Specific recommendations regarding plate size and number are addressed with individual fractures and arthrodeses as they are presented in later chapters.

DCP = Dynamic compression plate

DCS = Dynamic condylar screw plate

DHS = Dynamic hip screw plate

Self-compressing DCP:

1 Drill 3.2 mm hole 1 cm from fracture site
2 Measure depth
3 Tapping with 4.5 mm
4 Insert first screw loosely, displace plate toward fracture
5 Drill 3.2 mm hole on opposite side using yellow load guide
6 Measure depth
7 Tapping with 4.5 mm
8 Insert second screw
9 Tighten first screw 10. Apply other screws

Drill the first hole approximately 1 cm from the fracture plane.

2.4.1 Plate application

The technique for application of a plate will be described for the dynamic compression plate (DCP) as a self-compressing plate and with the tension device as used in certain circumstances, such as with a dorsal plate arthrodesis of the metacarpal phalangeal joint.

2.4.2 Self-compressing DCP

Video SCDCP

Video SCDCP: Animation about self-compressing DCP.

Two or more plates are often used. They should be placed at right angles to each other to optimize the inertial properties of the fixation.

Following contouring of the plate to the bone surface, a 3.2 mm hole is drilled through the bone approximately 1 cm from the fracture surface. The plate is placed over this hole, and the depth gauge is used to determine the length of the screw taking into account the thickness of the plate and the diameter of the bone. The hole is then tapped with the 4.5 mm tap and the correct length 4.5 mm screw is chosen and inserted. This screw is not tightened at this time but only inserted until the screw makes contact with the slotted hole in the plate. The fracture is reduced and stabilized with a bone-holding forceps or other means (lag screw, K-wires, etc.) and the plate is aligned with the long axis of the bone. The plate is slid toward the fracture line, from the side stabilized with the screw, until the screw engages the end of the oval hole. A second hole is drilled through the plate hole nearest the fracture in the other fragment using the yellow load guide in its proper position. the load guide has its hole placed off center, and the arrow on this guide should point toward the fracture line which positions the screw 1.0 mm up the inclined plane of the oval DCP screw hole. The position of this guide is very important since the insertion of the screw in this second hole will, upon screw tightening, move the plate over the bone surface and pull together the fractured ends of the bone in compression. Following drilling of this hole, it is measured, and tapped and the proper length screw is inserted. As this screw starts to engage the oval hole it should be tightened, alternating with the first. In this way both screw heads are drawn down equally. Any screw left with its head high on the inclined plane will be subjected to bending loads and may fail. The remaining holes on both sides of the plate should be drilled using the green neutral drill guide. This guide has the hole centered in the guide and its use will result in a hole that is 0.1 mm up the inclined plane of the oval hole. Therefore the neutral drill guide will still position the screw so as to exert a slight compressive effect. Overuse of the load guide can place all screws on the inclined plane of the DCP hole and expose them to destructive bending forces. Once the fracture fragments are in contact the neutral guide should be used for all other screws. The load guide can theoretically be used three times on each side of the fractures so the total distance that the fractured ends of the bone can be moved is 6.0 mm before the screw heads come to lie at the end of the oval holes. In this case it is necessary to loosen the screws previously placed prior to tightening any subsequent ones, since the tightened screw will make further movement of the bone fragments impossible. Obviously, it is preferable to achieve better reduction before plating, and not to use the definitive implant(s) for this purpose. Once the fixation is complete the screws should be checked for tightness from the center outward, since any change in tightness in one screw may shift the plate slightly and leave other screws loose. This tightening procedure should be repeated several times until all the screws are tight.

Slide the plate toward the fracture.

Reduction should be almost perfect before plate application.

If great force was necessary to bring the fracture fragments together the central two screws may be exchanged for new ones since the heads of these screws may have been weakened by bending during insertion.

2.4.3 Tension device with DCP

Video TDDCP

There are times when it is necessary or desirable to move the fragment ends more than the 6.0 mm allowed by the use of the DCP when used as a self-compressing plate. In these circumstances the tension device should be used.

Following contouring of the plate a 3.2 mm hole is drilled approximately 1 cm from the fracture. The plate is applied over this hole and the measuring device is used to select a screw of the proper length. The hole is tapped and the screw is inserted but not completely tightened. The fracture is reduced and held with a boneholding forceps, and the plate is aligned with the long axis of the bone. The guide for the tension device is placed in the last screw hole and a 3.2 mm hole is drilled in just one cortex. The hole is tapped and a short screw is placed through the tension device after it has been extended and hooked into the last hole in the plate. The tension device is tightened slightly to align the plate, and the first screw that was placed into the plate is tightened. At this time all the screws (at least four) should be added in the fracture fragment containing the first screw. This is accomplished by drilling with the neutral drill guide, measuring, and tapping as previously described. All the screws in this fragment are inserted and tightened. The tension device is now tightened using the socket wrench. A pin wrench is also available and may provide additional load on the fracture fragments. The holes on this side of the fracture are now drilled using the neutral drill guide. They are measured, and tapped and their screws are inserted and tightened. The tension device is then loosened, and removed, and the final screws are inserted into the plate after proper drilling, measuring, and tapping.

Video TDDCP: Animation about tension device DCP.

Use of the tension device was the standard method of plate application when round hole plates were used. These plates are rarely used today but may be applied as described above.

Use the tension device to move the fragment ends more than 6 mm.

2.5 Mechanics of plate fixation

Success with internal fixation using plates and screws comes with good technical ability and an understanding of the mechanics of plate fixation. Plates used for internal fixation are strongest in tension and compression. They are weakest in bending. They are also weak in torsion, but this is a result of the screws that fasten the plate to the bone. Therefore, plates should be applied to bones so that tensile forces are applied and bending forces minimized. To accomplish this, the plates should be applied to the so called “tension side” of the bone. This is the surface that in vivo weight bearing and theoretical studies have shown to be subject to mainly tensile forces. When the plate is applied to compress the bone ends together the plate is already placed under tension. The loads of weight bearing will increase the tension in the plate and therefore the compression in the bone. Since bones have a tension surface they must also have a compression surface. This means that bones bend and the bending force can be converted to a tensile force in the plate if the cortex opposite the plate is intact. If a gap is present due to comminution of bone then an unstable situation may occur. If the comminution is bridged by the plate then it may be stable but if the comminution is at the cortex opposite the plate (trans cortex), then the implant may be subjected to cyclic bending rather than tension, and fatigue failure may result (Fig. F2J). Some situations dictate that the plate be applied in a less than optimal location. This may occur based on soft tissue coverage, vascularity of the skin, and the shape and extent of the fracture or loss of bone stock. Even when implants are applied under the best of conditions, failures may occur. Certain techniques can be used to optimize internal fixations. These are enumerated in the following sections of this chapter and demonstrated on the videotapes corresponding to the individual fractures.

A plate should be contoured to the exact shape of the bone to which it is to be affixed.

Bones bend and the bending force can be converted to a tensile force in the plate.

2.5.1 Contouring and prebending

A plate should be contoured to the exact shape of the bone to which it is to be affixed. The importance and difficulty of this step cannot be overstated. Some of the shortcomings of inadequate plate contouring can be overcome by so-called luting (see below). Bending and twisting the plate may be necessary to make it conform. In general, plates should be bent in one direction and not back and forth since such cycling weakens them. A single bend may actually work-harden the material and does not affect the overall strength of the implant. A bending press, pliers, and bending irons are available to accomplish this task.

Avoid cycling a plate during contouring.

There is a 1.5–2 mm gap beneath a properly prebent plate.

Fig. F2J: a) Load transmission across the fracture line after anatomic reduction with resultant stability. b) Any inability to transmit loads across the fracture site will result in implant deformation. c) Persistent cyclic deformation will result in implant failure.

The work of Askew and others has shown that when a straight plate is applied to a straight bone and placed under tension (compression in the bone) a gap will form on the side opposite the plate (trans cortex) [3]. Thus, only a small area (1/5) of the bone will be in contact with resulting large stresses in the bone and implants. This phenomenon has been known for years, and the histology of Schenk showed the result of contact and gap healing using this model [4]. If bending is superimposed on the gap, stress concentrations develop that could induce fatigue failure of the implant. This problem can be addressed by using a technique known as “pre-bending.” Prebending involves making a small kink in the plate over the area of the fracture (Fig. F2K). This is accomplished after the plate has already been contoured to the bone. The prebending is done in the bending press to form a gap between the bone and the plate of 1.5–2.0 mm. When the plate is then attached to the bone the kink will be elastically straightened to allow contact between the plate and the bone, but the plate will be applying compression to the cortex opposite the plate\ (Fig. F2L). This technique can only be used when cortex contact is made between the fragment ends on the side opposite the plate. Insertion of all plate screws results in compression of the entire bone circumference (Fig. F2M). If a defect were present, plate prebending would cause malalignment. A lag screw can be applied across the fracture to augment the prebending technique (Fig. F2N). This is even possible in the fractures of very large bones as seen in the horse. Combining prebending the plate with lag screw compression will provide the best conditions for stability at the fracture site, regardless of subsequent loading characteristics.

Do not prebend the plate if there is a defect in the opposite cortex.

Fig. F2K: After contouring, a small ”kink“ or ”tent“ is put in the plate at the fracture site. It should separate the plate from the bone surface by ± 2 mm.

Fig. F2L: As the screws are tightened the plate is elastically straightened. As it tries to return to its prebent shape, it exerts compressive forces upon the fracture in the trans cortex.

Fig. F2M: Insertion of all plate screws results in compression of the entire bone circumference.

Fig. F2N: A lag screw can be combined with prebending the plate to maximize compression and stability.

2.5.2 Plate luting

The concept of placing a plate on the bone is similar to that of using a lag screw to attach two bone fragments. The plate is lagged to the bone just as two bone fragments are lagged to each other. Friction prevents the bone and plate from moving in relation to each other [5]. The screws are used to create a frictional force which amounts to 37% of the axial force generated by the screw/plate combination. It follows that a greater number of screws will provide a greater bone/plate frictional force. This in turn will allow larger weight bearing loads before shifting between the bone and the plate occurs. Since screws are strongest in tension and weak in bending and shear it is important to optimize bone/plate friction to minimize these damaging loading patterns. Contouring of the plate is a key factor in increasing bone/plate contact but the radius of curvature of the plate may still differ considerably from that of the bone to which it is to be secured.

This mismatch may result in a single line of contact between the bone and plate in the longitudinal plane or point contact in the transverse plane. The inappropriate contouring of the plate in the longitudinal plane may be further complicated by the spiraling and uneven nature of the bone.

Plate luting describes a technique that serves to optimize contact between bone and plate [6]. Polymethylmethacrylate (PMMA) is used as the interface between the bone and plate and between the screw heads and plate. The material acts to improve the contact area between bone and plate as well as between the screw head and the plate. This decreases the bending and shearing effects of weight bearing on the screw heads that occupy the oval holes of the DCP. In vitro mechanical tests showed that the cyclic fatigue life of bone-plate composites exposed to bending forces increased three to twelve-fold when plate luting was used. In vivo experiments and clinical experience have confirmed this advantage.

Plate luting begins with the completion of a normal internal fixation (see above). The screws are loosened to produce a ± 2 mm gap under the plate. When two plates are used, each is luted separately while the other provides stability. Surgical grade PMMA is mixed into a doughlike consistency and the material is pressed under the plate with the fingers. The screws are retightened and excess PMMA is removed as it is extruded from under the plate and around the screw holes. It is important that no PMMA penetrates between the fragment ends since this would inhibit healing.

Screws are strongest in tension and weak in bending and shear.

Plate luting optimizes the contact between the bone and the elements of the fixation.

2.6 Cancellous bone grafting

The use of a bone graft will be discussed here only in its relation to the mechanics of plate fixation. The use of axial compression in fracture fixation is only helpful if there is intact bone stock that will result in a stable situation under pressure. Many equine fractures are comminuted with oblique cracks and unstable segments. Where possible, interfragmentary compression using screws incorporated into the plate fixation will be helpful. There are times, however, when the fragments are too small to be stabilized and may have lost their blood supply. In these cases a gap is produced that can lead to stress concentration in the plate. Paradoxically, small gaps are potentially more devastating than large ones since they will cause greater concentrations of stress in the plate. Most surgeons will not hesitate to use a bone graft if there is a large defect, but many will neglect its use for ostensibly insignificant cracks or gaps. A bone graft will act as a portable callus or bridge, and the structural strength of the graft can be expected to increase rapidly after the first 10 days. Often the mechanical advantage contributed by a bone graft makes the difference between healing the fracture and premature breakage of the implants. If the need for a bone graft is ever questioned, the answer is... …use one!

Bone grafts contribute to structural strength after 10 days.

2.7 Cerclage wire

Wire fixation is used in both cerclage and tension band modes. Tension band wiring is perhaps best illustrated by the repair of olecranon fractures in young animals (chapter 16, Ulna (olecranon): tension band wiring). It can be accomplished with wire alone or with wires and pins to limit rotation. When pins are used they should be placed in pairs to prevent rotation. The wire should encircle both pins and should be tightened on both sides of the fixation. Single pins or screws should not be used. Screws may prevent the wire from compressing the fractured fragments and will often bend or break at the thread junction nearest the fracture site. Cerclage wires can be combined with tension band wires, or be used by themselves as in sesamoid fractures (chapter 9, Proximal sesamoids: tension band wiring). Wires and screws can be used successfully to retard growth across a physis and are often used in this way to correct angular limb deformities (chapter 25, Carpal and tarsal deviations).

2.8 References

1. Matthews LS, Hirsh C (1972) Temperatures measured in human cortical bone when drilling. J Bone Joint Surg [Am]; 54:297.

2. Perren SM (1976) Force measurements in screw fixation. J Biomech; 9:669–675.

3. Askew MJ, Mow VC (1975) Analysis of the intraosseous stress field due to compression plating. J Biomech; 8:203.

4. Schenk RK (1964) Zur Histologie der primären Knochenheilung. Langenbecks Arch Klin Chir; 308:440.

5. Hayes WC, Perren SM (1972) Plate-bone friction in the compression fixation of fractures. Clin Orthop; 89:236–240.

6. Nunamaker DM, Richardson DW, Butterweck DM (1991) Mechanical and biological effects of plate luting. J Orthop Trauma; 5:138–145.

2.8.1 Online references

See online references on the PEOS internet home page for this chapter: http://www.aopublishing.org/PEOS/02.htm

Principles of Equine Osteosynthesis: Book & CD-ROM

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