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
Gustave E. Fackelman
3.1 The day before surgery
The patient should arrive safely at your facility having been given appropriate first aid and having been carefully transported, the details of which are described elsewhere [1]. The following discussion is intended as a checklist of activities to promote ideal operative conditions and enhance the probability of a satisfactory outcome.
Completeness of biographical data should be carefully checked. The patient's name, age, breed, previous surgical/anesthetic history, known drug sensitivities, the dates of injury and of arrival, along with the owners name, address and telephone number would be the minimum. Gathering this data, as well as recording much of which is described below can be facilitated by the use of an appropriate computerized format such as the AO Equine Fracture Documentation System [2].
The owner should be made aware of the alternatives to surgery, the risks involved in any operative procedure, and any dangers that are specific to the intended surgery in this particular horse. A clear description of these risks should be part of a written document that details the operation and the recommended aftercare. This document is countersigned by the owner as having been read and understood. On the same page, the owner is provided with an enumeration of the costs
