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