FRANK J. MILNE STATE-OF-THE-ART LECTURE


screws are tightened, the plate is compressed to the surface of the bone to create friction between the underside of the plate and the bone. If the strength of the plate and screws balance with the integrity of the bone AND, most importantly, the forces on the entire construct are modest enough, the bone frag- ments remain in apposition well enough to allow healing in the desired anatomic reduction. Many horses have been saved with this traditional ap- proach to bone plating but many attempts have failed. Horses, even foals, apply enormous loads to the repaired fracture site and if those forces of weight bearing overcome the frictional forces of the plated bone, some degree of instability will ensue. Although rigid stability is NOT necessary (or truly even desirable) for the biology of fracture healing, stability IS required for our patients to stand and walk comfortably. Normal fracture healing is clearly inferior when there is too much rigidity so in human patients there has been a burgeoning em- phasis on LESS stable fixations as well as manage- ment techniques that allow alteration in deformation (strain) at the fracture site as it heals. We do not have that luxury in horses because we have so few tools to control their use of the fractured limb. Balancing biology and mechanics becomes absolutely essential to our equine patients but our solutions must be very different. Locking bone plates are mechanically different.


Fig. 6. A, This is a common complex configuration seen in com- minuted middle phalanx fractures involving both interphalan- geal joint surfaces. B, Even with an aggressive open approach that fully exposes the proximal surface of the fracture, it is impossible to have the understanding of the fracture afforded by a CT. C, Excellent accuracy of the implants is possible because the surgeon can plan exactly where screws should be placed. The planned glide holes are in red and the thread holes in yellow. D, Post-operative radiographs were taken through the cast but the stability is such that the cast can be removed relatively quickly, 4 weeks in this case. E, Three-month post- operative radiographs. The fracture and the fusion are well advanced.


The plate’s function is not dependent upon its com- pression to the bone surface. The head of the screw has threads that lock into the plate itself. This results in a rigid angle between the plate and the screw and that in turn results in more stability of the fracture fragments. The mechanical function of an external fixator or transfixation pin-cast is in- stantly recognizable; the transverse pin engages the bone and is securely fixed to a rigid element on the outside of the limb (side-bars or a cast). Weight bearing forces are then transferred to the pins and external supports. The locking plate is essentially functioning as an internal ex-fix. The locked screws are like half-pins and the plate is like the side-bar. The advantages, of course, are that ev- erything is under the skin, theoretically sterile and very close to the surface of the bone. The plate (“side-bar”) being close to the bone enormously en- hances the strength of the construct. The overall advantages of locking plates have caused most sur- geons to much prefer their use for the great majority of fractures.5–12 Because the locking plate does not have to be in


surgeons to achieve a better harmony of mechanics and biology. The original concept of bone plating is intuitive.


A surgeon takes a strong, but contourable, piece of biocompatible metal, puts some holes in a flattened piece of it, and attaches it to the outside of the bone with screws made of the same metal. When the


94 2019  Vol. 65  AAEP PROCEEDINGS


perfect apposition to the bone surface, it allows an entirely different approach to certain fractures. Plates can be placed in a more biologically friendly manner (i.e., with minimal disruption of the fracture or arthrodesis site). An extraperiosteal “tunnel” can be made with a simple device (Fig. 7, E–J, Fig. 8) and the locking plate slid into place under the skin. The screws are then inserted through stab incisions. This technique is absolutely NOT feasible in many


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