Metallic implants placed in the body should be corrosion-resistant, biocompatible, and must have adequate strength to withstand
functional stress. Corrosion of metal occurs because of the electrochemical ions in body fluid. Most metal implants currently
used in veterinary surgery are of 316L stainless steel. This is a low carbon steel with chromium nickel and molybdenum.
Surfaces of some implants are treated with a corrosion resistant film. Metallic implants include pins, screws, plates, wire,
and some prosthetic devices. There are others including Titanium alloys which are lighter and stronger than stainless steel
but more expensive, and Cobalt-chromium alloys which are used in prosthetic devices like artificial hips. Furthermore, bone
cement (methylmethacrylate) can be used to form around pre-placed screws where contouring of metal plates is difficult, e.g.
spine and acetabulum but should not be used for long bone fractures. There are also a variety of polymers used in joint replacements.
It is an easy concept to understand that all implants used in fracture repairs will fail at some point if the fracture does
not heal. This failure may occur in weeks or it may take months to years. One of the most common methods of failure is fatigue
failure of the metallic implant. Almost all implant failures are due to improper utilization, not a defective implant! Almost all implant failures occur at a point where there is an abrupt change in implant shape e.g. hole in the bone plate
or diameter-shape change in an intramedullary device (pin). These points are known as "stress risers," and the surgeon should
avoid putting them directly adjacent to the fracture site.
Common types of Implants
Intramedullary pins are the most commonly used fixation device in general veterinary practice today. Unfortunately they are
often overused on the premise that some fixation is better than no fixation. We will discuss indications for their use and
the common ancillary fixation devices used with IM pins.
Solid cylindrical stainless steel rods ⅛" to ¼" diameter, and 9-12" in length, available in three points; trocar, chisel,
and thread-trocar. The trocar point is most commonly used because it is readily inserted in cortical bone. Threaded-trocar
pins were originally designed to have improved holding power, but they have not been proven to hold better in the intramedullary
canal than smooth pins, and have the added disadvantage that they break at the junction between the shank and the last thread.
1. Steinmann pins gain their holding power through friction of metal surface to bone.
2. Steinmann pins are most useful in combination with cerclage wire fixation for the repair of long oblique and spiral
fractures. Steinmann pins should not be used in comminuted fracture repairs unless all pieces can be replaced.
3. Intramedullary pins interfere with medullary blood supply through their presence and the reaming effect of their introduction.
4. Single intramedullary pins provide axial alignment and resistance to bending, butgive poor control over rotational
stability and rely on friction between the bone and pin to resist tensile and compressive forces.