REDUCTION IN PLATE STRAIN BY ADDITION OF AN INTRAMEDULLARY PIN

Objective-The purpose of this study was to determine the strain sparin g effect of a bone plate and rod system compared with a bone plate alo ne. Study Design-Mathematical analysis and in vitro modeling of implan t-bone constructs. Implants were instrumented with uniaxial strain gau ges. Animals or Sample Preparation-Five pairs of canine femurs. Method s-Bone plates were instrumented with two 350-ohm strain gauges. The bo ne plates were used to bridge a simulated fracture gap in five pairs o f canine femurs. In one femur of each pair, a bone plate alone was use d to bridge the gap; in the opposite femur, a bone plate and intramedu llary rod combination was used. Each specimen was mounted on a custom jig and loaded in an axial servohydraulic testing machine. A constantl y increasing compressive load was applied at the rate of 0.7 cm/sec. S trains at 400.5 N were recorded and analyzed using Wilcoxon's signed r ank test. Mathematical modeling was done using parallel beam theory. R esults-Stress reduction in the plate and rod system was twofold compar ed with the plate alone (P = .059). As important, based on stress redu ction in the plate, the fatigue Life of the plate/rod system increased 10-fold over the plate system alone and was greater than 10-fold at h igher absolute stress values. Mathematical analysis of the plate/rod s ystem was similar to that seen with the in vitro analysis. Conclusions -The combination of a bone plate and intramedullary pin was superior i n reducing plate stress when compared with the plate alone and functio ned as two beams acting in concert. Clinical Relevance-Stabilization o f comminuted fractures by bridging the zone of fragmentation with a bo ne plate without anatomic reduction of each fragment is a useful metho d of managing this type of injury. Addition of an intramedullary pin r educes the stress applied to the plate and thereby extends the fatigue Life of the bone (C) Copyright 1997 by The American College of Veteri nary Surgeons.