Reduction in Plate Strain by Addition of an Intramedullary Pin

Objective —The purpose of this study was to determine the strain sparing effect of a bone plate and rod system compared with a bone plate alone.
Study Design —Mathematical analysis and in vitro modeling of implant-bone constructs. Implants were instrumented with uniaxial strain gauges.
Animals or Sample Preparation —Five pairs of canine femurs.
Methods —Bone plates were instrumented with two 350-ohm strain gauges. The bone plates were used to bridge a simulated fracture gap in five pairs of canine femurs. In one femur of each pair, a bone plate alone was used to bridge the gap; in the opposite femur, a bone plate and intramedullary rod combination was used. Each specimen was mounted on a custom jig and loaded in an axial servohydraulic testing machine. A constantly increasing compressive load was applied at the rate of 0.7 cm/sec. Strains at 400.5 N were recorded and analyzed using Wilcoxon's signed rank test. Mathematical modeling was done using parallel beam theory.
Results —Stress reduction in the plate and rod system was twofold compared with the plate alone ( P = .059). As important, based on stress reduction 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 higher absolute stress values. Mathematical analysis of the plate/rod system was similar to that seen with the in vitro analysis.
Conclusions —The combination of a bone plate and intramedullary pin was superior in reducing plate stress when compared with the plate alone and functioned as two beams acting in concert.
Clinical Relevance —Stabilization of comminuted fractures by bridging the zone of fragmentation with a bone plate without anatomic reduction of each fragment is a useful method of managing this type of injury. Addition of an intramedullary pin reduces the stress applied to the plate and thereby extends the fatigue life of the bone plate.