The Effect of Body Weight on Interfragmentary Fracture Strain in Plate Fixation of Distal Femur Fractures: A Finite Element Analysis.

INTRODUCTION: Distal femur fractures are difficult to successfully treat due to high rates of nonunion. Obesity is an independent prognostic risk factor for nonunion. Advances in finite element analyses (FEAs) have allowed researchers to better understand the performance and behavior of constructs at the bone-implant interface under a variety of conditions. The purpose of this study is to determine the impact of body weight on fracture strain in a lateral locking plate construct for supracondylar femur fractures and whether additional construct rigidity is beneficial to optimize fracture strain in high body mass patients.

HYPOTHESIS: We hypothesized that increased loads would produce a higher interfragmentary strain (IFS), which could be decreased by shortening the working length of the construct.

MATERIALS AND METHODS: A 3D finite element analysis was performed on two separate femur models with a comminuted supracondylar distal femur fracture fixed with a lateral distal femoral locking plate in bridging mode with Ansys software. Axial forces were varied to recreate the effect of load from normal and high body mass patients. Working length and screw density of the construct were varied for each condition. Measurements of interfragmentary strain and shear motion (SM) were compared.

RESULTS: Doubling the axial load from 70kg (control) to 140kg (high body mass) increased the interfragmentary strain by an average of 76% for the three working lengths (3.38% + 1.67% to 4.37% + 0.88% at the baseline working length (BWL), 1.42% + 1.00% to 2.87% + 2.02% at the intermediate working length (IWL) and 0.62% + 0.22% to 1.22% + 0.42% at the short working length (SWL)). On average, decreasing the working length in the 140kg load reduced the mean IFS to within 15% of the mean IFS of the 70kg load at the longer working length (2.87% + 2.02% at IWL 140kg versus 3.38% + 1.67% at BWL 70kg and 1.22% + 0.45% SWL 140kg versus 1.42 + 1.00% IWL 70kg).

DISCUSSION: Increased axial load increases interfragmentary strain in an AO/OTA 33A distal femur fracture fixed with a lateral distal femoral locking plate. Decreasing the working length of the fixation construct in the high body mass model decreased interfragmentary strain. Higher loading conditions reflective of high body mass patients should be considered in studies investigating optimization of fracture strain.

LEVEL OF PROOF: V; Finite Element Analysis (FEA).