Finite element mechanical analysis of rigid internal fixation selection for distal femur fractures

Background and Objective.Compared with lateral locking plate (LLP) fixation, dual-plate (DP) fixation has not demonstrated superior clinical outcomes in the treatment of distal femur fractures. We propose that this discrepancy arises from unclear indications. This study examining the influence of varying metaphyseal defect sizes on the mechanical behavior of the two fixation constructs, provides insight into defining their appropriate clinical indications.Methods.Finite element analysis was conducted to compare the biomechanical performance of LLP-only versus DP fixation in AO/ASIF type C2 fractures with varying metaphyseal defect sizes (0, 5, 10, 15, 20, 25, and 30 mm). Fourteen models were established and subjected to axial (700 N) and torsional (14 Nm) loading. We extracted six parameters, including the axial stiffness, the maximum displacement of the femur, the torsional stiffness and the torsion micromotion of the fracture, and the peak VMS stress on both the medial and lateral plates under both axial and torsional loadings.Results.As the metaphyseal defect size increased (0-30 mm), all measured parameters under axial compression and torsional loading exhibited consistent trends. Overall, changes were more pronounced in the LLP-only group. Under axial loading, increasing defect size led to a 95.2% reduction in axial stiffness, a 58.2-fold increase in axial displacement, and 7.7 times rise in the maximum equivalent stress of the LLP-only fixation. In contrast, DP fixation showed only a 9.4% decrease in axial stiffness, a 5.4-fold increase in displacement, and a 2.7% increase in lateral plate stress. Under torsional loading, LLP-only models exhibited an 87.4% decrease in torsional stiffness, a 26.9% increase in torsional displacement, and a 6.9% rise in lateral plate stress. Corresponding changes for DP models were a 64.7% decrease in torsional stiffness, a 13.5% increase in displacement, and a 57.4% increase in stress.Conclusions.This study demonstrates that larger metaphyseal defects result in decreased fixation stiffness, increased fracture-site displacement, and elevated plate stress, including in the medial plate of DP constructs. These changes are consistently less pronounced with DP fixation compared to LLP-only constructs. Biomechanically, a transitional range exists between the two methods, with a mechanical suitability threshold near a metaphyseal defect size of 15 mm.