Finite element analysis of donor site fracture risk after medial femoral condyle flap harvest: Implications for optimal harvest site and flap size

Abstract

Background: This study aimed to evaluate the risk of fracture with weight-bearing after medial femoral condyle (MFC) harvest with varying harvest size and location using finite element analysis (FEA).

Methods: FEA evaluated the potential impact of MFC harvest on donor femur strength. Donor flaps were modeled as cubes of corticocancellous defects within the descending genicular angiosome (DGA). Stress and safety factors were recorded during simulated single-leg stance as a function of harvest size and location within the DGA. An S-N curve was used to determine the number of cycles to failure due to stress.

Results: Among the 15 simulated harvest size and location combinations for each femur, the smallest (1x1x1 cm) corticocancellous flap centered 1 cm proximal to the medial epicondyle demonstrated the most favorable stress (34.95±9.81 MPa), safety factor (3.42±0.85), and fatigue (>10⁶ cycles) profiles during single-leg stance, but saw notably increased stress and lower safety factors at larger sizes. Flaps centered at the medial epicondyle and shifted 0.5 cm proximally exhibited consistent stress and safety factor values. The largest (2x2x2 cm) and most proximally based flap (2 cm proximal) resulted in a precipitous increase in stress (92.23±21.19 MPa) and fatigue profile (10²-10³ cycles).

Conclusions: Our results suggest that small corticocancellous flaps centered no more than 0.5 cm proximal to the medial epicondyle are the most favorable for harvest, resulting in physiologically tolerable stress values on the donor femur during single-leg stance. Caution and potential weight-bearing restrictions should be considered when harvesting larger and more proximally based flaps.

Level of evidence: Biomechanical study.

Keywords: Biomechanics; Finite element analysis; Iatrogenic fracture; Medial femoral condyle flap.