The axial stability of the femur after harvest of the medial femoral condyle corticocancellous flap: A biomechanical study of composite femur models

Purpose: Free bone or periosteal flaps from the medial femoral condyle are being employed for treatment of recalcitrant nonunions. When harvested in a corticocancellous fashion, these flaps have the potential to compromise the stability of the femur. This study is designed to test the axial stability of the femur after harvest of corticocancellous flaps using a standardized composite femur model. Methods: Corticocancellous defects of standardized width and depth (2 cm × 1 cm) were designed with increasing length (3-cm intervals extending from 3 to 24 cm) over the medial femoral condyle of five composite femur models. After harvest of each corticocancellous block, the femur was subjected to an axial force of 9100 N loaded and unloaded over one second using a Mini-Bionix load frame. During the application of force, load and deformation data were collected from the load cell and linear variable differential transducer. To determine changes in stiffness or deformation with increasing flap sizes, analysis of variance with repeated measures was used. If the main effect was found to be significant, a Tukey's test was used to determine differences between specific flap sizes. Results: There were no femur fractures in any femurs for any flap size. Deformation during load increased as the size of the flap increased (2.19 mm ± 0.062 mm for the 3-cm flap defect) to (2.33 mm ± 0.113 mm for the 24-cm flap defect). Post-hoc testing of deformation shows a statistically significant difference only between the 3-cm flap defect and the 15-cm flap defect (2.19 vs. 2.30 mm) (P = 0.002). The range of stiffness is between 4,339 and 4,697 N mm ^-1. Stiffness tends to decrease significantly (P < 0.001) with increasing flap size. Harvest of flap sizes greater or equal than 9 cm results in significantly lower stiffness compared to the 3-cm flap. Conclusions: In this composite femur model, when stressed with supraphysiologic forces, the femur retains its axial stability even after harvest of large corticocancellous flaps from its medial aspect. Statistical significance detected in deformation and stiffness may not be clinically relevant if the femur does not fracture after flap harvest. Such was the case in this experiment. The possibility exists of safely harvesting large flaps from this donor site. Corticocancellous flaps from the medial aspect of the femur may serve as an alternative to standard flaps used in medium and large osseous reconstructions. The size of flap that can be safely raised without compromising the stability of the femur has not yet been delineated.