The threshold force required for femoral impaction grafting in revision hip surgery

Background and purpose Femoral impaction grafting requires vigorous impaction to obtain adequate stability without risk of fracture, but the force of impaction has not been determined. We determined this threshold force in a preliminary study using animal femurs.

Methods Adult sow femurs were used because of their morphological similarity to human femurs in revision hip arthroplasty. 35 sow femurs were impacted with morselized bone chips and an increasing force was applied until the femur fractured. This allowed a threshold force to be established. 5 other femurs were impacted to this force and an Exeter stem was cemented into the neomedullary canal. A 28-mm Exeter head was attached and loaded by direct contact with a hydraulic testing machine. Axial cyclic loading was performed and the position sensor of the hydraulic testing machine measured the prosthetic head subsidence.

Results 29 tests were completed successfully. The threshold force was found to be 4 kN. There was no statistically significant correlation between the load at fracture and the cortex-to-canal ratio or the bone mineral density. Following impaction with a maximum force of 4 kN, the average axial subsidence was 0.28 mm.

Interpretation We achieved a stable construct without fracture. Further studies using human cadaveric femurs should be done to determine the threshold force required for femoral impaction grafting in revision hip surgery.