A Biomechanical and Finite Element Analysis of Femoral Neck Notching During Hip Resurfacing

Hip resurfacing is an alternative to total hip arthroplasty in which the femoral head surface is replaced with a metallic shell, thus preserving most of the proximal femoral bone stock. Accidental notching of the femoral neck during the procedure may predispose it to fracture. We examined the effect of neck notching on the strength of the proximal femur. Six composite femurs were prepared without a superior femoral neck notch, six were prepared in an inferiorly translated position to create a 2 mm notch, and six were prepared with a 5 mm notch. Six intact synthetic femurs were also tested. The samples were loaded to failure axially. A finite element model of a composite femur with increasing superior notch depths computed maximum equivalent stress and strain distributions. Experimental results showed that resurfaced synthetic femurs were significantly weaker than intact femurs (mean failure of 7034 N, $p<0.001$⁠). The 2 mm notched group (mean failure of 4034 N) was significantly weaker than the un-notched group (mean failure of 5302 N, $p=0.018$⁠). The 5 mm notched group (mean failure of 2808 N) was also significantly weaker than both the un-notched and the 2 mm notched groups (⁠$p<0.001$⁠, $p=0.023$⁠, respectively). The finite element model showed the maximum equivalent strain in the superior reamed cancellous bone increasing with corresponding notch size. Fracture patterns inferred from equivalent stress distributions were consistent with those obtained from mechanical testing. A superior notch of 2 mm weakened the proximal femur by 24%, and a 5 mm notch weakened it by 47%. The finite element analysis substantiates this showing increasing stress and strain distributions within the prepared femoral neck with increasing notch depth.