Introduction: Adequate bone in the femoral head and neck is a prerequisite in ensuring the longevity of a surface arthroplasty. The pistol grip deformity is one of the most common bony abnormalities of the femoral head encountered at the time of resurfacing. Severe flattening results in segmental bone loss requiring adjustments in the alignment of the femoral component to achieve optimal orientation. However, very little is known as to how the femoral implant positioning will be affected by increasing deformity. The purpose of this study was to classify the deformity of the femoral head to better understand how it influences the alignment of the femoral component during surface arthroplasty. This classification was then used to determine whether the femoral implant can be safely inserted with optimal alignment despite progressive deformity of the femoral head and neck.

Methods: The classification was developed using plain radiographs and computer tomography scans from 61 patients (66 hips) who presented with primary osteoarthritis prior to hip resurfacing. Surface arthroplasty simulations were generated with three-dimensional computed tomography to quantify the change in femoral component orientation from the neutral position that would allow optimal alignment. The biomechanical parameters were also calculated to determine the influence of the deformity on the final implant position.

Results: There were 47 men and 14 women, with a mean age of 50.3 years (range, 33 to 63 years). Three categories of femoral head deformity were created using a modified femoral head ratio (Normal ≥0.9, Mild = 0.75 – 0.9 and Severe < 0.75). There were a total of 32 normal hips (48%), 23 hips (35%) with mild deformity and 11 hips (17%) with severe deformity of the proximal femur. A severe deformity required significantly more superior translation of the entry point (p=0.027) and greater reaming depth (p=0.012) to allow safe insertion in relative valgus without notching. This could be achieved while preserving length discrepancy (p=0.17) and minimizing the component-head size difference (p=0.16), although femoral offset was significantly reduced (p=0.025).

Conclusion: A classification of femoral head deformity was created to better understand how progressive deformity influences the alignment of the femoral component during surface arthroplasty. This classification is simple and easily measured using standard AP radiographs of the hip. We found that the femoral component can be safely inserted with optimal alignment during surface arthroplasty by modifying the surgical technique in the face of severe deformity.