PATIENT FEMOROACETABULAR MORPHOLOGY SIGNIFICANTLY AFFECTS HIP RANGE OF MOTION AFTER TOTAL HIP ARTHROPLASTY

Abstract

Dislocation remains a major early complication after total hip arthroplasty (THA), and range of motion (ROM) before impingement is important in joint stability. Factors contributing to dislocation include design specific factors such as head-neck ratio, surgeon-related factors such as component placement, and patient-related factors such as bony anatomy. To study the relative importance of these factors, we analysed the effects of patient anatomy, implant design, and component orientation on hip ROM.

Femoral and acetabular geometry were extracted from CT scans of 20 hips. CAD models of four different THA component designs were virtually implanted in the 3D-CT reconstructed anatomic models. The major design differences were in head-neck ratio and neck-stem angle. A previously reported contact detection model (D’Lima, J Orthop Research 2008) was used to measure restriction in hip ROM due to prosthetic or bony impingement. The following patient parameters were measured on plain AP radiographs: acetabular inclination, acetabular depth ratio, the arc-length between the tip of greater trochanter and ilium, and the arc-length between lesser trochanter and ischium. Multiple linear regression was used to determine correlation between radiographic parameters and hip ROM in flexion, extension, adduction, abduction, and external rotation.

Mean head size was 51 ± 2mm, mean anatomic acetabular inclination was 41° ± 2, and mean acetabular depth ratio was 460 ± 60. When the cup and stem were implanted for best fit to the anatomy, mean hip ROM was 125° ± 8 (flexion), 57° ± 17 (extension), 29° ± 13 (adduction), 69° ± 7 (abduction), and 42° ± 13 (external rotation). Implanting the cup in “optimal” surgical alignment of 45° abduction and 20° anteversion reduced mean hip flexion, extension and abduction and increased adduction. Subject-to-subject variation was substantially greater than variation between CAD designs (differences in head-neck ratio) or component orientation (between ideal and anatomic). Hip flexion correlated moderately with acetabular abduction angle and the angle of the flare of the iliac wing (R2 = 0.59, p = 0.03). Hip abduction correlated moderately with the angle of the flare of the iliac wing and the length of the arc from the tip of the greater trochanter to the ilium (R2 = 0.50, p = 0.05).

A universal cup position that permits optimal range of motion in all patients may not be valid. Since patient-related factors overshadowed implant design, cup position should be tailored to the individual patient. Preoperative radiographs can help predict postoperative hip ROM although not as accurately as 3D-CT reconstructions. These results may lead to enhancements in surgical navigation techniques.