Bony Impingement Limits Design-related Increases in Hip Range of Motion

Factors affecting risk for impingement and dislocation can be related to the patient, implant design, or surgeon. While these have been studied independently, the impact of each factor relative to the others is not known. We determined the effect of three implant design factors, prosthetic placement, and patient anatomy on subject-specific ROM. We virtually implanted hip geometry obtained from 16 CT scans using computer models of hip components with differences in head size, neck diameter, and neck-shaft angle. A contact detection model computed ROM before prosthetic or bony impingement. We correlated anatomic measurements from pelvic radiographs with ROM. When we implanted the components for best fit to the subject's anatomy or in the recommended orientation of 45A degrees abduction and 20A degrees anteversion, ROM was greater than 110A degrees of flexion, 30A degrees of extension, 45A degrees of adduction-abduction, and 40A degrees of external rotation. Changes in head size, neck diameter, and neck-shaft angle generated small gains (3.6A degrees aEuro"6A degrees) in ROM when analyzed individually, but collectively, we noted a more substantial increase (10A degrees aEuro"17A degrees). Radiographic measurements correlated only moderately with hip flexion and abduction. It is feasible to tailor implant placement to each patient to maximize bony coverage without compromising ROM. Once bony impingement becomes the restricting factor, further changes in implant design may not improve ROM. Radiographic measurements do not appear to have value in predicting ROM.