Prosthetic hip range of motion and impingement. The effects of head and neck geometry

Two major design parameters to total hip arthroplasty which can be selected to optimize for anatomical variation are head diameter and neck length. By using a cadaveric pelvis mounted in a three-dimensional protractor, it was shown that increased head-neck diameter ratio results in increased range of motion, with decreased prosthetic impingement, and that increased neck-length results in increased range of motion, with increased prosthetic impingement. Head diameter has no range of motion effect in arcs characterized exclusively by bony impingement, and neck length has no range of motion effect in arcs characterized exclusively by prosthetic impingement. The surgeon is currently confronted with a variety of prostheses with different head-neck diameter ratios and neck lengths. When choosing the optimal prosthesis for a particular patient, the surgeon is often restricted by various anatomical constraints. While a large head diameter cannot always be chosen, an adequate range of motion without prosthetic impingement can still be achieved by ensuring that head-neck diameter ratio is optimum. A sufficient neck length to ensure adequate range of motion is desirable, but the neck should not be so long that neck-socket impingement is of concern. Optimum clinical range of motion, with minimal prosthetic impingement, may be achieved with a large head size and a medium neck length.