HARD/HARD BEARINGS – ARE THERE SPECIAL REQUIREMENTS FOR THE ASIAN MARKET?

Abstract

Modern Total Hip Replacement (THR) is in general one of the most successful surgical treatments although the functional requirements of modern patients are more and more demanding. Challenges arise from an extended life-span, a higher activity level requiring more sophisticated artificial materials, and a larger required range-of-motion (ROM) caused by the younger patients’ eagerness to continue a sporty lifestyle. The design criteria for modern THR resulting from these patient demands also depend on the anatomical conditions as well as the socio-cultural circumstances of the patients.

Asian people require in general a higher ROM due to their habit to squat during daily activities which is not common in western societies. The outcome of a THR regarding the ROM is influenced by the size of the bearing couple, the design of the acetabular component, the head-to-neck ratio, and the implantation angles. In the case of a wrongly designed or a misaligned component, e. g. a verticalised socket, subluxations and impingement might occur leading to edge-loading between the ball head and the insert. This leads in all material couplings to problems: in hard-soft couplings (ceramic or metal ball head and polyethylene insert) to strongly increased polyethylene wear, in hard-hard bearings (metal-on-metal or ceramic-on-ceramic) to point loading followed by stripe wear and, in the case of a metal-on-metal coupling, a much higher metal ion level in the blood. Therefore, an appropriate choice of the prosthesis design together with the necessary surgeon’s diligence is necessary to avoid this kind of complication.

Other important design challenges come from possible anatomical differences between different ethnical groups. It has been shown that the “asian knee” has a different mean thickness in anterior-posterior as well as medio-lateral direction compared to caucasian. As another example, an extensive study of mexican people has shown a significantly different femur geometry concerning the height of the Trochanter major compared to the cross section of the femoral axis and the neck axis. For asian people it is widely accepted that the mean femoral size is smaller. The nonobservance of these geometrical factors in implant design may again lead to higher wear rates or subluxation and impingement followed by dislocation.