Biomaterials for Artificial Joints

Abstract

SUMMARY Materials used in the rapidly expanding field of total joint replacement have been reviewed. For an all-alloy joint replacement the cobalt-chrome alloy (vitallium or similar composition) is the only acceptable metal of those currently used. Volumetrically the wear debris is low and probably lower than that possible with metal-polymer joints. However, wear of these joints depends on careful design and surface preparation. Severe wear of removed Lippmann prostheses and the cone clutch sticking of horseshoe bearings illustrate what can happen. Stainless steel 31, unalloyed titanium, titanium-6 aluminum-4 vanadium, and titanium-16 aluminum are unacceptable. The phenomenon of wear in these bearings is considered to be initiated by adhesion followed by abrasion and corrosion. Adhesive wear can be minimized by us-ing alloys with a hexagonal crystal structure. The alloy of 65 cobalt, 25 molybdenum and 10 chromium shows promise from the bearing standpoint, but refinement is needed to improve tissue compatibility. For the hip joint the metal-on-metal prosthesis with a larger ball diameter has the design advantage of stability but the disadvantage of a considerable impact force should the neck and socket rim impinge during motion of the hip. This impact may have a deleterious effect on the fixation at the bone-acrylic interface. Although the wear of these units has been demonstrated to be mild in removed or surgically reoperated cases, there are signs of adhesive and abrasive wear despite the fact that wear resistance has been improved by moving the contact bearing area away from the equator and toward the pole, and the sphericity and surface finish have been improved. The low friction and damping capacity of polymer materials make them attractive for joint replacement. High molecular weight polyethylene is a good bearing material with low creep under repetitive stress. It also has shown excellent chemical resistance to degradation over a five year period. Long term degradation, however, is still theoretically possible. The total hip joint replacement designed by Charnley has a low frictional torque and appears to have minimal wear in vivo. It has the disadvantage of less stability owing to its small, 22 mm. ball and thick socket wall. The range of motion is more restricted than with the McKee prosthesis when the prosthesis is inserted correctly to minimize stability. Despite this, many patients achieve a greater range of motion than is theoretically possible through a subluxation maneuver whereby the neck makes contact with the socket, and apparently the impact at impingement is much less than with the all-metal unit so that a very small percentage of loosenings at the bone-acrylic interface have been reported. A group of „second generation“ total hip joint replacements have been designed to improve motion without neck socket contact. They use a combination of metal on ultrahigh molecular weight polyethylene rather than all metal, despite the fact that there is no clear-cut indication of which will prove to be the most successful. Further research is desirable to minimize wear and to study the effects of wear debris. These developments raise the possibility of improved bearings in the future.