Ultra-high molecular weight polyethylene. The material and its use in total joint implants.

Abstract

Ultra-high molecular weight polyethylene is the current material of choice for use as a bearing surface in total joint replacements. Devices made of this material can function for more than fifteen years if they are well designed and well implanted [17,25,31,37,56,76]. However, there is a growing body of evidence showing that debris from ultra-high molecular weight polyethylene may limit its longer-term use [20,29,30,32,46,53,73]. Although, with only a few exceptions, no other bearing material has been used since the 1960's, only recently has attention been paid to the properties and performance of this material. Since the advent of total hip replacements, relatively few polymeric materials have been used in total joint replacements. These polymeric materials have included polytetrafluoroethylene, polyacetal, high-density polyethylene, polyesters, ultra-high molecular weight polyethylene, and carbon-reinforced ultra-high molecular weight polyethylene. Charnley originally chose polytetrafluoroethylene as a bearing material on the basis of its general chemical inertness and its low coefficient of friction. The types of polytetrafluoroethylene that he used were Fluon G1 and Fluon G2, made by Imperial Chemical Industries (London, England) [16]. The more familiar name, Teflon (manufactured by E. I. Dupont, Wilmington, Delaware), has been used widely and incorrectly for these polytetrafluoroethylene materials. Clinical failures with acetabular cups made of Fluon have generally occurred within one to two years; these failures have been attributed [72] to the low resistance to creep deformation of polytetrafluoroethylene resins and to their relatively poor abrasive-wear characteristics. After the poor clinical experience with polytetrafluoroethylene, Charnley turned to the use of other polymers [17,72]. Craven is credited with having tested a material called high-molecular-weight polyethylene that had been given to him by a salesman of plastic gears [72]. The first hip prostheses made of this material were implanted in 1962 [72]. This family of materials is still the one of choice for the bearing surface in total joint replacements. In the 1970's and 1980's, efforts were made to find a material with better wear properties than ultra-high molecular weight polyethylene. The two most notable efforts were the use of polyacetal (polymethylene oxide) in the Christiansen hip prosthesis and of Poly Two (carbon-fiber-reinforced ultra-high molecular weight polyethylene) (Zimmer, Warsaw, Indiana) in acetabular cups in total hip replacements, tibial components in total knee replacements, and patellar replacements [14]. Polyacetal has the potential advantages of higher yield strength, higher crystallinity, and ease of manufacturing compared with ultra-high molecular weight polyethylene. The commercial names for polyacetal are Delrin (E. I. Dupont) and Celcon (Hoechst/Celanese, Houston, Texas). Polyacetal can be formed into parts rapidly through injection-molding processes, which are much faster and cheaper than machining of the comparable parts. During the 1970's and 1980's, at least four types of polyacetal were implanted: Delrin 550, Delrin 150, Delrin 100, and Celcon. The performance of the devices was good for about five years. These early results were summarized and reviewed by Dumbleton [21]. It was estimated [47] that more than 10,000 devices containing Delrin were implanted between 1970 and 1986. However, by the mid-1980's, several groups of investigators had reported significantly higher rates of failure of these devices compared with the rates for the Charnley prosthesis (p = 0.000007), and their use was discontinued shortly thereafter [1,47,67]. Other than in rare clinical trials, Delrin is no longer used as a bearing surface in total joint replacement [12,13]. The current material of choice, ultra-high molecular weight polyethylene, has often (and incorrectly) been called high-density polyethylene in the literature [26,61,76].