Analysis of Ultrahigh Molecular Weight Polyethylene Failure in Artificial Knee Joints: Thermal Effect on Long-Term Performance Tai-Horng Young, 1 Cheng-Kung Cheng, 2 Ye-Ming Lee, 3 Li-Yen Chen, 1 Chun-Hsiung Huang 3 1 Graduate Institute of Biomedical Engineering, College of Medicine and College of Engineering, National Taiwan University, Taipei, Taiwan, Republic of China 2 Institute of Biomedical Engineering, National Yang Ming University, Taipei, Taiwan, Republic of China 3 Department of Orthopaedic Surgery, Mackay Memorial Hospital, Taipei, Taiwan, Republic of China Received 6 October 1997; accepted 24 April 1998 Abstract: The mechanism resulting in damage to and failure of ultrahigh molecular weight polyethylene (UHMWPE) tibial inserts was investigated on clinically retrieved components. The severity of the subsurface damage increased with the length of time that the component had been implanted. A theoretical analysis was developed to account for the generation of subsurface damage based on a heat transfer model. Friction generates surface heat during articulation of total knee systems. Due to the cooling effect of body fluid on the surface, the rise in temperature on the UHMWPE surface is lower than that below the surface. The peak temperature was estimated to occur on a plane positioned about 1 to 2 mm below the surface. This result was similar to the bulk temperature variation observed during in vivo and in vitro studies by other investigators. Although the difference in temperature on and below the surface is only a few degrees, the thermal effect becomes apparent after a long time and may be explained by the viscoelastic behavior of polymers: the temperature–time equivalence. It is therefore suggested that this thermal effect is another contributory factor to material damage, in addition to high stress and oxidative degradation (in appropriate cases). Therefore, any technological efforts aimed at improving the performance of artificial joint prostheses should minimize the thermal effects at the subsurface of the articular components. © 1999 John Wiley & Sons, Inc. J Biomed Mater Res (Appl Biomater) 48: 159 –164, 1999 Keywords: UHMWPE; subsurface yellowing; thermal effect INTRODUCTION The long-term performance of the articulating system and materials in the design of artificial joints is very important, particularly for younger, heavier, and more active patients. One of the most significant factors that leads to revision in total joint arthroplasty is the damage to and failure of ultra- high molecular weight polyethylene (UHMWPE). 1–4 Although many factors can affect the performance of UHMWPE in artificial joints, most investigators have adopted a purely mechanical force approach (e.g., estimation of stress distribution or trials to increase the strength of materials). 5–10 In addition, postirradiation oxidative degrada- tion of UHMWPE is also receiving attention as a potential factor influencing the performance of UHMWPE. 11–14 However, one aspect previously ignored with regard to long- term performance is the tendency for the articulating systems to generate frictional heat. A recent study performed by Tepic et al. showed that there was a tendency for the temperature in the femoral head and acetabular cartilage to increase by about 2.5°C in intact cadaver hip joints. 15 Davidson et al. showed that heat generated at the articulating surface increases the temperature of UHMWPE by 5 to 10°C in a simulator. 16 –18 In vivo measurements with a telemeterized hip implant by Rohlmann et al. showed an increase in the temperature of 3.8°C in the prosthetic neck of the hip joint when the patient walked outside for 90 min. 19 Because frictional heating can increase the creep, wear, and oxidation degradation of UHMWPE, the associated property changes and performance at elevated temperatures may be an important factor to be considered. However, a complete explanation of the effect of the frictional heating on UHMWPE and its implications seem to be lacking. Therefore, the objectives of this study were to summarize the experimental observations from retrieved UHMWPE tibial inserts and to present a theoretical explana- tion for these results. Correspondence to: Dr. Chun-Hsiung Huang (e-mail: thyoung@ntumc1.mc.ntu. edu.tw) © 1999 John Wiley & Sons, Inc. CCC 0021-9304/99/020159-06 159