Chain Entanglements and Radiation Crosslinking: Novel Approaches for Improving UHMWPE Wear and Fatigue Properties in Total Knee Replacements

人类的天然膝关节是一个复杂且负荷很大的关节。因此，出于功能的考虑，运动的自由和稳定性对人工膝关节来说是很重要的。由于这种需求，高应力放置在人工膝关节的关节。聚乙烯（具有非常高的分子量，称为超高分子量聚乙烯）已成为人工膝关节承重关节面的首选材料。因此，胫骨聚乙烯轴承的过度磨损和疲劳是常见的，并导致人工膝关节失效。聚乙烯相关的全膝关节故障限制了全膝关节置换术的使用寿命。然而，对全膝关节置换术的需求不断增加，现在占英国所有关节置换术的近一半。事实上，每年全世界大约有100万个超高分子量聚乙烯组件被植入。此外，55岁至64岁人群的手术数量也在显著增加。关节置换术在更年轻（< 64岁）和更活跃人群中的应用，加上预期寿命的普遍增加，迫切需要使用寿命更长、耐磨性和抗疲劳性更好的聚乙烯。该项目的重点是减少材料失效的发生率。本文的假设是UHMWPE膝关节部件材料失效的原因在于用未加工的UHMWPE粉末制造UHMWPE部件过程中所使用的精确的时间-温度历史。最近的研究表明，UHMWPE焊接界面的宏观韧性随焊接时间和温度的增加而增加。这表明超高分子量聚乙烯粉末颗粒界面的韧性随压缩成型时间和温度的变化有相似的变化。因此，需要优化加工条件，在应力强度高的胫骨部件位置提供具有足够韧性的界面。超高分子量聚乙烯（UHMWPE）用于人工膝关节的辐射交联仍存在争议，关于辐射交联是否有益存在争议。交联可以减少UHMWPE的磨损，但对材料进行后辐照处理以消除残留自由基的方法会影响材料在体内的长期性能。本计划将纳入辐射交联并进行评估，以解决有关其对磨损和疲劳影响的科学争议。拟议中的项目试图回答两个重要问题。1)界面粘合的改善是否提高了UHMWPE胫骨构件的耐久性，提高了多少？2)适当的附加辐射交联治疗是否有益？该项目采用了两种新颖的方法。一种是使用计算机辅助方法来提高材料的完整性，以产生一系列具有不同程度界面粘合的实际定制胫骨组件。另一种是将改进的材料完整性与先进的辐照后处理相结合，以降低磨损率，提高抗疲劳性。将根据磨损行为、磨损颗粒和疲劳强度的评估，对一系列非交联和辐射交联部件进行并排比较。优化生产条件的识别将降低目前较高的修正操作率。

The natural human knee is a complex and heavily loaded joint. Therefore, for functional reasons, freedom of movement and stability are important for artificial knee joints. Due to this demand, high stresses are placed on the articulation of the artificial knee joints. Polyethylene (with very high molecular weight, termed as UHMWPE) has been the material of choice for the load-bearing articulating surface in artificial knee joints. As a result, excessive wear and fatigue of the tibial polyethylene bearing are common and result in artificial knee joint failure. Polyethylene-related total knee failures have limited the lifetime of total knee joint replacements.However, there is an increasing demand for total knee replacement operations which now account for nearly half of all joint replacements in the UK. In fact, approximately one million UHMWPE components are implanted worldwide on a yearly basis. Also the number of operations carried out in those aged 55-64 has been rising significantly. The application of joint replacement to younger (aged < 64 years) and more active people plus the general increase in life expectancy make it an urgent need for longer lasting polyethylene with better wear and fatigue resistance. The Project is focused on reducing the incidence of material failure. The hypothesis to be examined is that the cause of material failure of UHMWPE knee-joint components lies in the precise time-temperature history employed during manufacture of the UHMWPE components from the virgin UHMWPE powder. Recent work has shown that toughness at macroscopic UHMWPE welded interface increases with welding time and temperature. This suggests that the toughness of UHMWPE powder particle interfaces may vary similarly with compression moulding time and temperature. Hence there is a need to optimise processing conditions to provide interfaces with adequate toughness at locations in tibial components where stress intensities are high. Radiation crosslinking of UHMWPE for artificial knee joints is still controversial and there is a debate over whether radiation crosslinking is beneficial. While crosslinking reduces UHMWPE wear, the method of post irradiation processing of the material to eliminate residual free radicals can affect the long-term performance of the material in the body. Radiation crosslinking will be incorporated and assessed in the Project in order to resolve the scientific controversy regarding its effects on wear and fatigue.The proposed project seeks to answer two important questions. 1) Does improved interface bonding improve the endurance of UHMWPE tibial components and to what extent? 2) Can an appropriate additional radiation crosslinking treatment be beneficial? The Project uses two novel approaches. One is to improve material integrity using computer-aided methodology for generating a range of actual customised tibial component with varying degrees of interface bonding. The other is to combine improved material integrity with state-of-art post irradiation treatment in order to reduce wear rate and improve fatigue resistance. A side-by-side comparison of a range of non-crosslinked and radiation cross-linked components, based upon evaluation of wear behaviour, wear particles and fatigue strength will be made. Identification of optimised manufacturing conditions will reduce the currently high rate of revision operations.