Tribology of total hip arthroplasty prostheses

Although total hip arthroplasty (THA) has provided good results for over forty years, the choice of the optimal bearing surface still remains controversial. Tribology is defined as “the branch of science and technology that deals with the study of friction, wear, and lubrication”.1 Tribology is fundamental to the function and long-term survival of orthopaedic implants. With the development of hip implants with metal-onpolyethylene (MoP) bearings in the early 1960s, cases of aseptic loosening of prostheses started to be seen towards the beginning of the 1970s which were not completely explicable at that time. Numerous hypotheses were put forward to try to explain these cases of aseptic loosening, especially one named ‘cement disease’, which subsequently led to the development of uncemented implants. Following several years of investigation, Willert et al were able to demonstrate that the observed cases of aseptic loosening could be explained by a concept they described as ‘particle disease’.2 According to these researchers, polyethylene (PE) debris particles are generated in the absence of fluid film lubrication between the femoral head and the acetabular cup, due to abrasive and adhesive wear caused by the relative movements between the two surfaces. Wear particles trigger a series of biochemical reactions, which change the dynamic balance between osteoblasts and osteoclasts. The enhanced osteoclast activity results in osteolysis in the areas affected by a high number of wear particles. This resorption of bone eventually leads to aseptic loosening of implants in the long-term. Osteolysis is directly correlated with the amount of debris particles.3-6 In order to maximize implant survival, articulating components should therefore minimize the generation of wear particles of a biologically-critical size, which is, according to Green et al, between 0.3 and 10 μm.7 The objective of this overview is to succinctly present the important elements to understand in the tribology of THA. A good understanding of tribological properties helps the orthopaedic surgeon to choose the most suitable bearing solution for each individual patient.