Modelling different mechanical damage mechanisms of ultra-high molecular weight polyethylene liners in total hip replacements

Over 1 million total hip replacements (THRs) are performed around the world every year. Advances in the polyethylene liner materials have improved the wear resistance and longevity of well-functioning THRs. However, THRs with malpositioned components and unbalanced tissue forces can generate loading to the edge of liners that may limit their durability. Maintaining very low clinical rates of liner fatigue or fracture related revisions is important for maximising THR longevity. The aim of this research was to investigate the types of damage that occur to polyethylene THR liners when subjected to edge loading conditions to better understand possible damage mechanisms and risk factors.
The research combined experimental hip simulator studies with finite element (FE) modelling to simulate edge loading in THR components. New geometric and microstructural characterisation methods using a co-ordinate measurement machine (CMM) and Raman Spectroscopy were developed to analyse the development of damage at the liner edge which was also visualised by MicroCT scans.
Investigations of a clinically available liner (5 mm thickness) did not produce signs of liner damage or potential failure after 4 million cycles (Mc) of edge loading in the hip simulator. Changes to the liner edge geometry were linked to plastic strain accumulation in the FE model but stabilised early during this testing with no evidence for any progressive damage accumulation.
Specially thinned liners (3 mm thickness) were subsequently tested in the hip simulator to accelerate the progression of damage. The study was dominated by two separate adverse events related to the disruption of liner fixation and resulted in large deformations to liners (> 1 mm). After the 3 Mc test, microCT scans revealed damage initiated in some liners at the liner backside due to gaps related to the component locking mechanism. FE modelling of thinned liners showed that stresses and plastic strains could be transmitted through the liner thickness and particularly in the case of unsupported polyethylene.
The results suggest that when polyethylene liners were subjected to edge loading the interaction between the liner and shell locking mechanisms had the most potential for damage initiation. Ensuring that liners remain sufficiently thick and well supported helped them to be more resilient to the effects of edge loading.