It has been shown that the contact motion path in artificial knee bearings is implant design dependent [1]. It is also known that multidirectional motion of artificial joints leads to higher wear rates compared to the application of unidirectional motion [2,3]. This phenomenon has been linked to “strain-hardening” of ultra-high molecular weight polyethylene (PE): the molecular chains in the surface layer align along the direction of shear. Since more energy is required to rupture the fibrils in their longitudinal direction than perpendicularly, less wear is generated if the direction of movement follows the molecular chain orientation [3]. Computer models could potentially become an important tool in wear prediction if this relationship could be quantified. A variable, defined as ‘cross-shear’ A/(A+B), with A being the fraction of motion along the molecular orientation and B being the fraction of motion perpendicular to the molecular orientation, takes this into account (Fig.1); however, pin-on-disc studies trying to validate this model found agreement for small cross-shear values only [4].

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