Investigation into the Engagement and Performance of Modular Taper Junctions in Total Hip Replacements

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Morse-type tapers were incorporated into total hip replacements to allow the use of a ceramic bearing material. Modularity also offers increased intra-operative flexibility during primary and revision surgery and, the ability to balance soft tissues in order restore the natural gait. However, moving from a mono-block to a modular design has meant fluid ingress and micro-motion at the interface can cause degradation via fretting-corrosion. Wear and corrosion products at the taper junction are associated with adverse local tissue reactions commonly presented in patients as pain followed by instability. The effect of fretting corrosion at the head-neck was highlighted in a recent study. Thirty-three (of 4581) patients required revision of a metal-on-polymer replacement due to adverse reaction to metal debris produced as the head-neck junction.Morse tapers were originally designed to allow machine parts such as drill bits and cutting tools in milling machines to be changed quickly without compromising torque transmission. The conical surfaces are designed to be highly conforming, smooth, hard, long and with a slight taper. Head-stem tapers on the other hand are much shorter with a much higher taper rate (i.e. shorter with a greater taper angle), often presenting a threaded finish and present a level of angular mismatch in order to create specific contact regions.Studies have investigated and found that the taper design influences the amount of fretting-corrosion. This has been seen in variations such as: the material couple, mechanical loading, surface texture of the interface, nominal contact area, presence of any stress concentrations and, impaction conditions. Many of these taper deign variations influence the engagement of the two mating taper interfaces. Previously, the geometrical form has been used to quantitatively evaluate macro-scale conformity using a maximum of three parameters providing only a limited insight into taper engagement. Some studies have investigated the surface topography to assess engagement on the micro-scale, neglecting the geometrical form. This project's aim is to assess taper engagement and performance due to geometrical form and local deviations.The aim has been divided into the following high level objectives:- Develop measurement and analysis techniques that allows taper interfaces to be appropriately characterised using precision engineering measurement devices such as Roundness and Coordinate Measurement Machines. - Assess how commercially available modular head and stem systems vary between manufactures and within each manufacturer.- Determine how these variations effect engagement between a head and a stem couple.- Finally, determine how these taper variations affect the performance of head stem systems in terms of taper mechanics, susceptibility to crevice corrosion and tribo-corrosion .This project will provide a better understanding of how fretting corrosion at the taper interface of medical devices can be minimised between each head and stem couple across a range of commercially available total hip replacements.