Panagiotidou, Anna; (2018) Assessment of Wear and Corrosion at the Taper Junction of Modular Total Hip Replacements. Doctoral thesis (Ph.D), UCL (University College London).

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Abstract

The concept of modularity in primary and revision total hip arthroplasty is well established. Whilst modern modular femoral components offer surgeons the flexibility to tailor the size, offset and biomaterials of the femoral head, neck and stem to the anatomy of the patients hip, they do present an additional interface and therefore a potential source of wear and corrosion. There is increasing global awareness of adverse reactions to metal debris and elevated serum metal ion concentrations following the use of third-generation modular metal-on-metal total hip arthroplasties. In some instances the high incidence of these complications can be attributed to corrosion at the head-neck interface. In 2011 published clinical data highlighted the early revision rate of thirdgeneration metal-on-metal modular hip replacements and consequently, in February 2012 the British Orthopaedic Association recommended that large head metal-on-metal hip replacements no longer be implanted. Although severe corrosion of the taper is identified most commonly in association with larger diameter femoral heads, there is emerging evidence of varying levels of corrosion observed in retrieved components with smaller diameter femoral heads and other material combinations such as metal on polyethylene and ceramic on polyethylene. In my thesis a systematic retrieval analysis has been completed in order to understand the contribution of material wear at the modular junction compared to the wear at the bearing surface. Furthermore in-vitro dynamic experimental approaches were adopted utilising in-situ electrochemistry in order to investigate the factors that contribute to enhanced wear and corrosion at the head-neck junction and ultimately lead to the early failure of modular hip replacements. Retrieval analysis of failed large head metal-on-metal prostheses (>36mm head diameter) indicated that about 1/3rd of total wear can be attributed to material loss at the taper junction. Experimental investigations indicated that neck taper rough surface finish and reduced engagement length contribute to enhanced fretting corrosion at the modular junction, a finding that points to concern associated with their current use in orthopaedics. Increased frictional torque and bending moment both of which can be associated with large diameter heads and high offsets may also increase the susceptibility of mechanically assisted crevice corrosion at the taper junction, an effect which may be diminished with the use of ceramic heads. Additionally it would appear that regardless of engagement length, surface roughness or material combination an assembly force of greater than 4kN reduces the amount of corrosion. From the findings presented in this thesis all tapers tested exhibited evidence of mechanically assisted crevice corrosion. Although this may be unavoidable it may be reduced if certain parameters associated with implant design are taken into consideration during implant selection.

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