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Title: Wear mechanisms of alumina hip prostheses
Author: Zeng, Peng
ISNI:       0000 0001 3577 0177
Awarding Body: University of Sheffield
Current Institution: University of Sheffield
Date of Award: 2008
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Although extensive research has been undertaken on wear of alumina there is still poor understanding about the mechanisms which can take place during the ill vivo wear of alumina, such as found in alumina hip prostheses. In particular, the origin of 'stripe' wear, a region of high wear dominated by fracture that is observed widely from retrieved ill vivo alumina hip prostheses, has not been clearly determined. Moreover, it has not been completely validated that laboratory hip simulators reproduce the same wear mechanisms as found in the body, although it is known that the stripe wear can only be replicated by the introduction of microseparation during the simulated walking cycle. The current study presents detailed analysis of the worn surface of alumina acetabular cup and alumina femoral head following ill vitro testing incorporating microseparation at the University of Leeds, as well as retrieved ill vivo alumina hip prostheses. Four different wear zones were identified on both the acetabular cup and femoral head for both ill vitro and ill vivo specimens, defined as: mild wear, wear transition, stripe boundary and stripe wear zones. Although sequence of events cannot be verified, the available evidence indicated the following wear process of alumina hip prostheses: rather than the final stage of the wear process, fracture occurred in the early stages due to the local impact associated with microseparation, leading to the region of stripe wear. FIB cross-section investigations showed that fracture was predominantly intergranular, with some transgranular fracture, and that in both cases, fracture was restricted to the outer layer of grains. In addition, plastic deformation and a 3rd body abrasion were also observed, with the dislocation activity also restricted to the outer grains. The wear debris liberated from the stripe wear region was believed to subsequently have resulted in wear in other parts of the joint that would otherwise have shown little evidence of damage. The stripe boundary zone, immediately adjacent to the stripe wear region, exhibited minimal wear, with a remarkably sharp boundary between the stripe wear and mild wear. The explanation for such an abrupt change from mild to severe wear were possibly abrupt change in lubrication from microseparation or small changes in the height of the alumina surface. Most of the remaining surface on the worn alumina-on-alumina THRs had experience only mild wear. Detailed analysis of this region showed that it contained extensive fine scale 3rd body abrasion, presumably from wear debris that had become much finer due to attrition between the articulating surfaces, originally coming from the stripe wear region. Cross-sectional TEM indicated that the abrasive grooves did result in surface dislocation activity which was restricted to the outer 100nm of the surface. In addition, differential wear between grains was observed in the mild wear zone, which was believed to be evidence of tribochemical wear. Some small additional wear scars, similar to the observations of the morphology of stripe wear and parallel to the stripe wear occurred near the stripe wear region on ill vitro and retrieved ill vivo alumina femoral heads in the mild wear zone. A wear transition zone outside the stripe wear zone was identified, comprising a highly pitted surface and an amount ofwear debris. The pits had largely arisen from intergranular fracture. The extent of this region was limited compared with other wear zones and was more obvious on the alumina acetabular cup than on the alumina femoral head. Detail TEM analysis showed of this region showed extensive dislocation activity on the pyramidal system. Three types of wear debris were observed: globular wear debris, nanocrystalline wear debris and needle shaped wear debris. The globular wear debris is most likely to have come from grains been pulled out and covered by tribochemical film as it rolls over the surface. The nanocrystalline wear debris was believed to arise from 3rd body microabrasion during the normal articulation and the needle shaped wear debris is believed to be a direct product of tribochemical wear.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available