Investigation of head-neck tapers in modular hip prostheses
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Corrosion at the head-neck junction of total hip replacements is a poorly understood phenomenon with an incidence of 1 - 2 %. Concerns around taper junction corrosion have focused on design factors including changes in taper surface topography and geometry as well as operating conditions such as high bearing surface friction and fluid ingress-egress at the taper junction. Hence, this thesis considered 3 aspects of the head taper junction namely: (1) frictional torque at the bearing surface and below the taper junction for varying head sizes and bearing material combinations, (2) Cobalt and Chromium ion release from CoCr/Ti taper junctions, (3) FE analysis of tapers utilising variables including taper length, material, angle, and clearance under loading conditions representative of walking, hip simulator profiles and stair climb. Bearing friction and the torque about the taper axis beneath the taper junction were positively correlated with the head size (R2 = 0.57 bearing friction, R2 = 0.88 torque) and average surface roughness (Ra) (R2 = 0.66 bearing friction, R2=0.79 torque) of the femoral head. Torque generated on large MoP bearings (0.93 ± 0.2 Nm) was found to be comparable to MoM (0.81 Nm). The median cumulative Cr release rate was at least 2 times greater than that of Co (0.0220 ppb/cycle Cr relative to 0.0109 ppb/cycle Co) due to the acidic environment utilised in the accelerated tests. No statistically significant difference in ion release was found, between the trunnions of different surface finishes. Finite element analyses showed that the largest gaps generated at the mouth of the taper, were associated with smaller taper contact areas. Clearances within ±0.1° enabled the tapers to engage over comparable lengths and therefore did not show differences in taper opening, showing this was influenced by the taper engagement length rather than location (proximal or distal) of contact. Stair climb loading generated the largest taper gaps (80 m) and surface stresses on the head taper (1200 MPa); these were greatest on the shortest trunnion. Although the stair climb loading condition is not currently mandated in testing THR devices, its use could provide a more accurate prediction of taper performance in vivo and may be beneficial to ‘beyond compliance’ initiatives to improve implant performance.
AuthorsRaji, Halimat-Shaddiya Yewande
- Theses