Periacetabular osteolysis is a significant complication following total hip replacement surgery. It is believed to be caused by wear debris and high fluid pressures within the joint capsule. To investigate the mechanisms by which high fluid pressures are generated in total hip replacement, a physical model of the hip joint was constructed, the Acetabular Pressure Transmission Rig (APTR).An aluminium chamber held the bone analogue, a polyurethane hemisphere with a 52mm acetabular cavity, and the prosthetic components, a 28mm femoral head and various uncemented cups, were inserted without press-fit to simulate cup loosening. A synovial fluid analogue was introduced into the chamber through an elevated reservoir. Rigid transmission tubes conducted fluid pressures from the cup-cavity interface to external transducers. The APTR was loaded under various conditions and the pressures produced by the loading regimes were analysed.Pressures over 35kPa, previously shown by other groups to cause osteolysis, were measured within the APTR, reaching a maximum of 131.3kPa measured at the pole of the cup. Changes in load application led to pressure changes within the APTR, with higher loading frequency and magnitude leading to higher median pressure amplitudes. The presence of different component features, such as screw holes in the metal shell, was also shown to affect periacetabular pressures. Tests with a fibrous rim interposed between the prosthetic cup and the test cavity showed an 88% reduction in periacetabular pressures, as the increased rim interference between cup and cavity prevented fluid ingress behind the cup. A larger initial separation between the loading head and the acetabular cup caused a significant increase in measured pressures, with a 0.15mm increase in head-cup separation producing a 53% increase in pressures measured at the pole of the cup. Pressure differentials between different transducer sites indicated the ability for fluid flow behind the cup, which can be related in vivo to the movement of particulate debris to periacetabular bone.The APTR was able to measure clinically significant pressures and to analyse the effects of modifying component and loading characteristics with currently available prosthetic components. This makes the rig useful in a clinical context, as it has been shown to be capable of testing a broad range of component types under a wide range of conditions. Its use will ensure new prostheses and fixation modes can be designed in such a way as to eliminate the damaging fluid pressures currently observed in artificial hip joint replacements.
|Date of Award||31 Jul 2013|
|Supervisor||Tony Miles (Supervisor) & Irene Turner (Supervisor)|
- hip replacement
- acetabular cup
- fluid pressure