Investigating Acetabular Cup Fixation in Cemented Total Hip Replacements

Abstract

The Total Hip Replacement is the second most successful surgical intervention of all time and is most commonly used to treat end stage osteoarthritis. The demand for this procedure is growing, with the most recent report from the National Joint Registry recording in excess of 93 thousand primary total hip replacements in 2016, a 3.5% increase from the previous year. The most common reason of failure is aseptic loosening of the acetabular cup. A review of the literature concluded that aseptic loosening is a multifactorial process with stress shielding as one of the contributing factors. The aim of this study is to further understand the changes in the mechanical environment of the cemented acetabular cup leading to early loosening. A range of investigation methods were used to approach this.This study presents a retrospective analysis of radiographs and other clinical data, comparing matched cohorts of revised and surviving acetabular cups in a young patient population. An in vitro investigation was conducted to determine the optimal size and number of acetabular keyholes to resist torsion. Finite Element Analysis specimen-specific models of the hemipelvis implanted with a cemented cup were generated and validated using mechanical tests of composite bone substitutes. The validated modelling methodology was implemented with human CT data to develop physiologically relevant models with the inclusion of muscle forces, greyscale dependent material properties and strain adaptive behaviour of bone. The physiological models were used for comparisons between the native and implanted joints, and to explore modifications.The data shows that progressive radiolucent lines in the Charnley acetabular zone I, as well as postoperative pain and poor function, were associated with failure of the cup through aseptic loosening. In the mechanical investigation, fewer acetabular keyholes with a larger diameter had an increased torsional resistance. The Finite Element models closely predicted the strains of the composite hemipelves, however there was significant interspecimen variability. The implantation of a cemented cup was associated with bone loss in the supero-anterior region of the acetabulum (Charnley zone I). The suggested introduction of a cement modification with a reduced stiffness could prevent the loss of trabecular bone in this region of interest. Decreasing the stiffness of the polyethylene cup was found to preserve acetabular cortical bone.

Date of Award	9 Nov 2018
Original language	English
Awarding Institution	University of Bath
Sponsors	James Dyson Foundation
Supervisor	Richie Gill (Supervisor) & Sabina Gheduzzi (Supervisor)