The hip joint is subjected to cyclic loading and motion during activities of daily living, which can induce micromotion of total hip replacements. High levels of micromotion inhibit bone formation, and hence osseointegration of cementless implants. Initial stability is therefore crucial to ensure successful osseointegration of cementless acetabular cups. Hence, it is important to be able to measure the micromotion of acetabular cups in vitro in order to predict if they will survive once implanted.There are no standardised methods to measure cup micromotion however there are numerous studies available in the literature. These studies have two main limitations: they only measure cup micromotion in the assumed dominant direction(s) of motion rather than in six degrees of freedom; and they overlook the effect of dynamic hip motion as the hip joint is held in a fixed position. Finally, most of these studies either use cadaveric pelvic bones or synthetic foam blocks with a hemispherical cavity, both of which have their advantages and limitations.A new robust methodology capable of measuring cup micromotion in six degrees of freedom under cyclic loading and hip motion was developed, as well as a more representative synthetic acetabular model which replicated the structural support in the acetabulum.A number of investigations were carried out using this protocol. The results indicated that hemispherical cavities used to model the acetabulum overestimate cup stability compared to the more representative model which replicated the natural acetabulum; there was a significant increase in cup micromotion under dynamic hip motion; and clinically relevant micromotions were present in all translations.This novel protocol provides a better understanding of the behaviour of an implanted press-fit cup and the basis for more representative protocol for future pre-clinical evaluations of new design features that can improve cup fixation, and hence its longevity.
|Date of Award
|12 May 2015
|Tony Miles (Supervisor) & Patrick Keogh (Supervisor)