Simulation of tissue differentiation in uncemented hip implants based on a mechanoregulatory hypothesis

I UNIVERSITY OF SOUTHAMPTON ABSTRACT FACULTY OF ENGINEERING, SCIENCE AND MATHEMATICS SCHOOL OF ENGINEERING SCIENCES Doctor of Philosophy SIMULATION OF TISSUE DIFFERENTIATION IN UNCEMENTED HIP IMPLANTS BASED ON A MECHANOREGULATORY HYPOTHESIS By Pramod Kumar Puthumanapully The proportion of uncemented implant procedures carried out annually worldwide is increasing, with improved clinical outcomes for young and old patients alike. However, their success depends largely on their ability to achieve good initial stability and long term bone ingrowth. There are a number of factors that govern this process and failure to integrate with surrounding bone could lead to loosening. Implant design factors are important with respect to implant/bone integration, and their shape, size and, if there is a coating present, the nature of coating, may be optimised to promote bone ingrowth around the implant. To this end, novel preclinical testing tools have been developed using computational methods, with the aim of predicting the response of bone to the implant. This thesis describes the development and application of a novel algorithm that simulates tissue differentiation around implants using finite element methods, to study design features that could promote bone formation at the interface. To illustrate the capabilities of the algorithm, a series of case studies were undertaken which focussed on the porous coating and implant geometries. The results were then compared to clinical data for corroboration. The effect of implant geometry was investigated by simulating tissue differentiation around short stemmed and long stemmed implants. For short stemmed implants, the beneficial role of shape, size and specific design features such as the lateral flare were highlighted. For conventional, diaphyseal filling long stemmed implants, tissue differentiation around the implant, and the influence of different porous coating lengths on bone formation at the interface was investigated. It was predicted that bone formation occurs more on the distal aspects of the stem and did not vary much with coating length. No added advantage in terms of the bone formation was observed for proximal coated implants. Given the relatively similar performance, using the porous coating over the entire length of the implant was favoured in light of the increased surface area available for ingrowth. As a final case study, the algorithm was employed in a microscale study of a sintered beaded coating, to investigate the effect of micromotion on tissue differentiation. The results suggested that micromotion had a strong influence on bone formation at the interface. In addition, it was found that a two layered porous structure appeared to promote more bone formation than the typically used three layered coatings for similar micromotion levels. The results obtained from the aforementioned case studies corroborated well with clinical data and clearly demonstrated the efficacy of the algorithm in simulating tissue differentiation around uncemented implants and highlighted the importance of design features in promoting bone ingrowth and securing a stable fixation.