Abstract

Purpose: Hip arthroplasty is a common surgical procedure to relieve pain caused by disease or hip fractions etc.. The durability of the procedure is finite, which makes treatment of younger patients challenging. Based on this, research with short femoral stems in order to preserve bone for multiple revisions have been made. One of these is the newly developed Primoris stem. This is fixated by bone compaction, which in theory provides a good initial stability. But the preconditions of the host bone are crucial for obtaining a successful result, however no studies have been made in order to investigate this. The aim of this thesis was to analyse the initial stability of Primoris, based on the compressive behaviour of bone tissue and thereby create representative mathematical models to analyse the effect of different preconditions of the host bone. Methods: As themechanical behaviour of bone tissue depends on the relative density ½r and additionally changes during compaction, a mathematical expression was derived for pr = 0.2 to pr = 0.6, in order to obtain results which rely on the true behaviour of the bone tissue. For analysing the stability by estimating the pull-out force, a Finite Element (FE) model was developed using LSDYNA. This method allows a simulation of the insertion process in 3D of where the impact of different preconditions of the host bone can be estimated. But as the reliability of the results fromthe FEmodel is difficult to determine, a simplified analytical model was created to provide a basic understanding of the process and thereby be able to develop the FE model. Results: The initial process of developing a mathematical expression in order to describe the compressive behaviour of the bone tissue, followed the trend found in the literature and provided the basis for the two models. The analytical model showed that the lowest contact pressure obtained was at pr = 0.2, where only 7 % compaction was obtained compared with 94.7 % at pr= 0.6. It was also found that the friction between the bone and stem had a major impact on the pull-out force. Both the analytical model and the FE model showed that various pr are of great importance for succes, as an increase in pr contributes with a higher pull-out force. They also showed higher pull-out force when changing the conical angle from µ=3.5°to µ=3.5°. Conclusion: There were a significant similarity between the results obtained from the models when changing the preconditions, of which it can be concluded that the FE model demonstrated the ability of analysing different preconditions and thereby estimating the initial stability. This can be useful for a qualitative prediction of the surgical procedure. The parameter effecting the stability the most was pr , as a lower pr resulted in lower pull-out force, and thereby an inferior stability. But it was also seen that the geometry of the stem had an impact of the result.

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A master thesis from Aalborg University

A mathematical analysis of the initial stability of a short femoral prosthesis