ABSTRACT of the Doctoral Thesis

Finite Element Based Investigation of Stress Fields and Bone Remodeling in and around Uncemented Tibial Prostheses

by Wolfgang Krach (1997)

Total knee replacements (TKR) have become increasingly necessary throughout the last years due to the increased life-time expectancy of human beings. Despite the progress that has been achieved over the last decades in developing new sophisticated TKR some failures still occur. Two thirds of these failures are caused by septic or aseptic loosening of the tibial part of the implant. The latter one being caused by a (functional) adaptation process of the bone called bone remodeling. The remaining possible scenarios are due to failure of device components. This raises the need for developing new tools that contribute to reduce the failure ratio of total joint replacements. The present work addresses some of the features that are necessary to improve the longevity of artificial joints.

An appropriate implant design first of all has to withstand the applied loads imposed to the device. Furthermore, the long-term success depends on the durability of the implant itself as well as on the quality of the underlying bone. Especially persons who are desired to receive an artificial joint replacement show a widely varying quality of the bone constituents due to the variety of possible diseases. Therefore, it is difficult to choose an appropriate design for an individual patient. Due to the complex geometry of human bones respectively bone/prosthesis arrangements and the sophisticated material characteristics of human bone numerical methods, e.g. the finite element method, are employed to solve such problems.

The present work captures the assessment of the patient-specific bone geometry and isotropic material data directly from quantitative computer tomography which is clinical routine. Utilizing standard image processing software the desired bone parts are extracted from the CT scans. The finite element model of the bone/prosthesis arrangement with position dependent material properties is set-up by using standard preprocessing software to keep the manual labor low in conjunction with a newly developed program code.

Furthermore, the basic mechanical characteristics of cortical and cancellous bone are discussed in the following chapter on the basis of a literature review. A discussion on the orthotropic material behavior of both cancellous and compact bone as well as some micro-mechanical models are included.

A more refined look on the modeling of cancellous bone as a crushable foam is carried out in chapter 3. The experimental work covers the elastic and inelastic regime of cancellous bone and consequently the Young's moduli and the yield limits of the uniaxial and hydrostatic behavior are measured and then disscused.

The first three chapters together cover the assessment of data which are necessary to set-up three-dimensional finite element models of a bone/prosthesis arrangement for an individual patient prior to the surgical treatment.

In a next step, the present study deals with the performance of three specific human total knee replacements. The possible failure modes of the tibial components of these implant devices are analyzed with both the tibial plateau made of a titanium alloy and the tibial insert made of polyethylene (PE) being addressed. Low-cycle fatigue due to accumulation of plastic deformation as well as high-cycle fatigue failure are covered in order to capture immediate and long-term failure scenarios of the titanium plateau and the PE insert.

Finally the study deals with the long-term stability of the implant concerning the 'bone remodeling' behavior which is caused by the altered stress/strain fields within the bone due to the insertion of an implant. It has been recognized for a long time that bone tissue has the ability to change its shape and internal architecture according to its functional requirements, i.e. the loading environment it is exposed to. Changes in the actual stress or strain pattern within the bone tissue stimulate pronounced cell activity resulting in a resorption of bone material in regions of low loading levels and {\it vice versa} in a deposition of bone tissue in highly stressed zones.

Chapter 5 first describes one of the numerous simulation tools currently available which was developed at the Institute of Lightweight Design and Structural Biomechanics and then shows the application towards the prediction of the density distribution in the neighborhood of the previously analyzed implant devices. Both two- and three-dimensional simulations show very good agreement with clinical observations which are based on X-rays. An evaluation of more than 60 patients contributes the basis of the comparison between the numerical simulation and the clinical evaluation.

revised 970305 (hjb)