Hua, Jia; (1994) Design and evaluation of CAD-CAM custom-made hip prosthesis. Doctoral thesis (Ph.D), UCL (University College London).

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Abstract

The clinical results of uncemented total hip replacement are strongly influenced by the geometry of the stem and its fit within the femoral canal. Particularly where the femoral canals are severely distorted such as in CDH or JRA, the femoral canal can not be accurately fitted by conventional hip stems. The condition of the femur on revision is highly variable, making it difficult to achieve consistently satisfactory constructs in each case. Therefore, there is a strong case for suggesting that custom-designed stems should play an important role in hip reconstruction. For this purpose, a Hip Design Workstation was developed, with the ability to design and manufacture custom femoral intramedullary stems for both primary and revision cases by using the technique of Computer-Aided-Design (CAD) and Computer-Aided-Manufacture (CAM). This system can three dimensionally reconstruct the femoral canal by digitizing A-P and M-L views of the femoral contour from plain radiographic images. In addition, the femoral canal can be reconstructed from CT scans. During reconstructing the femoral canal from plain X-ray films, special software was developed which can correct rotations of radiographs up to 30 degrees in the lateral view and 60 degrees in the frontal view. After the canal was reconstructed, the software then designed the stem for the individual canal with optimal design algorithms, allowing for multiple design options to be specified by the user. In order to evaluate the custom stems, several comparative studies with standard stems were carried out. The hypotheses for these studies were that custom femoral stems would produce a closer fit in critical regions, would be more stable on stem-bone interface motion, and would produce closer-to-normal stress distributions. In addition, the advantages of the custom stems would be reflected on the clinical results and radiographic appearances. The stem-canal fit study was conducted by analysing the gaps between stems and canals in four quadrants. Strains on the surface of a femoral bone before and after stem insertion was measured by using a photoelastic coating technique. Stem-bone interface motion under cyclic loading were quantified in four axes by using Linearly Variable Differential Transducers (LVDT). The data from six channels were simultaneously recorded by a linked computer. Radiographic studies including measurement of stem migration and bone density changes were conducted by using a newly developed technique and DEXA scan. In addition, a biological study of tissue osseointegration with titanium stems of different stiffness was carried out on rat femurs. The results from the studies showed that the custom stems were superior to the standard stems in terms of canal fit, strain distribution and interface motions. For the radiographic follow up, the results showed that 79% of custom stems were stable from zero to six months and 96% from six to twelve months, which was comparable with cemented stems. Bone mineral density changes were found to be less than 10% from zero to six months and from six to twelve months. In the biological study, the results showed that less stiffness of the implant could enhance the osseointegration onto the stems. This provides useful information for hip stem design. This study demonstrated that custom stems provide a better solution for uncemented total hip replacement. The CAD-CAM technique offers great accuracy and flexibility in designing and manufacturing individual stems. The method does however require additional time and expense compared with using an off-the-shelf stem. Nevertheless it is fully justified for many cases at this time, while a further reduction in cost will extend the indications to the more standard cases.

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