Metal implants in treatment of cartilage defects

Abstract

Localized cartilage defects in the knee are associated with disability and symptoms such as joint pain, locking phenomena and reduced or disturbed function. Moreover, these defects predispose to severe forms of osteoarthritis. The surgical treatment of localized cartilage defects is usually aimed at stimulation of biological repair, including subchondral perforation and autologous chondrocyte transplantation. Although the biological repair treatment modalities are well established, the have limitations. Often, fibrous tissue is formed, which is frequently followed by progressive joint degeneration. A proposed alternative for the treatment of localized cartilage defects is the use of defect-sized metal implants. The central aim of this thesis was to better understand the role and limitations of using small metal (femoral condyle) implants for the treatment of cartilage defects in the knee joint as well as to investigate the surgical possibilities for replacement of a larger cartilage surface (the medial tibial plateau). This was evaluated using different materials (cobalt-chromium and oxidized zirconium) in different animal models (rabbit and goat) by applying validated outcome tools. Both rabbit and goat studies investigating a metal implant as a treatment for a femoral condyle cartilage defect showed encouraging results and raise other research questions. First, the implants of either material were firmly osseointegrated and biocompatible. It was shown that a flush positioning is essential in a rabbit knee, but this has not been investigated in a goat knee. Perhaps a larger sized implant in a goat (or human) knee should be placed different. This is still to be investigated. Furthermore, it remains unclear which bearing material is the best performer against the opposing cartilage when used as a small implant, although previous in vitro studies were favorable to OxZr. Perhaps, the femoral condyle implant model is not sensitive enough to differentiate. Other, less hard materials (with equivalent friction characteristics) should be investigated as well. All in all, caution is warranted when using small metal implants as a treatment for localized cartilage defects in a human knee joints. A major finding in these studies was the generalized cartilage degeneration irrespective of the treatment. This degeneration was not only found after treatment with a metal implant, but also after treatment with the microfracture technique, which was included as a control group. This microfracture technique is one of the frequently used treatment modalities for a localized cartilage defect. This aspect of the microfracture technique has not been given full attention in previous research. Furthermore, the current tibial plateau replacement model might be a viable tool in the evaluation of the applicability of defined hemiarthroplasty implants. Since only one bearing material (cobalt-chromium) was investigated in this study, it remains unclear whether the model is sensitive enough to discriminate between candidate materials. The implant was firmly fixed and allowed for unlimited weight bearing and movement of the knee, but considerable opposing cartilage degeneration was induced, which raises caution as to its application in the human setting.