The fixation of joint prostheses. Aspects on mechanics and biology

Abstract: In the interface, between implant and bone or between the cement and bone, there will develop some amount of fibrous tissue. The synovial fluid, the synovium, the cartilage and the subchondral bone from the non-replaced knee compartment that had been exposed to wear particles were studied by histological analyses. These analyses showed polyethylene wear particles in all tissues except the cartilage. The analysed tissues showed frequently foreign body reactions. These reactions are a possible cause of bone resorption that may eventually lead to loosening of the implant. The lack of finding particles or foreign-body reactions in the cartilage is possibly an effect of the cartilage being without blood supply and thus less exposed to the immune system. In our second study we tried to enhance the possibilities for bone ingrowth in the porous coated surface of a knee implant. We tried by either using cement only in central, or in peripheral parts of the tibial bone bed. The results after 8 years did not differ between the groups. Minimising the shear forces at the implant-bone interface is a possibility to enhance the fixation, resulting in less migration, of the tibial implant. This was studied in a randomised clinical study where the patients were randomised to a group with fixed insert or a group with mobile insert.The study showed that the mobile inserts did move according to the design, however, no signs of improved migration characteristics during the first two years were found. Improving the fixation strength with hydroxyapatite coating of the implants surfaces facing the bone have been proven successful. A solution deposition technique of hydroxyapatite coating of the porous surface of the implant while preserving the porosities and also allowing more precise monitoring of the thickness was tested. Uncemented fixation of a knee prosthesis with or without this HA coating was studied. Initially the RSA results showed a more stable migratory pattern in the HA coated group but after two years there were no difference. In order to enhance fixation by minimising the bone resorption that occurres after the bone-cutting and implantation of the prosthesis, bisphosphonate may be used. We studied the effects of treatment started after surgery using uncemented fixation. The patients did either get the active substance, alendronate, or placebo. The follow-up using RSA did not show any differences regarding migration during the first two years