The results of total knee arthroplasty are inferior in younger patients. The challenge today is therefore to develop designs and concepts that will last at least 25 years. This thesis has evaluated the fixation to bone of modern designs of knee prostheses uring RSA analysis. Coating implant surfaces with hydroxy-apatite have proven to enhance fixation to bone. Addition of screws for fixation of the tibial component enhances the fixation, but has negative side effects such as osteolysis around the screws, in turn leading to a higher risk of component loosening. The magnitude and pattern of migration was studied in a randomized study of uncemented tibial implants coated with hydroxy-apatite with and without additional screw fixation in patients younger than 65 years. The uncemented implants migrated initially more than the cemented implants that constituted the control group. Both uncemented groups stabilized at 3 monthes with no further migration, while the cemented implants showed a continuous migration up to the 2 year follow-up, indicating continuous bone resorption at the implant-bone interface, a fact that might lead to an increased risk of late implant loosening. This may not be a problem in older patients, but may have consequences for long-term fixation in younger patients. There was no difference between the two uncemented groups indicating that screws do not improve fixation. Hydroxy-apatite coated knee implants might be well suited for younger patients. Mobile bearing total knee arthroplasty theoretically uncouples the forces at the implant-bone interface, thus improving fixation of the implant to bone. The magnitude and pattern of migration of a cemented mobile bearing knee arthroplasty and a fixed bearing total knee arthroplasty was compared in a randomized study. The results showed that mobile bearings did not improve fixation. Trabecular metal, a new material recently introduced for total knee arthroplasty, has several theoretical advantages. Trabecular metal tibial implants were evaluated in a randomized study in patients younger than 60 years. The implants displayed the typical migration pattern for uncemented implants with greater migration initially followed by early stabilization. The majority of the trabecular metal implants subsided into the bone with no lift-off. Lift-off has the potential of exposing the interface to joint fluid with the potential risk of bone resorption and late loosening, and is commonly seen in metal-backed implants. The finding of absence of lift-off is regarded beneficial for uncemented fixation. Trabecular metal tibial implants might be suited for younger patients. The optimal mode of fixation of the femoral component is yet to be established. Comparing cemented femoral components with uncemented femoral components in a randomized study in patients younger than 60 years revealed no differences of the magnitude or the pattern of migration. Uncemented femoral component seems equally as good as cemented components in younger patients.

[Truncated abstract] Unicondylar knee arthroplasty (UKA) has re-emerged as a successful treatment option for isolated single compartment tibio-femoral joint osteoarthritis. However despite its increasing use, controversy still remains over fixed or mobile bearing tibial components, as there is a lack to prospective randomised studies reported in the literature. In addition, the theoretical advantages of the mobile bearing for knee kinematics, kinetics and clinical outcome have not been evaluated in vivo. The aim of this research study was to explore the clinical and biomechanical outcomes of the fixed and mobile bearing UKA. . . . When the results for the both studies were combined, utilising the Preservation and MG fixed bearing prostheses, there was a significant relationship between knee adduction moment, and a poor prognosis predicted from RSA. Those patients with translation or rotation of the tibial component in any direction above 1mm and 1.5 degrees respectively were considered to have a poor prognosis for long term fixation. Of the 28 patients, the 8 patients considered to have a poor prognosis, had increased knee adduction moments post-surgery (mean difference = 1.66Nm.kg-1, p = 0.007). There was no difference between the groups for knee flexion moment (mean difference 0.16Nm.kg-1, p = 0.844). Pre-surgery gait was unable to predict the post-surgery outcome, due to the significant changes in gait from pre- to post-surgery. Care must taken when implanting the Preservation mobile bearing prosthesis, as long term outcome is questionable. The mobile bearing prosthesis also produced the worst clinical outcome, however the theoretical advantages of the mobile bearing does not affect gait. Gait analysis is a useful tool to identify patient who are overloading their prosthesis, leading to potential early failure. Identification of these gait patterns can allow for early intervention to reduce joint load, and possible extend the longevity of the prosthesis.