Total knee replacement (TKR) continues to increase in popularity, but satisfying patients who remain active for longer is a major challenge. Outcomes from TKR remain suboptimal, with as many as 20% gaining little benefit. An alternative approach to TKR is the use of partial or unicompartmental knee replacements (UKR). These are smaller and safer operations, but are arguably more difficult to perform and have been associated with a higher rate of revision surgery. Whilst a great deal is understood about medial UKR, relatively little is known about lateral UKR and this procedure forms the basis of this thesis. Outcomes of a lateral UKR implant, the Oxford Domed Lateral Partial Knee were assessed through upto 10 year follow-up of the largest independent series of this implant and demonstrated similar rates of revision and Oxford Knee Scores to previously published medial UKR series. Further assessment of the gait of patients with the same implant however failed to show a significant improvement over demographically matched patients with TKR. Accuracy of implant positioning is important for achieving optimal outcomes. One technology that may assist in delivering expert level skill is patient specific instrumentation (PSI). For the first time, the effect of changes in PSI guide design were explored, demonstrating the importance of achieving multiplanar bone contact. The use of PSI guides compared to traditional instruments was associated with superior femoral implant positioning and a trend for superior tibial implant positioning when used by inexperienced surgeons on a training course. The effect of tibial implant malpositioning on tibial strain was demonstrated using digital image correlation. Excessive posterior slope and tibial resection should be avoided to prevent higher tibial strains that may result in fracture. A pre-requisite for successful arthroplasty, including lateral UKR, is the establishment of osseomechanical integration. Enhanced osseomechanical integration may be better achieved with strontium-substituted bioactive glass (SrBG) than a hydroxyapatite (HA) coating as demonstrated in a lapine model. The data presented in this thesis demonstrates that lateral UKR is a challenging but effective operation, though further development is required to facilitate normal gait. Further improvements in function and outcomes may be achievable through the use of PSI and enhanced implant fixation with SrBG coatings.