The most frequent technical difficulty encountered at unicompartmental knee arthroplasty (UKA) revision to total knee arthroplasty (TKA) is filling in all bone defects. These bone defects can render difficult components positioning, mechanical axis restitution, and ligament balance assessment, which are the three most important parameters for successful TKA. We describe a computer-assisted technique which makes it possible to control these three parameters before removal of the implants that have caused the bone defects. Our study is based on a series of 20 cases, with a minimum follow-up of 2 years. The anatomical and clinical results were very satisfying and comparable to results of primary TKA. We recommend this computer-navigated technique, which is as simple as a primary TKA procedure.