When surgeons talk about Partial Knee Replacement (PKR) they may also use the term Unicondylar Knee Replacement (UKR).  This involves replacement of only the damaged part of the knee (usually the medial or inside compartment) 

Planning is an important part of the process because our knees vary in size.   In partial knee replacement it is especally important to get a good fit.  You can see how the plan below, matches the implant in the xray taken just after surgery