Different kinds of partial knee

The Freedom knee, formerly the Repicci, that I use is a minimally invasive RESURFACING implant with minimal bone removal. Most partial knees done today remove more bone and look like half of a total knee replacement, they are called RESECTION partial knees. Here are some xrays to illustrate the difference 

Repicci partial knee
Oxford resection partial knee
ZUK resection partial knee

In an arthritic knee the bone is typically quite dense under the joint surface. The bone gets more porous as one goes farther down from the joint. The Freedom knee only removes about 4 mm of bone from the surface of the tibia, so that the plastic component is inlaid into good hard bone. A resection partial knee sits on a flat cut at least 6 mm deep and must have pegs and a keel that go deeper into the bone.

The pictures show what the bone looks like after removal of the implants in the tibia, the lower bone. On a resurfacing partial knee very little bone is missing so a total knee replacement standard bone cut goes at or under where the implant was. With a resection partial knee there is missing bone, making conversion to a total knee replacement difficult.

Resection partial knee tibial implant
After removal of resection implant
Inlaied resurfacing tibial component
After removal of inlaied component

Thus if a resurfacing partial knee like the Freedom knee has to be revised, it is easily done with a first time total knee replacement  
If a resection partial knee is revised long stems and metal shims, called augments, are often needed. 

Total knee after resection PKR (left)
Primary total knee replacement after Repicci resurfacing PKR (right)