Osteolysis is a term to define bone loss. It's not uncommon after total knee replacements (TKRs). Bits of bone and debris from the back of the implant fleck off over time. The tibial plateau is referring to the lower leg implant. It's more of a flat surface (hence the term 'plateau') compared to the round end of the femoral (thigh) side of the implant. The medial side is the inside edge of the joint.
Scientists are studying this problem. They are trying to reduce the amount of osteolysis. So far they've found three risk factors. These include polishing the back of the implant (where it joins the bone). The second is using a different method of sterilization. The one risk factor that can't be changed is gender. Men are much more likely to have osteolysis after TKR compared to women.
With mild osteolysis you'll want to keep making regular visits to your surgeon for follow-up. X-rays taken over time will help make sure you get further treatment when it's needed sooner than later.