The McKeever hemiarthroplasty just replaces part of the joint surface. It's a unicompartmental implant for the tibial (lower leg) side of the knee joint. The bottom half of the knee is divided into two compartments. The inner edge is called the medial compartment. The outer edge is the lateral compartment.
The McKeever implant can replace either the medial or the lateral unicompartment. Most often the medial joint is affected and replaced. The McKeever is a thin piece of metal designed to replace the fairly flat tibial shelf. The femur (end of the thigh bone) slides and glides over the tibial plateau during knee motion.
This type of implant has been around since the 1950s. It's used mostly for patients who are too young, too active, or too heavy for a standard partial or complete joint replacement.