This usually occurs after an injury, where an area of articular cartilage (the solid lubricant inside the joint lining the bones) is damaged. It can present as pain, locking and/or giving way, and usually requires a MRI or knee arthroscopy (looking inside the joint with a camera) to accurately diagnose.
As the area is discrete, it falls short of true arthritis, and so a joint replacement should be avoided where possible, as it will significantly decrease function.
It can be treated with a procedure called autologous chondrocyte implantation (ACI), where the patient’s own cartilage cells are harvested, grown in the lab, then implanted back in to the patient to fill the defect.
While not always completely successful in removing pain, it has excellent results worldwide.
If you undergo this procedure, it is important to be prepared. You will have two operations, roughly 6-8 weeks apart. This procedure will be day surgery to harvest the cartilage cells (knee arthroscopy); it then takes 6-8weeks to grow the cells; then a second operation for implantation. This 2nd operation will almost always be an open procedure, and probably a 2-3 day stay in hospital.
If the defect is present on a weight-bearing area of the knee, you will be allowed to bend the knee almost normally but not take weight on the leg for around six weeks.
If the defect is on the patella (knee cap), you will be allowed to take weight on the leg, but will have to keep it straight in a brace for 2-3 weeks, with a graduated return to normal motion over the next three weeks.
Unfortunately (at the moment) this procedure (ACI) is not currently funded by Medicare, as we as surgeons fouled our own nest by doing too many of them on patients it was never going to work on. You can always pay for it yourself, but currently it is only funded by WorkCover Qld and the DVA.
The alternative mainstream treatment is a procedure called microfracture, which involves drilling lots of small holes in the defect in an attempt to trick the body in to thinking the whole bone is broken. The blood supply to the area will increase, as well as a clot forming where the microfracture was done. This allows the body to regrow a type of cartilage, that while not as good as the original articular cartilage, is better than a patch of bare bone.