When learning about knee arthritis, there is an important concept that one needs to understand. Doctors talk of three 'compartments' in the knee. These are not actual discrete pockets, but are three areas where the bones of the knee make contact with one another.
Two of the compartments are formed by the rounded ends of the femur (condyles) where they make contact with the flat top of the tibia bone. These are the medial and lateral compartment. The third compartment is where the patella (kneecap) makes contact with the femur bone below it. This is the patellofemoral compartment.
Medial compartment osteoarthritis (OA) is more frequent than lateral compartment OA and commonly follows damage to the meniscus ('knee cartilages'), the medial meniscus being more vulnerable to injury than the lateral meniscus.
Remove the whole meniscus on the medial side ('total meniscectomy') and the altered forces will eventually result in medial compartment OA, the joint space on that side will collapse, the bone will try to heal things by pushing out bridges of bone, further distorting the anatomy, the cartilage will become under stress and eventually break down. Bow-legs may result.
The same on the outer side (remember, the fibula bone is on the outer lateral side).
The same thing may happen on the lateral side, but this is less common due to biomechanical factors in the menisci. Take out the whole meniscus on the lateral (outer) side and eventually lateral compartment osteoarthritis will develop, with the leg will become knock-kneed - the opposite of medial compartmentOA.
OA of medial or lateral compartment may also follow an injury, where a chondral defect may have occurred - a chunk of joint surface knocked off into the joint, leaving a crater in the joint surface and a 'loose body' in the joint cavity. Or untreated cruciate ligament damage may cause joint instability and joint surface and meniscal damage - leading to OA.
The patellofemoral compartment - the joint between the undersurface of the kneecap (patella) and the femur.
Patellofemoral compartment OA usually follows problems with the alignment of the kneecap onto the underlying thighbone (femur). Joint surface damage can occur at the side where there is increased contact (too much pressure) and also at the side where there is decreased pressure (too little contact).