The knee is composed of two compartments: the medial compartment, which is the inside of the knee; and the lateral compartment, which is the outside of the knee. Bi-compartmental osteoarthritis involves deterioration of the knee joint in both the medial and lateral compartments.
For most people, osteoarthritis of the knee occurs primarily in the medial compartment, whereby the cartilage between the femur and the tibia weakens or deteriorates over time with normal wear and tear. As a result, the two leg bones that meet at the knee rub together, causing pain, swelling and stiffness [source: Kneeguru]. The entire joint space collapses when the medial meniscus becomes too weak or nonexistent, resulting in knee deformity and severe difficulty with knee movement [source: Kneeguru]. Bone spurs, which are little bone fragments, may also develop, leading to further pain and knee deformity [source: AAOS]. This deformity is called bow-leg.
Lateral compartment osteoarthritis involves deterioration of the lateral meniscus, which is similar in structure to the medial meniscus. Similarly, a weak or nonexistent lateral meniscus causes a complete collapse of the lateral compartment of the knee. This leads to a condition called "knock-kneed." [source: Kneeguru].
Healthy medial and lateral cartilage is slippery and smooth, allowing for absorption of shock and easy movement of the joint. As you can imagine, weak or deteriorated cartilage in both the medial and lateral compartments leads to problems with everyday tasks, such as bending, going up stairs, jumping, walking and running [source: AAOS]. As a result, bi-compartmental osteoarthritis, in its more moderate to severe form, causes disability. Normal, everyday activities are typically endured with pain or cannot be performed at all, due to stiffness or swelling [source: AAOS]. Unfortunately, total knee replacement surgery is the only permanently effective treatment option for moderate to severe bi-compartmental osteoarthritis.