Unicompartmental arthritis is joint destruction affecting only the one side of a knee, very often the medial (or inner) side.
The concept of compartments
The knee is roughly considered to have three 'compartments' - where the joint surfaces of the bones articulate or rub together. On the inner side is the medial tibiofemoral compartment, on the outer side is the lateral tibiofemoral compartment, and under the patella is the patellofemoral compartment.
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