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.
The Cost-Effectiveness of Surgical Treatment of Medial Unicompartmental Knee Osteoarthritis in Younger Patients: A Computer Model-Based Evaluation. Konopka JF, Gomoll AH, Thornhill TS, Katz JN and Losina E. J Bone Joint Surg Am. 2015 May 20; 97(10): 807–817.
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