Medial Femoral Condyle Cyst in Severe Varus Knee Osteoarthritis: A Finding Prior to Primary Total Knee Arthroplasty

Knee osteoarthritis (OA) often results in subchondral bone cysts (SBCs), which were initially identified as a concentric arrangement of trabeculae surrounding an enlarged marrow space on plain radiographs. Although the Anderson Orthopedic Research Institute (AORI) classification is commonly used, it lacks quantitative measures and is based on radiographs, which can underestimate the actual bone defect. There is a need for a more comprehensive classification system to achieve accurate preoperative planning for bone defect management. A 74-year-old male presented, complaining of bilateral knee pain that began seven years ago. Non-operative management failed to relieve his symptoms despite his ideal BMI of 23.6. Initial radiographs revealed severe bilateral OA (Kellgren and Lawrence grade 4) with a suspected cyst occupying the medial femoral condyle. A CT scan of the right knee confirmed the presence of a cyst in the medial femoral condyle. The authors believe that patients with severe knee OA (Kellgren and Lawrence grade 3 or 4) should not simply be treated as having a sequel of knee arthritis. Instead, a CT scan should be conducted to confirm the size and extent of any cyst.