## Osteoarthritis of the Knee: Compartmental Distribution **Key Point:** The medial compartment of the knee is the most common site of osteoarthritis, accounting for approximately 70–80% of cases. ### Anatomical Basis The medial compartment bears greater load during standing and walking due to the natural valgus alignment of the lower limb and the biomechanics of gait. The medial meniscus and medial femoral condyle are subjected to higher compressive forces. ### Compartmental Comparison | Compartment | Frequency | Reason | | --- | --- | --- | | **Medial** | 70–80% (most common) | Greater weight-bearing load; medial meniscus more prone to degenerative tears | | **Lateral** | 10–15% | Less common; often associated with varus deformity or lateral meniscal pathology | | **Patellofemoral** | 10–15% | Can occur in isolation or with tibiofemoral OA; related to patellar tracking | | **Tibiofibular** | <5% | Rare; usually secondary to primary tibiofemoral disease | **High-Yield:** Medial compartment OA is so common that it is the default assumption in knee OA unless otherwise specified. Progressive medial compartment disease leads to varus deformity ("bow-leg" appearance). **Clinical Pearl:** Patients with medial compartment OA typically report pain on the medial aspect of the knee, worse with prolonged standing or walking, and may develop a varus gait to offload the affected compartment. ### Why Medial Dominance? 1. **Weight-bearing axis:** In normal standing, the mechanical axis passes medial to the knee center, concentrating load on the medial compartment. 2. **Meniscal vulnerability:** The medial meniscus has firm attachments to the joint capsule and is more prone to degenerative tears with age. 3. **Cartilage thickness:** The medial femoral condyle cartilage thins more rapidly with age and use.
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