Association between medial meniscus extrusion and spontaneous osteonecrosis of the knee

Aim Medial meniscus tear has been proposed as a potential etiology of spontaneous osteonecrosis of the knee (SONK). Disruption of collagen fibers within the meniscus causes meniscal extrusion, which results in alteration in load distribution in the knee. Our purpose was to determine whether the extent of medial meniscus extrusion correlates with the severity of SONK in the medial femoral condyle. Methods Radiological stage of SONK and femorotibial angle (FTA) were determined on knee radiographs. Ellipsoid volume of SONK lesion and meniscal pathology (degeneration, tear and extrusion) were evaluated by magnetic resonance imaging. Results All the 18 knees with SONK in the present study showed substantial extrusion (≥ 3 mm) and degeneration of the medial meniscus. The extent of meniscal extrusion and FTA were strongly associated with the stage and volume of the SONK lesion. Multiple linear regression analysis revealed that medial meniscus extrusion and FTA were useful predictors of the volume of the SONK lesion. Conclusion There was high association of medial meniscus extrusion and FTA with the radiological stage and volume of the SONK lesion. Increased loading in the medial femoral condyle with greater extrusion of medial meniscus and varus alignment may contribute to expansion and secondary osteoarthritic changes of a SONK lesion.