Radial displacement of the medial meniscus in varus osteoarthritis of the knee.

The macroscopic and histologic findings for 31 medial menisci and medial tibial plateaus obtained during total knee arthroplasty were examined to clarify the etiology and progression of varus osteoarthritis. Medial menisci were preserved fairly well in cases of severe osteoarthritis in which the medial joint space had already disappeared. The anterior segment was preserved in 26 (84%) menisci and the posterior segment was preserved in 11 (35%). The medial meniscus may have been preserved because of its radial displacement. Exposure of subchondral bone of the medial tibial plateau occurred in all 31 knees. The exposure of subchondral bone was centered in the anterior, middle, and posterior in nine, 10, and 12 medial tibial plateaus, respectively. There was a mechanical inconsistency between the pattern of preservation of the medial menisci and the location of exposure of subchondral bone on the medial tibial plateaus. The inconsistency reflects that the segment of the medial meniscus on which the excessive load was considered to exist was preserved fairly well. The authors' hypothesis for explaining this inconsistency is that radial displacement of the medial meniscus precedes narrowing of the medial joint space during progression of varus osteoarthritis, so that the displaced meniscus is saved from severe degeneration or attrition.