A new arthroscopic classification of degenerative medial meniscus root tear that correlates with meniscus extrusion on magnetic resonance imaging

Background: To determine a new classification system for medial meniscus root tears (MMRT) based on arthroscopic findings. Methods: 24 knees (55%) belonged to the nondisplaced or overlapped group, and 20 knees (45%) to the widely displaced group. Absolute meniscal extrusion was defined as distance between outer edge of the articular cartilage of tibial plateau and meniscal outer edge. Relative extrusion was defined as extruded meniscus width divided by entire meniscal width, multiplied by 100. The proportion of knees with major (>. 3 mm) extrusion were compared in two groups, as were the severity of chondral wear and osteoarthritic change. Results: Absolute (4.6 mm vs. 3.7 mm, P = 0.006) and relative (46% vs. 39%, P = 0.042) extrusion of the medial meniscus were greater in widely displaced than in nondisplaced or overlapped group. Medial joint space width was significantly narrower in the widely displaced than in the nondisplaced or overlapped group (3.0 mm vs. 4.0 mm, P = 0.007). The widely displaced group had a 4° greater varus deformity, and higher rates of major extrusion (>. 3 mm), grade III or IV chondral wear in the medial femoral condyle (60% vs. 29%, P = 0.039) and medial compartment osteoarthritis (75% vs. 21%, P = 0.001) than did the nondisplaced or overlapped group. Conclusions: Widely displaced MMRT had greater meniscal extrusion and more severe chondral wear and osteoarthritis than did nondisplaced or overlapped MMRT. In this novel classification system, the stage of MMRT severity was associated with tear site displacement. Level of evidence: Case series (level IV).