Macroscopic and microscopic analysis of the structures inserted in the posterior segment of the medial meniscus. How is the semimembranosus tendon involved in ramp lesions?

OBJECTIVES: The anatomical description of the posterior segment of the medial meniscus is debatable. The aim of this study was to describe by macroscopic and microscopic analysis the histological nature of the posterior segment of the medial meniscus and the inserted structures (semimembranosus tendon and menisco-tibial ligament) METHODS: Fourteen fresh knees were dissected. For each specimen, a stable anatomical piece was taken en bloc, including the medial femoral condyle, the medial tibial condyle, the entire medial meniscus, the cruciate ligaments and the joint capsule, and the distal insertion of the semimembranosus tendon was preserved in its entirety. At this stage, a macroscopic analysis was performed. The blocks were cut along the sagittal plane in order to isolate the distal insertion of the semimembranosus tendon on the posterior joint capsule and the posterior segment of the medial meniscus in the same section. Histological slides were produced from these samples and were microscopically analyzed. RESULTS: In all patients, the macroscopic analysis showed direct semimembranosus tendon expansion and tendinous capsular expansion ending behind the posterior segment of the medial meniscus. It projected onto the joint capsule, on the meniscotibial ligament at the bottom and the meniscocapsular ligament at the top, but never ended directly in the meniscal tissue. On average, the tendon directly inserted 11 ± 2.8 mm below the articular surface of the tibial plateau. The length of the capsular expansion was 14.3 ± 4.4 mm. The meniscotibial ligament was inserted in the posterior-inferior edge of the posterior segment of the medial meniscus and the meniscocapsular ligament in the posterior-superior edge. There was a particularly vascularized adipocyte space delimited by the posterior segment of the medial meniscus, the meniscotibial ligament, the meniscocapsular ligament and the capsular expansion of the semimembranosus tendon. CONCLUSION: We repeatedly noted capsular expansion of the semimembranosus tendon that inserted behind the medial meniscus. There is an interposing zone between the tendon insertion and the body of the meniscus which creates a fragile zone. The capsular tendon expansion also inserts in the meniscotibial ligaments at the bottom and meniscocapsular ligaments at the top.