The semimembranosus muscle: A novel classification system of morphology and attachments with implications.

Introduction: The hamstring muscles consist of the biceps femoris, semitendinosus, and semimembranosus (SM). The SM is located on the posteromedial surface of the thigh and originates from a superolateral impression on the ischial tuberosity. Extensive research has been performed on the SM tendinous insertions into the posterolateral corner of the knee; however, limited data exist regarding the morphology of the main SM tendon. We hypothesized that distinct morphological types of the main SM tendon (SMT) exist and that their systematic classification can enhance clinical assessment, diagnostic accuracy, and surgical planning related to posterior knee structures. This study aimed to investigate the morphological variability of the SMT, focusing on its distal attachments.

Material And Methods: One hundred lower limbs from 50 formalin-fixed cadavers were examined. The SMT and its precise distal attachments were recorded and classified.

Results: The SM was present in all specimens (100 % constancy). A novel classification distinguished three morphological types: Type I (66 %), characterized by a single main tendon with three subtypes - Type IA (39 %), IB (12 %), and IC (15 %) - based on attachment shape and location; Type II (24 %), featuring a double main tendon with two subtypes - Type IIA (14 %) and IIB (10 %) - depending on attachment sites; and Type III (10 %), defined by three main tendons with distinct insertional bands. Considerable morphological variability in the number and configuration of accessory bands was observed.

Conclusion: The study provides a detailed anatomical description and an innovative classification of the main SMT morphology and distal attachments. This classification system may facilitate personalized surgical and rehabilitation strategies by improving anatomical understanding, aiding clinical assessment, and optimizing surgical planning for posterior knee interventions.