Collagen fibre and fibril ultrastructural arrangement of the superficial medial collateral ligament in the human knee

PURPOSE: The aim of the study was to investigate the collagen fibre ultrastructural arrangement and collagen fibril diameters in the superficial medial collateral ligament (sMCL) in the human knee. Considering sMCL's distinctive functions at different angles of knee flexion, it was hypothesized a significant difference between the collagen fibril diameters of each portion of the sMCL. METHODS: Fourteen sMCL from seven fresh males (by chance because of the availability) cadavers (median age 40 years, range 34-59 years) were harvested within 12 h of death. sMCLs were separated into two orders of regions for analysis. The first order (divisions) was anterior, central and posterior. Thereafter, each division was split into three regions (femoral, intermediate and tibial), generating nine portions. One sMCL from each cadaver was used for transmission electron microscopy (TEM) and morphometric analyses, whereas the contralateral sMCL was processed for light microscopy (LM) or scanning electron microscopy (SEM). RESULTS: LM and SEM analyses showed a complex tridimensional architecture, with the presence of wavy collagen fibres or crimps. TEM analysis showed significant differences in median collagen fibril diameter among portions inside the anterior, central and posterior division of the sMCL (p < 0.0001 within each division). Significant differences were also present among the median [interquartile range] collagen fibril diameters of anterior (39.4 [47.8-32.9]), central (38.5 [44.4-34.0]) and posterior (41.7 [52.2-35.4]) division (p = 0.0001); femoral (38.2 [45.0-32.7]), intermediate (40.3 [47.3-36.1]) and tibial (40.7 [55.0-32.2]) region (p = 0.0001). CONCLUSIONS: Human sMCL showed a complex architecture that allows restraining different knee motions at different angles of knee flexion. The posterior division of sMCL accounted for the largest median collagen fibril diameter. The femoral region of sMCL accounted for the smallest median collagen fibril diameter. The presence of crimps in the medial collateral ligament, previously identified in the rat, was confirmed in humans (taking into consideration differences between these two species). KEYWORDS: Collagen fibre array; Crimps; Human; Knee; Medial collateral ligament; Ultrastructure