POSTNATAL-DEVELOPMENT OF THE CRUCIATE LIGAMENT INSERTIONS IN THE RAT KNEE - MORPHOLOGICAL EVALUATION AND IMMUNOHISTOCHEMICAL STUDY OF COLLAGENS TYPE-I AND TYPE-II

The postnatal structural remodelling and calcification patterns in the insertions (entheses) of both cruciate ligaments were studied in a ra t model with histology and immunohistochemical analysis of collagens t ypes I and II. In the neonate, both ligaments which labelled only for type I collagen attached to epiphyseal cartilage which solely labelled for collagen type II. The entheses calcified between days 20 and 35, and a subchondral bone plate formed under the entheses between days 30 and 55. Thus, within a period of 35 days the tissue to which the liga ments attached increased multifold in stiffness. Interestingly, the pr ocess of enthesial calcification and formation of compact bone did not happen simultaneously in both ligaments, not even synchronous at both ends of the same ligament or within a single insertion. This asynchro nous calcification of the different knee ligament insertions may make the sudden change in mechanical environment at the entheses less drama tic for the ligaments and knee joint surfaces as anticipated from mech anical models. In addition, a fibrocartilaginous tissue, rich in colla gen type II, formed in the ligament at a time when the epiphyseal cart ilage was replaced by bone, and grew wider with time. The interpositio n of a fibrocartilaginous zone in the insertion may diminish the sudde n change in stiffness between ligament soft tissue and hard bone.