Postnatal Development of the Cruciate Ligament Insertions in the Rat Knee

The postnatal structural remodelling and calcification patterns in the insertions (entheses) of both cruciate ligaments were studied in a rat model with histology and immunohistochemical analysis of collagens types 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 ligaments attached increased multifold in stiffness. Interestingly, the process 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 asynchronous calcification of the different knee ligament insertions may make the sudden change in mechanical environment at the entheses less dramatic for the ligaments and knee joint surfaces as anticipated from mechanical models. In addition, a fibrocartilaginous tissue, rich in collagen type II, formed in the ligament at a time when the epiphyseal cartilage was replaced by bone, and grew wider with time. The interposition of a fibrocartilaginous zone in the insertion may diminish the sudden change in stiffness between ligament soft tissue and hard bone.