THE POSTNATAL-DEVELOPMENT OF THE INSERTIONS OF THE MEDIAL COLLATERAL LIGAMENT IN THE RAT KNEE

Bone soft tissue remodelling at the femoral and tibial insertions of t he medial collateral ligament (MCL) of the rat knee was monitored at r egular intervals from birth to 120 days of age in 40 Sprague Dawley ra ts. At birth the femoral insertion originated from the perichondrium o f the epiphysis. By day 8 the perichondrium within the insertion had t urned into fibrocartilage. Secondary ossification of the femoral epiph ysis had progressed in the region near to the insertion site by day 15 . The epiphyseal cartilage was entirely replaced by bone by day 40 exc ept for the fibrocartilage within the insertion. After that stage, no qualitative change in zonal insertion characteristics was observed, bu t only increase in size and decrease in cellularity. At birth, the tib ial ligament inserted onto the thin cortical bone of the metaphysis vi a periosteum. At day 8, osteoclasts started to resorb the thin cortica l bone at the ligament insertion, thus forming a metaphyseal depressio n between days 10 and 20. From days 20 to 120, the insertion remained qualitatively unchanged, showing three zones, the ligament, periosteum , and metaphyseal trabecular bone. The deep periosteal layer showed os teoclastic activity in the proximal part and osteoblastic activity in the distal part. The migration mechanism of the ligament insertion dur ing growth seems to be caused by this growth-related osteoclastic reso rption of the proximal metaphyseal bone and by simultaneous osteogenic activity, which successively cements the distal part of the ligament to bone. The persistence of the periosteal layer and the metaphyseal d epression for up to 120 days may be regarded as a sign of continuing g rowth in this animal model. This is the first investigation showing th at the formation of the metaphyseal depression is a purely postnatal e vent, and suggests that this process might be initiated by the change in mode of growth and joint biomechanics after birth, enabling ligamen t development and migration in a growing and increasingly loaded weigh t-bearing joint. The mainly resorptive process, which takes place duri ng development of the tibial MCL insertion, may account for the tensil e failure of this ligament that commonly occurs at this site during gr owth. The pronounced morphological differences between the chondral fe moral and the periosteal tibial attachment of the adult MCL are appare ntly caused by the different postnatal developmental processes at epip hyses and metaphyses.