Muscle loading and endochondral ossification are involved in the regeneration of a fibrocartilaginous enthesis during tendon to bone healing in rabbits

PurposeThe purposes of the present study are to investigate the effects of reduced muscle loading by prolonged immobilization on the regeneration of fibrocartilaginous enthesis through endochondral ossification in rabbits.MethodsForty-eight rabbits underwent standard partial patellectomy were randomly divided into the control group and the prolonged immobilization (PIM) group. The immobilized cast was only maintained for the first 4 weeks in the control group, while for the first 12 weeks or until euthanization in the PIM group. The Patella-patella tendon complexes were harvested for Micro-CT and histology at week 6, 12 and 18.ResultsThere was significantly lower bone volume in the PIM group than the control group at week 12, but not at week 6 and 18. At week 6, new bone was formed at the osteotomy site of the residual patella through endochondral ossification. At week 12, the chondrocytes in the tendon to bone interface were ordered and arranged in longitudinal rows separated by collagen fibres in the control group. While there were no visible fibers running continuously from tendon into bone in the PIM group. At week 18, a nearly normal fibrocartilaginous enthesis were regenerated in the control group. A similar fibrocartilaginous enthesis were also formed at the tendon to bone interface in the PIM group, but the four zones were not as distinct as that in the control group.ConclusionMuscle loading and endochondral ossification are involved in the regeneration of a fibrocartilaginous enthesis during tendon to bone healing in this partial patellectomy model.