Endochondral ossification

Last revised by Ciléin Kearns on 11 Jul 2024

Endochondral ossification is one of the two key methods of bone formation (the other being intramembranous ossification). Endochondral ossification occurs in ossification centers within a cartilaginous mold (anlagen) of a bone, progressively replacing that mold with bone 4. After primary ossification, endochondral ossification contributes to longitudinal growth during skeletal maturation until the physes (growth plates) close, and it is involved in healing fractures 2-6.

Development

Endochondral ossification forms the long bones of the axial skeleton, and the appendicular skeleton 3. Intramembranous ossification forms the flat bones such as the skull, mandible, and mid-clavicle.

Mesenchymal precursor cells form the anlagen, and at around 8 weeks gestation, chondroblasts drill a central medullary canal through this while osteoblasts deposit cortex deep to the periosteum 4. This forms the primary ossification center in the mid-diaphysis, from which ossification extends radially, replacing the anlagen. Secondary ossification centers form later at the epiphysis and apophysis.

Physes (growth plates) result from the trapped anlagen where bone formation from the primary and secondary ossification centers meet 4. Longitudinal growth continues at the physes until they fuse at skeletal maturity 4.

Pathology

Fracture healing can be by direct union (intramembranous ossification), or indirect union (primarily endochondral ossification, with some intramembranous ossification) 5,6. As direct healing requires anatomic reduction and stability (typically requiring open reduction and internal fixation to achieve), most bone healing is by indirect healing 5,6. Cartilage formation in the fracture gap helps to form a soft callus, increasing the stability at a fracture site 6. Chondrocyte apoptosis creates spaces for revascularization, facilitating conversion of the soft callus to hard (bony) callus, and remodeling to complete healing 6,7.

Updating… Please wait.

 Unable to process the form. Check for errors and try again.

 Thank you for updating your details.