Expansion
Intramembranous for flat bones, endochondral for long bones
Expansion
Two types of ossification
- Intramembranous: bone forms directly from mesenchyme. The flat bones of the skull, the clavicle and the mandible
- Endochondral: bone replaces a cartilage model. All long bones, the base of the skull and the vertebrae
Growth plate (physis) zones, from epiphysis to diaphysis:
- Resting (reserve) cartilage
- Proliferative - chondrocytes divide and stack in columns
- Hypertrophic - chondrocytes enlarge. The weakest zone, and where fractures and slipped upper femoral epiphysis occur
- Calcification
- Ossification - vascular invasion and bone deposition
“Growth in length at the physis, growth in width from the periosteum.”
Salter-Harris classification of physeal fractures, SALTR:
- I - Straight across the physis
- II - Above, into the metaphysis. The commonest
- III - beLow, into the epiphysis
- IV - Through both
- V - cRush injury. The worst prognosis and often diagnosed retrospectively
Types III to V involve the joint surface or crush the plate, so they risk growth arrest and deformity, which is why they need accurate reduction and follow-up.