Shear fractures through the capital femoral physis of the skeletally immature rabbit.

Shear fractures were created through the capital femoral physes of rabbits 2, 4, 6, 10, and 14 weeks of age. Biomechanically, the load to complete failure (i.e., fracture) increased with maturation. The load to failure increased with increasing area of the capital femoral physis. The shear stress increased with age and then began to plateau at 10 weeks of age. Histologically, the zone of failure involved the columnar zone in the specimens from rabbits 2-6 weeks old, progressed to the junction of the columnar and hypertrophic zones in the specimens from 10-week-old rabbits, and occurred within the zone of hypertrophy in the specimens from 14-week-old rabbits. Fractures progressed from a type I physeal fracture in the specimens from rabbits 4-6 weeks old to a type II physeal/metaphyseal fracture in the specimens from older animals, with the metaphyseal fragment increasing in size with increasing chondroosseous maturation of the specimen.