[Torsional strength of the epiphyseal plate and fracture patterns with aging (three-dimensional analysis with SEM)]

The tortional strength and fracture patterns of the proximal tibial epiphyseal plate in 4-, 8-, and 12-week-old rabbits were investigated (n = 11, 13 and 8, respectively). The strength was measured with a torsional machine, and then maximum torque, angular deformation and energy absorption were calculated from the load-deformation curve. The rotation axis was aligned to the long axis of the specimen, and a holding method for the specimen was devised to minimize positional error. The fracture surfaces were then examined both histologically and with three-dimensional scanning electron microscopy (SEM). The torsion test showed that maximum torque and energy absorption with the exception of angular deformation increased significantly with age. Histological examination by light microscope of sagittal sections passing through the midline of fragments revealed that epiphysiolysis commonly occurred between the calcification layer and the metaphysis in the anterior portion, as well as between the transitional part of the palisading layer and the hypertrophying layer in the posterior portion. These results indicate that epiphysiolysis is most likely to occur between adjacent layers with the greatest structural differences. Three-dimensional SEM micrographs revealed that fracture patterns became more varied as the torsional strength of the epiphyseal plate was increased with age. Additionally, SEM revealed that the density of the collagen network in the cartilaginous matrix surrounding cells in the proliferating layer also increased with age. One of the reasons for the increase in torsional strength is related to a morphological change such as the development of three-dimensional wave-like structure of epiphyseal plate. Another reason appears to be a qualitative change such as the variety of the fracture patterns and an increase in the collagen network density of the cartilaginous matrix with age.