Progression of congenital kyphosis in Ishibashi rats.

The development of congenital kyphosis was studied radiographically and histologically in 32 female and 34 male Ishibashi rats between the fourth and thirty-sixth week after birth. The major congenital spinal anomalies were defects of segmentation in the lumbar area. The malformation was classified into four groups according to the state of non-segmentation seen in radiographs. The progression of kyphosis was closely related to various changes at intervertebral spaces. The spinal deformity resulting from narrowing of the intervertebral spaces without bony fusion did not progress with growth, and the epiphyseal plates remained relatively intact unrelated to the existence of disc tissue. The deformity resulting from ventral bony fusion without radiographic bony anomalies showed histologically dysplastic epiphyseal plates on the ventral side. Progression of the kyphosis occurred in this type of deformity. The kyphosis resulting from bony fusion and ventral wedge-vertebrae progressed in the males. Progressive kyphosis appeared to be due to a difference in growth potential between the ventral and dorsal epiphyseal plates.