Changes in shape, ossification and quality of bones in children with spina bifida.

Changes in the cross-sectional shape, size, bone mass and amount of unmineralised osteoid tissue were studied in 17 dissected tibiae from spina-bifida babies who died with paralysis and foot deformities and in 14 tibiae from non-spina bifida controls of matching age. In addition, 12 tibiae from young experimental rats with myotomy of foot dorsiflexors and foot plantiflexors were double-labelled with bone-seeking markers and studied in order to find the role of experimental muscle imbalance in the dynamic remodelling of the developing long bones. It was found that in tibiae from spina-bifida children with paralysis the total area of cortical bone, its thickness, number of Haversian systems and number of large remodelling cavities are diminished. Significant changes in the cross-sectional shape of the midshaft of the tibia were found, ranging from the triangular shape seen in normal babies and in those with spina bifida and calcaneus-type foot deformity, to the circular shape of tibiae from babies with spina-bifida paralysis and no foot deformity or with spina bifida and equinovarus-type of deformity. Results of experimental myotomy on growing rats showed the direct influence of working muscles on the remodelling process of growing tibiae. On the side of myotomy the flat cortex resumed a bulging convex shape and the centre of gravity shifted towards the myotomised side. These principles cannot on their own explain the specific changes in the shape of human tibiae found during anatomical studies. There is, however, a common denominator in these apparently contradictory findings. This is the combined action of two factors previously reported: the combination of paralysis of the growing limb and mechanical intra-uterine pressure acting on it. The findings in the present study also indicate that they played a major role in the production of deformities. The total amount of osteoid tissue in spina-bifida paralysed bone is increased. This delay of mineralisation of newly laid-down bone matrix would lead to softening of the new bone matrix and osteoid-rich subepiphyseal and metaphyseal regions. This 'paralytic rickets', together with the diminished total bone mass found, could probably be the cause of the common spina-bifida fractures in these regions.