The Effect of Gradual Passive Static Posterior Compression with Hooked Rods on the Spine of Growing Dogs

Abstract

Spinal instrumentations have been practiced frequently for surgical stabilization of unstable segment, correction of the various deformities and maintenance of the correction. We performed this study to assess the effect of passive posterior static compression immobilization with hooked rods on the growth of spines of immature dogs. The immobilized bony vertebrae grew less than the nonimmobilized vertebrae above and below, and narrowing of the intervertebral disc spaces in the immobilized segments occurred in all dogs. The A–P diameter of the spinal canal in the immobilized segments increased up to 14% in comparison with the mobilized vertebrae above and below. Degenerative changes and narrowing of the facet joints developed gradually. From postop 16 weeks onwards, degeneration of the growth plate cartilage of compressed vertebrae developed. These areas of degeneration were sharply demarcated from the remaining normal cartilage layer of the growth plate. The direction of horizontally arranged fibers of annulus fibrosus began to distort and all the fibers of the annulus were vertically arranged completely at the end. The endplate cartilage gradually became thinner, and lastly, the profound degeneration occurred. Through this study, it was found that growth of the vertebral height, particularly in the posterior part, was suppressed by the growth retardation of the growth plate. The lordosis of the immobilized vertebral segments was caused mostly by disc changes and partly by the corporal growth suppression. It was also found that prolonged posterior instrumented immobilization and compression of the spine ultimately resulted in arthrosis, and that the unequal posterior axial compression of immature spine with static compression rod or rods could induce lordoscoliosis. In summary, this study clearly demonstrated that instrumented posterior static compression of the growing spine by hooked rods suppressed the posterior spinal growth, while there was less growth suppression on the vertebral body anteriorly. In conclusion, this study provided plausible explanation for the development of the crankshaft phenomenon after posterior static-instrumented compressive stabilization of immature animals with hooked rods.