Injuries in the adolescent porcine spine exposed to mechanical compression

Study Design. An experimental procine study in which functional lumbar spin al units were tested in compression to failure. Biomechanical, radiographic , magnetic resonance imaging, and histological characteristics are describe d. Objectives. To explain the different patterns of injury seen in adults and adolescents resulting from traumatic injury to the vertebrae and to explain the mechanism behind traumatic displacement of the ring apophysis seen in athletes. Summary of Background Data. Recent investigations of the spine in adolescen ts who have sustained trauma have shown injuries to the growth zone, wherea s studies of adults have shown injuries to the vertebral body. A higher fre quency of abnormalities in the discs, the vertebral bodies, the endplates, and the ring apophyses has been demonstrated in athletes with high loads on the spine. There is controversy over the etiology of these changes. Methods. Twelve functional lumbar spinal units (vertebra-disc-vertebra) obt ained from six young male pigs were tested in compression to failure. All u nits were examined with plain radiography and magnetic resonance imaging be fore and after compression. After the compression, histologic samples were taken from the injury site. Results. Identical traumatic changes were seen in all functional lumbar spi nal units, i.e., fracture in the endplate through the growth zone posterior ly and displacement of anulus fibrosus with a bony fragment at the point of insertion of the vertebra. The nucleus pulposus was ruptured and displaced through the fracture line in all cases. The injuries were not seen on radi ographs but were detected on magnetic resonance images, as confirmed on mic roscopic and histologic examination. Conclusion. This study shows that the weakest part of the lumbar spine of t he juvenile pig, when compresses, is the growth zone and the junction betwe en the point of insertion of the anulus fibrosus and the vertebra. This loc ation of weakness may explain the high frequency of disc degeneration and p ersisting apophysis seen in the spine of athletes.