MAGNETIC-RESONANCE-IMAGING AND BIOLOGICAL CHANGES IN INJURED INTERVERTEBRAL DISCS UNDER NORMAL AND INCREASED MECHANICAL DEMANDS

Study Design. An animal model was used to examine the short-term tissu e response to changes in the mechanical environment after the structur e (disc) is mechanically injured. Objectives. To observe changes in an injured intervertebral disc and the corresponding motion segment when the mechanical demands of the disc were increased by fusion of the ad jacent motion segments. Summary of Background Data. Disc degeneration has been modeled in animals by producing a tear in the anulus via lami nectomy, laparotomy, or posterolaterally. Methods of altering and quan tifying the mechanics of the intervertebral joint by use of internal f ixation and fusion in the canine have been developed. Methods. Eight d ogs divided into two groups (a study and a control group) had anular s tab wounds (L2-L3). The study group was surgically instrumented poster iorly from L3 to L7. Magnetic resonance imaging studies were conducted for all animals before and periodically after the surgical procedures . At the end of the study, the segments were processed histologically and biochemically. Results. Anular bulging was seen on magnetic resona nce imaging in all control animals 4 months after injury and did not p rogress out until 6 months after injury and did not progress out until 6 months after injury. Similar changes wee seen in study animals, but 75% were herniated by 6 months. Histologic changes correlated with ma gnetic resonance imaging changes. No significant difference in water o r proteoglycan content of the disc tissue between groups was found.Con clusions. Progression from the bulging of the anulus to herniation was not evident in damaged discs not subjected to adjacent fusions. No ch ange in water or proteoglycan content as a function of altered mechani cal state was found, suggesting the short-term effect of the altered m echanics is on the mechanical structure and not on the cells or extrac ellular matrix.