MECHANICAL MODULATION OF INTERVERTEBRAL DISC THICKNESS IN GROWING RATTAILS

Progression of scoliosis deformity during growth is thought to be caus ed by asymmetrical loading, resulting in asymmetrical growth with vert ebral and disc wedging in a ''vicious cycle.'' The purpose of this stu dy was to quantify the changes in disc thickness during growth in rat tails subjected to compression or distraction loading for 6 or 9 weeks , to investigate the hypothesis that disc growth is mechanically modul ated. Six-week-old Sprague-Dawley rats were studied with compression l oading (13 animals) or distraction loading (15 animals) applied to the ir tails, and there were 8 sham animals. Loading was applied to tail s egments by means of an external ring fixator. Radiographic measurement s of disc thickness were made at biweekly intervals. From the initial to final radiograph, compressed discs had reduced thickness averaging (+/-SD) 0.50 +/- 0.28 mm, distraction discs had average increased thic kness of 0.20 +/- 0.42 mm, and sham discs lost an average of 0.21 +/- 0.18 mm of thickness (analysis of variance p < 0.001). There was an '' initial change'' in disc thickness averaging 0.18 +/- 0.32 mm in nonlo aded discs, which was similar in magnitude to the elastic deformation and was attributed to disc swelling under anesthesia. These results in dicate that growth in disc thickness is mechanically modulated by axia l loading in growing rats.