MECHANICAL MODULATION OF VERTEBRAL BODY GROWTH - IMPLICATIONS FOR SCOLIOSIS PROGRESSION

Study Design. The authors developed a rat-tail model to investigate th e hypothesis that vertebral wedging during growth in progressive spina l deformities results from asymmetric loading in a ''vicious cycle.'' Objectives. To document growth curves with axial compression or distra ction applied to tail vertebrae to determine whether compression load slows growth and distraction accelerates it. Summary of Background Dat a. Progression of skeletal deformity during growth is believed to be g overned by the Hueter-Volkmann law, but there is conflicting evidence to support this idea. Methods. Twenty-eight 6-week-old Sprague-Dawley rats were assigned to one of three group: compression loading, distrac tion loading, or sham (apparatus applied without loading). Under gener al anesthesia, two 0.7-mm diameter stainless steel percutaneous pins w ere used to transfix each of two vertebrae. The pins were glued to 25- mm diameter external ring fixators. Springs (load rate, 35 g/mm) were installed on three stainless steel threaded rods that were passed thro ugh holes in each ring and compressed with nuts to apply compression o r distraction forces between 25-75% of bodyweight, Vertebral growth ra tes in mu m/day were measured by digitizing the length of the vertebra e images in radiographs taken 0, 1, 3, 5, 7, and 9 weeks Results. The loaded vertebrae grew at 68% of control rate for compressed vertebrae and at 114% for distracted vertebrae. (Differences statistically signi ficant, P < 0.07 by analysis of variance.) For the compressed vertebra e, the pinned vertebrae, which were loaded at one of their two growth cartilages, grew at a reduced rate (85%), although this effect was not apparent for the distraction animals. Conclusions. The findings confi rm that vertebral growth is modulated by loading, according to the Hue ter-Volkmann principle. The quantification of this relationship will p ermit more rational design of conservative treatment of spinal deformi ty during the adolescent growth spurt.