Spinal Growth Modulation in an Immature Bovine Model
• a - DePuy Acromed
Introduction: Spinal growth modulation by tethering has been proposed as a nonfusion alternative to bracing and/or instrumented fusion in the treatment of idiopathic scoliosis. The purpose of this study was to evaluate the effects of an anterolateral thoracic spinal tether in an immature bovine model.
Methods: 34 one-month-old calves underwent a right-sided thoracotomy and placement of spinal instrumentation. In 6 calves, two 6.5mm screws were placed in each vertebra from T6 to T9; two nontensioned flexible 3/ 16” cables, secured by setscrews, connected these levels (Double Tether). In 11 calves, one 6.25mm screw was placed in the same vertebrae and then connected by a cable (Single Tether). 17 calves had one 6.25mm screw placed at the same levels; no cable was placed (Control). Postoperatively, coronal and sagittal view radiographs were taken. After a 6-month survival period, the instrumented spines were resected. Coronal and sagittal radiographs were evaluated by Cobb measurements. The T6-T7 disc was analyzed for water content; the T7- T8 disc was analyzed by an S35 uptake analysis for proteoglycan synthesis. Statistical analysis was performed with one-way ANOVAs.
Results: In the Double Tether group, immediate postoperative radiographs revealed an average 0±2° of scoliosis and 2±1° of kyphosis. After 6 months, the calves had an average 39±8° of scoliosis (range: 33° to 53°) and 20±9° of kyphosis. The average vertebral wedging was 10±8°, with most wedging occurring in the central T7 and T8 vertebrae. In the Single Tether group, there was an average of 1±3° scoliosis and 1±2° kyphosis postoperatively. After 6 months, there was variable deformity with 11±10° scoliosis (range: 1° to 37°) and 7±6° kyphosis. Although one specimen had 37° of scoliosis, in most specimens, the screws ploughed through, levered out or pulled loose of the bone. The Control group had no significant change (postoperative: 1±2° scoliosis, 2±2° kyphosis; 6 months: 4±1° scoliosis (range -1° to 14°), 1±3° lordosis). Although only the Double Tether group had significantly increased coronal deformity (p<0 .001), both Single and Double Tether groups had increased kyphosis (p<0.001) when compared to the Control group. There was no difference in disc water content between (57.1±3.2%) (56.5±2.6%) (p="0.73)." The S35 uptake analysis revealed a significant increase proteoglycan synthesis group (3190±383 cpm/mg vs 2133±340 cpm/mg, p="0.427)." although there statistical concave convex sides of tethered discs (3098±448 cpg/mg 3282±318 cpm/ mg,
Conclusions: It was possible to substantially affect spinal morphology with the placement of a flexible mechanical tether. The results of the double screw group demonstrated that adequate bony fixation can result in consistent biplanar deformity. Increased proteoglycan synthesis in tethered discs suggests a response to the increased force across the motion segment without loss of disc hydration; however, further studies to assess disc viability and vertebral growth histology are necessary. Placement of a multilevel flexible tether on the convex side of a scoliotic curve may provide a mechanism for correction with growth while maintaining long-term spinal flexibility and intervertebral disc health.
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