Effects of Intraoperative Tensioning of an Anterolateral Spinal Tether on Spinal Growth Modulation in a Porcine Model

Study Design. In vivo analysis in an immature porcine model. Objective. To evaluate the effect of intraoperative tensioning of an anterolateral flexible spinal tether on growth modulation manifested as deformity creation, disc response, spinal motion, and screw fixation using radiographs, computed tomography, magnetic resonance imaging, biomechanical testing, and histology. Summary of Background Data. Spinal growth modulation using an anterolateral flexible tether has been proposed as a nonfusion surgical deformity correction strategy for idiopathic scoliosis and has been successfully demonstrated in a porcine model to create spinal deformity while maintaining disc viability. Methods. Twelve 7-month-old mini-pigs were instrumented with a screw-staple and polyethylene tether construct over 4 consecutive thoracic vertebrae (T8-T11). Intraoperative tensioning of the tether (250 N) was performed in alternate pigs (Pretensioned and Untensioned groups, n = 6 per group). Screws were coated with hydroxyapatite in half of the animals in each surgical group. Preoperative, postoperative, and monthly radiographs were evaluated, comparing deformity creation, vertebral body wedging, and disc wedging between the groups. Vertebral body shape was evaluated by computed tomography. Magnetic resonance and histology evaluated disc health. Biomechanical testing was performed to determine the effect of tensioning the tether on spinal motion and screw fixation. Results. Intraoperative tensioning produced immediate coronal deformity (8 degrees +/- 4 degrees vs. 2 degrees +/- 1 degrees in untensioned spines; P = 0.01) and apical disc (T9-T10) wedging, vertex on tethered side, (5 degrees +/- 2 degrees vs. 2 degrees +/- 1 degrees; P = 0.01). After 12 months, the groups were similar in coronal deformity (28 degrees +/- 18 degrees pretensioned, 27 degrees +/- 11 degrees untensioned, P = 0.88), sagittal deformity (25 degrees +/- 3 degrees vs. 22 degrees +/- 3 degrees; P = 0.14), vertebral body wedging (10 degrees +/- 5 degrees vs. 8 degrees +/- 3 degrees; P = 0.45), and disc wedging (-4 degrees +/- 1 degrees vs. -4 degrees +/- 3 degrees; P = 0.88). There was no radiographic evidence of screw loosening. One of the discs from each group had diminished T2 signal after 12 months of tethering. Tether pretensioning did not affect spinal stiffness or motion. Interestingly, screw fixation increased with pretensioning; however, there was no significant advantage with hydroxyapatite coating. Histology demonstrated normal-appearing discs. Conclusion. Pretensioning of the tether created immediate deformity without effecting ultimate vertebral or disc deformity creation. Spinal motion and stiffness were not altered by pretensioning; however, pretensioning increased the torque required for screw extraction.