Serial Study Of Shape Memory Alloy Based Scoliosis Correction

BackgroundTill now, the 3-demetional correction effects of severe rigid scoliosis were improved by whole segmental pedicle screws instruments. But, some shortcomings relate to the regular rigid rod, especially in the apex region of severe scoliosis, are obviously, such as the difficult of placing rigid rod into whole segmental pedicle screws, the concentration of stress, the pull-out of pedicle screws etc. On the other hand, shape memory spinal rod is characterized by its mallealbility at low temperatures and its ability to return to a preconfigured shape above its activation temperature. This process can be utilized to assist in the scoliosis correction. Though serial scoliosis patients were performed spinal correction with shape memory alloy from May,2002, no in vitro biomechanical analysis of the technique was performed, no systematic review was performed to envalue the clinical and radiographic effects, and no quantitative study was performed to evaluate the derotation effect.Objectives.1. A biomechanical analysis of shape memory alloy based scoliosis correction was performed by finite element analysis.2. A retrospective study of scoliosis patients that underwent shape memory alloy rod assisted correction was performed to evaluate the clinical and radiographic results of a temporary shape memory alloy rod in the correction of scoliosis, and the advantages of shape memory alloy rod were evaluated by comparing the clinical and radiographic results obtained from temporary using of shape memory alloy rod and those from rigid rod in the correction of severe scoliosis.3. A prospective study of shape memory alloy based scoliosis correction was performed to envaluate the derotation effects by comparing to single rod derotation.Methods.1. Shape memory alloy based scoliosis correction was simulated for 1 patient with idiopathic scoliosis using 3D finite element model (FEM) of the patient's entire spine. The geometry of the FEM was extracted from a 3D CT scan reconstruction, and mechanical properties were personalized from literatures. The main step of shape memory based scoliosis correction was simulated and the results were compared with the postoperative 3D CT scan reconstruction.2. From May 2002 to Sep 2006,38 scoliosis patients (range from 50°to 120°,22 cases over 70°) that underwent shape memory alloy assisted correction in our institute were reviewed. During the operation, a shape memory alloy rod served as a temporary correction tool. Following correction, the rod was replaced by a rigid rod. The correction rate, operative time, blood loss, and complications were documented. Patients with matched curve type, ages at surgery, operative methods, and fusion levels in our institute were instrumented with shape memory alloy rods (SMA) (n=14) and traditional correction techniques (n=16) were compared.3. From Sep 2007 to Mar 2009,30 patients with scoliosis were treated with segmental pedicle screw fixation were analyzed. The first group (n=14) was corrected by shape memory alloy, the major curve the patients located at thoracic spine in 8 patients, at lumbar spine in 6 patients. The second group (n=16) was treated by simple rod derotation, the major curve the patients located at thoracic spine in 9 patients, at lumbar spine in 7 patients. Both groups were evaluated for the deformity correction in coronal plane and sagittal plane, and the apical vertebral rotation was evaluated by computed tomography scans.Results.1. The results of simulation show that the shape memory alloy based scoliosis correction is a real 3D technique, in which the vertebrae were corrected in coronal plane, sagittal plane, and the axial plane.2. The major Cobb angle improved from an average 78.4°preoperatively to 24.3°postoperatively (total percent correction 71.4%). In 16 patients with a major curve <70°and flexibility of 52.7%, the deformity improved from 58.4°preoperatively to 12.3°postoperatively (percent correction 78.9%). In 22 patients with a major curve >70°and flexibility of 25.6%, the deformity improved from 94.1°preoperatively to 30.1°postoperatively (percent correction 68.1%). In the cohort study, in the SMA group, the pre-operative major curve was 92.6±13.7°with a flexibility of 25.5± 7.3% was corrected to 29.4±5.7°demonstrating a 68.4% immediate postoperative correction. In the traditional group, the pre-operative major curve was 88.6±14.6°with a flexibility of 29.3±6.6% was corrected to 37.2±7.3°demonstrating a 57.8% immediate postoperative correction. There was a statistic difference between the SMA group and traditional group in correction rate of the major thoracic curve.3. For patients with major curve at thoracic spine, in SMA group, the average preoperative apical vertebral rotation of 16.2°spine was corrected to 7.5°, showing 50.4% correction, whereas rod derotation group, the correction was from 15.3°to 9.9°(34.8%). There was statistically significant difference rotational correction (p< 0.05). For patients with major curve at lumbar spine, in SMA group, the average preoperative apical vertebral rotation of 26.2°pine was corrected to 13.7°, showing 47.6% correction, whereas rod derotation group, the correction was from 25.4°to 14.5°(42.8%). There was no statistically significant difference rotational correction (p > 0.05).Conclusions.1. The shape memory alloy based scoliosis correction is a real 3D technique.2. The temporary use of a shape memory rod is a safe and effective method to correct scoliosis, and the temporary use of shape memory alloy rod may reduce the operative time, blood loss, and decrease the failure rate of pedicle screws, while improve the correction of the coronal plane compared to standard techniques;3. Shape memory alloy based scoliosis correction can improve the derotation effect at thoracic spine, and can not at lumbar spine.