A biomechanical study on the effect of lengthening magnitude on spine off-loading in magnetically controlled growing rod surgery: Implications on lengthening frequency

Abstract

Purpose: To assess whether the magnitude of lengthening in magnetically controlled growing rod (MCGR) surgeries has an immediate or delayed effect on spinal off-loading. Methods: 9 whole porcine spines were instrumented using two standard MCGRs from T9 to L5. Static compression testing using a mechanical testing system (MTS) was performed at three MCGR lengthening stages (0mm, 2mm, and 6mm) in each spine. At each stage, five cycles of compression at 175N with 25min of relaxation was carried out. Off-loading was derived by comparing the load sustained by the spine with force applied by the MTS to the spine. Micro-CT imaging was subsequently performed. Results: The mean load sustained by the vertebral body before lengthening was 39.69N, and immediately after lengthening was 25.12N and 19.91N at 2mm and 6mm lengthening, respectively; decreasing to 10.07N, 8.31N, and 8.17N after 25minutes of relaxation, at 0mm, 2mm, and 6mm lengthening stages, respectively. There was no significant difference in off-loading between 2mm and 6mm lengthening stages, either instantaneously (p = 0.395) or after viscoelastic relaxation (p = 0.958). CT images showed fractures/separations at the level of pedicle screws in six spines and in the vertebral body's growth zone in five spines after 6mm MCGR lengthening. Conclusion: This study demonstrated MCGRs cause significant off-loading of the spine leading to stress shielding. 6mm of lengthening caused tissue damage and microfractures in some spines. There was no significant difference in spine off-loading between 2mm and 6mm MCGR lengthening, either immediately after lengthening or after viscoelastic relaxation.