Tether pre-tension within vertebral body tethering reduces motion of the spine and influences coupled motion: a finite element analysis

Anterior Vertebral Body Tethering (VBT) is a novel fusionless treatment option for selected adolescent idiopathic scoliosis patients which is gaining widespread interest. The primary objective of this study is to investigate the effects of tether pre-tension within VBT on the biomechanics of the spine including sagittal and transverse pa-rameters as well as primary motion, coupled motion, and stresses acting on the L2 superior endplate. For that purpose, we used a calibrated and validated Finite Element model of the L1-L2 spine. The VBT instrumentation was inserted on the left side of the L1-L2 segment with different cord pre-tensions and submitted to an external pure moment of 6 Nm in different directions. The range of motion (ROM) for the instrumented spine was measured from the initial post-VBT position. The magnitudes of the ROM of the native spine and VBT-instrumented with pre-tensions of 100 N, 200 N, and 300 N were, respectively, 3.29 degrees, 2.35 degrees, 1.90 degrees and 1.61 degrees in extension, 3.30 degrees, 3.46 degrees, 2.79 degrees, and 2.17 degrees in flexion, 2.11 degrees, 1.67 degrees, 1.33 degrees and 1.06 degrees in right axial rotation, and 2.10 degrees, 1.88 degrees, 1.48 degrees and 1.16 degrees in left axial rotation. During flexion-extension, an insignificant coupled lateral bending motion was observed in the native spine. However, VBT instrumentation with pre-tensions of 100 N, 200 N, and 300 N generated coupled right lateral bending of 0.85 degrees, 0.81 degrees, and 0.71 degrees during extension and coupled left lateral bending of 0.32 degrees, 0.24 degrees, and 0.19 degrees during flexion, respectively. During lateral bending, a coupled extension motion of 0.33-0.40 degrees is observed in the native spine, but VBT instrumentation with pre-tensions of 100 N, 200 N, and 300 N generates coupled flexion of 0.67 degrees, 0.58 degrees, and 0.42 degrees during left (side of the implant) lateral bending and coupled extension of 1.28 degrees, 1.07 degrees, and 0.87 degrees during right lateral bending, respectively. Therefore, vertebral body tethering generates coupled motion. Tether pre-tension within vertebral body tethering reduces the motion of the spine.