The Effects of Vertebral Body Tethering on the Intervertebral Discs and Facet Joints: A Numerical Analysis

Anterior vertebral body tethering (VBT) is a promising technique for the treatment of adolescent idiopathic scoliosis. However, the segments directly treated with VBT can experience substantial loads resulting from the tether pretension, which may alter internal stresses and potentially compromise structures such as the intervertebral discs (IVDs) and facet joints. We aim to investigate the effects of tether within the VBT on the L1-L2 IVD stresses and contact forces of the facet joints, using an extensively calibrated and validated finite element model of the T10-S1 spine. The implant was inserted on the left side of the T10-L3 and tensioned up to 300 N representing the tether pretension applied during surgery and the case of the postoperative neutral position. Subsequently, the spine was tested under an external pure moment of 8 Nm. The tether pretension resulted in a significant increase in the IVD stresses. In the neutral position, a gradual increase in intervertebral pressure (IDP) at the center of the IVD of 0.094, 0.181, and 0.267 MPa was observed after applying forces of 100, 200, and 300 N to the tether, respectively. The contact force of the left facet joint also increased with pretension. It was 12.5 N for the native spine and gradually increased to 49.5, 82.0, and 100.9 N for tether pretensions of 100, 200, and 300 N, respectively, during extension. These results indicate that tether pretension is a key parameter that increases the internal stresses of the IVD and the contact forces of the facet joints at the implant side.