Objective: A tether pedicle screw (TPS) enables individual stepless pretensioning and is placed at one or two levels above the upper instrumented vertebra (UIV+1 and UIV+2, respectively). This study aimed to evaluate a novel customized TPS for the prevention of proximal junctional kyphosis (PJK) and to investigate the potential to generate a smoother force transition from cranial to long fusion during trunk flexion, instead of an abrupt change at the UIV, following adult spinal deformity surgery.
Methods: A finite element model was designed based on an adult patient with spinal deformity instrumented from T10 to S1. Five different sagittal balance types and implant configurations were tested. The proximal range of motion (ROM) and intervertebral stress were examined, with a special focus on their respective discontinuities.
Results: Tension shielding at UIV/UIV+1 by the TPS was consistent irrespective of sagittal profiles. The use of TPSs at UIV+1 and UIV+2 increased the efficacy in reducing spinal ROM discontinuity at UIV/UIV+1, as compared with the use of TPSs at UIV+1 only. Through the use of two pairs of TPSs cranial to the UIV, the optimal tension configuration could be defined to avoid a reduction effect at UIV+1. Neither the addition of transition rods to the TPSs nor the use of transition rods in combination with standard pedicle screws improved the junctional mechanics when compared with TPSs at UIV+1/UIV+2.
Conclusions: A smoother motion discontinuity at the UIV can be achieved via implementation of a TPS strategy. This new technology shows favorable in silico mechanics for reducing the risk of PJK.
Keywords: PJK; adjacent-segment failure; adult spinal deformity; finite element model; proximal junctional kyphosis; tether pedicle screw.