Role of posterior ligamentous reinforcement in proximal junctional kyphosis: A cadaveric biomechanical study

Study Design: Cadaveric biomechanical study. Purpose: The purpose of this study was to biomechanically evaluate the effect of preserving or augmenting the interspinous ligament (ISL) and supraspinous ligament (SSL; ISL/SSL) complex between the upper instrumented vertebra (UIV) and UIV+1 using a cadaveric model. Overview of Literature: Adult spinal deformity is becoming an increasingly prevalent disorder, and proximal junctional kyphosis (PJK) is a well-known postoperative complication following long spinal fusion. Methods: Pure moments of 4 and 8 Nm were applied to the native and instrumented spine, respectively (n=8). The test conditions included the following: native spine (T7-L2), fused spine (T10-L2), fused spine with a hand-tied suture loop through the spinous processes at T9-T10, and fused spine with severed T9-T10 ISL/SSL complex. Results: The flexion range of motion (ROM) at T9-T10 of the fused spine loaded at 8 Nm increased by 62% compared to that of the native spine loaded at 4 Nm. The average flexion ROM at T9-T10 for the suture loop and severed ISL/SSL spines were 141% (p =0.13) and 177% (p =0.66) of the native spine at 4 Nm, respectively (p -values vs. fused). Conclusions: Transection of the ISL/SSL complex did not significantly change flexion ROM at the proximal junctional segment following instrumented spinal fusion. Furthermore, augmentation of the posterior ligamentous tension band with a polyester fiber suture loop did not mitigate excessive flexion loads on the proximal junctional segment. We postulate that the role of the posterior ligamentous tension band in mitigating PJK is secondary to the anterior column support provided by the vertebral body and intervertebral disc.