Proximal Junction and Transitional Mechanics and Effect of a Novel Tether Pedicle Screw in Long-Segment Spinal Instrumentation.

Abstract

Study design: A biomechanical study of ten human thoracolumbar (T7-L2) spine specimens was performed.

Objective: To analyse the biomechanical characteristics of a Tether pedicle screw (TPS) in long-segment thoracolumbar instrumentation in terms of proximal junction mechanics and transitional motion patterns.

Summary of background data: Adult spinal deformity correction carries a high junctional failure risk. A soft-landing construct at a rigid construct cranial end might reduce the proximal junctional kyphosis (PJK) and failure (PJF) risks. Therefore, a novel TPS was designed to mitigate the PJK/PJF risk. The pedicle screw is characterized by a tether between the threaded shaft and the screw head, enabling motion among parts.

Methods: For initial flexibility tests, three instrumentation patterns were tested. Representing conventional instrumentation, standard thoracolumbar pedicle screw-rod instrumentation at T10-L2 was used (STD group). The TPS was tested at T9 (TPS+1 group), one level above the upper instrumented vertebra (UIV), and at T9 and T8 (TPS+2 group). Flexibility tests (±5 Nm) in all three motion directions were performed and repeated after cyclic loading (250 cycles, 1-10 Nm). Finally, specimens in the STD and TPS groups were subjected to screw pull-out testing at the index level to analyse the TPS stress-shielding effects.

Results: The TPS+2 group demonstrated the largest range of motion (ROM) decrease at T9-10 in the flexibility tests, with a smaller effect in the second adjacent segment at T8-9. No significant change in ROM was observed in the uppermost segment (T7-8) among all instrumentation pattern studies. Pull-out testing revealed greater mean forces at the T10 end-level in the TPS+2 group than in the STD group.

Conclusion: The TPS effectively distributed the loads across three adjacent levels and softened the load transition compared to the rigid construct. The TPS also showed the potential to stress-shield the UIV (T10) and reduce the end-level screw loosening risk.