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

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.