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

Abstract

A biomechanical study of 10 human thoracolumbar (T7-L2) spine specimens was performed. To analyze the biomechanical characteristics of a Tether pedicle screw (TPS) in long-segment thoracolumbar instrumentation in terms of proximal junction mechanics and transitional motion patterns. 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 and proximal junctional failure risks. Therefore, a novel TPS was designed to mitigate the proximal junctional kyphosis/proximal junctional failure risk. The pedicle screw is characterized by a tether between the threaded shaft and the screw head, enabling motion among parts. For initial flexibility tests, 3 instrumentation patterns were tested. Representing conventional instrumentation, standard thoracolumbar pedicle screw-rod instrumentation at T10 to L2 was used [standard (STD) group]. The TPS was tested at T9 (TPS+1 group), 1 level above the upper instrumented vertebra, and at T9 and T8 (TPS+2 group). Flexibility tests (±5 Nm) in all 3 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 analyze the TPS stress-shielding effects. The TPS+2 group demonstrated the largest range of motion decrease at T9 to T10 in the flexibility tests, with a smaller effect in the second adjacent segment at T8 to T9. No significant change in range of motion was observed in the uppermost segment (T7-T8) 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. The TPS effectively distributed the loads across 3 adjacent levels and softened the load transition compared with the rigid construct. The TPS also showed the potential to stress-shield the upper instrumented vertebra (T10) and reduce the end-level screw loosening risk.