A biomechanical study of upper thoracic vertebras translaminar screws fixation

Abstract

Objective To compare the biomechanical pull-out strength (POS) of three different fixations in upper thoracic vertebras using translaminar screws (TLS), translaminar facet screws (TLFS), and transpedicle screws (TPS), respectively. Methods Nine fresh human cadaveric cervicothoracic junction spines specimens which including T1-T3 vertebras were harvested. The vertebras specimens were scanned using dual-energy radiograph absorptiometry for bone mineral density. Both of screw insertion techniques at each vertebrae was randomized. All the screw insertions were based on direct observation and the CT scan on the pedicles. The peak of insertional torque (IT) was recorded and axial pull-out testing was performed to simulate intraoperative failure of fixation. Results The mean peak IT of the TFLS, TPS and TLS were (0.43±0.01), (0.40±0.01), (0.35±).01) N·m, respectively. There was no statistically significant difference between the TFLS and TPS, and between the TPS and TLS was same. But the TFLS generated statistically greater peak 1T in comparison with the TLS(t=-13.86, P＜0.05). The mean POS of TLFS was (771±106) N,which had no statistically significant difference in comparison with the TPS(733±65) N. And the TLS (663±86) N was same. But the TFLS generated statistically greater POS in comparison with the TLS (t=9.907, P＜0.05). The peak IT showed a strong positive correlation with POS in three screw techniques. Bone mineral density correlation with POS in all methods of fixation. Conclusion It was not a significant difference to compare POS of TLS and TLFS to that of TPS respectively. TLS and TLFS appear to be a biomechanically sound alternative in the upper thoracic spine, and appear to be a safe and effective technique for instrumenting the upper thoracic spine. Key words: Thoracic vertebrae; Internal fixators; Biomechanics