Biomechanical evaluation of a novel translaminar rod construct across the cervicothoracic junction: a cadaveric study

Introduction: The cervicothoracic junction (CTJ) is a specialized biomechanical transition point linking the cervical to the thoracic spine. It is a transition in both alignment and biomechanics that can lead to increased biomechanical stress at the junction. Current CTJ deformities are treated with fusion across the junction, however, biomechanical flaws and tangible consequences have been noted prompting fusion optimization. This study aimed to evaluate the biomechanical stability across the CTJ between a standard two-rod fusion of C2-T1 and a novel two-rod C2-C7 fusion supplemented with an additional C2-T1 translaminar rod (TLR).

Methods: This study utilized 6 human cadaveric spines (C1-T2) instrumented sequentially with the two-rod and three-rod posterior constructs. Pure moments of 1.5 Nm were applied in Flexion-Extension (FE), Left/Right Axial Rotation (AR), Left/Right Lateral Bending (LB), Axial Rotation with Flexion (F+AR), and Extension (E+AR). Range of motion (ROM) at C7-T1 and T1-T2 were measured for each testing condition. Axis Angle at C7-T1 and T1-T2 were also measured to illuminate overall level stability between surgical conditions.

Results: In the 2-rod condition, the average C7-T1 ROM for LB, FE, and AR was 0.272°, 0.418°, and 0.569° whereas the ROM for the 3-rod condition was 3.486°, 1.510°, and 4.081°, illuminating a significant difference between the 2- and 3-rod conditions. There was no significant difference in the average T1-T2 ROM between the 2-rod and 3-rod conditions. Significant increase in Axis Angle was found at C7-T1 in all test directions except Extension (p=0.144).: For T2, there was no significant difference in Axis Angle between 2-rod and 3-rod conditions.

Conclusion : In this study, we sought to explore the biomechanics of the CTJ after fusion by comparing the relative stability of a standard C2-T1 2-rod fusion to a C2-C7 two-rod construct with an additional C2-T1 translaminar rod. Our study found, contrary to our hypothesis, that the standard C2-T1 2-rod fusion offered better fixation than the 3-rod construct. The 2-rod fusion approach may be more favorable when fixating across the CTJ.