Mechanical comparison of posterior instrumentation constructs for spinal fixation across the cervicothoracic junction

STUDY DESIGN. A biomechanical study comparing 4 different posterior implant configurations for instrumentation across the cervicothoracic junction. OBJECTIVE. To compare mechanical parameters during flexion bending and axial rotation testing among 4 different posterior cervicothoracic rod-and-screw constructs. SUMMARY OF BACKGROUND DATA. Several posterior rod constructs are available for instrumentation across the cervicothoracic junction. No studies have examined the mechanical properties of constructs of varying rod diameters and rod connector types. METHODS. Four different rod-and-screw-based constructs for posterior cervicothoracic instrumentation underwent flexion bending or axial rotation testing. Stiffness, ultimate and yield forces (torques) were compared using an analysis of variance. RESULTS. Significantly lower stiffness, ultimate and yield force (torque) was observed with a 3.5-mm rod construct compared with the 3 other constructs. No significant differences were demonstrated between a dual diameter rod (3.5 mm to 5.5 mm) and a solid domino connector extending between 3.5-mm and 5.5-mm rods. A hinged domino connector construct between 3.5-mm and 5.5-mm rods had similar stiffness but lower ultimate and yield force from either the dual diameter rod or the solid domino construct. CONCLUSIONS. The present results demonstrate that the 3.5-mm rod-and-screw construct is the weakest configuration for posterior fixation across the cervicothoracic junction. The dual diameter rod and fixed domino connector constructs were the strongest and demonstrated similar values for yield and ultimate force.