Biomechanical comparison of different rod-to-rod connectors to a conventional titanium- and cobalt chromium posterior spinal fixation system

INTRODUCTION: Several types of rod-to-rod connectors are available for the extension of spinal fixation systems. However, scientific literature regarding the mechanical performance of different rod-to-rod connector systems is lacking. RESEARCH QUESTION: The goal of this study was to evaluate the mechanical characteristics of axial and lateral rod connectors in comparison to a conventional pedicle screw rod (titanium and cobalt chromium) construct. MATERIAL AND METHOD: Six types of instrumentations were investigated in a standardized test model to quantify the mechanical differences: 1: titanium rod; 2: titanium rod with axial connector; 3: titanium rod with lateral connector; 4: cobalt chromium rod; 5: cobalt chromium rod with axial connector; 6: cobalt chromium rod with lateral connector. All groups were tested in static compression, static torsion and dynamic compression and statistically compared regarding failure load and stiffness. RESULTS: In static compression loading, the use of connectors increased the construct stiffness, but unaffected the yield load. The use of a cobalt chromium rod significantly increased by approximately 40% the yield load and stiffness in comparison to the titanium rod configurations. Under dynamic compression, a similar or higher fatigue strength for all tested groups in comparison to the titanium rod configuration was evaluated, with the exception of titanium rod with axial connector. CONCLUSION: Biomechanically, using rod connectors is a secure way for the extension of a construct and is mechanically equal to a conventional screw rod construct. However, in clinical use, attention should be paid regarding placement of the connectors at high loaded areas.