The influence of accessory rods and connectors on the dynamic response of spine fixation

Abstract Purpose This study presents a nondestructive technique to assess the influence of accessory rods and connectors on the dynamic response of spine fixation.Methods Eighteen spine specimens were divided into three construct groups such as group I (2 rods [2R]), II (2 primary rods + 2 accessory rods with 2 transverse connectors [4R+2TC]) and III (2 primary rods + 2 accessory rods with 4 transverse connectors [4R+4TC]). Anterior corpectomy was performed for all specimens. A custom test setup was built to assess the dynamic responses of constructs in flexion-extension (FE) and left-right lateral bending (LRLB) motions. This setup can slide in lateral direction, and it is excited with an electrodynamic shaker vibrated at band limited random frequencies. Accelerometer and reusable dynamic strain sensors were installed on constructs to monitor the dynamic responses. Quasi-static eccentric loading tests were performed to determine the range of motion (RoM).Results The results demonstrated that accessory rods significantly increase the resonance frequency (RF) and decrease the strain over standard 2R construction. Although 4R+4TC provided greatest reduction in rod strain over 4R+2TC and 2R, additional 2 connectors have no significant influence on dynamic response in FE motion.Conclusions An increase in the number of rods has a significant role on the improvement of the fixation's integrity in FE and LRLB motions. However, the additional transverse connectors have significant involvements only in LRLB motion. RF obtained from dynamic tests correlated with the RoM which indicates that the technique could be used as an addendum to the quasi-static test.