The Influence of Accessory Rods and Connectors on the Quasi-Static and Dynamic Response of Spine Fixation

Abstract

This study presents a nondestructive technique to assess the influence of accessory rods and connectors on the mechanical response of spine fixation. 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). 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. The findings suggest that 4R + 4TC has a role in the improvement of the fixation's integrity in FE and LRLB motions. However, the additional transverse connectors have significant involvements only in LRLB motion. Hence, the 4R + 4TC configuration is useful for a construct that is subjected to high LRLB moment, as well as excessive compressive stress and fatigue. Furthermore, 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.