Mechanical Analysis of 3 Posterior Fusion Assemblies Intended to Cross the Cervicothoracic Junction

Abstract

Objective: The objective of this study is to evaluate the mechanical properties of 3 posterior spinal fusion assemblies commonly used to cross the cervicothoracic junction.Summary of Background: When posterior cervical fusions are extended into the thoracic spine, an instrumentation transition is often utilized. The cervical rod (3.5 mm) can continue using thoracic screws designed to accept the cervical rods. Alternatively, traditional thoracic screws may be used to accept thoracic rods (5.5 mm). This requires the use of a 3.5-5.5 mm transition rod or a separate 5.5 mm rod and a connector to fix the 3.5 and 5.5 mm rod together. Fusion success depends on the immobilization of vertebrae, yet the mechanics provided by these different assemblies are unknown.Materials and Methods: Three titanium alloy posterior fusion assemblies intended to cross the cervicothoracic junction underwent static compressive bending, tensile bending, and torsion as described in ASTM F1717 to a torque of 2.5 Nm. Five samples of each assembly were attached to ultrahigh molecular weight polyethylene blocks via multiaxial screws for testing. Force and displacement were recorded, and the stiffness of each construct was calculated. Results:The 2 assemblies that included a 5.5 mm rod were found to be stiffer and have less range of motion than the assembly that used only 3.5 mm rods. Conclusions:The results of this study indicate that incorporating a 5.5 mm rod in a fusion assembly adds significant stiffness to the construct. When the stability of a fusion is of heightened concern, as demonstrated by the ASTM F1717 vertebrectomy (worst-case scenario) model, including 5.5 mm rods may increase fusion success rates.

Is It Necessary to Cross the Cervicothoracic Junction in Posterior Cervical Decompression and Fusion for Multilevel Degenerative Cervical Spine Disease? A Systematic Review and Meta-Analysis

“…The systematic search yielded 1528 articles, of which 553 were duplicates, and 942 were excluded by screening the title and abstract. After a full-text review, 18 studies were considered improper: six for including patients with diagnosis of trauma, infection, or tumour [13,14,[24][25][26][27]; five for non-comparative study (cervical group vs. thoracic group) [9,[28][29][30][31]; three for the absence of necessary outcomes [12,32,33]; three for partly duplicated cohorts [11,34,35]; and one for non-English study [36]. Finally, 15 studies were included in this systematic review and meta-analysis (Figure 1) [37][38][39][40][41][42][43][44][45][46][47][48][49][50][51].…”