Stability of multiple-rod constructs and dual-rod constructs in cadaveric thoracic spines

Purpose Rod fracture and pseudoarthrosis are common complications in spinal fusion surgery. Multiple-rod constructs have been shown to mitigate the risks of rod fracture and pseudarthrosis in the lumbar spine, but their effect on the thoracic spine is less studied. This work aims to compare the stability of two-rod (dual-rod) constructs (DRCs) to four-rod (multiple-rod) constructs (MRCs) in cadaveric thoracic spines. Methods Nine intact human cadaveric thoracic spines (T1-T12) were instrumented in a randomized manner with either DRC or MRC. Specimens were then statically loaded in six planes of motion (flexion/extension, rotation, and lateral bend). A 1-hour bodyweight simulation fatigue test (FT) was then conducted on each specimen, followed by another static loading test. Constructs were then alternated, and this same sequence of testing was performed again. Range of motion (ROM) during the static loading tests was recorded using a digital imaging correlation (DIC) system. Results Total ROM pre-FT and post-FT was similar between DRCs and MRCs. However, when comparing pre-FT to post-FT within each construct type, DRCs exhibited an increase in flexion/extension ROM (15.3° ± 4.4° vs. 12.8° ± 5.3°, p = 0.009) while MRCs did not (16.5° ± 4.9° vs. 14.1° ± 3.7°, p = 0.076). Conclusions In this biomechanical analysis of intact cadaveric thoracic spines, MRCs exhibited more resistance to cyclical loading than DRCs did after FT. This work supports the current literature in the spine surgery field of reduced nonunion and rod breakage with MRCs.