Dennis Hallager Nielsen MD, Martin Gehrchen MD/PhD, Benny Dahl MD/PhD/DMSci,. Jonathan Harris MS, Manasa Gudipally MS, Sean Jenkins BS, Ai-Min Wu ...
Authors: Dennis Hallager Nielsen MD, Martin Gehrchen MD/PhD, Benny Dahl MD/PhD/DMSci, Jonathan Harris MS, Manasa Gudipally MS, Sean Jenkins BS, Ai-Min Wu MD, Brandon Bucklen PhD Title: Does Anterior Column Support Following a Pedicle Subtraction Osteotomy Reduce Rod Strain? An In Vitro Biomechanical Investigation Summary: The present study investigated whether anterior column support (ACS) reduces rod strain, thus explaining reduced rates of rod fracture seen clinically following pedicle subtraction osteotomy (PSO). Cobalt Chrome rods (CoCr) significantly diminished strain, while the effect of ACS on motion-based changes in strain was minimal. Introduction: Recent literature reports decreased rates of rod fracture when ACS is used at the level of the PSO; however these results are poorly understood. Presently it is unclear whether ACS prevents disc collapse or reduces rod strain as a flexion blocker. The present study seeks to understand the effect of anterior support on rod strain at the PSO-level, thus explaining the results seen clinically. Methods: Five human specimens (T12-S1) underwent PSO at L3 with posterior pedicle screw stabilization from L1-S1. PSO was performed so that the final lordosis of all specimens was 70º. Specimens were subjected to 10 Nm in flexion-extension on a custom motion simulator at 1.5º/sec. Linear strain gauges, placed on the posterior surface of the rod at the level of the PSO, measured surface rod strain during motion. Dual lateral interbody spacers (S) were inserted at L2-L3 and L3-L4 following initial testing of the primary rods. Tested constructs include: 1) titanium two-rods (Ti 2-Rod); 2) CoCr 2-Rod; 3) Ti 2-Rod+S; 4) CoCr 2-Rod+S. Repeated measures ANOVA assessed significant differences between constructs (p≤0.05). Results: CoCr 2-Rod+S provided most resistance to surface rod strain relative to Ti 2-Rod, fig. 1. CoCr rods reduced strain by 28.1% compared to Ti (p≤0.05) while addition of ACS further reducing strain by 9.4% (p≤0.05). Alternatively, ACS with Ti construct increased strain at the PSO by 11.6% (p≥0.05). Conclusion: CoCr rods provided greatest reduction of rod strain at the PSO-level. ACS adjacent to PSO only alleviated rod strain in CoCr rods. While we cannot comment on the absolute strain state of the rod, it does appear the motion-induced strain is minimally affected by ACS. Therefore, interbody support may primarily act as an anterior column support.
Figure 1: Surface strain on primary rods at level of PSO, normalized to titanium two-rod construct per specimen (100%)
