Influence of Spinal Bridging Ossification on Mechanical Properties and Fracture Tolerance Under Flexion/Extension Loading

Purpose

Diffuse idiopathic skeletal hyperostosis (DISH) is characterized by “flowing” ossifications that form along the anterolateral aspect of contiguous vertebrae. While prior literature has identified that DISH patients are susceptible to spinal injury, the DISH-specific differences in mechanical response of human spines to applied bending have not been quantified. Therefore, the purpose of this work was to assess the effect of DISH on range of motion, stiffness, and fracture tolerance under flexion/extension loading.

Methods

Seven human cadaveric spines were dissected into specimens composed of three functional spinal units (3FSUs), spanning levels T1–T4, T5–T8, and T9–T12. For each 3FSU specimen, the presence of bridging DISH ossification was determined. Ten DISH-affected and eleven normal specimens were evaluated under flexion/extension range of motion followed by extension to failure/system limits.

Results

DISH-affected specimens had significantly less range of motion compared to normal specimens (1.8 deg vs. 3.2 deg, p = 0.0125) and were significantly stiffer (5.5 N·m/deg vs. 2.9 N·m/deg, p = 0.0167). All DISH-affected specimens fractured during extension to failure, whereas five normal specimens did not fracture under the test loading. Hazard ratio analyses indicated that DISH-affected specimens are four times more likely to fracture compared to normal specimens under a given torque and sixteen times more likely for a given extension angle. The normal specimen median energy at fracture was significantly greater than the DISH-affected group (301 Nm·deg vs. 81 Nm·deg, p = 0.006).

Conclusion

This study demonstrates a significant increase in the injury potential of DISH spines during bending loading and provides the first quantitative support for clinical observations of compromised fracture tolerance within the DISH population. Future work classifying the severity of DISH in the context of mechanical response may assist in radiological identification of otherwise asymptomatic patients with increased potential for injury.