Relative strength of thoracic vertebrae in axial compression versus flexion

Background contextNoninvasive strength assessment techniques are the clinical standard in the diagnosis and treatment of osteoporotic vertebral fractures, and the efficacy of these protocols depends on their ability to predict vertebral strength at all at-risk spinal levels under multiple physiological loading conditions.PurposeTo assess differences in vertebral strength between loading modes and across spinal levels.Study design/settingThis study examined the relative strength of isolated vertebral bodies in compression versus flexion.MethodsDestructive biomechanical tests were conducted on 30 pairs of donor-matched, isolated thoracic vertebral bodies (T9 and T10; F=19, M=11; 87+5 years old, max=97 years old, min=80 years old) in both uniform axial compression and flexion using previously described protocols. Quantitative computed tomography (QCT) scans were taken before mechanical testing and used to obtain bone mineral density (BMD) and “mechanics of solids” (MOS) measures, such as axial and bending rigidities.ResultsCompressive strength was higher than flexion strength for each donor by 940+152 N (p<.001, paired t test), and vertebral strengths in the two loading modes were moderately correlated (adjusted R2=0.50, p<.001). For both compression and flexion loading modes, adjacent-level BMD and MOS metrics had approximately half the predictive capacity as same-level measurements, and BMD and MOS values were only moderately correlated across spinal levels.ConclusionsThe results of this study are important in designing clinical test protocols for assessing vertebral fracture risk. Because vertebral body flexion and compressive strength are not strongly correlated and flexion strength is significantly less than compressive strength, it is imperative to investigate a patient's spinal structural capacity under bending loading conditions. Furthermore, our work suggests that clinicians using QCT-based measures should perform site-specific strength assessments on each at-risk spinal level. Future work should focus on improving the accuracy of densitometric measures in predicting vertebral strength in flexion and also on examining same- versus adjacent-level strength assessment for radiographic techniques with lower X-ray dosage, such as dual-energy X-ray absorptiometry.