The Difference in Hounsfield Units Between Vertebra Superjacent and Subjacent to the Level of Balloon-assisted Kyphoplasty Accurately Predicts Subsequent Fractures

Introduction: Balloon-assisted kyphoplasty (BAK) can augment vertebral height and correct focal kyphosis in osteoporotic patients with vertebral compression fractures (VCFs) but may also accelerate degenerative changes that lead to subsequent VCFs. Differences in Hounsfield units (HFU), a measure of radiodensity, between vertebra superjacent and subjacent to the index level may predict subsequent adjacent-level fracture after BAK.

Methods: Patients who underwent first-time BAK for osteoporotic VCFs, with preoperative computed tomography imaging and at least 6 months of follow-up, were evaluated. Relevant clinical, demographic, and radiologic data was collected. Variables independently associated with subsequent fractures were identified using stepwise, backward multivariable logistic regression. An HFU inflection point was identified at which subsequent adjacent-level fracture risk increased, and propensity score matching was used to quantify the increase in risk of fracture above that threshold.

Results: One-hundred eighty-eight patients met inclusion criteria and 32 (17.0%) patients developed a subsequent fracture after BAK. The average HFUs at superjacent and subjacent vertebra were 104.7±50.7 and 97.0±51.5, respectively. The average difference in HFUs between both adjacent levels was 29.4±35.6. There was no correlation between patients who developed a fracture and the absolute HFU of the superjacent and subjacent vertebra. There was a positive correlation between patients who developed a fracture and the percent difference in HFU between the adjacent levels (rho=0.7086, P=0.0005). On multivariable logistic regression, subsequent fracture was independently associated with BMI (OR=0.914 [0.840, 0.995], P=0.037), sex (OR=0.234 [0.064, 0.852], P=0.028), and percent difference in HFU between levels (OR=63.326 [8.385, 478.219], P< 0.001). After propensity score matching, a >25% difference in HFU between adjacent levels was calculated to cause a 25.5% absolute increase in risk of subsequent adjacent-level fracture (ATT=25.5% [9.1%, 41.8%], P=0.002).

Conclusion : The percent difference in HFUs between adjacent vertebra after BAK was found to accurately predict the development of subsequent adjacent-level compression fractures.