Effect of Vertebral Body Exclusion on TBS and FRAX Calculations

Abstract

Spinal degenerative changes are common and elevate DXA-measured BMD. However, existing data suggest trabecular bone score (TBS) is relatively unaffected by degenerative changes. Clinically, we observed patients in whom spinal degenerative changes appeared to substantially elevate not only BMD, but also TBS. The purpose of this study was to explore the hypothesis that vertebral body exclusion impacts TBS results and subsequently TBS-adjusted FRAX scores. The reports of DXA scans interpreted by one clinician (NB) over a 4-month time span were reviewed. Vertebral body exclusion was performed following ISCD recommendations. Data collected from those scans with vertebral body exclusion included the L1-4 BMD/TBS and lumbar spine BMD/TBS following vertebral exclusion. FRAX calculations were performed using L1-4 TBS and post-exclusion TBS values and the effect of vertebral exclusion was assessed by T-test. This study was determined exempt by the local IRB. Of 302 patients with TBS, 62 evaluable scans (57 female, 5 male) had vertebral bodies excluded. Patient mean (SD) age, BMI, and lowest T-score, were 71.9 (10.0) years, 24.7 (4.4) kg/m2 and -2.9 (1.1) respectively. Compared to L1-4, mean lumbar spine BMD and TBS declined by 10% and 3.6% respectively with vertebral exclusion (p<0.0001). TBS adjusted FRAX 10-year risk increased by 1% and 0.2% for mean osteoporotic fracture (MOF) and hip fracture respectively (p<0.0001). After vertebral body exclusion 37% of patients shifted into lower TBS categories. Lumbar spine BMD and TBS decline when excluding vertebral bodies per ISCD recommendations. This leads to higher fracture risk estimates using TBS adjusted FRAX. Additionally, a substantial number of patients move into lower TBS categories after exclusion. This study is limited by small sample size; however, these data suggest that vertebral body exclusion may be appropriate with TBS. Further research to determine if vertebral exclusion improves fracture risk prediction is needed.