Region-specific QCT thresholds for osteoporosis assessment in the thoracic spine: why lumbar cut-offs fail

Quantitative computed tomography (QCT) is increasingly used for preoperative bone quality assessment in spine surgery. However, established QCT osteoporosis thresholds are based on lumbar reference regions and are frequently extrapolated to the thoracic spine, despite anatomical and biomechanical differences.

To evaluate whether lumbar QCT thresholds are transferable to the thoracic spine and to derive provisional thoracic-equivalent thresholds for anatomically defined thoracic subregions by statistical translation of established lumbar American College of Radiology (ACR) thresholds.

In this retrospective single-center quantitative computed tomography (QCT) study, trabecular volumetric bone mineral density (vBMD, mg/cm³) was assessed in 227 patients. Thoracic vBMD was summarized as the mean of Th1-Th10, whereas Th11-Th12 were assigned to the lumbar reference region together with L1-L4 according to the American College of Radiology (ACR) QCT classification scheme. Segmental vBMD distribution was analyzed, and the thoracic-lumbar relationship was modeled using Deming regression. The overall Th1-Th10 conversion was retained as a secondary regional summary; provisional thoracic-equivalent thresholds corresponding to ACR lumbar thresholds (80/120 mg/cm³) were derived for upper (Th1-Th4), mid (Th5-Th8), and lower thoracic regions (Th9-Th10).

Thoracic vBMD was significantly higher than lumbar vBMD (102.5 ± 44.1 vs. 85.1 ± 44.6 mg/cm³). In patients with evaluable measurements in both regions (N = 101), thoracic vBMD exceeded lumbar vBMD by 14.1 ± 19.7 mg/cm³ (p < 0.001) and showed strong correlation (r = 0.890, p < 0.001). A pronounced cranio-caudal decline in vBMD was observed across the thoracic spine (mixed-effects slope - 3.96 mg/cm³ per level, p < 0.001). Deming regression yielded Th(1-10) = 0.956 × Lum(Th11-L4) + 18.0; the resulting overall Th1-Th10 conversion values of 94 mg/cm³ and 133 mg/cm³ are reported as a secondary regional summary. Provisional subregional thoracic-equivalent thresholds differed substantially: upper thoracic (Th1-Th4) 118/148 mg/cm³, mid thoracic (Th5-Th8) 94/132 mg/cm³, and lower thoracic (Th9-Th10) 90/132 mg/cm³.

Thoracic vBMD showed a systematic regional offset and substantial cranio-caudal variation. Because the vertebral-level composition of the overall Th1-Th10 mean varied between patients, the overall conversion should not be interpreted as a fixed anatomical thoracic threshold. The anatomically defined subregional values should be regarded as provisional thoracic-equivalent thresholds derived from lumbar ACR criteria and require prospective outcome-based validation before clinical implementation.