## 3 Results  
A total of 194 participants were identified, and after applying inclusion and exclusion criteria, 89 (45.8%) participants were excluded due to the following reasons: previous spinal surgery (n = 49, 55%), the absence of a non-contrast scan (n = 15, 16.9%), and technical limitations (n = 25, 28.1%). A summary of patients' characteristics and bone status is reported in Table 1. The mean value of participants' body mass index (BMI) was 26.6 (SD: 5; range 18.6-46.4). Overall, 49/105 participants (46.6%) had a positive history for at least one major fragility fracture. The mean T-score obtained by QCT was \(-3.4\pm 1.2\), with a final diagnosis of osteoporosis in 59/105 (56.2%), osteogenic in 36/105 (34.3%), and normal status in 10/105 (9.5%). On the other hand, the mean T-score obtained by DXA was \(-1.3\pm 1.6\), with a final diagnosis of osteoporosis in 25/105 (23.8%), osteogenic in 35/105 (33.3%), and normal status in 45/105 (42.9%). The direct comparison in terms of final diagnosis between DXA and QCT showed that in 42/105 (40%) cases, DXA and QCT provided the same final diagnosis (DXA-QCT matching), while in 63/105 (60%) the final diagnosis was different (DXA-QCT mismatching). A more detailed analysis of mismatching cases showed that DXA underestimated the disease in 58/63 cases (92.2%), while QCT underestimated the disease in only 5/63 (7.8%) cases, as listed in Table 2.  
The analysis of single cases in which QCT underestimated the disease demonstrated mean vBMD values very close to diagnostic thresholds, with little difference between the QCT-derived T-score values and the DXA T-score. On the contrary, in 49/58 (84%) of cases of underestimation by DXA, the radiologist stated the presence of degenerative changes in the lumbar spine, resulting in a fictitious increase in aBMD. An example of a mismatching case caused by spondylarthosis is shown in Figure 2.  
Regarding TBS, 35/105 patients (33.3%) presented normal TBS values (>1.310) corresponding to a normal indirect bone microarchitecture, 32/105 participants (30.5%) presented TBS values between 1.230 and 1.310 (partially degraded microarchitecture), and 38/105 participants (36.2%) presented a degraded indirect microarchitecture (TBS < 1.230). \begin{table}
\begin{tabular}{c c c c} \hline \hline  & **Male** & **Female** \\ \hline Sex & 18 (17.1\%) & 87 (82.9\%) \\  & Yes & No \\ Fragility Fracture & 49 (46.6\%) & 56 (53.4\%) & \\ QCT Diagnosis & Normal & Osteopenia & Osteoporosis \\  & 10 (9.5\%) & 36 (34.3\%) & 59 (56.2\%) \\ DXA Diagnosis & Normal & Osteopenia & Osteoporosis \\  & 45 (42.9\%) & 35 (33.3\%) & 25 (23.8\%) \\ TBS Diagnosis & Normal & Partially degraded & Degraded \\  & 35 (33.3\%) & 32 (30.5\%) & 38 (36.2\%) \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Summary of patients’ characteristics according to sex, presence of fragility fractures, and bone status diagnosis with quantitative computed tomography (QCT), dual-energy X-ray absorptiometry (DXA), and Trabecular Bone Score (TBS). \begin{table}
\begin{tabular}{c c c c} \hline \hline
**QCT** & **DXA** & **n** & **\%** \\ \hline Normal & Osteopenia & 1 & 1.6\% \\ Normal & Osteoporosis & 0 & 0.0\% \\ Osteopenia & Normal & 20 & 31.7\% \\ Osteopenia & Osteoporosis & 4 & 6.3\% \\ Osteoporosis & Normal & 16 & 25.4\% \\ Osteoporosis & Osteopenia & 22 & 34.9\% \\ Total mismatches & & 63 & \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Mismatching cases between vBMD by QCT and aBMD by DXA in the final diagnosis. In the majority of cases, DXA underestimated the diagnosis. Of note, about 25% of subjects classified as normal with aBMD were then classified as osteoporoporic using vBMD. The correlation between DXA-derived aBMD and QCT-derived vBMD was moderate (r = 0.446), as well as between TBS and vBMD (r = 0.524), while a good correlation was found between aBMD and TBS (r = 0.621). All correlations were statistically significant (\(p<0.005\)). The Bland-Altman analysis (Figure 3) showed a systematic bias between the methods.