Demographic and radiological characteristics of the participants
In the progress of retrospectively collecting the participants from the outpatient clinic (specializing in the treatment of scoliosis) from July 2016 to June 2023, a total of 297 subjects with the assessment results of the 2D photogrammetry of the posterior views of the whole body in standing position were initially collected. Among these subjects, 187 cases were excluded according to the inclusion criteria, including 74 cases under 10 years old or over 16 years old; 36 cases with leg length discrepancies; and 77 cases without the assessments results of the standing posteroanterior full-spine radiographs. Finally, a total of 110 participants without leg length discrepancies were involved and analyzed for this study. As shown in Table 2, among the 110 participants, there were 32 cases in single-curve group (13 ± 1 years, 156 ± 8 cm, 47 ± 12 kg, 19 ± 3 kg/m2, Cobb angle: 27 ± 10 degrees), 31 cases in double-curve group (13 ± 1 years, 156 ± 8 cm, 47 ± 9 kg, 19 ± 3 kg/m2, Thoracic Cobb angle: 31 ± 7 degrees, Lumbar Cobb angle: 29 ± 7 degrees) and 47 cases in the non-scoliosis group (12 ± 1 years, 156 ± 8 cm, 46 ± 9 kg, 19 ± 3 kg/m2). Except for significant differences in age among the three groups of participants (p < 0.01), there were no significant differences in height, weight, and BMI values among the three groups.
Reliability of the 2D photogrammetry measurement
As illustrated in Table 3, the inter-rater reliability of all postural indexes measured by rater A and rater B ranged from 0.80 to 0.93 (95% confidence interval: 0.72–0.95) in this study. The minimal detectable change (MDC) values in the inter-rater measurement were 0.72 cm, 1.54°, 2.77°, and 5.44° for the C7 deviation, shoulder alignment, scapula alignment, and waist angle discrepancy, respectively.
Clinical value of the measured postural indexes in scoliosis screening (i.e., scoliosis group vs. non-scoliosis group)
As shown in Table 4, the AIS participants were divided into three groups: (1) single-curve/C-shape group with Cobb angle been greater than 10° (defined as “single-curve/C-shape scoliosis group/participants”, n = 32), (2) double-curve/S-shape group with Cobb angle been greater than 10° (defined as “double-curve/S-shape scoliosis group/participants”, n = 31), and (3) non-scoliosis group with the angle of trunk rotation (ATR) appearing in the Adam’s test measured by the Scoliometer been smaller than 4° (defined as “non-scoliosis group/participants”, n = 47). The C7 deviation (F [2,107 = 3.5, p = 0.03), shoulder alignment (Welch F [2,50.4 = 12.0, p < 0.001), scapula alignment (Welch F [2,45.9] = 16.1, p < 0.001), waist angle discrepancy (Welch F [2,41.6] = 24.8, p < 0.001), and PSIS alignment (Welch F [2,52.5] = 8.7, p < 0.001) were significantly different among the three groups.
For AIS participants with single-curve/C-shape curve, the scoliosis participants have significantly 0.4 cm larger C7 deviation (95% CI 0.04–0.8, p = 0.02), 3.0° larger scapula alignment (95% CI 1.5–4.5, p < 0.001), 8.8° larger waist angle discrepancy (95% CI 4.8–13.0, p < 0.001), and 1.5° larger PSIS alignment (95% CI 0.3–2.8, p = 0.01) than the non-scoliosis participants.
For AIS participants with double-curve/S-shape curve, the scoliosis patients have significantly 1.1° larger shoulder alignment (95% CI 0.5–1.7, p < 0.001), 2.0° larger scapula alignment (95% CI 0.5–3.4, p = 0.04), 7.0° larger waist angle discrepancy (95% CI 3.5–10.5, p < 0.001), and 1.1° larger PSIS alignment (95% CI 0.3–2.0, p = 0.05) than the non-scoliosis participants.
Based on the comparison of the inter-rater minimal detectable change (MDC) values (Table 3), it was found that the differences of the scapula alignment (3.0°) and the waist angle discrepancy (8.8°) between the single-curve/C-shape scoliosis group and the non-scoliosis group exceeded the inter-rater minimal detectable change (MDC) values (2.77° for the scapula alignment and 5.44° for the waist angle discrepancy), but the estimates were too imprecise to exclude the possibility that the effect is trivial for the scapula alignment (95% CI 1.5–4.5) and for the waist angle discrepancy (95% CI 4.8–13.0). Additionally, the difference of the waist angle discrepancy (7°) between the double-curve/S-shape curve group and the non-scoliosis group exceeded the inter-rater minimal detectable change (MDC) values (5.44° for the waist angle discrepancy), but the estimate was also too imprecise to exclude the possibility that the effect is trivial for the waist angle discrepancy (95% CI 3.5–10.5). Meanwhile, the remaining differences of the C7 deviation (0.4 cm), the shoulder alignment (1.1°), scapula alignment (2°) and the PSIS alignment (1.5° and 1.1°) between the non-scoliosis group and the single-curve/C-shape or double-curve/S-shape curve group did not exceed the inter-rater minimal detectable change (MDC) values (0.72 cm for the C7 deviation, 1.54° for the shoulder alignment, 2.77° for the scapula alignment, 5.44° for the waist angle discrepancy, and 2.51° for the PSIS alignment).
No statistic difference existed between double-curve/S-shape curve scoliosis group/participants and single-curve/C-shape scoliosis group/participants in any of the five postural indexes in this study.
Clinical value of the measured postural indexes in scoliosis monitoring (i.e., correlation between Cobb angles and postural indexes)
As shown in Table 5, among all the three types of Cobb angles and in both scoliosis groups, the waist angle discrepancy had moderate to strong positive correlation with the lumbar or thoracolumbar Cobb angle for both the single-curve and the double-curve group (r = 0.4, p = 0.01; r = 0.8, p = 0.03; r = 0.7, p = 0.01). The shoulder alignment had moderate positive correlation with the thoracic Cobb angle of the single-curve group (r = 0.6, p = 0.03). No statistical difference existed in the remaining correlations between the postural indexes and the Cobb angles of the participated scoliosis patients.