In order to assess sagittal balance, clinical parameters are measured on lateral X-rays of the pelvis [1Roussouly. 2011.

Cliquez ici pour aller à la section Références, 2Vrotvec. 2012.

Cliquez ici pour aller à la section Références]. These radiographs require standard positioning of the patient during acquisition. The aim of this study was to estimate the effect of erroneous positioning of patients, during pelvis lateral X-ray acquisition, on the reliability and validity of sagittal pelvic parameters.

Helical pelvis CT-Scans (slice thickness - 0.6mm) of 8 children (3 F and 5 M, age - mean 12 - SD 2.2) and 9 adults (5 F and 4 M, age - mean 51 - SD 26) were considered. Lateral digitally reconstructed radiographs (LDRRs) were reconstructed from CT-Scans. Then, for each patient, axial rotation (AR) of the pelvis was simulated and the corresponding LDRRs were reconstructed at 5°, 10°, 15° and 20° of AR. Clinical parameters were measured digitally on each radiograph – sacral slope (SS), pelvic incidence (PI), pelvic tilt (PT), pelvic inclination (PIL), sagittal pelvic thickness (SPT), bifemoral distance (BFD) and pelvic depth (PD). Three trained operators repeated the measurements 3 times each, in each AR position and for each radiograph. Intraclass correlation coefficient (ICC) was evaluated for inter-observer agreement. Ninety-five percent confidence interval (95% CI) was calculated as 2SD of inter-observer reliability. The bias of each clinical parameter, in each AR position, was calculated as the absolute mean difference relatively to the 0° position.

Inter-observer agreement was shown to be very high (ICC>0.88) for all parameters and in all AR positions. In the absence of AR of the pelvises (0/730+), the 95% CI of the SS and the PI were lower than 4°, lower than 2° for PT and PIL and lower than 1cm for both SPT and BFD. Ninety-five percent CI increased with pelvic AR+ at 20° of rotation it exceeded 7° for SS and PI, 7° and 4° for PIL and PT respectively, and remained constant for SPT and BFD. All the parameters showed an increase in the bias during AR of the pelvis, where PI showed the greatest bias by reaching 6° at 20° position. Only SPT and PD exhibited a bias, which was minimal and constant (<0.5cm) for all AR positions. Even though insignificant (P>0.05), the bias on PI and SS was shown to be higher in children than in adults.

Clinical parameters measured on lateral radiographs were shown to be less accurate and less reliable when pelvic AR increased. Bias exceeded 10% of the normative values of each clinical parameter when the AR exceeded 10°. Bias could significantly affect orthopedic decision-making in patients with sagittal malalignment.