Sacral pelvic angle: a new pelvic anatomical parameter for assessing spino-pelvic sagittal balance

English Keywords:Sacral pelvic angle  Spine  Pelvis  Sagittal balance

English Abstract:

  【Abstract】 Objectives: To introduce a new spine-pelvic parameter, sacral pelvic angle(SPA), and to investigate the geometrical value of SPA and to evaluate its correlation, reliability and reproducibility compared with other parametres. Methods: SPA was defined as the angle between PR line and the line perpendicular to the sacral endplate. The relationship of SPA with pelvic angle(PA) and sacral slope(SS) was deduced with geometric method. Whole spine, standing radiographs of 111 Chinese adult volunteers were taken. Cobb angle and pelvic radius technique were adopted to measure the spino-pelvic sagittal parameters including SPA, thoracic kyphosis(TK), thoracolumbar kyphosis(TLK), lumbar lordosis(LL), SS, pelvic tilt(PT), pelvic incidence(PI), sagittal vertical axis(SVA), spino-sacral angle(SSA), PA, pelvic morphology(PR-S1) and total lumbopelvic lordosis(PR-T12). The correlations among all parameters were analyzed by Pearson correlation analysis. 80 radiographs were randomly chosen for SPA measurement by two independent spine surgeons for 5 times at first and another 5 times one month later. Coefficient of variation(CV) was calculated to evaluate the stability of SPA, and introclass correlation coefficient(ICC) was used to reflect the consistency of measurements by the same observer and between the two observers. Results: Geometric deduction confirmed that SPA=PA+SS=90°－PR-S1. There were statistically significant positive correlations of SPA with PI, PR-S1, PA, PT, SS, SSA(r=0.994, 1.000, 0.482, 0.538, 0.699, 0.465, P<0.05). Negative correlation was presented between SPA and LL(r=-0.532, P<0.05). CV of SPA was 0.0023±0.02 and 0.0085±0.053 for the twice mearurement. ICC between observers was 0.99(P<0.01) and 0.97(P<0.01), and ICC between twice measurements was 0.99(P<0.01). Conclusions: SPA is an anatomical parameter with good reliability and reproducibility. SPA=PA+SS. The correlation of SPA with other parametres was similar to that of PI with other parametres, which can be adopted to assess the spino-pelvic sagittal balance.