Anatomical Pelvic Parameters Using the Anterior Pelvic Plane: Normative Values and Estimation of the Standing Sagittal Alignment in Healthy Volunteers

Abstract

Introduction: In this study, we aim to estimate the natural standing sagittal alignment in patients with adult spinal deformity (ASD), firstly by investigating the normative values of anatomical pelvic parameters based on the anterior pelvic plane (APP) in a healthy population, and to clarify the relationships between the anatomical and positional pelvic parameters in standing position.

Methods: The images of biplanar slot-scanning full-body stereoradiography in 140 healthy Japanese volunteers (mean age, 39.5 years; 59.3% female) were examined. In addition to three-dimensional measurements including pelvic incidence (PI), sacral slope (SS), and pelvic tilt (PT), the APP angle (APPA; anterior tilting=positive) was measured as the angle between the APP and the vertical line using the two-dimensional lateral image. Anatomical SS and PT (aSS and aPT) were calculated as the angles of SS and PT in reference to APP.

Results: The mean (range) values of APPA, aSS, and aPT were determined to be 0.7° (-16.8°/15.5°), 36.8° (18.3°/64.9°), and 13.2° (-0.6°/28.7°), respectively. Moreover, SS was found to be significantly correlated with PI and aSS, while PT was significantly correlated with PI, aSS, aPT, and body weight. Also, PT was significantly larger in females than in males. Multiple linear regression analysis deduced the following equations: SS=0.404×aSS+0.203×PI+12.463, PT=-0.391×aSS+0.774×PI+1.950×sex (male=0, female=1)-12.971, wherein aSS had the greatest effect for predicting SS among the included factors and PI had the greatest effect for predicting PT. In addition, no significant differences were noted between PT/PI and aPT/PI.

Conclusions: As per the results of this study, significant correlations were noted among parameters and predicting models for positional parameters (SS and PT) using anatomical parameters (aSS and aPT) in a healthy population. This novel measurement concept based on the APP has been considered to be useful in estimating natural SS and PT in standing position using the anatomical pelvic parameters in patients with ASD.

Keywords: anatomical parameter; anterior pelvic plane; pelvic incidence; pelvic tilt; sacral slope; standing sagittal alignment.

Figure 1.

Standing biplanar slot-scanning full-body stereoradiography images of a 74-year-old female patient with adult spinal deformity. Anteroposterior images show bilateral severe hip osteoarthritis and mild lumbar scoliosis (A). The lateral images demonstrate the mismatch between pelvic incidence (PI, 80°) and lumbar lordosis (LL, 44°) (PI-LL=36°), which is compensated by pelvic retroversion (pelvic tilt=35°) (B).

Figure 2.

Anterior pelvic plane angle (APPA) measurement using two-dimensional lateral images of EOS imaging. APPA is the angle between the line connecting the midpoint of both anterior superior iliac spines (ASIS) and the pubic symphysis.

Figure 3.

Anatomical sagittal parameters of the pelvis based on the anterior pelvic plane. The anatomical sacral slope (aSS) is defined as the angle between the sacral plate and the line perpendicular to the anterior pelvic plane (APP). The anatomical pelvic tilt (aPT) is the angle between the line connecting the midpoint of the sacral plate to the hip axis and the line parallel to the APP.

Figure 4.

Correlations between sacral slope (SS) and anatomical pelvic parameters. SS was determined to be significantly, positively correlated with PI (A) and aSS (B) but not with aPT (C).

Figure 5.

Correlations between pelvic tilt (PT) and anatomical pelvic parameters. PT was noted to be significantly, positively correlated with PI (A), aSS (B), and aPT (C).

Figure 6.

Correlations between aPT/PI and PI/PT ratios. The aPT/PI ratio is significantly, positively correlated with the PT/PI ratio. aPT, anatomical pelvic tilt; PT, pelvic tilt; PI, pelvic incidence