Anatomical Pelvic Parameters Using the Anterior Pelvic Plane: Relationships with Standing Sagittal Spinal Alignment and Estimated Lumbar Alignment in Healthy Volunteers

Abstract

Introduction: To estimate natural standing sagittal alignment in patients with adult spinal deformity (ASD), we previously reported the normative values of anatomical pelvic parameters in a healthy population, based on the anterior pelvic plane (APP), and observed the relationships between anatomical and positional pelvic parameters in the standing position. As the second step, we aim to investigate the relationships between anatomical pelvic parameters and standing spinal sagittal alignment in a healthy population.

Methods: We analyzed biplanar, slot-scanning, full-body stereo radiography of 140 healthy Japanese volunteers (mean age, 39.5 years; 59.3% women). The APP was defined by bilateral anterior superior iliac spines and anterior surface of the pubis symphysis. Anatomical sacral slope (aSS) and anatomical pelvic tilt (aPT) were calculated as angles of the SS and PT regarding the APP.

Results: The APP was tilted anteriorly in the sagittal plane by an average of 0.7°. Anatomical pelvic parameters significantly correlated with standing sagittal parameters, except for cervical lordosis and T4-12 thoracic kyphosis (TK) (p<0.05). L4-S1 lumbar lordosis (LL) significantly correlated with aPT and aSS, but not with pelvic incidence (PI). In addition, T1-12 TK significantly correlated with aSS. Multiple linear regression analysis for lumbar alignment produced the following equations: L1-S1 LL (°)=0.588×aSS+30.522, L4-S1 LL (°)=0.165×aSS-0.248×aPT+32.825, lordosis distribution index (%)=-0.662×PI+102.8.

Conclusions: Novel relationships in a healthy population were identified between the anatomical characteristics of the pelvis and standing sagittal parameters not represented by PI. This novel measurement concept based on the APP may estimate natural standing sagittal alignments and proportions using anatomical pelvic parameters in ASD.

Keywords: anatomical pelvic parameter; anterior pelvic plane; lumbar lordosis; lumbar proportion; standing sagittal alignment.

Figure 1.

Anatomical sagittal parameters of the pelvis based on the anterior pelvic plane (APP). The APP angle was measured as the angle between the line connecting the midpoint of both the anterior superior iliac spines (a) to the anterior surface of the pubic symphysis (b), which was the APP, and the vertical line of the two-dimensional lateral image. The anatomical sacral slope (aSS) is the angle between the sacral plate and perpendicular line to the APP. The anatomical pelvic tilt (aPT) is the angle between the line connecting the midpoint of the sacral plate (c) to the hip axis (d) and the line parallel to the APP.

Figure 2.

Correlations between anatomical pelvic parameters and sagittal spinal alignment. Pelvic incidence (PI) significantly correlated with the sagittal vertical axis (SVA), T1 pelvic angle (TPA), and L1–S1 lumbar lordosis (LL) (A). Anatomical pelvic tilt (aPT) was significantly correlated with TPA and L4–S1 LL (B). Anatomical sacral slope (aSS) significantly correlated with the SVA, TPA, T1–12 TK, L1–S1 LL, and L4–S1 LL (C).