Radiographic Spinopelvic Parameters

Biplanar radiographs in a standing position were taken using the EOS imaging system (EOS Imaging). The subjects stood in a standardized erect posture, with knees and hips fully extended, shoulder anteriorly flexed by 90°, elbow extended, and hands resting on a support handle. Images were captured with minimal magnification and distortion as the system employs line detection of double-collimated X-ray beams. All radiographic parameters were measured by a senior orthopedist using the picture archiving and communication systems (PACS) software. The usefulness of these spinopelvic parameters were verified in previous studies by Tyrakowski et al. [16–18].

The following radiographic spinopelvic parameters were calculated (Figs. 1 and ​and22):

The graph shows how sagittal spinopelvic parameters were measured. PI = PT + SS. TK, thoracic kyphosis; PI, pelvic incidence; PT, pelvic tilt; SS, sacral slope; CFH, center of femoral heads; LL, lumbar lordosis; KE, knee extension

The graph shows how whole-body sagittal spinopelvic parameters were measured. SVA, sagittal vertical axis; CAM, center of acoustic meatus; SSA, spino-sacral angle; T1t, T1 tilt; T9t, T9 tilt; CFH, center of femoral heads

Pelvic incidence (PI): the angle between the line perpendicular to the sacral endplate and the line connecting the midpoint of the femoral heads to the midpoint of sacral plate [9].

Sacral slope (SS): the angle between the horizontal plane and the line tangent to the sacral endplate [9].

Pelvic tilt (PT): the angle between the vertical line and the line connecting the midpoint of sacral endplate to the midpoint of the femoral heads, namely the anterior pelvic plane (APP) [9].

Thoracic kyphosis (TK): the angle between the cranial T4 endplate and caudal T12 endplate [19, 20].

Lumbar lordosis (LL): the angle between the cranial L1 endplate and the cranial sacral endplate [9].

Sagittal vertical axis (SVA): the shortest distance between the posterosuperior corner of S1 and the C7 plumb line, which is the vertical line through the center of the C7 vertebral body [21]. (Fig. 2)

Center of acoustic meatus (CAM) plumb line: the distance between the plumb line through CAM and the C7 plumb line.

T1 tilt (T1t): the angle between the line tangent to the cranial endplate of T1 and a horizontal line.

T9 tilt (T9t): the angle between the line tangent to the cranial endplate of T9 and a horizontal line.

Spinosacral angle (SSA): the angle between the line connecting the center of the C7 vertebral body to the midpoint of the cranial sacral endplate and the line tangent to the cranial sacral endplate.

Knee extension (KE): the angle between the axis of femur and that of the tibia (Fig. 1). The average of bilateral angles was used in statistical analysis.