Comparison of Sagittal Spinopelvic Alignment in Patients With Ankylosing Spondylitis and Thoracolumbar Fracture

Abstract

This article is a comparative study. The aim of the study is to investigate the difference of sagittal alignment of the pelvis and spine between patients with thoracolumbar kyphosis secondary to ankylosing spondylitis (AS) and thoracolumbar fracture, and to evaluate the role of sacropelvic component in AS patients' adaption to the changes in sagittal alignment. Advanced stages of AS are often associated with thoracolumbar kyphosis, resulting in an abnormal spinopelvic balance and pelvic morphology, whereas thoracolumbar fractures may lead to major kyphosis with a potential compromise of the spinal canal, which can cause an abnormal spinopelvic balance. Until now, the comparison of that sagittal alignment between AS and thoracolumbar fracture is not found in the literature. This study included 30 cases of AS and 30 cases of thoracolumbar fracture. Sagittal spinal and pelvic parameters were measured from the standing lateral radiograph, and the following 11 radiological parameters were measured, including global kyphosis (GK), thoracic kyphosis (TK), C7 tilt (C7T), sagittal vertical axis (SVA), spino-pelvic angle (SSA), lumbar lordosis (LL), upper arc of lumbar lordosis (ULL), lower arc of lumbar lordosis (LLL), pelvic incidence (PI), sacrum slope (SS), pelvic tilt (PT), and T9 tilt (T9T). Analysis of variance was used in the comparison of each dependent variable between the 2 cohorts. The relationship between sagittal spinal alignment and pelvic morphology of AS patients was determined via Pearson correlation coefficient (r). Compared with the thoracolumbar fracture group, AS patients had significantly lower C7T, SSA, LL, LLL and SS (78.3° ± 9.3° vs 88.0° ± 2.7°, P < 0.001 for C7T; 91.6° ± 22.7° vs 119.1° ± 9.0°, P < 0.001 for SSA; 20.7° ± 21.0° vs 36.3° ± 16.8°, P = 0.001 for LL; 18.1° ± 11.9° vs 29.0° ± 9.7°, P < 0.001 for LLL; and 18.1° ± 11.9° vs 29.0° ± 9.7°, P < 0.001 for SS), whereas in terms of SVA and PT, AS patients had an obviously higher value than those of thoracolumbar fracture patients (94.5 mm ± 58.4 mm vs 8.0 mm ± 23.3 mm, P < 0.001 for SVA; and 26.5° ± 10.3° vs 17.5° ± 6.6°, P < 0.001 for PT). In AS patients, SS were found to be significantly correlated with SVA, SSA, and LL (r = -0.312, P < 0.05 for SVA; r = 0.475, P < 0.05 for SSA; r = 0.809, P < 0.001 for LL). In our study, there were significant differences in sagittal alignment of the pelvis and spine between patients with AS and thoracolumbar fracture, and changes in pelvic morphology compensated more in AS patients for a thoracolumbar kyphosis. These findings may be helpful for better understanding of sagittal alignment in patients with thoracolumbar kyphosis secondary to AS.

FIGURE 1

A 46-year-old man with ankylosing spondylitis. Sagittal spinal parameters were measured from the standing lateral radiograph. C7T is the angle between the horizontal plane and the line joining the center of C7 vertebral body and the center of the sacral endplate. SSA is the angle between the sacral endplate and the line joining the center of C7 vertebral body and the center of the sacral endplate. SVA is the distance between the C7 plumb line (C7PL) and the posterior superior corner of S1. GK is the Cobb angle between the upper endplate of the most tilted vertebra cranially and the lower endplate of the most tilted vertebra caudally. TK is the angle between the superior end plate of T5 and the inferior end plate of T12. LL is the angle between the superior end plate of L1 and the superior end plate of S1. ULL is the angle between the tangent line to the vertical axis at the apex of the lumber curve and the superior end plate of L1. LLL is the angle between the tangent to the vertical axis at the apex of the lumber curve and the superior end plate of S1. C7PL = C7 plumb line, C7T = C7 tilt, GK = global kyphosis, LL = lumbar lordosis, LLL = lower arc of lumbar lordosis, SSA = spino-pelvic angle, SVA = sagittal vertical axis, TK = thoracic kyphosis, ULL = upper arc of lumbar lordosis.

FIGURE 2

Sagittal pelvic parameters were measured from the standing lateral radiograph. PI is the angle between the line perpendicular to the sacral plate at its midpoint and the line connecting this point to the axis of the femoral heads. PT is the angle between the line connecting the midpoint of the sacral plate to the femoral head axis and the vertical axis. SS is the angle between the superior plate of S1 and a horizontal line. T9T is the angle between the vertical axis passing through the middle of both femoral heads’ centers and an axe passing through the center of T9 vertebral body. PI = pelvic incidence, PT = pelvic tilt, SS = sacrum slope, T9T = T9 tilt.

FIGURE 3

A comparison of sagittal spinopelvic alignment between AS patients (A) and thoracolumbar fracture patients (B). AS = ankylosing spondylitis.