A Comparison of Cobb Angle: Standing Versus Supine Images of Late-Onset Idiopathic Scoliosis

Abstract

Background: Scoliosis is traditionally evaluated by measuring the Cobb angle in radiograph images taken while the patient is standing. However, low-dose computed tomography (CT) images, which are taken while the patient is in a supine position, provide new opportunities to evaluate scoliosis. Few studies have investigated how the patient's position, standing or supine, affects measurements. The purpose of this study was to compare the Cobb angle in images from patients while standing versus supine.

Material/methods: A total of 128 consecutive patients (97 females and 21 males; mean age 15.5 [11-26] years) with late-onset scoliosis requiring corrective surgery were enrolled. One observer evaluated the type of curve (Lenke classification) and measured the Cobb angle in whole-spine radiography (standing) and scout images from low-dose CT (supine) were taken on the same day.

Results: For all primary curves, the mean Cobb angle was 59° (SD 12°) while standing and 48° (SD 12°) while in the supine position, with a mean difference of 11° (SD 5°). The correlation between primary standing and supine images had an r value of 0.899 (95% CI 0.860-0.928) and an intra-class correlation coefficient value of 0.969. The correlation between the difference in standing and supine images from primary and secondary curves had an r value of 0.340 (95% CI 0.177-0.484).

Conclusions: We found a strong correlation between the Cobb angle in images obtained while the patient was standing versus supine for primary and secondary curves. This study is only applicable for patients with severe curves requiring surgical treatment. It enables additional studies based on low-dose CT.

Keywords: Scoliosis; Spine; Supine Position.

Figure 1

The left image shows a traditional whole spine radiograph. The right image shows the scout picture obtained with low-dose CT in the same patient. The images were taken on the same day before planned corrective surgery.

Figure 2

The linear correlation between the Cobb angles measured while patients were standing and in a supine position for the 128 primary curves, both thoracic and lumbar. y=0.91*x – 5.96; r=0.899; r²=0.809. The blue bars indicate predicted values.

Figure 3

The linear correlation between Cobb angles measured while the patient was standing and in a supine position for all secondary curves, both thoracic and lumbar. y=0.91*x – 4.21; r=0.933; r²=0.870. The blue bars indicate predicted values.

Figure 4

A Bland and Altman diagram showing the difference between Cobb angle measurements obtained while the patient was in a supine position and standing on the y-axis, and the mean between the Cobb angle measurements taken while supine and standing on the x-axis. Both axes are given in degrees.