Reliability of Vertebral Pelvic Angles in Assessment of Spinal Alignment

Abstract

Study design: Reliability analysis.

Objectives: Vertebral pelvic angles (VPA) are gaining popularity given their ability to describe the shape of the spine. Understanding the reliability and minimal detectable change (MDC) is necessary to determine how these measurement tools should be used in the manual assessment of spine radiographs. Our aim is to assess intra- and interobserver intraclass correlation coefficients (ICC) and the MDC in the use of VPA for assessing alignment in adult spinal deformity (ASD).

Methods: Three independent examiners blindly measured T1, T4, T9, L1, and L4PA twice in ASD patients with a 4-week window after the initial measurements. Patients who had undergone hip or shoulder arthroplasty, fused or transitional vertebrae, or whose hip joints were not visible on radiographs were excluded. Power analysis calculated a minimum sample size of 19. Both intra- and interobserver ICC and MDC, which denotes the smallest detectable change in a true value with 95% confidence, were calculated.

Results: Out of the 193 patients, 39 were ultimately included in the study, and 390 measurements were performed by 3 raters. Intraobserver ICC values ranged from .90 to .99. The interobserver ICC was .97, .97, .96, .95, and .92, and the MDC was 5.3°, 5.1°, 4.8°, 4.9°, and 4.1° for T1, T4, T9, L1, and L4PA, respectively.

Conclusion: All VPAs showed excellent intra- and interobserver reliability, however, the MDC is relatively high compared to typical ranges for VPA values. Therefore, surgeons must be aware that substantial alignment changes may not be detected by a single VPA.

Keywords: adult; observer variation; radiography; reproducibility of results; scoliosis.

Figure 1.

Vertebral pelvic angle (VPA) defined as the angle formed between lines drawn from the center of the femoral head to the center of the T1, T4, T9, L1 and L4 vertebral bodies and to the midpoint of the superior S1 endplate.

Figure 2.

The hip joints are marked with a circle on the software tool and their centers are automatically identified on the PACS system. The center of the superior S1 endplate is also automatically detected by the software once the endplate is traced.

Figure 3.

(A) A description of how the distance between the target vertebral body and the center of the hip joint affects the area where the measurement error ±2° (dotted line) from the ground truth (GT, black line) can be obtained. The acceptable range for the T1 vertebral body is larger than that for the L4 vertebral body. (B) The magnified image of Figure 2A focused at the level of the T1 vertebral body. (C) Compared to the acceptable range of ±2° measurement error for the line used for T1PA, the acceptable range of ±2° measurement error for the line along the superior endplate used for the Cobb angle technique of the T1 vertebral body is narrow.