Computer Assisted Cobb Angle Measurements: A novel algorithm

Abstract

Background: The standard for evaluating scoliosis is PA radiographs using Cobb angle to measure curve magnitude. Newer PACS systems allow easier Cobb angle calculations, but have not improved inter/intra observer precision of measurement. Cobb angle and its progression are important to determine treatment; therefore, angle variability is not optimal. This study seeks to demonstrate that a performance equivalent to that achieved in the manual method is possible using a novel computer algorithm with limited user input. The authors compared Cobb angles from predetermined spinal levels in the average attending score versus the computer assisted approach.

Methods: Retrospective analysis of PA radiographs from 58 patients previously evaluated for scoliosis was collected. Predesignated spinal levels (e.g., T2-T10) were assigned for different curves and calculated by Cobb method. Four spine surgeons evaluated these Cobb angles. Their average scores were measured and compared to formulated values using the novel computer-based algorithm. Literature reports inter-observer reliability is 6.3-7.2degrees. Limits of accuracy were set at 5 degrees of average orthopedic surgeons' score.

Results: The computer-based algorithm calculated Cobb angles within 5 degrees of orthopedic surgeons' average with a standard deviation of 3.2 degrees. This result was based on a 95% confidence interval with p values <0.001. The computer algorithm was plotted against average angle determined by the surgeons, with individual determinations and linear regression (r² =0.90). The average difference between surgeons' measures and computer algorithm was 0.4 degrees(SD= 3.2degrees, n=79). There was a tendency for the computer algorithm program to overestimate the angle at larger angles, but difference was small with r² = 0.09.

Conclusions: Our study showed the novel computer based algorithm was an efficient and reliable method to assess scoliotic curvature in the coronal plane with the possibility of expediting clinic visits, ensuring reliability of calculation and decreasing patient exposure to radiation. Level of Evidence: III.

Keywords: cobb angle; computer algorithm; scoliosis; scoliotic curvature.

Fig. 1

Original, localized brightness, and enhanced images prior to curvature measurements.

Fig. 2

(A) Computer based calculation of Cobb angle based on pre-designated spinal levels. (B) Surgeons’ average calculation on this patient was 23.6 degrees with a standard deviation of 2.8 degrees.

Fig. 3

Computer algorithm program-based (PB) determined angle plotted against the determined surgeons’ average angle. Individual determinations are indicated by the symbol o; solid line is the linear regression (r² =0.90) and dotted lines are the 95% prediction intervals.

Fig. 4

Bland-Altman Analysis. Difference in angles measured by the two techniques versus the mean angle.