Bracing in Adolescent Idiopathic Scoliosis Trial (BrAIST): Development and Validation of a Prognostic Model in Untreated Adolescent Idiopathic Scoliosis Using the Simplified Skeletal Maturity System

Abstract

Study design: Prognostic study and validation using prospective clinical trial data.

Objective: To derive and validate a model predicting curve progression to ≥45° before skeletal maturity in untreated patients with adolescent idiopathic scoliosis (AIS).

Summary of background data: Studies have linked the natural history of AIS with characteristics such as sex, skeletal maturity, curve magnitude, and pattern. The Simplified Skeletal Maturity Scoring System may be of particular prognostic utility for the study of curve progression. The reliability of the system has been addressed; however, its value as a prognostic marker for the outcomes of AIS has not. The BrAIST trial followed a sample of untreated AIS patients from enrollment to skeletal maturity, providing a rare source of prospective data for prognostic modeling.

Methods: The development sample included 115 untreated BrAIST participants. Logistic regression was used to predict curve progression to ≥45° (or surgery) before skeletal maturity. Predictors included the Cobb angle, age, sex, curve type, triradiate cartilage, and skeletal maturity stage (SMS). Internal and external validity was evaluated using jackknifed samples of the BrAIST data set and an independent cohort (n = 152). Indices of discrimination and calibration were estimated. A risk classification was created and the accuracy evaluated via the positive (PPV) and negative predictive values (NPV).

Results: The final model included the SMS, Cobb angle, and curve type. The model demonstrated strong discrimination (c-statistics 0.89-0.91) and calibration in all data sets. The classification system resulted in PPVs of 0.71-0.72 and NPVs of 0.85-0.93.

Conclusions: This study provides the first rigorously validated model predicting a short-term outcome of untreated AIS. The resultant estimates can serve two important functions: 1) setting benchmarks for comparative effectiveness studies and 2) most importantly, providing clinicians and families with individual risk estimates to guide treatment decisions.

Level of evidence: Level 1, prognostic.

Keywords: Adolescent idiopathic scoliosis; Curve progression; External validation; Internal validation; Natural history; Prognostic model.

Copyright © 2019 Scoliosis Research Society. Published by Elsevier Inc. All rights reserved.

Figures

Figure 1.

Receiver operating characteristic (ROC) curves for identifying patients with a poor prognosis using the model. The c-statistic (area under the curve) and 95% confidence intervals were 0.91 (0.86, 0.97) in the model development (BrAIST) dataset, 0.89 (0.83, 0.95) in the internal and 0.90 (0.84, 0.95) in the external validation datasets. The reference line indicates no discrimination.

Figure 2.

Calibration plot for the Sanders model when applied to the development (BrAIST, blue line), internal (green line) and external validation (red line) samples. The distribution of predicted probabilities from each sample was divided into deciles and the average predicted probability per decile (x-axis) was plotted against the percentage of patients in each decile observed to have a poor prognosis (y-axis). Loess smoothing was used to generate the calibration lines. Perfect calibration is represented by the dotted line.

Figure 3.

Model predictions for curve patterns involving thoracic vertebrae by SMS and Cobb angle. Yellow shaded area indicates the subjects would be classified as high risk for a poor prognosis (predicted probability ≥0.31).

Figure 4.

Model predictions for single lumbar or thoracolumbar curve patterns by SMS and Cobb angle. Yellow shaded area indicates the subjects would be classified as high risk for a poor prognosis (predicted probability ≥0.31).