Tangent Approximation Formula to Determine Height Loss in Scoliosis Patients by a new Trigonometric Model
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Background
The problem of predicting loss in body height due to scoliosis especially in adolescent idiopathic scoliosis (AIS) has been investigated for over 50 years. The corrected height is required to make appropriate lung function and other anthropometry assessments. Most existing formulae estimate height loss through different regression models.
Method
A new trigonometric model was developed based on the Cobb angle (α) and the apical vertebral translation (AVT) in the coronal plane. By using this model, a formula was derived using the tangent of the triangle to estimate height loss in Scoliosis.
As measurement of AVT is not commonly performed, a mean value was obtained from patients series reported by Şarlak et al. It was found that AVT is affected by the Cobb angle, which was significantly different between patients with Cobb angle >= 50 and < 50.
Results
Validation of this new Tangent formula was carried out with two datasets, one from Tyrakowski et al 2014, and another from 58 operated Scoliosis patients in a local hospital. The mean bias was the lowest with this novel tangent formula compared to Bjure, Kono, Stokes formulae. A paired t-test confirmed that these differences were significantly better with the tangent formula. The trigonometric approach has the advantage that it is valid over a larger range of Cobb angle as most of the regression models cannot be extrapolated outside the range of available data points.
The Tangent formula is made available at the website for easy access and height loss can be estimated by inputting the Cobb angle with or without the AVT: https://tangentformulae.scoliosisdevices.com/
Conclusion
The results indicate the use of the Tangent formula with simple measurements and trigonometric calculation yield results comparable with other formulae used in the past to estimate height loss. The application of this new formula can prove useful in clinical settings to allow more accurate testing and better care for patients.
