Thoracic kyphosis assessment in postmenopausal women: an examination of the Flexicurve method in comparison to radiological methods

The Flexicurve ruler is an alternative method to radiographs for measuring thoracic kyphosis (curvature), but it is not certain that it is comparable. This study shows that Flexicurve can estimate radiographic vertebral centroid angles with less error than Cobb angles but that its accuracy would be inadequate for most clinical purposes.

The Flexicurve ruler provides a non-radiological method of measuring thoracic kyphosis (TK) that has moderately strong correlations with the gold-standard radiographic Cobb angle method, while consistently underestimating the TK angle. Cobb angles can include measurement errors that may contribute to poor agreement, particularly in older populations. The vertebral centroid angle could be a better radiographic reference method for the validation of Flexicurve. Using two separate radiographic measurements of TK, we examined the validity of Flexicurve. We aimed to ascertain the level of agreement between measures and to empirically explore reasons for between-method differences.

TK angles determined using Flexicurve and radiographic Cobb and vertebral centroid methods were compared using data from 117 healthy postmenopausal women (mean (SD) age 61.4 (7.0) years). Bland and Altman plots were used to assess differences between methods. Age, bone mineral density and body mass index were examined as characteristics that might explain any differences.

Flexicurve angles were scaled prior to analysis. There was no statistically significant difference between angles produced by Flexicurve and vertebral centroid methods (MD - 2.16°, 95%CI - 4.35° to 0.03°) although differences increased proportionally with TK angles. Flexicurve angles were significantly smaller than radiographic Cobb angles and depending on the scaling method used, systematic error ranged between - 2.48° and - 5.19°. Age accounts for some of the differences observed (R² < 0.08, p < 0.005).

TK measured using the Flexicurve shows better agreement with the radiographic vertebral centroid method, but inaccuracy of the Flexicurve increases with increasing angle of kyphosis.