REEXAMINATION OF THE COBB AND FERGUSON ANGLES - BIGGER IS NOT ALWAYS BETTER

In scoliosis, the Cobb measure of curve severity has been recommended over the Ferguson method because it had greater magnitude and appeared more sensitive to changes during progression and after treatment. Thi s study made comparisons between the Cobb and Ferguson measures in rad iographs of patients with idiopathic scoliosis to test whether the met hods were really different, and to compare their precision. In 138 obs ervations of 77 untreated patients there was a very high correlation ( R2 = 0.98) between Cobb and Ferguson angle, with Cobb angle averaging 1.35 times greater. For sequential measures (mean interval 10 months), the percent changes agreed closely (R2 = 0.5). The relationship betwe en Cobb and Ferguson angles remained the same in measurements of 24 pa tients wearing a brace compared with the unbraced condition and in 18 patients measured before and after Harrington rod surgery. Repeated me asurements were made by three observers with the apex and end vertebra e premarked and held constant. For Cobb angle, the greatest range of m easurements on any film was 8-degrees (pooled SD = 1.3-degrees). For F erguson angle the greatest range was also 8-degrees (pooled SD = 1.8-d egrees). Ferguson angle was slightly more sensitive to incorrect selec tion of end vertebrae. It was concluded that both methods can be usefu l for measuring curve magnitude. Ferguson angle should be measured and then adjusted by multiplying it by 1.35 in situations where Cobb angl e measurement is technically difficult or invalid. Ferguson angle is b etter suited to automated measurement.