Purpose of the study: To analyse post-operative imbalance after C.D.I. (Cotrel Dubousset Instrumentation) for idiopathic scoliosis according to the fused area, particularly the lower level of fusion. To recall a classification for determination of fusion area based on pre-operative standing coronal radiograph.

Patients and methods: To be included in this study the patients had to have an adolescent idiopathic scoliosis, at least two years of post-operative follow up. One hundred and twenty-two patients met the criteria; mean follow-up was three years, five months (minimum two years, maximum nine years). Scoliotic curves were classified as single structural (81), double structural (41). Balance was clinically analysed by plumbline, radiographically by a plumbline dropped from C7 to the sacrum and measuring deviation from the midpoint of the sacrum in centimetres. A curve with a deviation of 10 mms or less was considered as balanced.

Results: Imbalance in single structural curves was 70% when using stable vertebra (King) or “other vertebra” (beyond stable vertebra or one or two levels upper stable vertebra). Using end vertebra (J.MOE), (elected vertebra – C. Salanova) imbalance was 10%. In double structural (41 cases) imbalance was 50% using stable, or “other vertebra” 10% when elected vertebra was fused.

Conclusion: In this study there was a strong statistical relationship between the lower level of fusion and imbalance.