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A radiographical follow-up and analysis.To identify appropriate radiographical parameters for measuring the extent of distal adding-on and to discuss criteria for determining the onset of distal adding-on.There is no consensus on how to determine the onset of distal adding-on in Lenke 1A scoliosis. Such questions as: “Which radiographical parameters should be used for measuring the extent of distal adding-on?” and “What criteria should be applied in determining the onset of distal adding-on?” need to be answered.We reviewed all the AIS cases surgically treated in an institution from 2003 through 2009. Inclusion criteria were as follows: (1) patients with Lenke 1A curves who were treated with selective thoracic fusion; (2) age less than 30 years; (3) 2-year radiographical follow-up. Eight radiographical parameters were tested to see if they are potential instruments in the detection of distal adding-on.Fifty-three patients met the inclusion criteria. No pseudarthrosis or crankshaft phenomenon was observed in the current cohort. Five out of 8 radiographical parameters: thoracic Cobb, LIV-CSVL distance, LIV + 1-CSVL distance, thoracic AV-CSVL distance and LIV + 1 tilt angle, in the 2 years after surgery, showed significant increase. The remaining 3 parameters: LIV tilt angle, T1-CSVL distance and number of vertebrae within Cobb, however, did not show significant increase. In regard to the 5 parameters that have the potential to detect the onset of distal adding-on, we found a high correlation between every 2 of them. The correlation coefficients range from 0.504 to 0.962 (P = 0.001), suggesting that all of them are in a positive linear relationship. Regarding the criterion for determining the onset of distal adding-on, an increase of more than 10 mm in LIV-CSVL distance in the postoperative period can be considered as a the main criterion because it is unlikely to be induced by measurement errors.LIV-CSVL distance could be an ideal parameter for measuring the extent of distal adding-on. Distal adding-on can be determined when the LIV-CSVL distance increases by 10 mm in the postoperative period.