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A retrospective study by an independent observer of a consecutive series of 67 cases of adolescent idiopathic scoliosis presenting with a King II curve pattern.To demonstrate the validity of a selective thoracic fusion as a treatment of King II curves with special attention to immediate postoperative and long-term trunk balance in the coronal and sagittal planes.The literature has been fairly controversial in terms of the recommended treatment of King II curve patterns in adolescent idiopathic scoliosis. The main confusion appears to be whether the thoracic curve alone or both curves should be instrumented and fused.Sixty-seven patients were identified as having had a selective posterior thoracic spine fusion with instrumentation between 1961 and 1994. None of these cases had a fusion of the lumbar spine. Preoperative radiographs were analyzed for determination of the appropriate fusion level using the criteria of the stable and neutral vertebra. Follow-up radiographs were evaluated for balance in the coronal and sagittal planes using the central sacral line on posteroanterior radiograph and the C7 sacral promontory line on lateral film.At 2-year or greater follow-up, the unfused lumbar curve remained equal to or less than the corrected thoracic curve in 63 patients (94%). No patient required extension of fusion. Frontal plane balance analysis showed that 47 of the 67 patients had the T1 plumb line within 2 cm of the midline for an average decompensation of 8.7 mm. In no patient was the loss of balance greater than 3.8 cm. Sagittal plane balance analysis showed that only one patient had inferior junctional kyphosis greater than 10°. This did not require extension of fusion. There were no cases of superior junctional kyphosis.The concept of selective thoracic fusion in the King II curve pattern appears to be valid. These findings suggest that arthrodesis of the lumbar spine can be avoided when this pattern is properly diagnosed and appropriately treated. Proper identification of the stable and neutral vertebra and of the appropriate level of fusion are important to achieve good postoperative balance. Successful preservation of lumbar motion segments is important to long-term satisfactory outcome in adolescent idiopathic scoliosis.