To evatuate interactions betvveen corrective forces applied by segmentary spinal systems and fusion levels along with coronal and sagittal balance, x ray radiographs of 126 scoliosis patients operated in SSK İstanbul Edu-cation Hospital llnd Orthopaedics and Traumatology Clinics betvveen November 1990 and May 1995 were exam-ined retrospectively. Except 20 patients with atypical or left sided curves, 106 right sided adolescent thoracic idio-pathic scoliosis were classified according to King's criteria and patients with King Type II and Type III comprising almost 2/3 of the group vvere included in the study (66 patients). Of these 47 patients were reevaluated in Decem-ber 1995 (9 boys, 38 giriş, mean age 14.9years). Preoperatively, average coronal thoracic Cobb angle was 52.4 andlumbar was 34.6 degrees (mean follow-up period 31.7 months). For 12patients CD instrumentations patients MSS instrumentations vvere used postoperatively, average correction in coronal Cobb measurement for thoracic curves vvas 59.9% and for lumbar was 63.5%. King Type II and Type III scoiiotic curves are evaluated separately, and further subgrouped in relation vvith lovvest instrumented vertebra (LIV) relative to stable vertebrae (SV). Coronal decompensation vvas observed in 8 (36%) Type II patient, and 3 (12%>) Type III patlent.

Data obtained from this study shovved us that Type II curves are more prone to decompensation than Type III curves. Lovvest instrumentation level is not prognostic for postoperative decompensation. Patient based modifica-tions are more raiiable than strict guidelines for selection of distal fusion level since application of corrective forces to each segment at each sides is possible vvith posterior derotation systems. Only prognosis factor for Type II curves for postoperative decompensation is preoperative decompensation. Type II curves vvith preoperative decompensation are more prone to develop postoperative decompensation. Type III curves can be managed safely ending fusion at SV, and ending short of SV does not obviously results vvith decompensation but rather vvith ad-ding on phenomena. Extension of fusion in Type III curves does cause detoriation of compensation but rather more effectively corrects coronal curve.

Keywords: Balance, Fusion Levels, Scoliosis Surgery.