Coronal and sagittal balance in Lenke 5 AIS patients following posterior fusion: important role of the lowest instrument vertebrae selection

Lenke 5 AIS is a kind of three-dimensional deformity and literature reported it usually accompany with coronal or/and sagittal imbalance. However, the postoperative coronal and sagittal balance in these patients has rarely be analyzed previously and the predict factors for postoperative trunk balance are still unclear. To synthetically analysis coronal and sagittal balance of Lenke 5 AIS patients simultaneously and found out predict factors for postoperative coronal or/and sagittal imbalance. Fifty-six Lenke 5 AIS patients who underwent posterior surgery and be followed up more than 2 years were included in this study. Coronal parameters included main curve Cobb angle, lumbosacral hemi-curve Cobb angle, preoperative LEV/LIV tilt and translation and C7-CSVL distance; While sagittal parameters included pelvic incidence(PI), sacral slope(SS), pelvic tilt(PT), lumbar lordosis(LL), thoracic kyphosis(TK), and sagittal vertical axis(SVA). Coronal imbalance was defined as C7-CSVL> 20 mm, and sagittal imbalance defined as (1) SVA > 40 mm or (2) PT 20° or (3) PI-LL > 10°. And relative parameters were compared between balance and imbalance group to find out predict factors. All seven final coronal imbalance patients occurred in LIV = L5 group. Preoperative LIV tilt(11.4°) and translation(5.2 mm) in coronal imbalance group were abnormally lower than balance group (21.7° and 15.7 mm respectively). Eighteen patients performed final sagittal imbalance. The PI in these patients (37.7°) was significantly lower than balance group (48.0°). And most of finial sagittal imbalance patients also occurred in LIV = L5 group. LIV = L5 as a threshold point, represents higher risk of postoperative coronal and/or sagittal imbalance. Besides, large LEV-S1 curve in reduce-bending film and small PI is directly related to final coronal imbalance and sagittal imbalance respectively.