Coronal imbalance in degenerative scoliosis with type A coronal alignment: an amendment to the Nanjing coronal imbalance classification

OBJECTIVE The purpose of this study was to propose a refined coronal classification with subgroup analysis of degenerative scoliosis (DS) patients with a type A coronal pattern and to ascertain its implications on postoperative coronal imbalance (CIB). METHODS A total of 239 DS patients who underwent spinal correction surgery were recruited for this study. Patients were divided into types A, B, and C based on the Nanjing CIB classification system. Patients with type A CIB were further divided into three subtypes according to the coronal balance distance (CBD) and the trunk inclination tendency: type A a , CBD ≤ 1 cm; type A b , CBD > 1 cm and C7 plumb line (C7PL) shifted to the concave side of the curve; and type A c , CBD > 1 cm and C7PL shifted to the convex side. Scoliosis Research Society–22 questionnaire scores were analyzed, and the incidence of postoperative CIB was compared across groups. RESULTS The incidence of postoperative CIB was 23% (32/139) in the type A group, 18% (11/60) in the type B group, and 58% (23/40) in the type C group (p < 0.001). Among patients with type A coronal alignment, 54 patients had type A a , 46 had type A b , and 39 had type A c . After surgery, 32 patients had postoperative CIB, with 5 (9%) patients in the type A a group, 5 (11%) patients in the type A b group, and 22 (56%) patients in the type A c group. Postoperative coronal malalignment was found to be more prevalent in type A c patients (p < 0.001). CONCLUSIONS Patients with type A c CIB are at greater risk of postoperative CIB following surgery compared with patients with type A a or A b alignment. The modified CIB classification highlights the high risk of CIB in type A c , similar to the type C CIB pattern. These findings provide a more comprehensive delineation of coronal alignment phenotypes and introduce a refined system for stratifying the risk of postoperative CIB.