Global Sagittal Alignment and Proximal Junctional Kyphosis in Adolescent Idiopathic Scoliosis

Study design: Case-Control Study Objectives: To analyse global sagittal alignment including the cranial center of mass (CCOM) and proximal junctional kyphosis (PJK) in Adolescent Idiopathic Scoliosis (AIS) patients treated with posterior instrumentation. Summary of background data: PJK plays an important role in the global sagittal alignment in AIS patients. Maintaining the head above the pelvis allows for a minimization of energy expense in ambulation and upright posture. Numerous studies have been performed to understand PJK phenomena in AIS patients. However, to our knowledge, no study performed on AIS patients included the head in the analysis of global sagittal alignment and PJK. Methods: This study included 85 AIS patients and 51 asymptomatic adolescents. Low-dose bi-planar X-rays were acquired for each subject preoperatively and at two years follow up. Two global sagittal alignment parameters were calculated, i.e. the angle between the vertical and the line joining the center of the bi-coxofemoral axis (HA) and either the most superior point of the dentiform apophysis of C2 (OD) or the cranial center of mass (CCOM). Results: Among normal adolescents, the average OD-HA and CCOM-HA angles were -2.3°±2° and -1.5°±1.8°, respectively. Among AIS patients the average OD-HA and CCOM-HA angles were respectively -2.3°±1.9° and -1.3°±1.8° preoperatively and -2.8°±1.7° and -1.9°±1.7° at the last follow up. 13% of the patients developed PJK postoperatively. Case-by-case analysis showed that adjusting the thoracic kyphosis and the compensations required to maintain this constant could provide explanatory elements. Conclusions: OD-HA and CCOM-HA angles remain almost constant among normal group and patients, pre- and post-operatively, whether PJK or non-PJK. Five patients without PJK and only one patient with PJK produced abnormal values relative to the asymptomatic subjects. Therefore, PJK could be a compensation mechanism, which allows for CCOM-HA and, to a lesser extent, OD-HA to remain invariant. Level of Evidence: III BiomecAM chair on musculoskeletal modeling, with the support of Cotrel and ParisTech Foundations, Société Générale, Covea and Proteor.