Thoracolumbar Kyphosis Is a Risk Factor for Proximal Junctional Kyphosis After Isolated Posterior Surgery for Lenke-5C Curvature

Background/Objectives: This study aimed to examine the occurrence and risk factors for proximal junctional kyphosis (PJK) in relation to preoperative sagittal alignment, particularly the shape of thoracolumbar kyphosis (TLK) and the proportion of lumbar lordosis. Methods: We recruited 38 consecutive patients with adolescent idiopathic scoliosis (AIS) who underwent isolated posterior fusion. Participants were categorized according to the presence or absence of PJK at 1 year postoperatively (PJK+ or non-PJK) and by preoperative TLK status (positive or negative; pre-TLK+ or pre-TLK, respectively). We compared spinal parameters preoperatively, immediately postoperatively, and at 1 year postoperatively between groups. Results: Among the 38 participants, PJK occurred in 21 patients (55.3%). The PJK group had significantly larger preoperative TLK and LDI values and decreased postoperative TLK and LDI. Simple linear regression revealed a moderate positive correlation between ΔPJA and preoperative TLK as well as a fair positive correlation between ΔPJA and changes in TLK and LDI. The prevalence of PJK was high (83.3%) in the pre-TLK+ group (24 patients), and preoperative LDI was significantly larger compared with the pre-TLK group. However, TLK and LDI were significantly decreased after surgery in the pre-TLK+ group. Conclusions: Patients with Lenke-5C curvature who exhibit positive preoperative TLK are at a very high risk of developing PJK after isolated posterior surgery. Preoperative sagittal alignment should be considered when planning the extent of sagittal correction.