How to make a more optimal surgical plan for Lenke 5 adolescent idiopathic scoliosis patients: a comparative study based on the changes of the sagittal alignment and selection of the lowest instrumented vertebra

BACKGROUND: The treatment of patients with Lenke 5 adolescent idiopathic scoliosis (AIS) is closely related to the pelvic because the spine–pelvis is an interacting whole. Besides, the choice of fusion segment is a significant issue; with the optimal choice, there will be fewer complications and restoring the pelvic morphology to some extent. This study aims to analyze the impact of changes in sagittal parameters and selection of the lowest instrumented vertebra (LIV) on spine and pelvic morphology for better surgical strategy. METHOD: Ninety-four patients with Lenke 5 AIS who underwent selective posterior thoracolumbar/lumbar (TL/L) curve fusion were included in the study and grouped according to pelvic morphology and position of LIV. Spinopelvic parameters were measured preoperatively, postoperatively, and at the latest follow-up. The patient’s preoperative and last follow-up quality of life was assessed with the MOS item short-form health survey (SF-36) and scoliosis research society 22-item (SRS-22). RESULT: Patients being posterior pelvic tilt had the oldest mean age (P = 0.010), the smallest lumbar lordosis (LL) (P = 0.036), the smallest thoracic kyphosis (TK) (P = 0.399) as well as the smallest proximal junctional angle (PJA) while those being anterior pelvic tilt had the largest PJA. The follow-up TK significantly increased in both groups of anterior and normal pelvic tilt (P < 0.039, P < 0.006) while no significant changes were observed in the posterior pelvic tilt group. When LIV is above L4, the follow-up PJA was larger than other groups (P = 0.049, P = 0.006). When LIV is below L4, the follow-up TK and PT were larger and LL was smaller than other groups(P < 0.05). The SF-36 and SRS-22 scores were better in the LIV = L4 group than in other groups at the last follow-up (P < 0.05). CONCLUSION: The correction of TK and LL after surgery can improve pelvic morphology. Besides, LIV is best set at L4, which will facilitate the recovery of TK, the improvement of symptoms, and the prevention of complications and pelvic deformities. Level of evidence Level III.