Adolescent idiopathic scoliosis (AIS) is a common spinal deformity in adolescents requiring surgical correction in progressive cases. Body mass index (BMI) may influence perioperative outcomes; however, the impact of low BMI, as classified by World Health Organization (WHO) criteria, remains unclear. BMI is an important factor influencing perioperative outcomes in AIS surgery. A total of 328 scoliosis patients who underwent posterior spinal fusion (PSF) surgery. PSM was performed with a match tolerance of 0.05. Patients were classified into two groups: Group 1 [Thinness/Underweight (Below 5th percentile)] and Group 2 [Normal weight (5th to <85th percentile)]. Forty-three matched patient pairs were analyzed. Patient demographics and perioperative outcomes were compared. The primary outcome measures included the number of screws used, duration of surgery, postoperative haemoglobin levels, intraoperative blood loss, blood transfusion requirements, length of hospital stay, total patient- controlled analgesia (PCA) usage, and complication rates. The cohort consisted of 80.2% female and 19.8% male patients, with a mean age of 14.5±1.8 years. Majority had Lenke 1 curves (39.5% in Group 1 and 48.8% in Group 2, p=0.761). There was no significant difference observed for preoperative Major Cobb angle (67.0±19.9° and 67.7±19.0°, p=0.882), correction rates

(72.7±10.2% and 70.4±9.8%, p= 0.281), screw density (1.3±0.1 and 1.4±0.2, p=0.508) and length of hospital stay (3.3±1.0 and 3.0±0.3, p=0.062). Surgery duration was comparable between groups (134.3±33.8 minutes vs. 147.6±45.4 minutes, p=0.076). Intraoperative blood loss was higher in Group 1 (771.1±300.0 mL) compared to Group 2 755.7±295.6 mL) (p=0.751), however the difference was not statistically significant. The overall complication rate was 3.5% (p=1.000), with two complications in Group 1 (4.7%

-Anterior perforation and surgical site infection) and one complication in Group 1 (2.3% -Ulnar nerve Neuropraxia due to positioning), however it was not statistically significant. Previous literature suggests that patients with low BMI face increased perioperative risks compared to those with normal BMI. Our study found a higher risk of perioperative complication rate, but the difference was not statistically significant.