Comparative Study of Post-Surgical Outcomes in Pain, Disability, and Health-Related Quality of Life for Adult Spinal Deformity in Patients Aged above and below 75 Years

(1) Background: Adult spinal deformity (ASD) surgery is known to improve clinical and radiological parameters; however, it may also cause more complications in elderly patients. The purpose of this study was to compare the outcomes of ASD surgery, specifically regarding pain, disability, and health-related quality of life (HRQOL) in patients aged 75 years and over and patients aged under 75 years; (2) Methods: A total of 151 patients who underwent ASD surgery between August 2014 and September 2020 were included. Patients were divided into two groups based on whether they are 75 years and over or under. Radiological parameters measured included sagittal vertical axis (SVA), pelvic tilt (PT), and pelvic incidence (PI)- lumbar lordosis (LL). Data were collected 3, 6, and 12 months after surgery; (3) Results: At 12 months postoperatively, visual analog scale (VAS) for low back pain (p = 0.342), Oswestry disability index (ODI) (p = 0.087), and EuroQol 5-Dimensions (EQ-5D) (p = 0.125) did not differ between patients under 75 years and those 75 and above 75 group. PT (p = 0.675), PI-LL (p = 0.948), and SVA (p = 0.108) did not differ significantly 12 months after surgery in the two groups. In the entire patient group, compared to preoperative data, significant improvements were demonstrated for clinical and radiological parameters 12 months after surgery (all p < 0.001). The rate of medical complications did not correlate with age, but the rates of proximal junctional kyphosis (PJK) and proximal junctional failure (PJF) did (p = 0.638, p < 0.001, and p = 0.001, respectively); (4) Conclusions: In terms of clinical and radiological improvements, ASD surgery should be considered for patients regardless of whether they are younger than or older than 75 years. The clinical and radiological improvements and the risk of complications and revision surgeries must be considered in ASD patients who are 75 years or older.