Changes in sagittal spinopelvic parameters following S2-alar-iliac screw fixation in adult degenerative scoliosis: A one-year radiographic analysis of 80 patients.

BACKGROUND

S2-alar-iliac (S2AI) screws are widely used for spinopelvic fixation due to their biomechanical advantages. Although pelvic incidence (PI) is traditionally considered fixed after skeletal maturity, emerging evidence suggests that surgical procedures involving the sacroiliac joint may alter pelvic parameters. This study aimed to evaluate the effect of S2AI screw fixation on spinopelvic parameters and analyse variations based on preoperative PI values.

METHODS

A retrospective review was conducted of 80 consecutive patients (160 screws) who underwent spinopelvic fusion for adult degenerative scoliosis between January 2020 and December 2023. Standing radiographs obtained preoperatively and at one-year follow-up were analysed for PI, pelvic tilt (PT), sacral slope (SS), and lumbar lordosis (LL). Patients were classified by preoperative PI as low (<40°), normal (40°-60°), or high (>60°). Changes >6° were considered clinically significant. Statistical analyses used paired t-tests and chi-squared tests (p < 0.05).

RESULTS

PI significantly decreased from 50.45° ± 11.42°-46.18° ± 10.81° (p < 0.001), and PT decreased from 21.46° ± 10.45°-16.50° ± 9.19° (p < 0.001), while SS remained unchanged (p = 0.403). PI-LL mismatch improved from 18.26° ± 11.30°-8.07° ± 5.88° (p < 0.001). Clinically significant PI reduction (>6°) occurred in 40 % of patients, most frequently in those with high preoperative PI (64.7 % vs. 36.5 % normal, 18.2 % low; p = 0.043). ODI scores improved from 77.2 ± 12.1 to 33.9 ± 15.0 (p < 0.001), representing a 56 % reduction in disability, with all patients achieving the minimal clinically important difference. Hardware failure occurred in four screws (2.5 %) without related symptoms or need for revision, and alignment was maintained.

CONCLUSION

S2AI screw fixation significantly modifies spinopelvic parameters, challenging the concept of fixed PI after skeletal maturity. Greater PI reduction in patients with high preoperative PI highlights its relevance for surgical planning in adult spinal deformity correction.