Is more better? Multicenter analysis of the incidence and mechanisms of multiple pelvic fixation failure in adult spinal deformity surgery.

BACKGROUND CONTEXT: High mechanical stress at the lumbosacral junction (LSJ) contributes to elevated failure rates in long-segment adult spinal deformity (ASD) fusions. To enhance fusion across the LSJ and preserve correction after ASD surgery, pelvic fixation has become a critical component of surgical constructs. However, conventional techniques that use a single point of fixation per side remain prone to implant failure. Biomechanical studies indicate that multiple-point pelvic fixation (MPF) may improve load distribution and construct stability, yet supporting clinical evidence remains limited.

PURPOSE: The purpose of this study was to assess the incidence, mechanisms, and potential protective strategies for pelvic fixation failure (PFF) utilizing multiple pelvic fixation (MPF) constructs in adult spinal deformity (ASD) surgery.

STUDY DESIGN/

SETTING

Multicenter retrospective-prospective cohort study conducted across 4 tertiary spine centers from 2010 to 2024.

PATIENT SAMPLE: A total of 229 ASD patients (age ≥ 18 years, minimum of 5 instrumented levels) with pelvic fixation (S2-alar-iliac [S2AI] and/or iliac screws) with a minimum 1-year follow-up, consecutively collected (2010-2024). MPF was defined as having more than two pelvic fixation points per patient, with at least one sacral-alar-iliac (SAI) screw per side.

OUTCOME MEASURES: The primary endpoint was establishing the incidence of pelvic fixation failure requiring reoperation in MPF constructs. The secondary endpoint was to determine the mechanisms underlying PFF in MPF constructs.

METHODS

PFF endpoints included implant failure requiring reoperation (RR), all-modality failure RR, all-modality failure not requiring reoperation (NRR), screw loosening (NRR), and rod breakage. Patient information including demographic data and health history (age, sex, BMI), instrumented levels (IL), 3-column osteotomy (3CO), interbody fusion (IBF), screw (iliac, S2AI, length, diameter), rod (diameter, material), rod pattern (number crossing lumbopelvic junction), pre- and postsurgery (pelvic incidence, pelvic tilt, T1 pelvic angle (TPA), PI-LL, and sacral slope parameters were collected. Failure rates were compared with single-fixation benchmarks from Eastlack et al (2022) using one-sample z-tests for proportions. Radiographic parameters and implant characteristics were analyzed between the failure and nonfailure groups.

RESULTS

From the 229 patients analyzed (52.4% female, mean age 67.2, mean IL 11.3, 3CO 27.9%, L5-S1 IBF 45.8%, L4-L5 IBF 34.1%) 3.1% (n=7) experienced PFF requiring reoperation after 1 year. Mean screws per patient were 3.6; 77.7%. MPF implants had a mean length of 88.7 mm and a mean diameter of 9.53 mm. Mean rods across the LS junction were 3.37 with 50.7% cobalt chrome. PFF due to mechanical failure requiring reoperation after 1 year was 2.2% (n=5). The MPF rate without reoperation was 6.6% (n=15). Further mechanisms of failure included rod breakage (2.2%; n=5) [2 (0.9%) requiring reoperation] and screw loosening (5.2%; n=12). Failures tended to be associated with greater preoperative deformity, evidenced by preoperative pelvic tilt (29.6° vs 26.2°) and TPA (30.4° vs 27.4°), as well as shorter screw length (86.7 vs 88.8 mm).

CONCLUSIONS

Multiple-point pelvic fixation is associated with lower and reduces the risk of PFF in long-segment ASD surgery. Compared with multicenter reports using single- or limited-point fixation strategies, the observed failure rate, including both revision and nonrevision cases, was lower in constructs with multiple pelvic anchors. Load distribution across multiple fixation points enhances construct durability and provides a more stable foundation for long-segment fusion, mitigating implant failure and the need for revision surgery.