Utilization of s1 foraminal hooks for augmentation of s1 screws in adult spinal deformity surgery: Comparative study with iliac screws

Objective: To compare the outcomes of S1 foraminal hooks and iliac screws regarding fusion rate at the lumbosacral junction and protective effects on S1 screws. Methods: From January 2017 to December 2019, consecutive patients who underwent long fusions (uppermost instrumented vertebra at or above L1) to the sacrum for adult spinal deformity were enrolled. Patients were divided into S1 foraminal hook group and iliac screw group. Radiographic parameters and the incidence of pseudarthrosis and instrument failure at the lumbosacral junction were compared between the groups. Results: Twenty-nine patients (male:female = 1:28) with a mean age of 73.6 ± 6.8 years were evaluated. Sixteen patients (55.2%) had S1 foraminal hook fixation and 13 patients (44.8%) had iliac screw fixation. Lumbar lordosis, sacral slope, and sagittal vertical axis did not differ between the groups preoperatively and postoperatively. The rate of L5/S1 pseud-arthrosis was significantly higher in S1 foraminal hook group (5 of 16, 31.3%), compared to iliac screw group (0 of 13, 0%; p = 0.048). Instrument failure at the lumbosacral junction trended toward a higher rate in S1 foraminal hook group (6 of 16, 37.5%) than in iliac screw group (1 of 13, 7.7%), without statistical significance (p = 0.09). Proximal junctional kyphosis/failure occurred less often in S1 foraminal hook group (2 of 16, 12.5%) than in iliac screw group (3 of 13, 30.8%) without statistical significance (p = 0.36). Conclusion: Treatment with S1 foraminal hooks achieved equivalent satisfactory sagittal correction with proportioned alignment compared to that with iliac screws. However, S1 foraminal hooks did not provide enough structural support to the lumbosacral junction in long fusions to the sacrum.