Evaluation of Risk Factors for S1 Screw Lucency via Radiographic Assessment

Introduction: Correcting lumbar and thoracolumbar spinal deformities often involves instrumentation at the L5-S1 segment. There is a susceptibility for S1 screw failure in these constructs thought to be due to significant shear forces or variations in bone quality at the sacrum. We aim to determine the extent and risk factors associated with S1 screw lucency after elective lumbar spinal surgeries.

Methods: A retrospective radiographic chart query was conducted to identify patients who underwent elective spine surgeries involving the S1 level that resulted in screw lucency from January 2018 to January 2023. Patients were assessed for maximum S1 screw lucency diameter in relation to HU at L1 and S1 on the first post-operative CT demonstrating screw lucency. A multiple regression was conducted to control for race, age, and gender and assess the association between HU at L1 or S1 and screw lucency.

Results: A total of 207 patients were included in this study. The mean age was 67 years and 56% were male. The mean screw lucency for this population was 9.89, and the mean HU at L1 and S1 were 14.8 and 17.2, respectively. Our analysis revealed no significant association between maximum S1 screw lucency and Hounsfield Units (HU) at L1 or S1 (B=-0.04, CI -.08-0.01, p=0.14 and B=0.03, CI -.02-0.07, p=0.30, respectively). Maximum S1 screw lucency was also not associated with baseline patient demographic factors including race, age, or gender.

Conclusion : Our findings suggest that S1 screw lucency may not correlate with factors such as bone quality via HU measurements, age, race, or gender. Various other modifiable patient risk factors may play a more critical role in this relationship. Future investigation is warranted.