Incidence of Suboptimal Fluoroscopic Outlet Imaging of the Sacrum and Pelvic Retroversion Necessary for Optimal Views

Abstract

OBJECTIVES: To estimate the prevalence of suboptimal fluoroscopy of sacral outlet images due to anatomic and equipment dimensions. Pelvic retroversion is hypothesized to mitigate this issue. Design: In silico simulations using retrospectively collected computed tomography (CT) data from human patients. Setting: Level I trauma center. Patient Selection Criteria: Adults with AO/OTA 61 pelvic ring disruptions treated with posterior pelvic fixation between July and December 2021. Outcome Measures and Comparisons : C-arm tilt angles required to obtain three optimal fluoroscopic sacral outlet images, defined as vectors from pubic symphysis to S2 and parallel to the first and second sacral neural foramina, were calculated from sagittal CT images. A suboptimal view was defined as collision of the C-arm radiation source or image intensifier with the patient/operating table at the required tilt angle simulated using the dimensions of five commercial C-arm models and trigonometric calculations. Incidence of suboptimal outlet views and pelvic retroversion necessary to obtain optimal views without collision, which may be obtained by placement of a sacral bump, was determined for each view for all patients and C-arm models. RESULTS: CT data from 72 adults were used. Collision between patient and C-arm would occur at the optimal tilt angle for 17% of simulations and at least one view in 68% of patients. Greater BMI was associated with greater odds of suboptimal imaging (standard outlet: OR 0.84, CI 0.79-0.89, p<0.001; S1: OR 0.91, CI 0.87-0.97, p=0.002; S2: OR 0.85, CI 0.80-0.91, p<0.001). S1 anterior sacral slope was associated with suboptimal S1 outlet views (OR 1.12, Cl 1.07-1.17, p<0.001). S2 anterior sacral slope was associated with suboptimal standard outlet (OR 1.07, Cl 1.02-1.13, p=0.004) and S2 outlet (OR 1.16, Cl 1.09-1.23, p<0.001) views. Retroversion of the pelvis 15-20° made optimal outlet views possible without collision in 95%-99% of all simulations, respectively. CONCLUSIONS: Suboptimal outlet imaging of the sacrum is associated with greater BMI and sacral slope at S1 and S2. Retroversion of the pelvis by 15-20° with a bump under the distal sacrum may offer a low-tech solution to ensure optimal fluoroscopic imaging for percutaneous fixation of the posterior pelvic ring. Level of Evidence: Therapeutic Level IV. See Instructions for Authors for a complete description of levels of evidence.