The obturator foramen “safe zone” for anterior pelvis internal fixation by trans-obturator cerclage

Objectives: To quantify the morphology of an avascular “safe zone” within the obturator foramen to assess the risk of neurovascular injury with trans-obturator foramen cerclage. Design: Retrospective chart review. Setting: Level 1 trauma center and tertiary academic center. Patient Selection Criteria: Adults ≥18 years without prior pelvic pathology and with pelvic computed tomography angiography (CTA) demonstrating complete visualization of the obturator arteries. Outcome Measures and Comparisons: The minimal distance from the medial cranial border of the obturator foramen to the obturator artery (DOA) and the obturator foramen area (OFA) bounded by the obturator artery and pubis were measured on CTA in the plane of the obturator foramen. Associations of OFA and DOA with demographic and anthropometric variables were analyzed by multivariable linear regression. The “safe zone” for passage of trans-obturator cerclage instrumentation was determined by the 5.0mm outer diameter of a commercially available cable passer. Results: The sample included sixty adults, 35 men (58.3%), with a mean age of 56.2±16.4 years. The mean DOA (left 6.8±1.9mm, right 6.9±1.6mm, p=0.724) and OFA (left 195.1±63.3mm 2 , right 190.3±55.4mm 2 , p=0.657) did not vary by laterality. DOA was >3.2mm for all patients but <5.0mm in 23.3% of patients. Multivariable analysis indicated that DOA was associated with patient height (β=0.04, p=0.019) but not sex (β=0.01, p=0.987) or weight (β=0.01, p=0.253). Conclusions: A “safe zone” defined by the obturator artery and cranial and medial obturator foramina exists for internal fixation of the anterior pelvic ring by trans-obturator foramen cerclage. While a cerclage device ≤3.2mm in diameter appears safe, passage of a cerclage wire through the cranial and medial obturator foramina for internal fixation anterior pelvic ring disruption with typical instrumentation may injure the obturator neurovascular bundle without additional surgical dissection. Level of Evidence: Prognostic Level III.