Does 2.5 cm of symphyseal widening differentiate anteroposterior compression I from anteroposterior compression II pelvic ring injuries?

Objective: It has been proposed that 2.5 cm of diastasis of the symphysis pubis corresponds with injury to the anterior sacroiliac ligament and differentiates Young-Burgess anteroposterior compression Type I and II pelvic ring injuries. We hypothesized that if a pelvis has greater than 2.5 cm of symphysis pubis diastasis, the anterior sacroiliac ligaments are disrupted and the pelvic floor has failed. Methods: Pure torsional moment was applied to cadaveric human pelves with the hemipelvis either unconstrained (n ≤ 10) or constrained to move only in the plane of rotation (n ≤ 10). We recorded displacement of the symphysis pubis and sacroiliac joint and the applied torque that corresponded with failure of the anterior sacroiliac ligaments. Results: Average symphysis pubis diastasis at the point of anterior sacroiliac ligament failure was 2.2 cm (n ≤ 20; range, 1-4.5 cm); however, 80% of the values were outside the range of 2 to 3 cm. Symphysis pubis diastasis in male specimens averaged 2.5 cm and in female specimens, 1.8 cm (P ≤ 0.06). The sacrospinous and sacrotuberous ligaments that make up the pelvic floor were not injured in unconstrained testing (zero of 10 specimens) but were at least attenuated in constrained testing (10 of 10 specimens), either simultaneously or after anterior sacroiliac ligament failure. Conclusions: We were not able to confirm 2.5 cm of symphysis pubis diastasis as a valid differentiation point between anteroposterior compression I and II injuries because significant morphologic variation seems to exist. Our data support that anterior sacroiliac ligament disruption is likely for displacement greater than 4.5 cm and unlikely for values less than 1.8 cm. Our study suggests that sacrospinous and sacrotuberous ligaments might not rupture at the same time as the anterior sacroiliac ligament.