How much vertical displacement of the symphysis indicates instability after pelvic injury?

BACKGROUND: Measures of pubic symphyseal widening are used by at least two classification systems as determinants of injury grade. Recent work has challenged the commonly used parameter of 2.5 cmof pubic symphysis as an accuratemarker of pelvic injury grade and has suggested a role of rotation in the flexion-extension plane as a determinant of pelvic stability.We investigated pelvic stability in the flexion-extension plane to determine a threshold of rotational displacement of the hemipelvis above which the potential for instability exists. METHODS: Cadaveric specimenswere mounted onto a servohydraulic biaxial testing machine and subjected to a vertically directed flexion moment. Position of hemipelvis was recorded using a three-dimensional motion capture system and video recording. Displacement of the pubic symphysis and changes in length and position of the sacrospinous and sacrotuberous ligaments were recorded. Amount of force applied was measured and recorded. Ayield point was determined as the first point at which the force plot exhibited a decrease in force and was correlated to the corresponding displacement. RESULTS: The mean vertical displacement of the pubic symphysis at the yield point was 16 mm (95% confidence interval, 11-22 mm). Mean sacrospinous ligament strain at yield point was 4% (range, 1.0-9.5%). CONCLUSION: Pelves with vertical rotational symphyseal displacement of less than 11mmcan reasonably be expected to have rotational stability in the flexion-extension plane. Those with displacement of greater than 22 mm can be expected to have lost some integrity regarding resistance to pelvic flexion. These values may allow clinicians to infer pelvic stability from amount of vertical symphyseal displacement.