BIOMECHANICAL INVESTIGATION OF THE PUBIC SYMPHYSIS - WHICH FORCES INDUCE MOBILITY OF THE SYMPHYSIS UNDER PHYSIOLOGICAL CONDITIONS

For estimation of the loads of fixation devices, physiological movemen ts and acting forces at the symphysis pubis are examined. Experimental ly, Walheim determined the movements of the pubic symphysis under phys iological conditions (one-leg stance, walking): there is no effective mobility in the horizontal direction. During the one-leg stance he obs erved symphyseal mobility of up to 2.6 mm (O 1.2 mm) in the vertical d irection and 1.3 mm (O 0.6 mm) in the sagittal direction. During walki ng he found symphyseal mobility of up to 2.2 mm (O 0.9 mm) in the vert ical direction and 1.3 mm (O 0.6 mm) in the sagittal direction. Until now the forces leading to symphyseal mobility have not been estimated either experimentally or mathematically. In our experimental study we examined ten fresh cadaver anterior pelvic rings by means of a multidi rectional force-mobility measurement. Maximal physiological movements, as determined by Walheim, were increasingly induced in ten equal step s, and the forces required were measured for every single step. Out of the resulting force-mobility curves for the ten cadaver specimens the mean force required (with standard deviation) was calculated for each of the ten measure points, separately for the vertical (y-) direction and sagittal(z-) direction. The values were graphically transferred, and a regression curve was created. This curve allows the acting force to be estimated for every movement of the pubic symphysis: During the one-leg stance the mean force to induce mean mobility is 169 N in ver tical direction and 148 N in sagittal direction; for maximal mobility a force of 398 N in the vertical direction and 148 N in the sagittal d irection is necessary. During walking, the force required to induce me an movement of the symphysis pubis is 120 N in the vertical direction and 68 N in the sagittal direction; for maximal mobility 333 N is requ ired in the vertical direction and 136 N in the sagittal direction. Fo r mobilization of patients with a symphyseal rupture (type Tile B1) wi th partial weight-bearing, neutralization of the mean acting forces du ring the one-leg stance (169 N in the vertical direction, 68 N in the sagittal direction) must be achieved by an adequate fixation device. I n cases where full weight bearing is desirable, such as in patients wi th limited compliance, stability can only be reached by neutralization of the maximal acting forces during walking (333 N in the vertical di rection, 136 N in the sagittal direction).