Constrained acetabular cups - A cadaveric biomechanical evaluation

Reports indicate that constrained acetabular cups may reduce range of motion and catastrophically fail, although the biomechanics of dislocation have not been reported. We measured the available motion in 6 constrained cups (from 2 manufacturers) when anatomically placed in cadaver pelves. We measured the torque and rotation necessary to dislocate the hip. Range of motion was measured using a custom jig and revealed a functional range of motion with all cup positions. Extension was limited to 0degrees with anteversion of 11degrees to 29degrees. Thus, anteversion of <10degrees is recommended. Torque and rotation to produce dislocation was 7 ft-pounds and 11 ft-pounds and 82degrees and 72degrees, for each manufacturer. Torques were reduced with subsequent dislocations. We recommend consideration of polyethylene replacement if dislocation occurs. Dislocation occurred at the ball-polyethylene interface without catastrophic failure.