A new device for external fixation.

Using a ball-and-socket joint model, the optimal mechanical design of a simple universal ball joint was established. The maximum resistive moment was 70-72 Nm for a tightening torque moment of 5 Nm, their ratio being 14. The corresponding ratios for the Hoffmann universal ball joint and Orthofix are 1.0-4.3 and 1.1-1.6, respectively. A complete unilateral frame consists of two ball joints and one square connecting tube. Full-frame behavior under external loads was tested in the three principal planes and in torsion. Compared with a Hoffman quadrilateral system, the new system was four times more rigid in the anteroposterior plane, similar in axial load and lateral bending, and 64 percent more rigid in torsion.