Hoffmann half-frame external fixation rigidity and its relationship to universal joint slippage.

Hoffmann half-frame external fixation device configurations often fail under minimal loads secondary to joint slippage. In these experiments improved universal joints that were developed in an earlier study were tested on Hoffmann half-frame assemblies. The rigidity of selected half-frame configurations was tested in four modes (axial compression, torsion, medial-lateral and anterior-posterior four-point bending). These results were compared to those of an earlier, similar study looking at the standard Hoffmann half-frame. No changes in overall rigidity were noted, but significant increases in yield loads and loads to frame failure were achieved. Such improvements will increase the reliability and usefulness of the Hoffmann device to the orthopaedic community.