Measurement of fracture gap motion in external fixation.

A testing machine to study fracture gap motion ex vivo was devised. The apparatus permits direct measurement of bending and shear at a fracture site under different loading conditions. Using this device the single and double frame Hoffmann external fixateurs were compared to a Küntscher medullary nail. Shear displacements of (0.4-1.7 mm) were observed for the fixateurs as opposed to 0.3 mm for the intermedullary nail. Changing from a single to a double frame halved hinge motion at the fracture site and reduced shear by 25%.