Three-dimensional movement at externally fixated tibial fractures and osteotomies during normal patient function

Three-dimensional movements at four externally fixated tibial fractures and four leg-lengthening osteotomies were measured using the Oxford micromovement transducer. By measuring movements in 6 degrees of freedom during normal patient activity, both shear and angulatory displacements have been found to be of the same order as axial displacements, and are often more dominant. The magnitude of such movements may be as high as 2 mm and 2 degrees in early healing to a small fraction of this in later stages, regardless of whether the fractures are initially reduced and whether they are oblique or transverse. Also, the main bars of certain fixators have torsional 'backlash' looseness, causing significant torsional movement at the fracture and shear perpendicular to the plane of the bone screws. Further interesting findings concern the movement of fractures during walking, 'dynamization' (telescopic movement of the fixator bar) and muscle activity. Axial, angular, and shear movements reverse direction in phase with the 'heel-strike' and 'toe-off' sequence of walking. During dynamization (where the fixator bar is 'loose'), fracture movements at a given load are often less than when not dynamizing (where the fixator bar is 'locked'). Active muscle response can cause movements of 5-6 times greater than movements arising from the transfer of force through the tibia during weight bearing. Also, the tensile stiffness during the above activities has been shown to be more than an order of magnitude lower than the apparent compressive stiffness.