An Instrumented Spacial Linkage for Monitoring Relative Three-Dimensional Motion Between Fracture Fragments

Abstract

A mechanical linkage with electro-magnetic sensors (a displacement transducer) is described, which may be used to measure accurately the relative motion at a bony junction such as a fracture. The linkage may be fixed to bone screws of externally-fixated fractures during routine patient activity, to measure three-dimensional inter fragmentary displacements arising from dynamic loading. Movements of the linkage are monitored by six Hall Effect devices for the six degrees of freedom (three orthogonal translations and three rotations about the translating axes). Measurements are made within error bounds of ±0.025 mm and ±0.025 deg over a range of 5 mm for the two orthogonal transverse translations, 8 mm for axial translation and 8 deg for the three rotations. Movements at the linkage, remote from the fracture, are then translated mathematically to the fracture site, assuming rigid screw contact with the bone. Displacements of the distal fragment in relation to the proximal, at the fracture center, can then be expressed anatomically through anterior, medial, and distal translations, and rotations in the sagittal, coronal, or transverse planes.