The stability of union in tibial shaft fractures: its measurement by a non-invasive method.

The stability of the union in tibial shaft fractures was followed by repeated non-invasive radiographic measurement of a deflection induced in the area of the fracture. By means of a Shift Comparator the difference in the angle between the two fragments before and during application of the deflecting force can be measured. The relative measure of the strength of the union is furnished by a quantity, the deflection ratio, which is based on the quotient of the induced deflection by the magnitude of the applied bending moment. When this ratio falls below a certain value the union is considered to be stable enough for full weight bearing during walking. A plot of the deflection ratio against time, with logarithmic scales on both axes had a negative slope and was more or less linear. This graph can serve as an aid in detecting the course of union, in deciding when the cast should be removed, and in detecting any irregularity in the process of union. In the management of 207 fractures of the tibial shaft 508 such deflection measurements were performed.