Stiffness in healing fractures.

Methods are available for direct measurement of stiffness in fracture healing. The methods are generally dissimilar in their technique and in some cases the applied assumptions are contrary to the principles of basic mechanics. External fixation provides the potential for direct measurement of fragment end movement, and techniques associated with this type of treatment have been applied in Belgium on more than 500 patients. These results are generally presented as decreases in flexure of the fixator bar due to constant loading, representative of increases in fracture stiffness. The same procedure is adopted to internal fixation plates and also direct strain gauge application to the healing bone--there are obvious complications due to electrical connections in the latter case. The biomedical changes occurring during healing are illustrated by the work at Cardiff where a biphase characteristic has been identified. The method used has the disadvantage in requiring the removal of the fixator for the application of the stiffness sensor recording deflection and rotation. The advantages of the technique include the more precise modeling of the bending characteristic of the tibia (the results refer to this particular skeletal element). The experimental studies at Oxford show evidence of enhanced osteogenesis when controlled intermittent deformation is applied in a cyclic manner. In a group of some 50 patients treated with induced micromovement, there is evidence that the rate of increasing fracture stiffness would allow removal of the fixator at 15 weeks compared with 24 weeks in the group with rigid fixation. The exact technique of measuring fracture stiffness has to be compared with other noninvasive test methods which include radiological examination, ultrasound, resonant vibration, and modal analysis. Many of these methods are based on linear elastic relationships which are inadequate to describe the anisotropic behavior of bones. The vibration analysis techniques are showing potential as clinical tools, and collaboration within the European community is directed towards a correlation between invasive and noninvasive methods of fracture monitoring.