Non-invasive determination of tibia stiffness: Development, validation, and clinical application of a vibration technique.

摘要

The development of the ability to determine non-invasively long bone functional properties such as stiffness and strength has been pursued by many researchers. I have explored the viability and application of one such technique through theoretical, experimental, and clinical analyses. With mechanical response tissue analysis (MRTA), a mathematical model of the bone, skin, and end conditions is used to determine physical parameters, including lateral bending stiffness, from the bone frequency response. In this work, the currently used mathematical model is validated with tests in vivo and ex vivo. While this model has previously shown efficacy in human ulnas and monkey tibias, it is not sufficiently complex to fully describe the behavior of the human tibia. A new model of the tibia is proposed that accounts for two uncoupled single-bending modes, and the appropriateness of simplifying assumptions is discussed. An important potential use of this or any such mechanical testing technique is for the monitoring of long bone fracture healing. Conventional radiographic evidence is poorly correlated with degree of healing, and a quantitative mechanical measurement could be useful in identifying problems and guiding treatment. Skew bending theory is used to examine the effect of a healing fracture on bending stiffness, torsional stiffness, and resonant frequency for a variety of loading and boundary conditions. All values increase very rapidly with healing. Whole bone stiffness is more sensitive to changes in callus stiffness than is resonant frequency, but neither is sensitive in the later stages of healing. Finally, tibia bending stiffness and bone mineral mass were measured in a group of ambulatory and a group of spinal cord injured (SCI) subjects. Tibia bone mass and stiffness were significantly reduced in the SCI relative to the ambulatory subjects. SCI subjects who had sustained lower extremity fractures following their SCI had lower bone mass but not bone stiffness. It appears that MRTA can be an effective and powerful tool for evaluating bending stiffness of the human tibia. This value is functionally significant as it is highly correlated with bone breaking strength. Further development is, however, necessary before introduction for clinical use.