Dynamic Characterization of a Human Tibia Including Soft Tissues: Experimental and Numerical Analysis.

The tibia is one of the largest bones which characterize the leg. It is important for body weight supporting and for the mechanics of lower leg joints. Muscles surrounding it, through their activation, are relevant into locomotion, maintenance of posture and balance. One of the worst damages for the tibia is the fracture caused by injuries due to vehicles accident, falls, sports or disease such as osteoporosis. Fractures are classified on the basis of location, pattern and tissues damages. A simple fracture divides bone in two parts, a wedge fracture has a third segment instead the complex fracture has more than one fragments. In this last case is necessary a surgical treatment. On the basis of damages into surrounding tissues is called open or closed fracture. In case of shaft fracture, located al level of the diaphysis, is applied the external fixation, a structure characterized by pins and screws near fracture site attached to an external bar outside of skin. A subject with a tibial fracture has to undergo, during the 6 months of healing, to a high number of radiographic investigations in order to assess if there is a union, non-union or a delay in healing process. It is necessary to understand when is completed the hard bone callus formation to remove the fixation. X-rays drawbacks are the patient exposure to this dangerous rays and the high cost for the sanitary system. During the years were developed different solutions involving excitation with vibrations and measurement by accelerometer. After some improvements to avoid interference on the signal was used as, transmission medium, the external fixation. Using as starting point these studies the present work is based on the dynamic characterization of the tibia including soft tissues to highlight from FRF its mode of vibrations. The experimental test for the FRF evaluation, before fracture, is characterized by hammer excitation and accelerometer measurement both at level of the tibial shaft. From the results arise a changing for the resonant frequencies and width of the peak at which occurs the first mode for the driving point in horizontal and vertical plane, when added mass and external fixation. Furthermore is validated a numerical model of tibia including soft tissues comparing its results with the experimental ones. At the end are observed the index healing curves in a fracture simulated by a 2 mm cut through a handsaw and the bone callus formation reproduced by a bi-component cement. During fracture simulation, the fixation is used as a transmission medium in which a pin is excited by a shaker and on the other pin is applied an accelerometer. From this last part arises that after more than one cut on the tibia and in case of adding mass there is the lack of repeatability and the healing process is not more accurate.