[Mechanobiology of fracture healing part 2 : Relevance for internal fixation of fractures].

Clinical studies do not allow a quantitative correlation between stability of fracture fixation and outcome of bone healing. This limits the biomechanical improvement of fracture fixation techniques. The most practical quantitative parameter to describe the stability of a fracture fixation is the stiffness. This can be determined for several types of fixation through biomechanical methods and in some clinical studies in vivo. By using numerical fracture healing models, it is now possible to use the tissue differentiation rules found in basic research to calculate optimal stiffness parameters for various fixation techniques. For a tibial fracture as an example the possibilities of a numerical fracture healing simulation have been demonstrated. The effects of the diameter of an intramedullary nail, type of fracture, fracture gap size and nail material on healing could be demonstrated. To circumvent complex and time consuming calculations for several fixations a map was calculated which shows the expected bone healing quality as a function of the axial stiffness and the shear stiffness of the fixation device. By comparing the stiffness of various fixation techniques with the stiffness map it becomes evident that the methods most often used (e.g. unreamed nail, plate and external fixator) have a low shear and/or rotational stiffness that is too low to achieve the optimal healing outcome. The high axial stiffness of plates next to the plate surface can lead to very low tissue strain directly adjacent to the plate and can delay the bone healing process at this location.