Assessment of the stability of fracture fixation systems: Mechanical device to investigate the 3-D stiffness in vitro

Different fixation systems are used for fracture and defect treatment. A pr erequisite for complication free healing is sufficient mechanical stability of the osteosynthesis. In vitro investigations offer the possibility of bo th analysing and assessing the pre-clinical fixation stability. Due to the complex loading environment in vivo, stiffness analysis should include a co mplete determination of the stiffness under standardised conditions. Based on a mathematical procedure to calculate the 3-D stiffness, a mechani cal testing device for the 3-D loading of fixation systems was designed and integrated in the existing test set-up. The set-up consisted of a material testing machine to produce the necessary loads and an optical measurement device to detect the resulting inter-fragmentary movements. To validate the testing device, the 3-D stiffness matrices of different Ilizarov fixator c onfigurations were determined and compared. The good reproducibility of the test was reflected in the small intra-indiv idual variability of the stiffness components. A distinct direction depende nce of the fixator stiffness was observed. Increasing the number of rings l ed to a stiffness increase of up to 50%, especially in bending. The present ed testing device allows a complete standardised determination of the stiff ness of different fixation systems. It considers the direction dependence o f the stiffness and creates a prerequisite for a more direct implant compar ison.