In vivo characterization of respiratory forces on the sternal midline following median sternotomy

Pai, Shruti

Etd

In vivo characterization of respiratory forces on the sternal midline following median sternotomy

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The development and clinical adoption of more effective fixation devices for re-approximating and immobilizing the sternum after open-heart surgery to enable bony healing has been limited, in part, by the lack of in vitro test methods used to evaluate these devices which precisely emulate in vivo loading of the sternum. The present study is an initial effort to determine the loading parameters necessary to improve current in vitro and numerical test methods by characterizing the direction, magnitude, and distribution of loading along the sternotomy midline in vivo using a porcine model. Changes in forces incurred by death and embalming were also investigated to estimate the applicability of cadavers as chest models for sternal fixation. Two instrumented plating systems were used to measure the magnitude, direction, and distribution of forces across the bisected sternum in four pigs during spontaneous breathing, ventilated breathing, and coughing for four treatments; live, dead, embalmed, and refrigerated. Forces were highest in the lateral direction and highest at the xiphoid. An important finding was that the magnitude of the respiratory forces in all directions was smaller than anticipated from previous estimations, ranging from 0.37 N to 43.8 N. No significant differences in force were found between the four treatments, most likely due to the very small magnitude of the forces and high variability between animals. These results provide a first approximation of in vivo sternal forces and indicate that small cyclic fatigue loads should be applied for long periods of time, rather than large quasistatic loads, to best evaluate the next generation of sternal fixation devices.

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