Characterization of the In Vivo Forces on the Sternum in Pigs

Approximately 15,000 Americans suffer complications associated with inadequate sternal fixation after open-heart surgery each year [1]. Although alternative fixation methods exist, limitations of current device evaluation systems have led to uncertainty about the relative increase in stability that these novel devices provide, thereby diminishing their widespread clinical use. Sternal closure techniques are typically evaluated in situ where estimated sternal forces (180–400N) are applied to an intact chest [2] or in vitro on isolated sternal models [3]. The mechanical stability afforded by each technique is quantified as the resultant separation along the bisected sternal midline. This displacement is assumed to reflect micro-motion that would occur at the wound site under physiological loading, a critical factor during bony healing [4]. However, the loading in these studies is hardly physiological; it is generally simplified to a single direction and applied quasistatically to only a few discrete locations along the sternum without regard for the in vivo distribution of forces. It is also unclear whether the transfer of loads from sternum to fixation device during in situ tests [5](cadavers) accurately reflects loading in vivo due to the potential effects of rigor mortis and/or embalming. To improve the accuracy of current in situ and in vitro sternal fixation test methods it is essential to advance our knowledge of in vivo dynamic multi-directional sternal loading.