Y. V. Baril, V. Brailovski, P. Terriault, Ecole de technologie superieure, Montreal, QC, Canada; R. Cartier, Montreal Heart Institute, Montreal, QC, Canada
To reduce a risk of sternal dehiscence of the patients subjected to median sternotomy, a new sternal closure device is developed. This device prevents the cut in and through the sternum bones during aggressive post-surgery physiological activities as coughing, deep breathing and sudden movement. Furthermore, the new device is capable of maintaining a nearly constant pressure between sternum halves during rehabilitation of such patients, thus improving the bone healing conditions. The principal element of such a device consists in a tubular mesh-like structure made of superelastic Nitinol, which tends towards a flat form when it is in contact with the sternum. Finite element modeling and in-vitro laboratory testing of the tubular superelastic device confirm a 30% reduction in contact pressure exerted by this device on the sternum bones, and a 25% increase in compressive pressure between the sternum halves, as compared to the conventional sternal steel wire closure device.
Summary: Still today, a median steronotomy is one of the most important surgical procedures used to gain access to the thoracic cavity. In 2002, there were 709 000 open heart surgeries practiced in the United States. The particularly major drawback of the conventional suture technique using from 8 to 10 steel wires is that post-operative stresses on the closure loops (coughing, sudden movement, etc.) may cause the thin wires to cut into and through the bone of the sternum. To reduce the risks of sternum dehiscence, a median sternotomy closure by a superelastic tubular braid is proposed. The use of SMA suture allows the principle of safe springback and adaptive geometry to be exploited and the compression to be maintained at the interface between two sternum halves in case of any geometrical redefinition caused either be healing process or by local indentation of the closure system into the sternum.
Finite element modeling allows the net benefit of using superelastic material instead conventional steel suture to be demonstrated. As an example, it is numerically proven for the sternum of a density of 0.24 g/cm3 that superelastic suture having an equivalent diameter of 0.22 mm continues to keep compressive forces at the sternum interface after an external disturbance as high as 145 N, which is 60% greater than a 0.78 mm diameter steel suture can sustain. This gain is obtained however at the expense of a larger sternum opening: after an identical external disturbance of 90 N, superelastic suture allows an opening of 1.1 mm, while steel suture, of 0.8 mm.
The results of the experimental testing with sternum simulators in the 0.24 to 0.48 g/cm3 density range, confirm that the new braided superelastic suture maintains approximately 30% greater residual forces than the steel suture, regardless of the installation technique (peristernal or transsternal), the character of external force (single impulses imitating severe coughing or cyclic loading imitating deep breathing).
For the remainder of the project, a specialized device capable of calibrated pretensionning of the suture during installation will be developed. The fatigue properties of the SMA braid will also be determined. These two steps are essential for the completion of the engineering part of the project and for the preclinical study to be started.
