In silico analysis of Superelastic Nitinol staples for trans-sternal closure

Superelastic Nitinol staples, utilized routinely in foot surgeries, are proposed to be used for sternal closure application in this study. It is hypothesized that the shape memory induced superelasticity will allow multiple staples placed along the sternum to promote fast and safe recovery by maintaining constant clamping pressure at the sternotomy midline.

Two different Nitinol staples of different alloying compositions, one representing the metal formed wire geometry and, the other, powder metallurgy manufactured rectangular geometry, are chosen from the literature. Austenite finish temperatures of both materials are confirmed to be appropriately below the body temperature for superelastic shape memory activation. The adopted finite element superelasticity model is first validated and, via design optimization of parametrized dimensions, the staple geometries for producing maximal clamping forces are identified. The performances of the optimized staples for full trans-sternal closure (seven staples for each) are then tested under lateral sternal loading in separate computational models.

The optimized metal formed staple exerts 70.2 N and the optimized powder metallurgy manufactured staple exerts 245 N clamping force, while keeping the maximum localized stresses under the yield threshold for 90° leg bending. Testing the staple-sternum constructs under lateral sternal loading revealed that the former staple can be utilized for small-chested patients with lower expected physiological loading, while the latter staple can be used for high-risk patients, for which high magnitude valsalva maneuver is expected.

Computational results prove that superelastic Nitinol staples are promising candidates as alternatives to routinely performed techniques for sternal closure.