Design Considerations for Nitinol Bone Staples

The use of Nitinol in orthopedic staples has a long, successful history beginning with their first commercial introduction in the early 1980s. Nitinol bone staples are generally inserted through a process of being chilled, opened, and inserted into predrilled holes. Upon insertion, they recover their preprogrammed shape either through springing back after removal of a constraint (using superelasticity) or thermal triggering (using thermal shape memory). In general, Nitinol bone staples fall into one of three categories: (1) those which are superelastic at room temperature, (2) those which recover their shape upon heating to body temperature, or (3) those which recover their shape upon heating above body temperature with the application of an external heat source. These three different design approaches—room temperature superelastic, body temperature activated, and heat activated—each have different performance characteristics. A version of the heat activated staple that uses a controlled heat source appears to have the best combination of clinical forces and procedural control.