Current Nitinol Staple Technology in Orthopedics

Shape Memory Alloy staples made of nitinol have been used in orthopedics for over two decades due to their biocompatibility, good corrosion resistance, super-elasticity, and shape-memory properties. Early versions of the implants were typically fashioned out of wire and were either body-temperature activated requiring freezer storage, or above-body-temperature activated requiring activation after implantation. With the recent development of below-body-temperature activated implants, ease of implantation has improved markedly. Nitinol implants are manufactured in the “closed” position, such that the legs are toed-in, and then loaded in the open/active position on an insertion device used for implantation. Upon release from their “active” position, the implants immediately impart compression across the fusion site as the implant attempts to return to its “closed” state. Furthermore, with advances in manufacturing techniques, the amount of compression is tunable, allowing for applications across various bones of the body.

Several studies have been published showing that nitinol implants produce compression across the fusion site. Another key advantage for nitinol staples is that they provide continuous compression; after repetitive loading, compression is maintained at the fusion site. Recent studies at our laboratory have shown compression recovery after loading. Results were compared to plate and screw constructs and showed that compression was maintained for all nitinol staple constructs, while plate and screw constructs had a reduction in compression with plantar gapping evident. Furthermore, 4-point bend and torsion results were comparable. These nitinol characteristics have translated to high fusion rates clinically, with non-union rates as low as 1.5%.