Bone staples have been used in the U.S. since the early part of the 20th century, as they are widely believed to be an effective internal bone fixation method. Bone staples gained popularity and widespread usage on account of their ability to reduce the overall time of surgery and the trauma attached to it. These factors are also directly linked to quicker healing1 and substantial relief in post-operative pain.2 For foot & ankle osteotomy and fusion procedures, bone staples are also more low profile compared with plates and screws, reducing patient irritation due to prominence under the skin.
While traditional foot staples offered some improvements over bone plates and screws, they still left room for improvement. Traditional bone staples made from stainless steel were successful in maintaining proper bone alignment. However, they resulted in frequent loosening, were bulky, and lacked the ability to hold compression post-surgery reducing the stability of the bone-staple construct. As staple designs evolved, several design variables, including metal composition, were explored to address these problems. Developed out of the Naval Ordinance Laboratory, nickel-titanium alloy, better known as NiTiNOL, has emerged as a frontrunner for foot staples in recent years.3 NiTiNOL has the ability to change its shape and stiffness in response to temperature or mechanical force. Initially, NiTiNOL staples were shape-memory and changed in response to body temperature, providing the ability to activate upon implanting in the body4. However, these staples were prone to pre-mature activation and required storage in cold temperatures, resulting in manufacturers and surgeons shifting to pseudoelastic (or superelastic) NiTiNOL staples that could be activated instantaneously by mechanical force. Superelastic NiTiNOL staples are provided pre-loaded on an applicator5 in their temporary constrained shape (legs open or parallel) and then activated upon release from the applicator to return to their initial configuration with the legs compressing into the bone.
The success of an osteotomy or fusion procedure depends on the compression and repair stiffness6 provided by the implant. These factors allow the bony fragments to stabilize and resist plantar gapping, eventually leading to successful union. Compared with other fixation methods, NiTiNOL foot staples offer a large contact area and uniform, dynamic compression that is maintained during bony healing by easily adapting to changes in the joint or osteotomy due to resorption or movement. Each of these factors allow NiTiNOL bone staples to provide superior internal fixation7.
1Malal, J. G., & Kumar, C. S. (2018). THE USE OF MEMORY® STAPLES IN FOOT AND ANKLE SURGERY. British Orthopaedic Foot Surgery Society, 90(B). doi:10.1302/0301-620X.90BSUPP_II.0900231
2Weltmer JB, Jr, Cracchiolo A., 3rd The use of the powered metaphyseal stapler for reconstructive procedures in the adult foot. Foot Ankle. 1990;11:12–5.
3Russell, S. M. (2009). Design Considerations for Nitinol Bone Staples. Journal of Materials Engineering and Performance, 18(5-6), 831-835. doi:10.1007/s11665-009-9402-1
4Choudhary, R. K., Theruvil, B., & Taylor, G. R. (2004). First metatarsophalangeal joint arthrodesis: A new technique of internal fixation by using memory compression staples. The Journal of Foot and Ankle Surgery, 43(5), 312-317. doi:10.1053/j.jfas.2004.07.003
5Hoon, Q. J., Pelletier, M. H., Christou, C., Johnson, K. A., & Walsh, W. R. (2016). Biomechanical evaluation of shape-memory alloy staples for internal fixation—an in vitro study. Journal of Experimental Orthopaedics, 3(1). doi:10.1186/s40634-016-0055-3
6Aiyer, A., Russell, N. A., Pelletier, M. H., Myerson, M., & Walsh, W. R. (2015). The Impact of Nitinol Staples on the Compressive Forces, Contact Area, and Mechanical Properties in Comparison to a Claw Plate and Crossed Screws for the First Tarsometatarsal Arthrodesis. Foot & Ankle Specialist, 9(3), 232-240. doi:10.1177/1938640015620655
7Mureau, T., & Ford, T. (2006). Nitinol compression staples for bone fixation in foot surgery. Journal American Podiatry Medical Association, 96(2), 102-106. Retrieved January 20, 2019, from https://www.ncbi.nlm.nih.gov/pubmed/16546946.