Nitinol Implants

Douglas A. Campbell, Jonathan Dwyer, Boyko Gueorguiev, Andrew K. Sands

Nitinol implants offer a new fixation technology in orthopedic trauma and reconstructive surgery

In 2016, DePuy Synthes acquired BioMedical Enterprises Inc (BME) a leading manufacturer of nitinol orthopedic implants for small bone fixation. BME, founded in 1991, was one of the first US manufacturers of
nitinol implants for musculoskeletal fixation. Nitinol is a metal alloy of nickel and titanium with two elements present in roughly equal atomic percentages. The word Nitinol is derived from its composition and place of discovery: Nickel TitaniumNaval Ordnance Laboratory. Nitinol alloys exhibit two closely related and unique properties: shape memory effect and superelasticity.

When nitinol is cooled down below its transformation temperature its crystal structure changes from austenite to martensite. In the latter phase it can be easily deformed to a new shape, however, when the alloy is heated again through its transformation temperature it reverts to its normal austenite crystal structure and recovers the predetermined shape (or memory shape) originally defined by forging and heat treatment. This process is known as shape memory effect. If the recovery of the original shape is prevented by constraining the alloy in the deformed shape, a large amount of stress can be generated. In the surgical setting, this stress can be used for generating a compressive or distractive force when a nitinol implant is released from its constraint. The transformation temperature can be adjusted by alloying and processing. For clinical applications it is chosen well below room temperature.

Nitinol also shows a superelastic behavior, a very springy and rubberlike elasticity, if it is deformed in its normal crystal structure (above the transformation temperature). At a certain stress, a large elastic deformation of up to 8% is possible without plastic (permanent) deformation. The undeformed shape is recovered when the stress is removed. In a postoperative setting, this behavior results in the implant recompressing the fracture or fusion site even after the bone fragments have been subjected to outside forces.

Both nitinol material properties (ie, shape memory effect and superelasticity) offer unique benefits in medical applications. Moreover, nitinol is highly biocompatible and corrosion-resistant, which makes it very attractive
for use in orthopedic implants. At the time of the BME acquisition, Ciro Rmer, the Company Group Chairman of DePuy Synthes, stated that the BME portfolio will be integrated into our trauma platform, where we will be able to expand the availability of these solutions, increase the pace of innovation in this area, and reach more patients around the world. The product portfolio of BME includes Nitinol Staples, implants made of nitinol in staple shape that provide continuous active compression, due to the above-mentioned properties, throughout the healing process.

These implants are produced and applied as shown in Fig 1: Each implant is machined from nitinol in its final shape, with converging staple legs, and heat-treated to maintain this shape at room temperature (austenite crystal structure). Subsequently the implant is cooled down below its transformation temperature to become easily malleable (martensite crystal structure). The legs are then pulled back out to parallel orientation and the implant is loaded onto a sterile insertion tool. Following this, the implant is activated in the insertion tool while reaching room temperature (higher than the transformation temperature). Activated means that the implant tries to regain ist original shape with converging legs but is constrained by the insertion tool. When inserted into bone and then released from the insertion tool, the implants legs are constrained by the bone fragments and a compressive force is effectively transferred from the insertion tool to the bone fracture or fusion site. The implant behavior is not dependent on the temperature in the operating room or the human body because of the much lower transformation temperature.

Compared with screws, Nitinol Staples offer the advantage that due to their superelastic properties compression is kept even in case of some bone resorption or under repetitive motion. There are a variety of implants and inserters available, depending on the geometry and compression strength required. The implants come ready to use on a sterilized, preloaded, fully disposable insertion tool. The insertion technique is simple, fast, and repeatable. The system offers very accurate implant placement with small surgical exposure.

After the acquisition of BME, Depuy Synthes approached the Foot and Ankle Expert Group (FAEG) and the Hand Expert Group to investigate potential applications of Nitinol Staples. The first Nitinol Staples products with limited AOTK approval are the following (Fig 2): Speed, Speed Shift, Speed Arc, Speed Titan, Speed Triad, Elite.

Evaluation by the Nitinol Staple Task Force

A special task force was appointed to evaluate the new technology based on the nitinol mechanical features and the specific products offered by BME, as well as the current clinical experience from a variety of clinical applications. The task force came to the following conclusions.

Principles for the application of compression staples resulting from their mechanical features and functions:

  • Compression staples should be chosen based on their form/shape to provide optimal compression in a given situation
  • Uniaxial compression staples can only provide compression in the direction of their axis and lead to immediate loss of compression if placed obliquely to their axis of loading
  • Uniaxial compression staples provide almost no resistance against shear forces
  • Bi- or multi-directional compression staples can only provide limited resistance against shear forces or bending out of their primary axis of (pre)loading
  • Compression staples do not provide the compression force at the bone surface where they are placed, but at the depth of their legs (indeed, at the tips of the legs)
  • Compression staples provide limited resistance against pullout

As a result, the axis of Nitinol Staples should be oriented more or less perpendicular and through the center of the contact area between the fragments, where the compression should be applied. Compression staples should not be applied, where shear and/or bending forces are applied during weightbearing (lower limb) or functional movement (upper limb). Compression staples alone cannot provide long lasting stable fixation between fragments under multiaxial loading.

Indications and contraindictions

Indications for using compression staples include the following:

  • Compression staples are indicated to support reduction and temporary fixation
  • Compression staples are indicated to support stabilization by limiting movement between fragments and providing compression in distinct directions.

Contraindications include:

  • Compression staples are not indicated as primary stabilization/fixation implants where multiaxial movement between the fragments is possible/expected
  • Compression staples are not indicated in bone stock with poor quality, especially in the areas around the tips of the legs; the legs are effectively unicortical
  • Nitinol, the material of the compression staples, is based on a high percentage (55%) of nickel, which can lead to complications in patients with nickel allergy

Clinical need

  • Compression staples may reduce operation time due to their easy and fast application
  • Compression staples can support primary fixation, by a secondary application of compression.

Alternative fixation concepts are available for most configurations, but they might be more difficult to apply and/or more time consuming.

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