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The use of nickel titanium (NiTiNOL) in large orthopaedic devices has been stunted for decades by technological challenges in basic materials and device design. In November 2012, MedShape announced the first surgical procedure with its DynaNail Ankle Arthrodesis Nail, an intramedullary (IM) nail that contains the alloy. MedShape received 510(k) clearance on its newly-developed nail and patented a new surgical method to activate NiTiNOL inside the body.
We queried Jeremy Blair, MedShape’s team leader for the NiTiNOL program, on how the company overcame the challenges of using the alloy, how it works and its possible future uses.
Q. How does NiTiNOL work?
A. NiTiNOL exhibits shape memory in that it can be stretched up to nearly 10% of its initial length and then recover to its initial length.
Q. Until now, NiTiNOL hasn’t easily translated as an alloy that could be used in orthopaedic devices. What were the previous challenges? How did you overcome those challenges? 
A. The primary challenges arose in making an alloy that was readily machinable into complex geometries using conventional machining; finding a surgical procedure for activating NiTiNOL inside the body and combining NiTiNOL with titanium in a way to harness the advantages of both materials.
To address these challenges, MedShape was able to develop a method to machine NiTiNOL by leveraging some of its unique thermodynamic properties. Activation inside the body occurs by inserting the nail into a bony site contacting multiple bone segments. Once the nail is in place, the element is stretched and fixed to the bony segments, all within the body. MedShape has patented both the machining and in vivo activation methods. DynaNail comprises an internal NiTiNOL element that is housed within a titanium nail body. The internal element provides the nail with the shape change ability to adapt to changes in loading across the bone site, while the external titanium body offers rigidity to the device.
Q. You mentioned that NiTiNOL has the ability to change shape and to recover that shape. What are the benefits of this feature?
A. NiTiNOL has the ability to change shape and recover that shape (shape memory), even in the face of opposing forces. It can recover up to ten times more strain compared with traditional metals. Shape memory provides NiTiNOL with more deformability compared with traditional alloys, and thus, the ability to better respond to changes in loading. Another benefit is the ability to recover its stored shape in response to changes in loading, thus enabling a method to apply continuous load across a bone site. Shape memory also allows for implants to be delivered in the body in one “easy-to-insert” shape and then be activated into a more functional form in the body. In some cases, as is the case with DynaNail, the materials can continue to change shape in the body in response to changes in local environment. This has been realized already with cardiovascular stents.
Q. What is the significance of having shape memory characteristics during and after the implant is placed?
A. Shape memory provides DynaNail with the ability to adapt to local changes at the joint site, such as bone resorption, and to apply sustained compression across the joint. Compression of the joint site facilitates healing and bone fusion. In addition, maintaining compression ensures better torsional stability such that the bones remain in close apposition. The design of the nail with the dynamic element also allows load to be effectively transferred to the bone, preventing any stress shielding that could cause bone resorption.
Q. MedShape used NiTiNOL in the DynaNail Ankle Arthrodesis Nail. How do you see NiTiNOL being used in orthopaedic implants in the future?
A. The shape memory characteristics of NiTiNOL could be leveraged in other fusion devices for fracture repair in which compression is needed across the fracture site. The DynaNail technology could be translated to other IM nails to repair long-bone fractures, such as the tibia, femur and humerus.
Q. What else should readers know about DynaNail and its use of NiTiNOL?
A. DynaNail is the first IM nail to harness the shape memory properties of NiTiNOL. DynaNail’s ability to apply compression mirrors the performance of external fixation frames, surpassing that of traditional IM nails.
Jeremy Blair (MBA/BSME) serves as the Team Leader for MedShape’s NiTiNOL program. In this role, he has led the development and commercialization of the DynaNail Ankle Arthrodesis Nail as well as future products that will leverage MedShape’s NiTiNOL platform. Blair has more than ten years of experience as a product development engineer in the medical device industry, accumulating six pending and issued patents. He can be reached for comment at jeremy.blair@medshape.com.
This article initially appeared in ORTHOPRENEUR® at www.orthopreneurpub.com.
