A great deal of force can be produced by preventing a deformed piece of NiTiNOL from returning to its original shape—sometimes as much as 100, 000 psi. The atoms in the nickel and titanium (NiTi) that make up NiTiNOL work extremely hard to get back into their original, very specific locations.
And it doesn’t even require heat. Just the right implant design.
Atlanta, Georgia-based MedShape Inc. is introducing an tibial-talo-calcaneal (TTC) fusion nail made of this remarkable shape memory metal at this week’s American Foot and Ankle Society Trauma (AOFAS) course in Durham, North Carolina.
Called DynaNail, this intramedullary nail is made of NiTiNOL which has been encased in a rigid titanium body. When implanted, the NiTiNOL inner core exerts constant compressive force to accommodate implant loosening and/or resorption and drive fusion.
How cool is that?
Think about it. Because of the shape memory metal core, this implant will maintain compression throughout the bone healing and fusion process. Those atoms of nickel and titanium are working very hard to get back to their original locations so this implant keeps a tight fit, constantly working to stabilize the bone as the patient puts on the loads and stresses from walking, stepping, pivoting—in short, daily life.
NiTiNOL
The shape memory alloy that is at the base of DynaNail is NiTiNOL—an increasingly common medical grade implant. Its greatest successes have historically been for cardiovascular use, but MedShape has innovated several ways to adapt shape memory implants to the unique demands of orthopedics.
Unlike orthopedics, cardiovascular applications require very small forms—like stents, guidewires or embolic protection filters. Orthopedic implants, however, have more complex performance requirements—like load bearing, metal bone interactions, sheer and torsional stresses.
NiTiNOL is also a difficult alloy to fabricate. Making a small embolic filter is very different from a bone plate or intramedullary nail.
MedShape has come up with a clever solution by housing NiTiNOL in a titanium shell. DynaNail embodies the best of both metals—titanium strength with NiTiNOL shape memory.
Nickel and titanium is not an obvious alloy. The inventor, William J. Buehler, stumbled across it while working on nose cones for the Navy’s Polaris re-entry vehicle. About 50 years ago he discovered that hot and cold forms of the nickel/titanium alloys had different properties.
His theory was that nickel and titanium, when combined as an alloy, underwent some sort of atomic structural transformation and became, in effect, elastic at different temperatures. In fact, he found, self-forming elastic!
Buehler found that NiTiNOL “remembers” and recovers its original shape when it warms up. In engineering terms, the alloy transforms at the atomic level from martensite (the lower temperature phase) into austenite (the higher temperature phase).
Superelasticity
The “killer app” was that NiTiNOL was superelastic. That means that the alloy would change dynamically and dramatically from austenite to martensite and back again repeatedly, back and forth, back and forth, applied stress, and released stress, again and again. The same every time.
It was SUPER elastic. In fact, NiTiNOL turned out to have roughly ten times the deformation recovery capability of a typical stainless steel.
In the 1980s, NiTiNOL was a hit for cellular telephone antennas and eyeglass frames. The two original landmark NiTiNOL-based medical devices were the Homer Mammalok for breast tumor localization, developed by Mitek in Westwood, Massachusetts, and the Simon NiTiNOL Filter, which traps potentially deadly blood clots in the venous system, developed by NiTiNOL Medical Technologies in Boston, Massachusetts.
Since the late 1990s, NiTiNOL medical devices were predominately peripheral vascular products like stents and guidewires. Johnson & Johnson’s (JNJ) Cordis unit released their version of the NiTiNOL self-expanding stent with the 1998 release of the SMART Stent. Total worldwide sales of the SMART Stent are estimated to be about $1 billion.
More recently, NiTiNOL mesh inferior vena cava filters and embolic protection devices became commercially successful products. In 2006, NiTiNOL-based devices accounted for over $750 million of the U.S. peripheral vascular device market. The extended worldwide market for these devices has been estimated by some analysts to be as large as $1.1–1.5 billion.
Tough to Fabricate
Imagine the head scratching that comes with this request: “Frank, melt down some NiTiNOL and make a shape like this.” To start with, the basic metallurgical mechanisms in NiTiNOL make such activity very susceptible to impurity contamination. Too many impurities and NiTiNOL’s functionality is severely compromised.
Even after the high purity requirements are met, an even higher manufacturing hurdle remains. NiTiNOL is an extremely difficult material to machine into final shape. It is for the reason that most orthopedic implants, given the requirements for reasonable cost relative to, say, cardiovascular implants, are of very simple geometries such as a staple, and are most often formed from wire.
MedShape’s engineers, including its CEO and Chairman Kurt Jacobus, (who has a Ph.D. in mechanical engineering from the University of Illinois and sits on the advisory board for the Georgia Tech College of Engineering) and CTO Ken Gall (a Georgia Tech professor and world renowned expert in shape memory materials) bring advanced processing technology to their implants. “We have discovered unique ways to process the NiTiNOL to make it as easy to machine and grind as free-machining steels. And in the process we have created an economical path to create complex shapes like the NiTiNOL element that powers DynaNail”, say Jacobus. And this processing brings with it an additional bonus: better strength and durability than normal NiTiNOL. So DynaNail is only the start of the potential orthopedic applications.
Trauma Applications
DynaNail can be used for the following indications:
- Post-traumatic and degenerative arthritis
- Post-traumatic or primary arthrosis involving both ankle and subtalar joints
- Revision after failed ankle arthrodesis with subtalar involvement
- Failed total ankle arthroplasty
- Non-union ankle arthrodesis
- Rheumatoid hindfoot
- Absent talus (requiring tibiocalcaneal arthrodesis)
- Avascular necrosis of the talus
- Neuroarthropathy or neuropathic ankle deformity
- Neuromuscular disease and severe deformity
- Osteoarthritis
- Charcot foot
- Previously infected arthrosis, second degree
Because of its NiTiNOL core, DynaNail sustains compressive loads during bone resorption/implant loosening. Most other nails lose all of their compression after 1mm of resorption and often behave as distracton devices. DynaNail sustains its compression for up to 7mm of resorption at a sustained load of 600N. And in the process behaves like an external fixator embodied in an intramedullary nail.
Shape Memory Polymers?
In addition to NiTiNOL, MedShape also has an entirely new class of smart materials—shape memory polymers. MedShape’s shape memory polymers can deform up to 400% and still recover their original shape without losing their mechanical integrity. What is especially interesting about these polymers is that they can be triggered by heat or light or mechanical force.
MedShape is the only company with an FDA cleared shape memory polymer implant based on PEEK or PMMA chemistries. Medshape’s PEEK Altera is a biomedical polymer which can be triggered to deploy after implant by the physician into the precise geometry for fixation. MedShape’s PEEK Altera is biocompatible, biostable, radiolucent and MRI safe.
Outside of the medical device industry, shape memory polymers are being tested as automobile fenders which could self-repair using a common household hairdryer to remove dents.
MedShape has three product families based on its proprietary shape memory polymers: ExoShape, Eclipse and Morphix.
ExoShape is a soft tissue fastener for fixing soft tissue grafts on the tibial side of the knee joint during ACL (anterior cruciate ligment) reconstructions. It is made from MedShape’s proprietary shape memory polymer, PEEK Altera. ExoShape is delivered pre-compressed in a low profile geometry that inserts between the graft bundles.
Eclipse is a simple soft tissue anchor for surgical use between the tendon and the bone and, of course, since it is made from shape memory polymers, it stays in place and applies a constant compressive force to hold the soft tissue grafts in place.
Morphix is a very small polymer anchor for foot and ankle and sports medicine surgeries. It is designed to anchor into the cancellous bone beneath the cortical shelf. Again, because it utilizes a shape memory polymer, the Morphix anchor applies a constant level of force against the bone. Most suture anchors, by contrast, can pull out after about 1, 000 cycles at loads which are less than 50% of initial pull out strength. Morphix, on the other hand, keeps its initial pull out strength.
MedShape
MedShape is one of these small, innovative manufacturers are innovating its way into the orthopedic industry. CEO Kurt Jacobus, who spent about five years at McKinsey & Company focusing on developing start-up businesses, joined MedShape in 2006 and has built the firm to the stage where, this year, it should generate around $10 million in sales. He and the MedShape team have done this in a very rough economic environment and in an unusual way: they have financed the business privately, meaning the company has taken no venture capital financing. At six years old, MedShape is one of the more interesting young companies successfully bringing innovation to orthopedic surgeons.
So, when you’re at AOFAS this week look up MedShape and ask for Kurt. Ask him to show you how these shape memory materials work and see if he’ll give you a sample to take back to the hospital so you can amaze your friends and colleagues.
