The early days of nitinol wire

Dr. Larry White interviews Dr. Patrick Brady

Modern orthodontics has experienced some significant developments over the past 75 years that have resulted in the current state of the art. The first was, of course, Dr. Edward Angle’s development of the edgewise bracket which has endured with only supplemental changes such as the twin-bracket design by Dr. Brainerd Swain, addition of rotation wings by Dr. Paul Lewis, and the Uni-Twin™ (3M Unitek) bracket developed by Dr. Tom Creekmore along with the self-ligation brackets of late. Certainly the change initiated by Rocky Mountain^® Orthodontics from customized gold appliances to preformed stainless steel brackets, bands, and wires presented orthodontists and their patients with another large improvement in therapy.

Perhaps nothing contributed more to patient comfort than the development of direct bonding begun by Dr. George Newman, which eliminated the fitting and cementing of multiple bands. Dr. Larry Andrews’ introduction of the Straight Wire Appliance™ captured the imagination and endorsement of the profession in the early 1970s. Simultaneously, Dr. George Andreason, Chairman of the Orthodontic Department at the University of Iowa Dental College, had begun to work with nitinol wire, whose eponymous name derived from its content — nickel and titanium (NiTi) and its development at the Naval Ordnance Laboratory (NOL).

In this issue of Orthodontic Practice US, Dr. Larry White interviews Dr. Pat Brady of Dallas, Texas, who, as an orthodontic resident at Iowa, worked closely with Dr. Andreason in those early investigations and published one of the first articles on nitinol wire. Dr. Brady will share some of his experiences in those early days with nickel-titanium wires.

By the time you arrived at Iowa, the chairman of the orthodontic department, Dr. George Andreason, had already established himself as a first-rate researcher and orthodontic scholar. What was it like to work with him? What was his style?
George was a very humble and quiet person. He led by knowledge and patience. He treated all students equally. In my time as a resident, I never heard him raise his voice in anger to any resident or colleague. He was one of the most intelligent persons that I have ever known. He had an engineering degree and was also a Rhodes Scholar. He was extremely sensitive to peoples’ needs and helped as much as possible with clinical theses and with any personal problems of the residents. I found him to be very competitive, however. He and I played racquetball a lot, and he was relentless in his efforts to win. I worked on my thesis at his home in the garage, and this was great for both of us. We got to know each other well, and his great humor came through. It was actually fun working together and wasn’t a chore at all.

When, where, and how did you get the idea that nickel-titanium wire might be useful in orthodontics?
I was discharged from the United States Air Force Dental Corps in June 1969 and had started my orthodontic residency at the University of Iowa Dental College. One day when I was in the orthodontic conference room, Dr. Andreason asked me to read an article in a national news magazine about a couple of men, William Buehler and Frederick Wang, at the Naval Ordnance Laboratory who were working with a wire alloy made of almost equal parts of nickel and titanium. Dr. Andreason thought that this wire, which had unusual superelasticity and shape memory, was being used by the Naval Ordnance Laboratory as antennae in space satellites and might have an orthodontic use.

The wires that the Naval Ordnance Laboratory used were cooled, folded, and packaged in satellites. In space, the package heated and opened because of its shape memory, and the satellites functioned. We felt that perhaps a cooled nitinol wire might warm in the mouth and align the teeth. This idea evolved into my Master’s Thesis, A Use Hypotheses for 55 Nitinol for Orthodontics, which was published in the April 1972 issue of The Angle Orthodontist.

Even in 1969, many orthodontic residents were still doing theses based on cephalometric studies. Did it excite you for Dr. Andreason to ask that you work with him on the only nonferrous orthodontic metal since stainless steel became popular 30 years earlier?
George definitely thought “outside of the box.” When he asked me to work on this project, I jumped at it. At the time, I had no idea that nitinol wire would be in the future of orthodontics. I always liked to do things off the beaten path — e.g., motocross racing, handball, playing the accordion. This project kind of fit my personality. I had so much confidence in George’s abilities as a thinker and clinician that I was completely taken with this challenge.

What were the features that made nickel-titanium wires so inviting to you and Dr. Andreason?
Nitinol wires have two closely related properties — shape memory and superelasticity. Shape memory is its ability to undergo deformation at one temperature and then recover its original shape upon heating above its transformation temperature range (TTR). Superelasticity refers to the wire’s enormous flexibility that is 10 to 30 times more than comparable stainless steel or twist flex wires. These unusual features derive from its reversible crystalline transformation between the high temperature austenite (parent phase) and the martensite (daughter phase) that occurs when the alloy cools. We were interested in the amount of force that occurred by the crystalline change during the transformation temperature range (TTR). We couldn’t come up with a simple plan to measure this force during leveling and alignment of teeth, but we realized that the wire would stretch about 8% of its length. This led us to measure the force of recovery after stretching and subsequently heating the wire, which then returned to its original length. This suggested the possibility of using it as a closing mechanism much like Alastik chains or stretched coil springs. This idea proved stillborn because we could not keep the wire cool enough while attaching it to the molars before it passed through its TTR. Even though the space closure idea didn’t work out, it provided a simple way of testing the shape memory force of the wire.

Since nickel-titanium wire comes in two forms, martensite and austenite, what type did you work with originally, and what size wire did you use?
The Naval Ordnance Laboratory supplied us with two types of straight martensitic wires of .020″ diameter. One wire had a TTR of 16°-27°C, and the other had a TTR of 32°-42°C. Both of these wires were cold-worked martensite, and they were almost like a wet noodle, so we didn’t feel that they would be very effective in leveling and alignment, and that caused us to consider it as a stretched closing mechanism.

One conclusion of your study was that the wire’s stiffness was too low to act as a leveling and alignment wire, and that it should not be used as a main archwire, but did it not, in fact, become exactly the main archwire and useful in leveling and alignment?
Yes, it did, but that didn’t happen until a few years later after Dr. Andreason and Unitek began to collaborate on the manufacture of a clinically useful wire, but even those first nitinol wires displayed extreme brittleness and poor formability and often broke between the premolars and molars where the mastication force was greatest. Dr. Rohit Sachdeva noted that the early nitinol wire had a soft outer surface that would notch from the biting pressure and work-harden, making them susceptible to fracture. It was some time before nitinol wires achieved some formability and more fracture resistance. This was done by altering the processing temperatures and the alloys used in their manufacture.

How difficult was nickel-titanium wire to make when you first started with it?
Apparently, it was quite difficult and expensive to manufacture and needed to pass through several companies before a useful clinical wire could be made. That was the reason the wires we finally got in the late 1970s and early 1980s were so expensive. They started by melting, combining, and processing large nickel-titanium ingots that kept being refined, heat treated, and drawn through ever finer dies. Fortunately, for doctors and patients, these improved nickel-titanium wires proved scalable, and the price has reduced dramatically.

Nickel is a known allergen and considered carcinogenic. What prevents leakage of nickel ions from the nickel-titanium wires?
When nickel and titanium combine, a stable titanium oxide forms on the surface of the wire and prevents any leaching out or ion exchange. In fact, nitinol releases nickel more slowly than stainless steel, and this stability also prevents the wire from corroding.

George’s engineering degree, his vision, and creative genius in the clinic allowed us to have one of the most useful tools ever to straighten teeth.

With the development and ubiquitous use of nickel-titanium wires, Dr. Andreason must have greatly profited from his work. Did that happen?
No, it did not. George was a generous and thoughtful person, and he assigned all financial proceeds from his arrangement with Unitek Corporation to the Iowa University College of Dentistry. I, of course, have no way of knowing how much that amounted to, but it had to have been a lot. Unfortunately, George died from a brain tumor soon after the development of these wires. He was a remarkable, highly intelligent person who had a passion for his work, and his death robbed orthodontics of a seminal intellectual force that we sorely miss.

Reflecting back, what did you learn from your experience, and how has it affected your professional life?
This project taught me how unique our profession is. This research pointed out how important our history is to our profession. George’s engineering degree, his vision, and creative genius in the clinic allowed us to have one of the most useful tools ever to straighten teeth. I’m very fortunate to have known and worked with him. Because of this, I feel that because of his mentorship that I was lucky to have helped improve and be a part of orthodontic history. His vision of the shape-memory mechanism has also helped improve other fields in the health profession and engineering. Every time that I tie in a piece of nitinol wire, I have to admit that I’m proud to have been a part of its pioneering efforts and thankful for George’s unrelenting obsession with bringing this wire to ortho-dontic clinicians.

Orthodontic Practice US thanks you for taking the time to remind us how the nitinol age began for orthodontics and your participation in it.