Introduction:
Orthodontic tooth movement in mechanotherapy depends on the ability of a clinician to use controlled mechanical forces to stimulate biologic responses within the tissues surrounding the roots of treated teeth, most commonly by way of an edgewise bracket sliding along an archwire. Frictional forces at the interface of the bracket, archwire, and ligature can hinder these sliding mechanics. The problem has become more pronounced with the increase use of soft titanium alloys in recent years. Titanium has a strong tendency to gall, owing to its chemical reactivity and the ease with which it forms alloys. The specific aim of this research was to identify and develop the best performing, friction reducing coating for use on orthodontic devices specifically concentrating on the archwire/bracket/ligature interface. Representative nitride and inert hard metal coatings were examined.

Sample Preparation:
CrN was deposited onto both stainless steel brackets and beta-titanium wires by an outside vendor (Multi-Arc, Inc., Rockaway, NJ) using a commercially established cathodic arc process. The use of an outside vendor alleviated the need for the determination and optimization of a number of parameters, such as operating pressure, current, substrate bias, nitrogen flow, and substrate temperature. Iridium was deposited using the ion assisted deposition method of magnetically unbalanced magnetron sputtering onto both stainless steel brackets and beta-titanium wires using a process established for this application. Substrates were pre-cleaned using an argon glow discharge.

The coatings were applied to triangular "pins", used to simulate brackets, as well as to actual stainless steel orthodontic brackets. Beta-titanium wire (Beta III, composition 80% Ti, 11.5% Mo, 6% Zr, 4.5% Sn, from Ultimate Wireforms, Inc., Bristol, CT) was similarly coated. The temperature of such thin, small substrates as wires is difficult to control, and overheating of thermally sensitive materials such as titanium-molybdenum alloy (TMA) was acknowledged as a possibility in both cases.

Testing & Results:
A series of wire pull tests through actual orthodontic brackets was performed to assess the friction-reducing characteristics of the coatings. Conditions of no lubrication (dry) and lubricated with saliva (Oralube artificial saliva, Preventative Dentistry Support Center, VA Medical Center, Houston, TX 77030) were examined. A series of test were performed on the coated pieces and controls including diamond scratch testing to determine coating adhesion, wire pull testing using a tribometer specifically designed to determine friction and the degree of stick-slip on archwires, and brittleness testing.

Iridium and chromium nitride coatings exhibited adequate adhesion, as evaluated by diamond scratch testing. The overall coefficient of friction and the "stick-slip" were significantly reduced against stainless steel brackets under both dry and wet conditions. The qualitative term "stick-slip" is a way to describe the adhesive component of friction, which is due to the formation and rupture of interfacial bonds. Such bonds are the result of interfacial interatomic forces that depend on the degree of interpenetration of asperities and surface composition. Commercial nitride-coated TMA wires (Ormco, San Diego, CA) exhibited an average slip comparable to those measured for iridium-coated wires against bare stainless steel brackets. Both performed significantly better than the control bare titanium wire. This was true whether the wires and brackets were dry or wet with artificial saliva. However, wires deposited with chrome nitride became extremely brittle and could not be easily bent or formed without breaking. Iridium coatings could be applied to TMA wires without embrittling them, making iridium attractive as a wire coating material on a commercial scale. Results of friction testing with coated brackets and uncoated TMA wires yielded friction and stick-slip that was not significantly different from control tests, regardless of the coating applied to the bracket. Further, coated wire tests have far less "stick-slip" than coated bracket tests.

Conclusions:
Coated brackets were discovered to afford no particular improvement in frictional properties of a stainless steel bracket/TMA wire couple. However, coating the wire with a hard, inert metal like iridium was found to dramatically improve the frictional properties of the bracket/wire couple. Coefficients of friction and stick-slip were greatly reduced. Wires coated with the nitride coating similarly reduced friction and stick slip motion, but were embrittled by the process used to coat them.

Funding for this research was granted by the National Institutes of Health (NIH).