Evaluation of stress-relief methods on cobalt-chromium orthodontic wires.

Stress-relieving heat treatment is reported to accelerate the elastic memory process in order that the configuration of cold-worked orthodontic wires becomes more stable. In this experiment, 3 methods of stress-relieving O.036-inch cobaltchromium orthodontic wires used in the fabrication of spacemaintaining appliances were evaluated. The results showed that stress relief in a dental oven, by the electric current of an orthodontic spot welder or by the flame of a dental soldering unit, does appear to accelerate the expression of elastic memory in cold-worked, cobalt-chromium wires. However, after an initial deformation, the stress-relieved wires tended to stabilize while untreated wires continued to deform. The experiment reinforced the need to check stress-relieved wires and appliances before inserting them into the mouth and to reshape them as required. When an orthodontic wire is bent, internal stresses are created within that wire. These stresses are created by irregularities in the atomic lattice structure and may cause the newly shaped wire to deform and seek its original shape. This progressive change in shape is known as elastic memory and may result in unwanted tooth movement. At ordinary room (or body) temperatures, elastic memory is a gradual process. Stress-relieving heat treatment accelerates the elastic memory process so that the wire becomes more stable (Thurow 1982; Phillips 1982). The effects of heat treatment on the physical properties of cobalt-chromium orthodontic wires, such as flexural yield strength, modulus of stiffness, fracture resistance, hardness, etc. are well documented (Mahler and Goodwin 1967; Martin et al. 1984). However, little research has dealt directly with the dimensional stability associated with stress-relieving heat treatment. Durr et al. (1986) showed that 3 common methods of stressrelieving heat treatment using a dental oven, electric current, or a soldering flame, did not reduce the dimensional change which occurred in U-shaped 0.036-inch cobalt-chromium orthodontic wires as compared to nonstress-relieved wires. However, almost all of the dimensional change occurring in the experimental groups took place during the stress-relief treatment after which there were only insignificant changes. Dimensional changes in nonstress-relieved wires continued throughout the 4-week experimental period. The experiment demonstrated that the 3 methods of stress relief accelerated the elastic memory process. However, the U-shaped wires used in the study were not soldered to orthodontic bands, nor were they heat treated on working casts. Thus, the experiment did not accurately represent procedures commonly used to fabricate interceptive orthodontic or space-maintaining appliances. The purpose of the present study was to evaluate the same 3 methods of stress relief on 0.036-inch, cobaltchromium orthodontic wires under conditions which more accurately reflect clinical use. Materials and Methods Cobalt-chromium 0.036-inch-diameter orthodontic. wire (Blue Elgiloy ® -Rocky Mountain Orthodontics; Denver, CO), all from the same batch, was cut into 80 pieces, each 10.0 cm in length, using a jig and wire nippers. The wires were bent to a U-shaped form using finger pressure and an edgewise arch former (Rocky Mountain Orthodontics; Denver, CO) and then soldered to orthodontic bands which were stabilized on jigs (Fig 1, next page). The outermost width of each shaped wire was measured immediately after soldering with the wire still on the jig (coded as "baseline" measurement). Measurements were made to the nearest 0.1 mm using a Boley gauge (Fig 2, next page). The 80 wires were divided into 4 groups each containing 20 wires: Group A -Untreated Control Group. These wires were removed from the jigs after soldering and initial measuring, immediately remeasured with the Boley gauge Pediatric Dentistry: September, 1988 N Volume 10, Number 3 205 FIG 1. If-shaped 0.036-inch cobalt-chromium orthodontic wire soldered to orthodontic band material on jig. FIG 3. Storage rack for hanging wire during experimental period. Fi G 2. Outermost width of each U-shaped wire measured with Boley gauge. FIG 4. Stress-relieving heat treatment using electrodes of an orthodontic spot welder. (coded as "off-jig" measurement), and then placed on a storage rack (Fig 3). Group B — Oven-Treated Experimental Group. These wires were kept on the jigs and placed in a previously calibrated dental oven (Ney-Barkmeyer model 260 — J.M. Ney Co; Bloomfield, CT) at 950° F for 8 min as per a method described by the manufacturer of the wire. Group C — Electrode-Treated Experimental Group. These wires were stress relieved on the jigs using the electrodes of an orthodontic spot welder (Model 660 — Rocky Mountain Orthodontics; Denver, CO), according to the directions accompanying the spot welder. The standardized method was to stress relieve short segments of wire (Fig 4), each segment being about onefifth of the total length of the wire, for about 8 sec and then to proceed on to the next segment. Temper-indicating paste (Rocky Mountain Orthodontics; Denver, CO) which ignites at 950° F was used to show when the desired temperature was achieved. Group D — Flame-Treated Experimental Group. These wires were stress relieved on the jigs using the flame of a dental soldering unit (Hydroflame® — Unitek Corp; Monrovia, CA). The temper-indicating paste was placed on the wire, and the flame was passed over the wire at a uniform distance of 1.0 cm (Fig 5) until the paste ignited, indicating that the proper temperature of 950° F for stress relief had been reached. Immediately after the stress-relief treatment, wires in the experimental groups were remeasured on the jigs (coded as "post-tx" measurement), then removed from the jigs, measured again (coded as "off-jig" measurement), and placed on storage racks. The wires in all 4 groups were remeasured after: 1,2, 3, and 4 hr; 1,2, 3,4, 5, and 6 days; and 1, 2, 3,4, 5, and 6 weeks. These time intervals were chosen on the basis 206 STRESS RELIEF OF ORTHODONTIC WIRES: Durretal. of the results of the earlier experiment by Durr et al. (1986). At the end of the 6-week observation period, the forces generated by the lateral expansion of the U-shaped wires were measured using a dial tension gauge (Model AG30 — John Chatillon & Sons Inc; New York, NY) to deflect the wires back to their baseline widths. Then the wires were readjusted to their baseline widths. Expansion after readjustment was measured at 1day and 1-week intervals. Summary statistics were calculated for the control and experimental groups at each time interval. An analysis of covariance followed by multiple comparisons testing was used to investigate treatment differences over time. These comparisons were based on the least square means which differ from raw means in that they are adjusted for any differences that may exist among baseline widths of the U-shaped wires (Winer 1971). Statistically significant differences were reported if P values less than 0.05 were found.