Improving alignment by modulating archwire force

Drs. George F. Schudy and Larry White explore acceleration of tooth movement
From the beginning of the profession to the present time, orthodontists have been best known for aligning or straightening teeth. While our work involves much more — e.g., skeletal correction, TMJ harmony, bite correction, and so forth — it all begins with alignment. This is not only the phase for which we are best known, but alignment precedes the other phases. Frequently, our pivotal phases cannot be addressed properly before the alignment phase. Ironically, this perfunctory step stands time-wise between us and the more complex and time-consuming aspects of our treatment. Any acceleration in this phase can be applied directly to our end goal of facilitating the other phases of treatment.

Literature — how fast do teeth move?
Jones, et al.,1 and O’Brien, et al.,2 in benchmark alignment studies in 1990 tested .014 NiTi and .015 multistrand stainless steel (S.S.), .016 Nitinol and Titanol, respectively. Each wire was in place 35 days. Their average rates of tooth movement for their fastest alignment wires were 1.4 mm/month. Eighteen years later, Scott, et al.,3 were motivated to study Damon® self-ligating (SL) alignment versus synthesis conventional ligation (CL). The rate of alignment was measured through the use of three wires: .014 NiTi, .014 x .025 NiTi, and .018 x .025 NiTi. Alignment continued until placement of a S.S. rectangular wire was possible. The rate of alignment was greatest during the use of the .014 NiTi — i.e., 3.6 mm/month for (CL) and 3.0 mm per month for (SL). Overall, however, the rate dropped significantly. Total rate was slightly less (243 days) with the conventional ligation (CL) and 253 days for Damon (SL). The (CL) had an alignment rate of .056 mm per day, and (SL) had an alignment rate of .047 mm per day. For a 28-day month, this was a rate of 1.5 mm per month for (CL) and 1.3 mm per month for (SL).

Ong, et al.,4 did a similar study to compare initial alignment of Damon self-ligation (SL) versus conventional ligation — Victory series (CL). In this study, two wires, .014 NiTi and .014 x .025 NiTi, were left in place for a fixed time interval of 10 weeks each. As with Scott, et al., the conventional ligation (CL) alignment produced faster tooth alignment than self-ligation (SL). In the mandible, the (CL) alignment (or reduction in irregularity) for the first 10 weeks was 8.4 mm of the initial 12.5 mm of irregularity and (SL) reduced irregularity 6.5 mm of 10.8 mm. This produced a rate of 3.3 mm/28-day month for (CL) and 2.6 mm/month for (SL). In the maxilla during the first 10 weeks, the (CL) reduced at a rate of 2.7 mm/month and the (SL) was 2.6 mm/month.

During the second 10 weeks in the Ong, et al., study, the rates dropped as they did in the Scott study. The (CL) rate was .5mm/month, and the (SL) was .3mm/month. This fall off in rates in the second 10 weeks, when averaged with the first 10 weeks, slows the overall (20 weeks) rate of alignment. The (SL) was 1.5 mm/month for upper and lower and (CL) averaged 1.8 mm/month for upper and lower.

Sebastian in 20125 studied alignment efficiency in the mandibular arches of 24 patients. Wires tested were .016″ NiTi (single strand) versus .016″ NiTi (woven, coaxial). Tooth movement was measured at 4 weeks, 8 weeks, and 12 weeks. The braided .016 NiTi produced the fastest alignment with average first month alignment of 4.9 mm/month. The second and third months decreased to 3.6 and 3.1 mm per month respectively. Overall rate over 3 months was 3.9 mm/month. These rates were faster than those observed in past alignment studies by a factor of 2.6. The single strand .016″ NiTi had an initial rate of 1.5 mm/month, and this decreased in the second and third months.

Efforts have been made recently to increase the rate of our tooth movement. Some of these are vibration therapy, osteoperforation, infrared laser, and photo biomodulation. Most of these high-tech efforts are expensive, and some have not yet had control studies. Others have been studied relative to space closure and not initial alignment efficiency.

Osteoperforation has been shown by Teixeira, et al.,6 to increase the rate of tooth movement in rats. The small tissue traumas elicit the regional accelatory phenomenon that occurs as a healing response to trauma or surgery. These punctures physiologically mimic tissue trauma and bring about the release of cytokines and prostaglandins that are known to be associated with increased bone turnover and faster tooth movement.7-10 This concept has only had a controlled clinical trial evaluating the effects in cuspid retraction.11 This showed a 130% increase over a 28-day period. It has also been shown in one patient study to increase Invisalign® movement by 28%.12

Infrared laser has had a controlled evaluation that showed a 38% increase cuspid retraction.13 Vibration therapy has had one university administered study14 that showed initial alignment improvement of approximately 30% in the mandible compared to benchmark studies (Jones, et al.; O’Brien, et al.). This study also showed maxillary alignment to be 3 mm/month or 100% more than the early studies.

 

 

 

Photo biomodulation or the use of near-infrared light is known to stimulate more cytochrome oxidase c production15 which mediates an increase in ATP. It is assumed that more ATP would facilitate tooth movement. Exposure in the facial area over the alveolar processes at a 850 mm wavelength for 60 minutes per week resulted in a 120% increase in the rate of alignment.16 The control rate was 1.9 mm per month, and the test sample rate was 4.4 mm per month.

Modulating wire protocol review
This dual-arch initial wire protocol as described in a previous article17 starts treatment with a round NiTi archwire, the force of which is modulated by an underlying annealed wire. Figure 1A shows an .014 annealed wire by itself in place in the maxillary arch and an .012 annealed wire in place in the mandibular arch (Figure 1B). This wire has no spring but is usually pressed lightly into the interproximal areas to assure passivity. Figure 2 shows a maxillary arch (2A) and mandibular arch (2B) with both wires in place — maxillary .014 NiTi with a .014 annealed wire and a mandibular .012 NiTi with an annealed .012 wire. After 3 to 5 weeks, the annealed wire is removed. Removal is demonstrated in Figure 3. Ties are removed from one to three teeth, and the annealed wire is clipped usually between the centrals. The patient bites tightly on a cotton roll, and the clinician pulls the wire through the brackets with a Weingart pliers. The patient experiences no discomfort in this procedure, and it takes only 1-2 minutes to perform. Teeth move remarkably fast during ligation with the dual-archwires despite increased friction. But after removal of the annealed wire, the teeth move at a particularly rapid pace.

Examples of alignment during dual-arch use and after
Each of the following patients shows the initial arch irregularity (as per Little’s18 Irregularity Index), followed by the change while the two archwires are in place together and finally the change after the annealed wire was removed. The duration and rates of movement are noted.

The etiology of rapid alignment
At this point, one can only conjecture about the mechanism that results in such rapid alignment, but it does call to mind the biological system known as Regional Acceleration Phenomenon (RAP), which was first described by Frost19 and noted earlier. Researchers have learned that RAP occurs with many provocations — e.g., tooth extractions, gingival flap surgery, mini-implant insertion, orthognathic surgery, etc. — and this causes a more rapid movement of teeth than otherwise might happen.20-22 Melsen23 has even suggested that intrusion of incisors resulted in RAP with dense woven bone as proof of its involvement. So perhaps, unwittingly, the addition of dual wires as in this experience has resulted in RAP, which accelerates the movement of teeth.

 

Another possibility is that the resultant force to the periodontal membrane of this protocol may impart a force that dips below that of traditional wire bracket mechanisms and is more physiologic. Schwartz24 stated that capillary pressure of 25 gm would be the most physiologic with which to initiate tooth movement. Thilander, et al.,25 contend that most traditional initial orthodontic wires have the potential to impede tooth movement by causing sterile necrosis or hyalinization. Many orthodontists begin treatment with a NiTi wire of approximately .014 in size. Our initial hope and assumption were that the force produced by this protocol modulated the .014 NiTi and the .012 NiTi (mandibular) to result in substantially less force.

In vitro testing
Drs. Freudenthaler and Pseiner at the University of Vienna Orthodontic Department tested this dual-wire protocol. They used a 3D testing device developed by Dr. Hans-Peter Bantleon. This device (Figure 12) is capable of simulating a three-tooth segment and testing the force from a deflection or rotation of the middle tooth. Testing was done for rotation of the maxillary canine. Brackets for the lateral, canine, and first premolar were placed, and the maxillary canine rotated 20°. In Figure 13, the test setup is shown with a twin bracket on the lateral and single brackets on the canine and first premolar. The lateral slot was .016 in., and the canine and first premolar were .018 in. The interbracket distances were set to simulate average sized teeth and were 7 mm. Figure 13A shows the setup with one traditional .014 NiTi in place and Figure 13B with a .014 NiTi wire in combination with the .014 annealed wire.

The results of the testing for the maxilla are seen in Figure 14. For a 20° rotation, the .014 NiTi applied 722.6 cNmm and the .014 NiTi with the .014 annealed wire yielded 300.8 cNmm. This is 103.1 gms for the .014 alone and 42.8 gms when the annealed wire is present. The standard deviations were ± 56.1 and ± 44.2, respectively. Mandibular testing was done in exactly the same manner as the maxillary testing but with a .012 NiTi and a .012 annealed wire. With a 20° rotation, the .012 NiTi alone applied 588.1 (SD, 48) cNmm of force and the .012 NiTi with the annealed wire applied 221.5 cNmm (SD, 59) of force. This is 84.9 gms for .012 NiTi when applied alone and 31.5 gms when slowed by the annealed wire. Applying the standard deviations, the modulated .014 NiTi applied between 35-50 gm. The modulated .012 wire applied 23-40 gm. The mandibular test results are depicted in Figure 15.

The .014 by itself applies 2.4 times the force of the same wire when restrained by an annealed wire, and the .012 alone applies 2.7 more force when not restrained by an annealed wire. It may be that these very low forces are close to the true physiologic force and launch the cellular cascade of periodontal changes in nearly an ideal way. The cells of the periodontal membrane may be preconditioned, so to speak, to allow more efficient cellular activity. The force is extremely low, but also it is dispensed to the teeth at a gradual rate given the blocking nature of the annealed wire with the increased friction and resistance. The very instant that the annealed wire is removed, all patients report a sensation of increased force on their teeth. A preconditioned periodontal membrane may be able to embrace this greater force and allow for more physiologic development and activity of end stage cells (osteoclasts and osteoblasts). This would promote and/or allow more efficient and therefore more rapid movement.

Discussion
This initial alignment technique produces greater rates of tooth alignment than any other previously reported studies. We have measured a group of 17 patients. Basically, these represent all recent patients that fit the criterion of significant arch irregularity that would have previously been aligned with a continuous arch without coil springs. Certainly, a controlled study of this technique in the future would be appropriate and desirable. However, there is significant consistency in these patients with dramatic rates of movement. Table I shows the measured results of our patients. Table II shows a summary of results from previous alignment studies. The average rates of alignment from all previous studies is 2.6 mm/month. For this average, only the studies from 2006 forward were used. Where standard deviations were available, it was possible to calculate the fastest one rate measured in each study; these are also listed in Table II.

These maximum rates range from 2.49 mm/month to 5.7 mm/month. Of the 17 patients we measured, only four had rates less than the maximum rate of all previous studies. Figure 16 shows a graph of the relationship between the average and maximum rates of previous studies and those of the dual-wire method. The average rate of our study group is 1.56 times the maximum average of all former studies, and the dual-wire average rate is 3.12 times faster than the average rate of other investigations. The maximum rate measured with the dual-wire protocol is 14. mm/month. This rate is 2.45 times the maximum rate from all previous studies, which was 5.7 mm in the Sebastian study.

As you can see from Table I, this protocol produces consistently faster rates of tooth alignment. However, some patients do not respond as dramatically as others. Maxillary teeth align somewhat faster, and age seems to affect the outcome. Some adults do not align as quickly as the average adolescent. The specific geometry of each irregularity, as well as individual variation in physiology, also plays a significant role in the alignment response. These resistance factors have not been examined enough to accurately predict which patients’ teeth will not respond as efficiently as the average patient.

What about friction?
One of the anathemas for contemporary orthodontics has been friction. This has been encouraged by varied ways of securing wires in the brackets to produce less friction and binding. These are the “self-ligating” type brackets. The passive self-ligating brackets do allow easier sliding and apparently less friction and binding. If such movement is essential to initial alignment, and friction reduces this movement, then friction would be undesirable.

Dr. Dwight Damon stated in 2012,26 “What I also find interesting in Dr. Burrow’s paper is that in utilizing small round wires (.014, for example), friction (not binding) constitutes more than 95% of the total resistance to sliding, and that is exactly why low-friction passive ligation is more efficient than conventional ligation during the alignment phase.” Dr. Damon indicates that his experience and observations concur with the notion that friction is bad for tooth movement and especially in the early alignment phase. A large segment of the profession has pretty much accepted this presumption.

Our clinical observations and the finding of this investigation and others would render this fear of friction invalid. The studies of Scott 200833 and Ong 201034 noted above found self-ligating (SL) brackets slightly slower, not faster in initial alignment. The dual-wire protocol described herein produces more friction than any technique known. The annealed wire plus the active wire combine for a total width of .028 in. in the maxillary arch and .024 in. in the mandibular arch. This overfills the .018 x .025 bracket slots. Then the annealed wire moves randomly into the interproximal area touching wings and tooth surfaces, which causes more restrictions. Additionally, we used elastomers and stainless steel ligations, which produce even more friction. However, the rates of movement accelerate significantly.

Treatment advantages

Treatment time saved
The graph in Figure 17 shows a comparison between the rate of traditional alignment methods and this protocol for different levels of arch irregularity. Obviously, clinicians save more time with patients who have greater initial arch irregularity. From 5 to 19 mm irregularity, the time saved ranges from 1.3 months to 5.4 months. In patients with less arch irregularities, the dual-arch technique helps clinicians finish earlier. In patients with more arch irregularities, their treatments do not exceed the estimated dates of completion. In each case, patients and orthodontists experience more pleasant orthodontic therapy. More importantly, the dual-arch technique affords clinicians the flexibility to spend more time to detail tooth positions and finish with a higher level of quality. There are a number of dental, societal, and economic forces at work currently that provoke less quality of orthodontic therapy. This procedure affords us the opportunity to improve our results within a reasonable time.

Less patient discomfort
As was reported in a previous article in Orthodontic Practice US,35 this wire configuration dramatically reduces the patient discomfort experienced in the initial early alignment phase of our treatment. Of course, the early weeks and months of treatment build the foundational ties in a doctor-patient relationship that we all hope will have positive and mutually beneficial results. Patients and parents have an excitement during this early phase, and anything we can do to make the experience more positive (or less negative) produces pluses for our practice. Reduction of discomfort and allaying the apprehensive child certainly does this.

Decreased root resorption
Root resorption is an age-old nemesis for orthodontists and patients. Few initial alignment studies measure root changes, but Scott,3 Mandall,27 and Linge28 included this in their studies. Scott measured resorption at the mandibular central incisor. Sixty patients were divided between Synthesis™ (Ormco) brackets and Damon® 3 (Ormco) brackets. Root resorption was measured at 1.21 mm (SD 3.39) and 2.26 mm (SD 2.63) for Synthesis and Damon 3, respectively, during the initial 18 weeks. The difference between appliances was not statistically significant. Mandall noted an average of 1.18 mm (SD 1.4) central incisor root resorption during early alignment. This represented 18% of patients studied. Linge and Linge noted similar resorption (1.5 mm) on 16.5% of their patients.

While Scott, Mandall’s, and Linge’s levels of root resorption would typically not reach clinical significance, it is not insignificant as a treatment sequela. The extremes of resorption measured by Scott were 4.89 mm for Damon and 4.60 mm for Synthesis. These would have to be considered quite significant particularly for a lower incisor. Depending on the patient’s periodontal aging pattern, these could easily become clinically significant.

This dual-wire protocol has been used exclusively for more than 20 years. During this time, our root resorption level has been extremely minimal and less than when we used a traditional one-wire treatment initiation. Of the patients presented in this study, one adult experienced slight mandibular incisor resorption while the other 16 had no root resorption.

Universality of use
This arrangement of wires should work with all bracket systems. Obviously, at this point only one bracket arrangement has used the dual-arch system, but it should enhance the alignment characteristics and reduce patient discomfort with any system.

Summary
The profession has struggled for more than 90 years to produce more efficient tooth movement. The gold wires of Angle offered little hope in this pursuit. Stainless steel wires with greater resiliency and flexibility offered less force and more efficiency, but they did not increase patient comfort. Nickel-titanium archwires began to push orthodontics in the right direction by delivering wires with less force, more durability, and a greater range of action. However, this great advancement may have produced a generalized professional complacence.

We have neglected the research of Schwartz24 and Reitan29 in our contentment over the amazing range and duration of action of the NiTi wires. While these wires work continuously, they still apply too much initial force to achieve more efficiency and a more comfortable level of tooth movement. In 2007 Berger and Waram30 did a test of the force levels of all initial NiTi wires. Of the 43 different NiTi wires measured, only one produced less than 4 gm of unloading force when deflected 1 mm. None yielded less than 57 gm when deflected 2 mm. Not surprisingly, that wire was the same woven NiTi wire studied by Sebastian. His study had the highest average rate of movement (3.6 mm) prior to the movement reported here.

The efforts of 10 orthodontic30 supply companies in the development of numerous NiTi wires has not produced clinical forces in the 25 to 30 gms range suggested by Schwartz and Reitan. This modulating wire technique has produced forces at or near the target set by these pioneers. And efficiency has jumped by a factor of 3 times. Apparently, the initial alignment phase is more about force levels than friction.

We have presented a new protocol for initial irregularity alignment that uses two juxtaposed initial wires that reduce the net force to the teeth to levels lower than currently possible with one wire. With one very small and inexpensive adjustment in initial clinical procedures, orthodontists, patients, and orthodontics generally receive several significant benefits.