The Science Behind Orthodontic Retention

For this column, I wanted to share the history and science of orthodontic retention, so I knew I needed to contact Brian Gray, DDS, for my conversation. He is a fellow in 3 academies and a master in the Academy of General Dentistry (AGD). At AGD, he was awarded the Lifelong Leadership and Service Recognition Award—fewer than 1% of dentists have received this award since its inception. Dr Gray is chief dental officer at Candid, and when he took on this role, one of his goals was to evolve a next generation of retainers. Candid is a dental company that provides CandidPro clear aligner solutions. Our discussion on the science of retention leads to understanding why the right materials and protocols are needed for lasting orthodontic results.

Lou Shuman (LS): Why did you start looking at a next-generation retention solution as an important part of the whole orthodontic process?

Brian Gray, DDS: In the past, when patients finished orthodontic treatment, they got their retainer and returned to their general practitioner [GP] for their regular dental checkups. If the patient didn’t adhere and noticed that a tooth was starting to relapse, the GP would usually send them back to the orthodontist. That extra appointment was just another hassle in their day, so often they would let it slide, and the teeth would shift even more. Retention must be for life. But it must be based in science to produce lasting results. GPs now represent such a significant part of aligner treatment that understanding retention has become an integral part of long-term patient care.

LS: How has retention evolved over time?

BG: We want a retainer to act as a gentle reminder for the teeth to stay in the final position after treatment. The true standard of care for removable retention has been the Hawley retainer—it has a long life span unless it gets lost or the dog eats it—and it can be adjusted in the case of simple relapse. A Hawley maintains tooth position because the scalloping of the acrylic on the lingual surface locks teeth in place. But the problem is, when you wear that type of retainer, it is like going back to high school. After patients have had their teeth straightened with a clear aligner system, they don’t like to wear a Hawley because it looks and feels strange.

LS: So, from a scientific point of view, what makes a good retainer?

BG: An ideal retention appliance has 2 important components: It needs to fit very well and maintain its shape. In other words, a clear plastic retainer needs to be precise and know where to stay. In the past, some doctors would use the final aligner as a retainer. The plastic would fatigue very easily, and then the teeth would start to shift a bit in the retainer. If the plastic is not holding the teeth in position, they tend to move in a transverse direction, sideways. But the proper retainer material changes the game.

Studies showed that certain first-generation aligner and retainer plastics would start to fatigue in as little as 1 week after being worn for 8-hour days. Based on that information, Candid started searching for a new, more reliable material. We saw improvement but still observed too much fatigue with second-generation materials. For efficient retention, patients had to update their aligners about every 6 months. So we continued analyzing the data to deliver the best retention plastic available. And we nailed it. We chose the latest clear, stain-resistant Zendura FLX material because of how well it resists cracking and deformation.

Candid performed rigorous stress analysis tests on plastic materials to see which one was going to have the best memory and last the longest. We ran fatigue tests for strength, memory, and rigidity. We then ran a double-blinded clinical trial to verify the best trim and thickness that patients found to be most comfortable.

LS: Why not use an aligner as a retainer?

BG: There is a big difference between clear aligners and retainers. Clear aligners are designed to move teeth. Retainers are designed to keep teeth from moving—to maintain a specific position. The fit needs to be highly intimate with the teeth, so they have less of an opportunity to start movement under the plastic. A poorly made retainer will allow the teeth to drift or shift, but a well-made retainer will hug the teeth and hold them in place.

Also keep in mind that a retainer will wear out more quickly if the patient is a heavy grinder or bruxer. This causes the plastic to flex and not hold on to the sides of the teeth as well.

It is important to know that a precise-fitting retainer relies on 2 components of accuracy for success. First is the veracity of the impression or scan. Second is the validity of the model used to thermoform the clear retainer. The scan, which gives us data on tooth position, is turned into a solid lithograph model. When the models are fabricated, they must be made with a highly accurate printer. We found that Carbon printers were the best in this regard. Super-precise models allow our thermoforming units to deliver the fidelity we were looking for when we started this mission.

The most important part is the type of plastic used to form over the model. The material needs to be able to [reach] into the interproximal spaces to help hold the teeth. Without that, the teeth will be allowed to shift. When Candid came up with the design of our next-generation retainer, we looked for these key things: the proper material, being able to obtain the best data from the scan, and the highest accuracy of printing.

The next generation of retainers Candid just launched does not rest on the gingiva, and this is very important. It is designed to be supragingival, so it does not impinge at all on the soft tissue. But because it is extended lower, the retainer can now leverage physics to grab on to more teeth and lock into the interproximals between the teeth to keep them from moving. And this helps to prevent that transverse movement.

LS: What about rotational movement?

BG: Teeth actually relapse by wanting to return to their previous position. This has a lot to do with the periodontal ligament. Without getting too deep, there are 3 leading theories on strain-based bone remodeling in orthodontic movement. They all pretty much correlate on relapse rotation, which may occur because round circular fibers act like a rubber band. [Before orthodontic treatment], the tooth is out of position, but the circular fibers are passive. Now we move the tooth, and even though it is in an ideal position, those circular fibers are stretched and want to return to their original passive pre-rotated position. The longer the aligned tooth is held in its ideal position, the fibers start to relax and become passive in their new position. This change occurs over a lengthy period, which is why adherence and the best retentive device are so critical. It is also why Candid spent so much research and development time to create a retainer to allow the dentition to remain ideally positioned.

LS: What does the future hold?

BG: The exciting part of working at Candid is their state-of-the-art [research and development team]. The company is quick, agile, and has a ton of game-changing innovations in progress. The next 18 months will see much disruption and change in our incredible profession. Candid already has a retainer update coming in July, adding some extra features. You do have to be a CandidPro provider to purchase retainers through the CandidPro website.But patients don’t have to have gone through CandidPro aligner treatment to get PermaForm retainers.

Conclusion

The research and science that went into the creation of this next generation of retainers from CandidPro provide significant upgrades in fit, comfort, and longevity. Patients spend much time and money on straightening their teeth, and Dr Gray and Candid are determined to keep those teeth in place.