Emerging Principles of Modern Clear Aligner Mechanics by Dr. Clark Colville

by Dr. Clark Colville

For much of the past decade, I felt that clear aligner treatment had plateaued. In my practice, the share of chair time going to aligners wasn’t climbing, and outcomes didn’t feel like they were improving at the rate the industry wanted me to believe.

I don’t think that was a failure of orthodontists using the technology. Instead, I see the stagnation as a limitation of the technology itself, and specifically as a consequence of the mechanical approach that has dominated the category: force-driven mechanics.

In a force-driven system, we trust attachments to mimic tooth movement forces well established in fixed-appliance biomechanics.^1,2 This system has proven to be moderately effective at the expense of attachment proliferation and high refinement rates.^3,4,5 The result increases chair time and frustrates patients, doctors, and team members alike.

Over the past few years, I have been treating patients with a displacement-driven aligner system (Oliv), and I have been involved in its development. The experience changed how I think about aligners and led me to question whether our preconceived notions about orthodontic forces have stalled one of the most progressive tools at our disposal.

How clear aligners got stuck in their own past
The first commercially available clear aligners were limited by the manufacturing techniques of the day. Early aligners weren’t effectively engaging undercuts on the clinical crown, so aligners alone didn’t reliably move teeth. As a result, manufacturers added engineered purchase points (attachments) and pressure areas designed to recreate bracket-like movements and forces on specific tooth surfaces. Designs also emphasized retractive movements, which studies show are more predictable.⁴

Attachments were a smart solution to a manufacturing problem. The first generation of these aligner systems started with root-torque pressure areas, and computer-designed attachments proliferated from that point forward.

The trade-off is complex attachment libraries. Movement of any given tooth depends on a single engineered surface on a small attachment, which has two clinical implications. The first is a loss of precise control. Tooth-movement studies on early force-driven systems have reported prediction accuracy of 50% or lower for several movements, with rotation and extrusion the hardest to control.^4,5

The second implication is human intervention. Clinicians often choose to remove computer-designed attachments at a patient’s request or on clinical judgment. In those circumstances, the programmed movement then relies almost entirely on displacement-driven mechanics, which a sculpted trim line and aligner fit were never engineered to support.

A systematic review of clear aligner studies reached a similar conclusion: Force-driven aligners perform best in simple malocclusions, while extrusion, rotation of rounded teeth, and other complex movements remain less predictable across studies.⁶ Though further research is needed, most of us are using hybrid systems in which force-driven and displacement-driven mechanics share the load. In most systems, the displacement half is not engineered to carry it.

Fig. 1: Illustration based on heatmap images from laboratory testing demonstrating how an extended trim line engages multiple points of contact on the tooth. For illustrative purposes only.

Engineering better displacement-driven control
Early aligner plastic and sculpted trim lines could not reliably move teeth, so manufacturers added attachments and prioritized retractive protocols to compensate. The question now is whether aligners can be built that don’t lean so heavily on those workarounds, and whether displacement-driven mechanics can be made to work on their own terms.

In my opinion, the answer depends on several manufacturing variables working together. Here are some of them.

• Print fidelity: High-resolution 3D printers deliver more accurate models than legacy technology could produce. This makes it possible to create better-fitting aligners that effectively engage undercuts and exert meaningful pressure over large areas of the tooth surface.

• Material properties: Layered thermoplastics must deliver forces within the range required for consistent tooth movement while limiting force decay. Most aligner manufacturers use these plastics today.

• Trim line innovation: An extended trim line that minimizes gingival blockout can increase the undercut the aligner captures on the clinical crown. This design makes more surface area available for effective control of tooth movement without relying on an attachment to do the work.^5,7,8

Together, these variables create multiple points of contact on the tooth, enabling predictable displacement-driven mechanics instead of depending on a single surface of an engineered attachment (Fig. 1).

Though I am unaware of any peer-reviewed studies comparing modern displacement-driven mechanics to traditional force-driven systems, I can speak from experience. Since I implemented a displacement-driven system in my practice, my attachment count has dropped substantially, and molar attachments have become nearly unnecessary.

These observations are anecdotal, but the pattern is consistent enough across my patients that I think other clinicians should test it in their own practices.

Building predictability into digital treatment planning
Attachments are only part of the story. The way we craft digital setups by default follows the standard principles of orthodontic tooth movement, and the rules behind those setups deserve as much scrutiny as the appliance.

A protocol is essentially a set of rules that determines how a clear aligner digital treatment is planned before a human designer touches it. Different malocclusion types call for different protocols, each built using insights from successfully treated patients. For example, a protocol may automatically create small spaces around a difficult rotation rather than rotating the tooth while maintaining intimate contact with adjacent teeth. Experience shows that creating that space facilitates a more predictable movement, so the logic can be built into the protocol itself.

In my experience, three protocol-level decisions matter most:

• Movement guardrails: Software can produce overly optimistic simulations, and a setup that displays unrealistic goals is not a helpful one. Protocols that lack appropriate movement guardrails set us up for refinements and disappointed patients. Building movement limits into protocols, based on what’s actually achievable, keeps the initial setup honest and reduces the time we spend manually correcting movements that would never happen.

• Finish with one refinement, not zero: When we plan to get it right the first time and then keep rerunning the same plan when it doesn’t work, we set ourselves up for an endless cycle. The first aligner series should focus on fundamentals like establishing arch form, rotation correction, and leveling. The second aligner series should be distinct and prioritize the details that make for a great finish: small space closures, posterior seating, and settling the occlusion. In my experience, this produces fewer total refinements than treating every patient as a one-and-done attempt and improvising when that fails.

• Keep attachments simple: In a displacement-driven system, the attachment functions as an extra purchase point that creates an undercut for retention. If that is its role, extensive attachment libraries serve little purpose.⁵ A retrospective study comparing optimized and conventional attachment designs found rotational accuracy around 63% and extrusive accuracy under 48%, with no significant difference between attachment types.³ As a result, my attachment library has been pared down to a single design that can be manually adjusted to suit the treatment goal.

Fig. 3: Space closure with Class II elastics using displacement-driven aligner mechanics. Post-treatment radiograph shows widening of the periodontal ligaments along the full root length, indicating root-driven movement.

Building better protocols with granular data
Traditionally, protocols have been built and refined based on measured outcomes data. Today, the methods of gathering that data provide more nuance than ever.

For the last several years, I have used a remote monitoring platform that has helped me assess progress, anticipate adjustments, and improve outcomes. The platform captures intraoral photo records at every aligner change and analyzes the images to determine adherence to the treatment plan. That data can then feed the protocols and improve initial setups by prioritizing the most predictable movements.

I believe this granular data gathering has benefited my outcomes, but it hasn’t solved everything. In my observations, extruding upper lateral incisors remains a challenge regardless of system (Fig. 2).⁶ That is exactly the kind of insight this kind of monitoring is meant to surface.

In my practice, displacement-driven mechanics have worked well with these standardized protocols. Over time, my confidence in the system has increased, and I have significantly reduced both attachments and refinements for my patients.

Observations in practice
The following examples from my own practice illustrate these principles.

One treatment that changed how I think about the efficacy of displacement-driven mechanics involved a patient who needed significant space closure (Fig. 3). I used Class II elastics with buttons on the lower first molars. On the panoramic radiograph taken at the end of the initial series, the periodontal ligaments of the lower molars are visibly widened along the length of the root, not just at the crown, indicating that the aligner engaged the undercuts and facilitated root-driven movement rather than tipping.

The next patient was a 23-year-old with Class I spacing (Fig. 4). She needed eight upper and four lower attachments in her initial series of 24 active and three passive stages. That series left the alignment of L7 and posterior intercuspation incomplete. A finishing series of 12 upper and lower aligners, with three finishing-touch stages targeting posterior seating and no additional IPR or elastics, completed the treatment.

Finally, a 17-year-old with a maxillary skeletal deficiency in the transverse dimension and midline deviation (Fig. 5) needed four upper and three lower attachments through a 24-stage initial series with IPR to correct her Bolton discrepancy. A 12-aligner upper-only finishing series with Class II elastics reduced the midline discrepancy and corrected the final molar occlusion to Class I. Cephalometric comparison showed a reduced interincisal angle and improved lip posture.

Fig. 4: Class I spacing. Eight upper and four lower attachments over 24 active and three passive stages. A 12-stage finishing series improved posterior seating.

Fig. 5: Maxillary skeletal deficiency with midline deviation. Four upper and three lower attachments over 24 stages. IPR corrected a Bolton discrepancy. A 12-aligner upper-only finishing series with Class II elastics.

Clinical takeaways

• Displacement-driven systems can predictably control tooth movement given modern advances in aligner manufacturing technology.

• Research on attachment efficacy suggests attachments may not be as reliable as we assume.^3,4,5

• Gathering granular progress data can help identify systematically unpredictable movements and improve protocols.

• Advances in aligner manufacturing, materials, and design protocols can reduce the need for attachments and produce controlled movements using displacement-driven mechanics.^5,7,8 Comparative studies are still needed.

• Data-driven guardrails for specific treatment types can reduce unnecessary tooth movement, lower the overall number of aligners, and minimize the time doctors spend on design modifications.

• Thoughtful planning of two distinct aligner series, each with its own prioritized objectives and with the finishing series built on dedicated finishing protocols, may result in fewer refinements and shorter overall treatment times.

Disclosure: Dr. Colville has a financial interest in Oliv Orthodontics, the manufacturer of the aligner system referenced in this article, and serves as its clinical lead. The views and treatment selections are his own.

References
1. Proffit WR, Fields HW, Larson BE, Sarver DM. Contemporary Orthodontics. 6th ed. Elsevier; 2019.
2. Graber LW, Huang GJ, Vig KWL, Fleming PS. Orthodontics: Current Principles and Techniques. 7th ed. Elsevier; 2023.
3. Karras T, Singh M, Karkazis E, Liu D, Nimeri G, Ahuja B. Efficacy of Invisalign attachments: A retrospective study. Am J Orthod Dentofacial Orthop. 2021;160(2):250-258. doi:10.1016/j.ajodo.2021.04.028
4. Kravitz ND, Kusnoto B, BeGole E, Obrez A, Agran B. How well does Invisalign work? A prospective clinical study evaluating the efficacy of tooth movement with Invisalign. Am J Orthod Dentofacial Orthop. 2009;135(1):27-35. doi:10.1016/j.ajodo.2007.05.018
5. Weir T, Meade MJ. Optimising clear aligner therapy: what current evidence says about materials, attachments, and protocols. Semin Orthod. 2026;32(2):376-380. doi:10.1053/j.sodo.2025.07.007
6. Rossini G, Parrini S, Castroflorio T, Deregibus A, Debernardi CL. Efficacy of clear aligners in controlling orthodontic tooth movement: a systematic review. Angle Orthod. 2015;85(5):881-889. doi:10.2319/061614-436.1
7. Elshazly TM, Salvatori D, Elattar H, Bourauel C, Keilig L. Effect of trimming line design and edge extension of orthodontic aligners on force transmission: a 3D finite element study. J Mech Behav Biomed Mater. 2023;140:105741. doi:10.1016/j.jmbbm.2023.105741
8. Nakomnoi T, Sirirodjanakul W, Chintavalakorn R, Santiwong P, Sipiyaruk K. The biomechanical effects of clear aligner trimline designs and extensions on orthodontic tooth movement: a systematic review. BMC Oral Health. 2024;24:1523. doi:10.1186/s12903-024-05274-7

Dr. Clark Colville graduated from UT Health San Antonio School of Dentistry in 1989 and received a certificate from St. Francis Hospital and Medical Center the following year. In 1993, he completed his graduate orthodontic training at the University of Texas Houston School of Dentistry. Over a career of more than 30 years, he has worked with many industry partners and has lectured nationally and internationally, primarily on clear aligner treatment. Colville maintains a private orthodontic practice in Seguin and San Marcos, Texas, and serves as clinical lead at Oliv Orthodontics.