Biomechanical Issues with Orthodontic Aligners: Understanding the Challenges of Tracking

Clear aligner therapy has transformed the orthodontic landscape. Patients love the convenience, comfort, and aesthetics. Dentists appreciate the digital workflow and the perception that aligners can simplify treatment. Yet, behind the sleek marketing and glossy simulations lies a truth every clinician must confront: aligner biomechanics is complex, and aligner tracking failures are common.

In my experience, many of the frustrations clinicians encounter with aligners—teeth not moving as planned, spaces opening, rotations stalling—stem from a lack of understanding of biomechanics. Aligners are not magic; they are simply another force delivery system. Just as with fixed appliances, success depends on applying sound biomechanical principles.

In this article, I will explore the biomechanical issues unique to aligner therapy and explain why tracking problems occur so often. My goal is to equip clinicians with insights that help them anticipate, prevent, and manage these challenges.

What Do We Mean by “Tracking”?

In aligner therapy, tracking refers to how closely the teeth follow the movements programmed into the virtual treatment plan. When teeth move as predicted, aligners fit snugly. When they do not, gaps form between plastic and enamel, aligners stop seating fully, and subsequent movements are compromised.

Tracking problems are the most common reason for refinements, prolonging treatment and frustrating both patient and clinician.

Biomechanical Realities of Aligner Force Systems

Unlike braces, which deliver force through a rigid wire–bracket interface, aligners apply force via the elastic deformation of plastic. Each aligner is thermoformed with a small incremental offset (usually 0.25–0.33 mm) from the current tooth position. When inserted, the aligner flexes and pushes the tooth toward the programmed position.

However, this system has several inherent limitations:

1. Force decay – Plastic loses force rapidly as it relaxes intraorally. What begins as an effective force may diminish significantly within hours or days.

2. Short duration of force application – Unlike braces, which exert continuous forces until adjusted, aligners rely on patient wear compliance (ideally 22 hours/day). Any reduction in wear time reduces force effectiveness.

3. Poor root control – Because the aligner contacts primarily the crown, transmitting torque or root movement through plastic is inherently inefficient.

4. Non-uniform force distribution – Contact points vary across teeth, meaning force expression is unpredictable compared to wire-and-bracket mechanics.

These biomechanical realities set the stage for tracking issues.

Common Clinical Issues Leading to Tracking Problems

1. Excessive Staging and Unrealistic Tooth Movements

Digital setups often prescribe movements that are too great per stage. For example:

• Extrusion beyond 0.25 mm per aligner.

• Rotation corrections greater than 2–3 degrees per aligner.

• Torque movements of incisors or premolars exceeding biomechanical limits.

Aligners simply cannot deliver such movements predictably. When staging exceeds biological and mechanical thresholds, teeth fall behind, and the aligners stop fitting.

Clinical pearl: Respect the limits—0.25 mm linear, 2–3° rotational, 1–2° torque per aligner. Anything more requires auxiliaries or staging refinements.

2. Rotations of Round Teeth

Rotations of canines, premolars, and lateral incisors are notorious for poor tracking. Their rounded morphology provides little undercut for aligner grip, making it difficult to generate the couples needed to derotate effectively.

Attachments help, but even optimised designs often fall short. Without adequate purchase, aligners tend to “slip” rather than rotate the tooth.

Clinical pearl: Expect refinements for canine and premolar rotations, and consider auxiliaries (buttons and elastics, sectional wires) if precision is critical.

3. Extrusion Movements

Extruding teeth with aligners is one of the least predictable movements. Plastic can push teeth in but cannot easily pull them out. Aligners lack vertical vectors of force, and their intimate fit resists extrusive movement.

This is particularly problematic for extruding lateral incisors in deep bite cases or for levelling gingival margins.

Clinical pearl: Extrusions often require bonded auxiliaries, attachments with vertical bevels, or even limited fixed appliances. Relying on plastic alone rarely succeeds.

4. Torque and Root Control

Controlling root position—especially torque of incisors—is one of the greatest biomechanical challenges for aligners. Because forces are applied mostly to the crown, achieving a couple strong enough to move the root bodily within bone is very difficult.

As a result, patients often end up with flared incisors or insufficient root uprighting despite apparent alignment. This compromises esthetics, stability, and periodontal health.

Clinical pearl: Overcorrection is often necessary in the digital plan. Auxiliary torqueing mechanics (power ridges, sectional wires, or TAD-supported mechanics) may be required.

5. Arch Expansion and Intercuspation

Digital setups often prescribe broad arch forms, but aligners primarily tip teeth buccally rather than translating them bodily. Expansion achieved this way is unstable, prone to relapse, and may produce posterior open bites due to lack of true occlusal seating.

Clinical pearl: Differentiate between dentoalveolar expansion and true bodily movement. Plan expansion conservatively, and combine with auxiliaries or fixed phases when skeletal change is needed.

6. Posterior Open Bites

Posterior open bites are a common side effect of aligner therapy. They occur because:

• Plastic thickness between posterior teeth prevents full intercuspation.

• Uncontrolled anterior intrusion or posterior tipping disturbs occlusion.

• Bite-block effect of aligners alters vertical dimension.

Clinical pearl: Monitor posterior occlusion carefully, use precision cuts or occlusal coverage selectively, and be prepared for finishing with elastics or fixed appliances.

7. Anchorage Loss and Reciprocal Effects

Aligners, like any orthodontic appliance, obey Newton’s third law. Moving one tooth creates reciprocal forces on others. Unfortunately, aligner systems often underestimate anchorage demands. For example, retracting incisors may lead to unwanted premolar mesial drift if anchorage isn’t reinforced.

Patient-Related Causes of Tracking Issues

Biomechanical challenges are compounded by patient factors:

• Poor compliance – Wearing aligners fewer than 20–22 hours/day drastically reduces force effectiveness.

• Skipping aligners – Some patients advance too quickly, creating a backlog of incomplete movements.

• Failure to seat aligners fully – Without chewies or attention to fit, aligners cannot deliver intended forces.

Even the best biomechanics cannot overcome poor compliance. Patient education and motivation remain critical.

Managing Tracking Failures

When tracking fails, clinicians have several options:

1. Refinements – The most common solution, involving new scans and revised staging. Effective but time-consuming.

2. Auxiliaries – Buttons, elastics, sectional fixed appliances, or TADs can supplement aligners for difficult movements.

3. Overcorrection in planning – Building extra torque, rotation, or extrusion into the digital setup to counter under-expression.

4. Hybrid approaches – Using short phases of fixed braces for problematic movements, then transitioning back to aligners.

The key is recognising limitations early and managing patient expectations from the outset.

Conclusion

Clear aligner therapy is here to stay. Its popularity with patients is undeniable, and its digital workflow appeals to clinicians. But we must be honest: aligners are not inherently more predictable than braces. In fact, from a biomechanical standpoint, they present unique limitations that often lead to tracking problems.

As orthodontists, our role is to understand these limitations, respect the biology of tooth movement, and apply biomechanics wisely. Aligners are simply one tool in our armamentarium. Success comes not from the plastic itself but from the clinician’s ability to design, monitor, and adapt force systems intelligently.

If we do this, we can harness the benefits of aligner therapy while minimising the frustrations of poor tracking—and most importantly, deliver the high-quality outcomes our patients deserve.

Kind regards,

Dr Geoffrey Hall
Specialist Orthodontist 

B.D.Sc(Melb) Cert.Orth( Uni Of Penn) MRACDS (ORTH)
Director OrthoED Institute
Ph: +613 9108 0475
E: geoff@orthoed.com.au