Evolution of orthodontics: from fixed appliances to clear aligners. An enhanced narrative review

Historical evolution of orthodontics

The roots of orthodontics date back to the 18th century, when Pierre Fauchard developed the bandeau, an appliance used to expand the dental arch. However, the specialty evolved in the late 19th century through Edward H. Angle, who described malocclusions and developed the edgewise appliance that remains the conceptual foundation of fixed orthodontics (4).

In the first half of the 20th century, gold and precious alloys were used in orthodontics because of their workability and biocompatibility. In the 1960s, stainless steel replaced gold due to its cost-effectiveness and durability (5). The introduction of direct-bonding methods made it possible to eliminate full bands, thereby improving patient comfort.

The next decades were focused on aesthetics and patient satisfaction. Lingual orthodontics, introduced by Fujita in 1979, hid the brackets behind the teeth, and ceramic and sapphire brackets provided transparency (6). Nickel-titanium (NiTi) wires, with shape memory and superelastic properties, delivered light, continuous forces that were biologically compatible with tooth movement (7).

These developments culminated in the straight-wire appliance developed by Andrews in 1979, which incorporated torque and angulation values into the design of the brackets–a precursor to the pre-programmed and computer-controlled systems of the present day (1,8).

Material science evolution

Material science has influenced orthodontics in terms of force systems and biological responses. The transition from metallic alloys to thermoplastics reflects the shift from mechanical methods to biologically adaptive strategies (Table 1).

The development of multi-layer thermoplastics helped to reduce stress peaks while improving flexibility and load retention. These material advancements have allowed orthodontics to move closer to delivering biologically efficient forces (7,13).

Digital orthodontics and the emergence of clear aligners

The digital revolution brought significant changes to orthodontics in the 1990s with the integration of CAD/ CAM technology, 3D imaging, and stereolithography. The idea of sequential aligners, first proposed by Kesling in 1945, became a practical reality with the launch of Invisalign® in 1997 (9).

This innovation introduced a workflow involving 3D intraoral scanning, digital simulations of tooth movement, and the fabrication of thermoplastic aligners using 3D printing. Each aligner moves the teeth incrementally—typically by 0.25 mm per step— following a carefully planned biomechanical sequence. Initially suitable only for mild cases, advancements in digital planning tools, attachment designs, and materials science have expanded its suitability.

Mechanisms of action

Clear aligners function as flexible shells that apply force across the entire tooth surface, rather than targeting isolated points as traditional systems do. On average, they deliver forces of 0.25–0.35 N per tooth for tipping and 0.15–0.25 N for intrusion. Finite element analysis confirms that this force decreases exponentially, with about 40% of the original force remaining after 48 hours (12).

To address limitations in torque and rotational control, attachments and auxiliary elastics are used to enhance effectiveness by increasing moment arms. Digital staging also improves precision by dividing overall tooth movement into small, physiologically manageable steps.

Clinical performance and quantitative evidence

Recent research has measured the effectiveness of clear aligners with growing precision. Systematic reviews and randomized controlled trials (RCTs) report accuracy rates between 55% and 70%, with higher predictability for tipping and intrusion, and lower for rotation and extrusion (13–17) (Table 2).

Treatment duration is comparable to that of fixed appliances for mild to moderate malocclusions: 15 ± 3 months vs 14 ± 3 months (18).

Periodontal and TMJ Health

Clear aligners support periodontal health due to their ease of cleaning and reduced mechanical irritation. Studies report a 32% reduction in the plaque index and a 28% reduction in the gingival index compared with fixed appliances (21).

Temporomandibular joint (TMJ) discomfort is also less common among aligner users, as occlusal interference and heavy interarch mechanics are minimized.

Biomechanics of clear aligners

aligners work through material deformation. The way forces are distributed is influenced by factors such as aligner thickness, elasticity, and the geometry of contact with the teeth. Finite element studies have shown that the stress–strain response is nonlinear and that attachment positioning significantly affects the delivered force (22).

Force Decay Over Time:

• 0h: 100%
• 12h: 68%
• 24h: 54%
• 48h: 42%
• 96h: 31%

This pattern supports the common clinical recommendation to replace aligners every 10–14 days to maintain effective, biologically safe levels of force.

Biological response

Tooth movement occurs via remodeling of the periodontal ligament (PDL) and alveolar bone. Clear aligners produce low, continuous forces that help reduce PDL hyalinization. Inflammatory markers such as IL-1β, TNF-α, and prostaglandin E2 rise temporarily but tend to normalize faster than with fixed appliances (23).

Additionally, the risk of root resorption is reduced by approximately 30%, owing to lower stress magnitudes. Aligners also preserve vascular integrity and promote overall periodontal health, supporting biologically compatible tooth movement.

Artificial intelligence and predictive orthodontics

artificial intelligence (AI) is transforming orthodontic treatment planning. Machine learning models are trained on extensive clinical datasets to improve the accuracy of tooth movement staging and to predict the need for refinements (26).

AI-assisted planning has been shown to increase movement accuracy by approximately 18% compared to traditional manual sequencing. Additionally, smart sensors embedded in aligners enable real-time tracking of patient compliance, with an average wear time of 19.2 ± 2.1 hours per day (27).

Smart materials and 4D printing

Next-generation aligners use shape-memory polymers that can change stiffness at body temperature, allowing the application of adaptive, continuous forces throughout treatment (28).

The advent of 4D printing introduces materials that respond to time and environmental changes — gradually adjusting their shape and mechanical properties during use.

Aligners enhanced with nanocomposite materials, such as graphene or hydroxyapatite nanoparticles, have demonstrated improved mechanical strength and antimicrobial performance. Moreover, drug-eluting aligners that release fluoride or chlorhexidine have shown a 20–30% reduction in bacterial load after two weeks of wear (29–30).

Long-term stability and relapse

post-treatment stability remains a critical concern in orthodontics. Two-year follow-up data indicate a relapse rate of approximately 11% after clear aligner therapy, which is comparable to that observed with fixed appliances (31).

The use of precision retainers and extended nighttime wear protocols contributes significantly to long-term stability. Furthermore, machine learning models are under development to predict relapse risk based on variables such as initial malocclusion, aligner wear time, and type of tooth movement (32).

Ethical and Clinical Considerations

Clear aligners have significantly expanded access to orthodontic care, particularly among adult patients, due to their discreet appearance and improved comfort. However, the high costs associated with digital workflows, proprietary software platforms, and advanced materials raise socioeconomic and ethical concerns (33).

Additionally, the rise of remote monitoring systems and teleorthodontics introduces new challenges for data privacy, patient autonomy, and the need for continuous clinical supervision. These evolving models of care necessitate updated regulatory frameworks to ensure patient safety and professional accountability (34).