Mechanical, Physical And Optical Properties Comparison Between Direct Printed And Thermoformed Aligners - A Systematic Review

RESULTS 

The PRISMA 2020 flow diagram summarizes the study selection process. A total of 162 records were identified through electronic databases, of which 9 duplicates and 6 records for other reasons were removed, leaving 147 unique records for screening. During the screening phase, 131 records were excluded because they did not provide a direct comparison between thermoformed (TFA) and direct printed aligners (DPA), many focused only on treatment outcomes, clinical case reports, or unrelated orthodontic materials, while others examined properties of 3D printers rather than aligners.

Sixteen full-text articles were sought for retrieval, of which two could not be obtained, leaving 14 articles for detailed eligibility assessment. At this stage, four studies were excluded (three did not match the topic and one investigated 3D printers), resulting in 10 studies that fulfilled all criteria and were finally included. These included studies directly compared the mechanical, physical, or optical properties of TFA and DPA, ensuring that the final synthesis was restricted to the most relevant and clinically meaningful evidence.

Study characteristics

The general information of the 10 included studies are enlisted in the characteristic Table 3. The ten included studies, consisting of eight in vitro studies and two in vivo studies, comprehensively evaluated the mechanical, physical, and optical properties of DPAs and TFAs, focusing on parameters such as geometric deviations, load versus displacement behavior, dimensional accuracy, thickness, gap width, force application and optical properties.

The parameters in the included studies were assessed using different experimental methods and equipment. Thickness and gap width were reported in three studies,^10-15 with measurements taken using profilometers and micro-CT to assess material uniformity and dimensional consistency. Force decay and relaxation properties were analyzed in four studies^12-15 using customized intraoral simulation setups to evaluate sustained force delivery over time. Tensile strength and elastic modulus were examined in two studies, using dynamic mechanical analysis (DMA) and Instron systems, highlighting differences in flexibility and durability between DPAs and TFAs.^13,15 Hardness properties were assessed in two studies^13,14 using nano-indentation and Vickers micro-hardness testing, while optical properties, including transparency and visual appearance, were evaluated in one study.¹³ The diverse range of testing methods and outcome parameters provided a comprehensive comparison of DPAs and TAs, emphasizing their respective advantages and limitations in clinical applications.

Risk of bias in the included studies

The risk of bias in the included studies was assessed using the RoBDEMAT tool across four domains. Overall, the included studies exhibited a moderate risk of bias, with sufficient reporting in aspects such as control groups, testing procedures, and outcome measurements, but consistent deficiencies in randomization, sample size justification, and operator blinding. While all studies adequately reported control groups and identical experimental conditions, randomization of samples and sample size justification were insufficiently reported in 4 studies,^10,16-18 but sufficiently reported by 2 studies.^6,12 Standardization of samples and materials was well-documented except in et al.,¹⁰ which lacked material sourcing details.

Testing procedures and outcome measures were sufficiently reported across all studies. However, none reported operator blinding, introducing potential bias. Statistical analysis methods were detailed only in Koenig et al.,⁶ and Sayahpour et al.,¹⁴ though all studies sufficiently reported outcomes. Randomization of samples, sample size justification, operator blinding, and material sourcing details were not sufficiently reported in several studies, while statistical analysis methods were inadequately detailed in most studies except for Koenig et al.,⁶ and Sayahpour et al.¹⁴

Table 4. Risk of Bias assessment of the included studies (RoBDEMAT Tool)

The studies by Sayahpour et al.,¹⁴ were assessed for risk of bias using the ROBINS-I tool. Both studies demonstrated a low risk of bias for most domains. However, there was a moderate risk of bias in the classification of interventions due to the lack of blinding and in the measurement of outcomes. Overall, both studies were judged to have a moderate risk of bias, suitable for inclusion with minor limitations. The Table 4 highlights areas of methodological strength and potential bias, providing a comprehensive assessment of study quality.

Dimensional Accuracy and Geometric Deviations

DPAs demonstrated superior dimensional accuracy and stability compared to TFAs, with smaller geometric deviations and narrower gap widths. The precision of additive manufacturing minimizes distortions, whereas the thermoforming process in TFAs introduces variability. Dimensional accuracy and geometric deviations were assessed in three studies.^6,11,16

Koenig et al.,⁶ demonstrated superior precision in DPAs, with trueness values of ± 0.08 mm compared to ± 0.15 mm for TFAs. Jindal et al.,¹⁷ reported smaller geometric deviations in DPAs (0.12 mm) compared to TFAs (0.20 mm), indicating better dimensional stability.¹⁶ Park et al.,¹¹ found that DPAs exhibited narrower gap widths of 0.08 mm, whereas TFAs showed greater deviations with gap widths of 0.15 mm. These findings suggest that DPAs may provide better aligner fit, which could contribute to their potential suitability for precision driven orthodontic treatments, al-though further comparative studies with TFAs are warranted.

Thickness

Thickness variation, a critical physical property, was evaluated in three studies Bandic et al.,¹⁰ and Park et al.,¹¹ Sayahpour et al.,¹² and Shirey et al.,¹⁵ using micrometers, profilometers and micro-CT.

Sayahpour et al.,¹⁴ reported that DPAs maintained an average thickness of 0.85 mm during intraoral use, while TFAs exhibited slight reductions, with average thicknesses of 0.56 mm and 0.42 mm for two thermoformed groups.12 Bandic et al.10 reported that DPAs demonstrated minimal thickness deviations of ±0.05 mm, while TFAs exhibited greater variability at ±0.12 mm under mechanical stress. Park et al.,¹¹ observed that DPAs had a median thickness of 687.53 µm, compared to 509.54 µm for TFAs.

In addition, Shirey et al.,¹⁵ directly measured aligner thickness in the facial aspects of all the samples (maxillary left: canine, first premolar, second premolar, and first molar) using Miltex Ilwanson Caliper and reported that DPAs had a significantly greater and more consistent thickness (0.76 ± 0.02 mm) than TFAs (0.72 ± 0.04 mm), with the difference being statistically significant (p< 0.05). These results indicate that DPAs maintain superior thickness uniformity during the fabrication process, ensuring material consistency.

Load versus Displacement Behavior

The load versus displacement behavior, reflecting mechanical performance, was examined in three studies.^16,12,18Two out of the three studies concluded that TFAs exhibited better and more stable load-deflection behavior, with consistent force profiles observed during incremental loading and simulated intraoral conditions.^18,12 In contrast, one study reported that DPAs demonstrated a higher load-bearing capacity (662 N compared to 540 N for TFAs) but exhibited less predictable load-deflection behavior.¹⁶

These findings suggest that while DPAs exhibit superior strength, TFAs offer more predictable and stable mechanical performance.

Force Decay and Relaxation

Four studies evaluated and compared the force decay and relaxation properties of DPAs and TFAs.^12-15 Two studies reported that TFAs retained 94.2% of their initial force after one week, compared to 87.7% for DPAs, indicating higher force decay in DPAs.¹²

Atta et al.,¹³ demonstrated that DPAs exhi-bited significant force decay at room tem-perature but achieved excellent shape recovery of 86.1% at 45°C, compared to minimal recovery of 1.5-2.9% in TFAs. These findings suggest that while TFAs offer better force retention over time, DPAs excel in shape recovery under elevated temperatures. Tensile Strength and Elastic Modulus

Two studies by Shirey et al.,¹⁵ Atta et al.,¹³ assessed tensile strength and elastic modulus. Shirey et al.,¹⁵ reported that DPAs exhibited reduced tensile strength and greater stress relaxation under wet conditions, while TFAs maintained superior mechanical stability. The study found that direct-printed aligners (DPAs) exhibited significantly greater stress relaxation under wet conditions than thermoformed aligners (TFAs).¹⁵ Within 60 minutes, DPAs lost 40-50% of their initial stress, while TFAs relaxed by only 20-30%. This suggests TFAs provide more consistent force retention over time (p< 0.05). Atta et al.,¹³ found that DPAs demonstrated greater flexibility but slightly lower tensile strength compared to TFAs. These findings indicate that while DPAs offer enhanced flexibility, TFAs exhibit superior durability and stability, particularly under simulated oral conditions.

Hardness

Hardness, which determines material resistance to wear, was evaluated in two studies by Sayahpour et al.,¹⁴ and Atta et al.,¹³ observed a 15% reduction in hardness for DPAs after intraoral wear, whereas TFAs retained their hardness with minimal changes. Atta et al.,¹³ reported higher hardness values for TAs at room temperature compared to DPAs, suggesting greater resistance to mechanical wear over time. These findings highlight the superior mechanical stability of TFAs during prolonged clinical use.

Optical Properties

Although optical properties were not widely evaluated, one study reported on material transparency and visual appearance.¹³ TFAs demonstrated better optical clarity at ro-om temperature compared to DPAs, which exhibited slight cloudiness under mechanical stress. This suggests that TFAs may offer superior aesthetic outcomes in clinical applications.