This blog has been written by Shanya Kapoor, as part of a new orthodontic blog Orthobrief, An excellent resource to keep up to date with digestible updates in orthodontics. There is a monthly newsletter. Subscribe below (I have no conflict of interest, just keen to read more!) Subscribe Orthobrief
The trimline marks the boundary of the aligner and plays a crucial role in its fit against the gingiva and teeth. The three primary trim line designs used in orthodontic aligner therapy:
Traversa 2024 delves into the biomechanical performance of clear aligner therapy (CAT) across three teeth—central incisor, canine, and first molar—during three translational movements, using the Orthodontic Force Simulator (OFS) It also evaluates the impact of two flat trimline designs trimmed at the gingival margin (TL0) and extended 2.0 mm below (TL2).to refine aligner effectiveness and improve clinical outcomes in orthodontic treatments.
The findings through Welch’s ANOVA unveiled significant biomechanical variations influenced by tooth location, trimline height, and movement type. Molars exhibited greater forces and moments during mesial, palatal translations and intrusions compared to canines and incisors, highlighting distinct biomechanical demands across teeth.
Aligners with the TL2 extension consistently demonstrated better control over movements than TL0, (F values 20.54; 290.94; 2.09 ;7.31 for buccopalatal, mesiodistal, buccopalatal moment and buccopalatal force respectively) suggesting extended trimlines enhance precision in force and moment application for effective tooth alignment.
In summary, this research identifies critical factors influencing CAT efficacy, including force patterns across translational movements. By elucidating these biomechanical dynamics, the study informs the refinement of aligner designs and treatment strategies, aiming to optimize orthodontic outcomes through tailored approaches that account for individual tooth biomechanics.
Overall better translational movements – Straight Trimline >> Scalloped Trimline
Elshazly 2022 utilized Fuji pressure-sensitive films to assess the pressure distribution across the facial surface of an upper right central incisor with various aligner trimlines: scalloped, scalloped extended, straight, and straight extended.
It was found that scalloped trimline enhances flexibility but also weakens material strength at the margins, resulting in reduced force application( 3.8 ± 1.5 N as compared to 5.6 ± 1.3N for straight trimline)near the gingival area and a lower likelihood of achieving complex movements like bodily translation.
Conversely, aligners with straight extended designs exhibited increased stiffness, better retention, and improved adaptation to the gingiva. These designs applied higher forces at the gingival area, enhancing the probability of accomplishing complex movements. Moreover, aligner trimlines closer to the center of resistance were noted to potentially improve movement control during treatment. The study also observed non-uniform contact between the aligner and tooth surface, affecting force distribution across different regions—cervical, central, disto-incisal, and mesiocervical—where straight extended trimlines consistently exerted higher forces compared to scalloped designs, indicating their potential superiority in achieving effective tooth movements.
Scalloped trimming leads to higher flexibility and material weakening at the marginal areas, which results in reduction of the amount of force applied on the tooth near the gingival area whereas Trimming of the aligner in (straight extended) design is a way to increase stiffness, retention, and gingival adaptation of the aligner, and consequently to achieve better stress distribution and better control of tooth movement.
Overall maximum active forces – Straight extended trimline
Lyu 2023 explored the biomechanical impacts of clear aligners with varying thermoplastic material thicknesses and trimline design proposing a computer-aided optimization procedure for aligner design. A finite element analysis (FEA), on orthodontic simulation model was evaluated for 0.75 mm and 0.50 mm thicknesses in scalloped and straight gingival trimlines at heights of −2 mm, 0 mm, and 2 mm.
Regarding trimline designs, aligners with margins set at −2 mm height exhibited significantly lower PDL stress compared to higher margin heights. Straight trimline margin designs generally induced higher stress than scalloped designs, although differences diminished at 2 mm height.
For molar, the R/C ratio of 0.75-mm-thick aligner with a 2 mm-straight margin was 0.38, which was able to control the teeth movement better. For incisors, it was 0.75-mm-thick aligner with 0 mm height and straight shape (R/C ratio was 0.31). Whereas for canine, the R/C ratio of aligner with 0.75-mm-thick and 0 mm-scalloped margin was 0.34, leading to the least tendency of tooth tipping.
These insights provide a foundation for refining aligner trimline designs to optimize treatment outcomes in orthodontic practice, ensuring effective and controlled tooth movement during extraction treatments.
Optimized 0.75-mm-thick aligner with differential trimline designs (0-mm-height straight trimline for incisors, 0 mm-scalloped trimline for canines, and 2 mm-straight trimline for posterior teeth) was recommended on the basis of the biomechanical calculation results, which facilitated the efficiency and control of tooth movement for multiple teeth.
|
Tooth |
Trimline Design |
Height Of Trimline |
|
Incisor |
Straight |
0 mm |
|
Canine |
Scalloped |
0 mm |
|
Molar |
Straight |
2 mm |
Cowley 2012 involved conducting pull-off tests using a Universal Testing Machine to evaluate the retention capabilities of orthodontic aligners with different margin designs. Six groups were tested, including scalloped line, straight line at the gingival zenith, and straight line positioned 2mm above the gingival zenith, each fabricated with and without attachments. To determine the difference in pull off forces analysis of variance with Bonferroni post hoc testing was utilized.
Specifically, aligners with straight margins and attachments exerted pressures of 15,571 and 22,122 psi, respectively, significantly higher ↑ than those without attachments, which registered pressures of 9,947 and 16,726 psi. The results indicated that the aligner trimming line design with a straight line 2mm above the gingival zenith, particularly when equipped with first-premolar attachments, demonstrated the highest retention strength.
The study also highlighted that flexibility of scalloped trimline at the gingival margin affects the aligner’s force distribution near the gumline, potentially limiting its ability to achieve certain tooth movements (mainly torqueing)
Aligners with straight gingival margins were found to reduce flexibility, thereby enhancing retention and facilitating more complex movements such as torquing and extensive tooth alignment throughout the treatment duration.
Overall retention ability – Straight Trimline >> Scalloped Trimline
Nakornnoi 2024 systematic review aimed to evaluate how different trimline designs and extensions in clear aligners influence the biomechanics of orthodontic tooth movement. Studies published between January 2000 and August 2024 were analyzed using a structured search strategy.
Twelve studies met the inclusion criteria, all of which were rated as having a low to moderate risk of bias. In this review, the term “trimline” was used broadly to cover all design types, without differentiating between specific styles or conventional models. This generalization method has also been used in past systematic reviews and is considered a valid strategy for identifying relevant research.
Key findings show that aligners with straight and extended trimlines deliver superior biomechanical effects, enhancing force magnitude and direction, particularly for complex tooth movements. These designs also improved aligner retention, which is crucial for consistent force application. However, most studies assessed trimline effects in isolation, without considering how factors like material type, attachments, elastics, or miniscrews interact with trimline design
Trimline design summary
|
Trimline design |
Flexibility |
Stiffness |
Active force maximum at |
Force level |
Retentive capability |
Ability to perform complex movements |
|
Scalloped |
↑ |
↓ |
– |
3.8 N (least) |
↓↓ |
Compromised |
|
Straight |
↓ |
↑ |
– |
4.8 N |
↑ |
Better |
|
Straight Extended |
– |
– |
Distoincisal, Centre & Mesiocervical |
5.5 N (max) |
↑↑ |
– |
|
Scalloped extended |
– |
– |
Cervical region |
5.0 N |
↓ |
– |
By leveraging insights from trimline design research, we can effectively plan and achieve the desired tooth movements.
Contents: Shanya Kapoor, a member of the orthodontics in summary team and curator of a new blog, Orthobrief blog Subscribe Orthobrief
