Removable thermoplastic appliances modified by incisal cuts show altered biomechanical properties during tipping of a maxillary central incisor
Brockmeyer et al. Progress in Orthodontics
Removable thermoplastic appliances modified by incisal cuts show altered biomechanical properties during tipping of a maxillary central incisor
Phillipp Brockmeyer 0
Katharina Kramer 2
Florian Böhrnsen 0
Rudolf Matthias Gruber 0
Sarah Batschkus 4
Tina Rödig 1
Wolfram Hahn 3 4
0 Department of Oral and Maxillofacial Surgery, University Medical Centre Goettingen , Robert-Koch-Str. 40, 37075 Goettingen , Germany
1 Department of Preventive Dentistry, Periodontology and Cariology, University Medical Centre Goettingen , Robert-Koch-Str. 40, 37075 Goettingen , Germany
2 Department of Medical Statistics, University Medical Centre Goettingen , Humboldtallee 32, 37073 Goettingen , Germany
3 Private practice , Goettingen , Germany
4 Department of Orthodontics, University Medical Centre Goettingen , Robert-Koch-Str. 40, 37075 Goettingen , Germany
Background: The present study aimed to evaluate the force delivery of removable thermoplastic appliances (RTAs), modified by different sized incisal cuts, during tipping of a maxillary central incisor in palatal and vestibular direction. Methods: Forty-five RTAs from three different materials (Biolon®, Erkodur®, Ideal Clear®) of the same thickness (1 mm) were used. Analysis was performed on a separated maxillary central incisor which was part of a resin model with a complete dentition. In 15 RTAs, of different material, a cut was inserted at the incisal edge of tooth 11. In 15 other appliances, the cut was extended to teeth 12 and 21. Fifteen aligners remained uncut. The experimental tooth was tipped starting from the zero position in 0.05° steps to a maximal deflection of ± 0.42° of the incisal edge in vestibular and palatal direction, after positioning the RTA onto the model. Results: The horizontal (Fx) and the vertical (Fz) force components were decreased by approximately half with increasing cut size. Fz values changed during palatal tipping from a weak intrusive force, for aligners without cut, to an extrusive force with increasing cut size. Compared to both other materials used (Erkodur® and Ideal Clear®), the Biolon® aligners showed significantly higher Fx and Fz values (p < 0.0001, respectively). Conclusions: RTAs modified by different sized incisal cuts show altered biomechanical properties and an inversion of the vertical force component, during tipping of a maxillary central incisor.
Removable thermoplastic appliances; Biomechanics; Incisal cut
Removable thermoplastic appliances (RTAs) can be used in
patients as a less visible alternative to conventional, fixed,
orthodontic treatment appliances [
malocclusions, slight arch expansions, and corrections of a deep
overbite are indication ranges . Bodily tooth movements
are restricted, as aligners primarily transfer forces produced
by point contact between the appliance and the tooth [
This results mainly in tipping and intrusive movements .
Forces required for tooth movement can be generated by
local and whole body deformation of the aligner when it is
placed onto the dental arch. This deformation results from
a discrepancy between the actual and the intended tooth
position incorporated into the appliance [
appliance manufacturing process increases material stiffness by
incorporating bends and edges into the aligner. This
increase in stiffness is mainly located at the incisal edge [
Therefore, it might be appropriate to attenuate the
generated force component by inserting cuts into the material in
Force components were analysed using a measurement
device containing a nano-17 sensor (ATI Industrial
Automation, Apex, USA), as described in prior
]. The sensor was connected to a separated
maxillary central incisor as part of a standardized resin
model (Frasaco GmbH, Tettnang, Germany). Analysis
was carried out in an incubator at 37 °C. An impression
(Tetrachrom®, Kanidenta, Herford, Germany) of the
model with the measuring tooth placed on the
measuring device was made. Subsequently, a plaster model (GC
Fujirock® EP, GC GERMANY GmbH, Munich, Germany)
was fabricated. Two model duplicates were produced by
using Adisil® blau 9:1 (SILADENT Dr. Böhme & Schöps
GmbH, Goslar, Germany). Following, an impression was
taken from both models and five model duplicates were
manufactured, as described above, respectively.
Fortyfive RTAs (15 for each material) with a similar extension
of 2.5 mm beyond the gingival margin were produced
from three different materials (Biolon®, Dreve Dentamid
GmbH, Unna, Germany; Erkodur®, Erkodent® Erich Kopp
GmbH, Pfalzgrafenweiler, Germany; Ideal Clear®, Dentsply
GAC, Gräfelfing, Germany). For the thermoforming
process of the Biolon® aligners, the Drufomat-TE appliance
(Dreve Dentamid GmbH, Unna, Germany) was used. Ideal
Clear® appliances were prepared with the ‘Vacuum Forming
Machine’ 202 (Dentsply GAC, Gräfelfing, Germany) and
Erkodur® aligners with the Erkoform RVE device (Erkodent®
Erich Kopp GmbH, Pfalzgrafenweiler, Germany). Appliance
edges were trimmed along the marginal sulcus with HSS
twist drills and Lisko-S polishing discs (Erkodent® Erich
Kopp GmbH, Pfalzgrafenweiler, Germany). The resulting
forces were evaluated in 15 aligners without cut, 15 aligners
with an incisal cut on the measuring tooth 11 (z11; the
incisal edge of tooth 11 was exposed), and 15 aligners with
an incisal cut from tooth 12 to 21 (z12–21) (Fig. 1). To
produce cuts of the same size, their length and depth were
marked using a ruler and a pen. These cuts were again
prepared by HSS twist drills and Lisko-S polishing discs. The
inner surface of the aligners was moistened using artificial
saliva (University Pharmacy, Goettingen, Germany). Before
analysis, all forces were set to zero. With the aligner in
place, the experimental tooth was tipped in vestibular and
palatal direction from 0° to 0.42° (24.9 arcminutes) and
back to 0° in 0.05° (2.7 arcminutes) steps. Data was
recorded five times after each movement. To protect the
sensor from overloading, the incisal edge was maximally
deflected up to ± 0.151 mm from the initial position in all
cases. This activation range is comparable to the lowest
activation range value documented in the literature for
thermoplastic aligner systems [
Data was analysed using a mixed, multifactorial analysis
of variance (ANOVA) with repeated measurements.
Since there was no normal distribution, observation
ranks were used. All tests were performed at a
significance level of α = 5%, using the statistical software SAS
(SAS Institute Inc., Cary, NC/USA).
Horizontal force component
Regardless of the tipping direction, the highest
horizontal force component (Fx) values were observed for the
Biolon® aligners; the lowest values were found during
palatal tipping for the Erkodur® and vestibular tipping
for the Ideal Clear® aligners (Table 1). Mean values
during palatal tipping ranged from 2.56 N (SD 0.57 N)
to 4.03 N (SD 0.36 N) and during vestibular tipping
from 3.05 N (SD 0.80 N) to 5.41 N (SD 0.74 N).
Aligners with cut on the incisal edge of tooth 11 (z11)
Table 1 summarises the Fx values for z11 at a deflection
of ± 0.151 mm. The lowest values were measured during
both tipping directions for the Erkodur® aligners. The
highest values were found for the Biolon® appliances.
Mean values during vestibular tipping ranged from 2.47 N
(SD 0.74 N) to 4.07 N (SD 0.26 N) and during palatal
tipping from 1.94 N (SD 0.48 N) to 3.0 N (SD 0.40 N).
Aligners with cut on the incisal edge from tooth 12 to 21
Table 1 shows the mean Fx values and SD for z12–21 at
a deflection of ± 0.151 mm. The highest values could be
observed for the Biolon® aligners, whereas the lowest
values were found for the Ideal Clear® appliances.
Overall, the force values during vestibular tipping were
higher than those during palatal tipping. Mean values
during palatal tipping ranged from 1.48 N (SD 0.59 N)
to 1.70 N (SD 0.12 N) and during vestibular tipping
from 1.72 N (SD 0.89 N) to 2.09 N (SD 0.40 N).
Mean (N) SD (N)
Correlations and interactions between material, deflection
distance, and incisal cut for Fx
The ANOVA revealed a significant effect of the
deflection distance (p < 0.0001), the material (p < 0.0001)
and the incisal cut (p < 0.0001) on the Fx component.
A significant interaction between the material and the
incisal cut was found (p = 0.0429). No statistically
significant interactions between the deflection distance
and the material (p = 0.6479), the deflection distance
and the incisal cut (p = 0.4201), and the deflection
distance and the material and the cut (p = 0.8381) could
be observed. Since the analysis revealed a significant
interaction between the material and the cut, analysis
was performed separately for each material (Biolon®,
Erkodur®, Ideal Clear®, respectively) (Fig. 2).
A statistically significant correlation between the
deflection distance (p < 0.0001) and the incisal cut
(p < 0.0001) could be observed. No significant
interaction between the deflection distance and the cut was
found (p = 0.1995).
A significant effect of the deflection distance (p < 0.0107)
and the incisal cut (p < 0.0001) was found. No interaction
between the deflection distance and the incisal cut
(p = 0.7348) could be observed.
(SD 0.13 N) and during vestibular tipping from −
1.0 N (SD 0.29 N) to − 2.55 N (SD 0.46 N). The sign
of Fz values reflects the effective force direction. A
negative sign indicates an intrusive Fz, whereas
positive values indicate an extrusive Fz component.
No significant effect of the deflection distance (p < 0.2087)
could be observed. The analysis revealed a significant
influence of the incisal cut (p < 0.0001) on the measured Fx
component. The interaction between deflection distance
and the incisal cut was not significant (p = 0.6635).
Vertical force component
Mean Fz values during palatal and vestibular tipping
are summarized in Table 2. Overall, Fz values were
higher during vestibular than palatal tipping.
Regardless of the tipping direction, the highest Fz values were
measured for the Biolon® aligners. The lowest values
were found during palatal and vestibular tipping for
the Ideal Clear® aligners. Mean values during palatal
tipping ranged from − 0.02 N (SD 0.22 N) to − 0.16 N
Aligners with cut on the incisal edge of tooth 11 (z11)
The lowest Fz values at a deflection distance of −
0.151 mm were observed for the Erkodur® aligners;
whereas for a deflection distance of + 0.151 mm, the
lowest values were measured for the Ideal Clear®
appliances. The highest Fz values were found
independent of deflection direction for the Biolon® aligners.
Mean values during vestibular tipping ranged from −
0.58 N (SD 0.20 N) to − 0.69 N (SD 0.31 N) and
during palatal tipping from 0.10 N (SD 0.35 N) to
0.19 N (SD 0.59 N).
Aligners with cut on the incisal edge of teeth 12 to 21 (z12–21)
The highest Fz values at a deflection distance of −
0.151 mm were measured for the Biolon® aligners;
the lowest values were found for the Ideal Clear®
appliances. For a deflection distance of + 0.151 mm,
the analysis revealed the lowest values for the
Biolon® and the highest for the Erkodur® aligners. Mean
values during vestibular tipping ranged from −
0.35 N (SD 0.18 N) to − 0.48 N (SD 0.19 N) and
during palatal tipping from 0.26 N (SD 0.33 N) to
0.51 N (SD 0.16 N).
Correlation and interactions between materials, deflection
distance, and incisal cut for Fz
The ANOVA revealed statistically significant effects of
the deflection distance (p < 0.0001) and the incisal
cut (p < 0.0001) on the Fz component. No significant
effect of the material (p = 0.4672) was found. In
addition, no significant interactions between the
deflection distance and the material (p = 0.8743), the
deflection distance and the incisal cut (p = 0.3741),
the material and the incisal cut (p = 0.1590), and the
deflection distance and the material and the cut
(p = 0.6148) could be observed (Fig. 3).
The measuring device used in the present investigation
is comparable to the one already described previously
]. A rigid connection between measuring tooth
and sensor was used. So far, it has been impossible to
simulate the periodontal ligament (PDL) using a
measuring device [
]. Therefore, the measured forces must
be interpreted as an immediate and transient sequence,
as no significant tooth movement can be expected at this
point due to the PDL’s visco-elastic properties [
The sign of Fz values reflects the effective force direction. A negative sign
indicates an intrusive Fz, whereas positive values indicate an extrusive Fz
The results confirm a direct dependency between the
aligner material and the magnitude of force delivery, as
has been described in preliminary investigations [
The Fx and Fz values for the Biolon® aligners were
significantly higher than those for the two other materials tested
(Erkodur® and Ideal Clear®). Apart from different material
properties comprising individual elastic moduli , the
manufacturing process plays a crucial role in force delivery
strength. The Biolon® appliances were prepared by using
6 bar overpressures; while in the Erkodur® and Ideal Clear®
appliances, a 0.8 bar under pressure was used.
Thermoforming using high positive overpressure leads to more
precise fitting of the appliance on the individual oral
structures. Depending on the degree of better play of the
appliance in the anterior region, where a tooth is tipped,
complete deformation of the aligner results. Superior fit
and, consequently, higher friction generate higher
restoring force components. A space holder foil of
0.05 mm was additionally used for Erkodur® appliance
preparation. Friction was decreased, resulting in a
reduced force application compared to the two other
The incisally cutted aligners exhibited reduced values
for the horizontal (Fx) and the vertical (Fz) force
components with increasing cut size. As has been described
], an important component of the high force
values occurring during tooth movements using aligners
is caused by the increase in material stiffness due to
incorporating bends and edges into the appliance during
thermoforming. When the appliance is cut at the incisal
area, this stiffening is reduced. This could be due to an
altered stress–strain relationship in the modified
aligners. With increasing cut size, the aligner could be
warped more easily near the tooth contact area (below
the incisal edge). This results in less force against the
tooth to be treated (Fig. 4).
For the first time, the present investigation
demonstrated a possible inversion of the Fz component
during palatal tipping as Fz values changed from a weak
intrusive force for aligners without cut to an extrusive
force for those with a cut of increasing size. One
reason for this might be the different morphology of the
vestibular (convex) and palatal (both convex and
concave) surfaces of the experimental tooth, resulting in
a unique interaction between inclination and shape of
the interior aligner surface and the anatomy of the
tooth in the contact area during force generation.
The contact point between appliance and tooth,
where the force is generated in uncut aligners, is
located near the incisal edge at an inclined inner
surface of the appliance [
]. During tipping, this
inclined surface results in a single point force
application. With incisal cut, the aligner material can move
more horizontally at the contact area. In this case,
the force components that are generated are less at
an inclined surface, which results in reduced intrusive
force values (Fz). What is more, during palatal
tipping, the incisal parts of the tooth slip relatively in an
apical direction on the palatal aligner surface with
high friction. This could explain the extrusive Fz
values (Fig. 4). Compared to uncut aligners, the
material at the area around the contact point of
appliances with a cut can be more deformed with
lower resulting forces.
The amount of orthodontically induced
inflammatory root resorptions has been shown to be directly
correlated with the force magnitude applied [
By using removable aligners, less inflammatory root
resorptions are caused, even with higher forces [
which have also been reported by Barbagallo and
colleagues during premolar tipping [
modifying the aligner by an incisal cut could reduce
even more the risk of orthodontically induced
inflammatory root resorptions.
The conclusions of the study are as follows:
Removable thermoplastic aligners modified by
incisal cuts exhibit altered biomechanical properties
and a possible inversion of the vertical force
Incisal cuts decrease the horizontal and vertical
force components significantly.
This could be used to reduce the number of aligners
during clinical application.
Availability of data and materials
The dataset supporting the conclusions of this article is available over the
All authors share in all steps of this study. All authors read and approved the
The authors declare that they have no competing interests.
Springer Nature remains neutral with regard to jurisdictional claims in
published maps and institutional affiliations.
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