1. Introduction
In recent years 3D printing technology endured significant technological improvements[
1,
2,
3,
4,
5,
6,
7,
8,
9,
10]. Whereas SLA, DLP and LCD based 3D printers improved[
6,
7,
10,
11,
12,
13,
14], 3D printing resins proved to be bio-compatible[
3,
9,
12,
14,
15,
16,
17,
18] and being able to endure masticatory forces[
5,
8,
11,
14,
18,
19,
20]. This enabled orthodontists to set up their own 3D lab and design and print a variety of orthodontic appliances. Molar distalizers exist in a variety of solutions. As there are appliances as Hilgers pendulum[
21], distal jet appliance[
22], modified slider[
23], intraoral bodily molar distalizer[
24], J-Molar Distalizer[
25], Greenfield lingual distalizer[
26] or simplified molar distalizer[
27], which are fixed appliance connected to a plastic pad in contact with the palatal ruggae, with springs or rotary devices to distalize molars. These have the disadvantage of disto-rotating the mandible thus increasing the anterior facial height[
28,
29]. Others like the Hybrid hyrax Distalizer are printed in metal, and skeletally anchored in the palate with the use of mini-implants, thus avoiding the disto-rotation of the mandible [
30,
31]. The appliance proposed in this article is an in office designed distalizer comparable to Carriere® Motion 3D™ (Henry Schein Orthodontics, Carlsbad, Ca, USA)[
1,
32,
33,
34,
35,
36,
37,
38,
39,
40,
41,
42,
43,
44,
45,
46], consisting of a rigid bar connected with a pad on the canine attached to the anterior third of the clinical crown, with a mesial hook attached to it and pivoting in a ball-and-socket joint with a pad bonded to center of the clinical crown of molar facilitating the distalization en derotation of the molar. The activation of the device is done with elastics attached to the medial hook on the canine. Measuring the results are shown using CBCT [
37,
45,
47,
48], cephalometric superimposition and model [
1,
49] or overlay of the STL files before and after treatment [
1,
35]. The advantage of using a distalizer is its insertion at the beginning of the treatment, when compliance is still high. A distalizer is a low invasive, with easy placement and removal technique advantages. The doughnut in a socket design on the molar pad is tipping and derotation the upper first molar. The hook on the canine pad allows Class II elastics to be worn attached to the lower first or second molar [
32].
2. Materials and Methods
1. Digital Design
Digital bio-compatible Additive Manufacturing (AM), brings 3D printing and manufacturing closer to a orthodontic practice, and allows for manufacturing of distalizers, power-arms, retainers and other perceivable orthodontic auxiliaries[
14,
17,
18,
50] .
The bio-compatible resin chosen, Dental LT Clear, has been applied with success in orthodontic manufacturing [
3,
5,
8,
9,
11,
12,
18,
20,
50]. For 3D printing we opted for a Formlabs 3B+ printer. There has been extensive research comparing 3D print technology [
4,
6,
7,
10,
12,
13,
14,
51]. These conclude that model position [
13], anti-aliasing, grey-scale and blur [
7,
52] are the most influencing parameters. Positioning parallel to the 3D printer tray [
14] is favourable and we used the anti-aliasing, grey-scale and blur settings proposed by the manufacturer.
We start the design process with drawing a cylinder with the desired shape, we design a spline to sweep the cylinder over to form the arm. Finally we make the rotary part of the molar hinge
Figure 1. This concise of a sphere with a diameter of 2.52mm, here we make indentations on the side of this ball with spheres with diameter 10mm. These indentations will allow the arm to click in place in the molar base. The software used is Fusion360 and 123D Design both from Autodesk™[
53].
Form a hemisphere with the desired size and attach to the arm to form the canine attachment
Figure 2. Another hemisphere is being created on top of the canine attachment to create the hook for the elastics. A cutout is made with a round edged box and finally the top edge of the cutout has been rounded off
Figure 3.
We start with a hemisphere ⊘ 7.5mm to form the molar attachment. The molar pad will be flattened on the side and a large sphere ⊘ 35mm is used to form the concavity of of the molar pad. Then a cutout is made first in 90º then a second cutout in -18º
Figure 4. A copy of the existing arm is enlarged 1.1 and used to make the cutout in the molar pad. After this the entrance is enlarged to facilitate the rotation of the arm
Figure 5.
In
Figure 6A the completed molar hinge is shown. In
Figure 6B the completed arm is shown.
2. Patient Selection
Patients were chosen by the following selection criteria:
Class II occlusion with no skeletal Class II values
Skeletal cephalometric values of SNA = 81± 3º , SNB = 78 ± 3º , ANB = 3 ± 2º ;
Patients were compliant with dental monitoring on a monthly basis;
3. Printing
The bio-compatible resin chosen, Dental LT Clear, has been applied with success in orthodontic manufacturing [
3,
5,
8,
9,
11,
12,
17,
18,
20]. For 3D printing we opted for a Formlabs™Form 3B+ , as it has been validated in FDA-cleared workflows. There has been extensive research comparing 3D print technology [
4,
6,
7,
10,
12,
13,
14,
51,
54,
55]. These conclude that model position [
9,
13], anti-aliasing, grey-scale and blur [
7,
52] are the most influencing parameters. Positioning parallel to the 3D printer tray [
14] is favourable and we used the anti-aliasing, grey-scale and blur settings proposed by the manufacturer.
4. Post-Processing after 3D Printing
After printing the distalizer
Figure 7, it must be washed, and the support removed. We used a FormWash filled with a concentration of 99% IPA, to comply with bio-compatibility regulations [
56]. 3D printed parts require post processing in order to ensure their optimal performance and bio-compatibility of the 3D printed dental appliances. Parts 3D printed with the Dental LT Clear V2 Resin should be first washed for 15 minutes, then soaked in fresh isopropyl alcohol for the remaining 5 minutes. Leaving the parts in the IPA for longer than 20 minutes will result in lower quality of the parts due to excessive solvent exposure[
56].
Curing was done with the help of the FormCure curing chamber (60 minutes at 60 ºC).This cure setting ensures that it achieves both bio-compatibility and optimum mechanical properties
Figure 8 [
57].
5. Testing
We build testing equipment to test the distalizers resistance to force. The first tool was a jaw that could open and close, this each time 1200 cycles and this repeated as much as needed. Software was written for an arduino uno with a LCD screen and keypad, the moving force was produced through a servo motor. The whole tool was 3D printed with a Ultimaker 3 and PETG filament. The upper jaw in the tool had places prepared to glue human teeth into. Two human teeth were bonded to the tool, and the distalizer attached to them.
Figure 9.
The second testing tool was built using an Arduino Mega and a touchscreen. The software used to drive this was written by ourselves. A stepper motor and stepper motor driver board were being attached. A worm drive was connected to the stepper motor, moving a sled on which the distalizer was attached. A load-cell of 10kg was used to measure the maximum load applied using 1/4’ 6oz. and 1/4’ 8oz. elastics
Figure 10. The tool was 3D printed with a Ultimaker 3 with peg filament. No distalizers were damaged using this testing process.
6. Clinical Application
The 3D printed distalizers were prepared, sandblasting the canine and molar bonding pads. Then Transbond™ XT (3M Unitek)primer was applied to the pads and light cured [
58]. Transbond™ XT (3M Unitek) light cure paste was used to bond the distalizer to the teeth, after the teeth were cleaned, etched and Transbond™ XT (3M Unitek) primer applied [
59]. Before light curing excessive remnants were being removed. The activation of the molar distalizers was done with class II elastics, during testing from beginning until the end with 1/4 inch (6.35mm), 6 oz (170 g) elastics were connected from the canine pad to a button on teeth 46 or 47 and 36 or 37 [
60,
61,
62,
63].
7. Measurements The 3D scans were made with the 3Shape TRIOS® 4 and measurements were done within the software of Medit-Link with the Medit design app (©MEDIT corp. 8, Yangpyeong-ro 25-gil, Yeongdeungpo-gu, Seoul, Republic of Korea) [
64,
65]. The measurements were done independent by 2 researchers, each two times with 7 days between both measurements. The distalisation was measured on an overlay of the STL-file at the beginning and at the end, where a fixed point on the canines and molars in both STL’s was used. A line connecting respectively canines and molars was drawn on which the same point on canines and molars in the overlay STL were measured in distance perpendicular to the lines in the first STL.
The derotation was measured by comparison of the angulation of a line between distinct point on the vestibular side and a distinct point on palatal side, wereas each researcher decided independently [
35,
46,
49].
3. Results
The objective was to propose a method of designing a molar distalizer library, and forthcoming manufacture the molar distalizers in-office as described in the digital-design part.
The next objective was the evaluation of failure, both breakage and debonding and thus evaluating the relative strength in vivo.
Table 1 shows the overview of 16 patients treated with these 3d printed molar distalizers. Assuming a moderate effect (d = 0.7) and a alpha error of 0.05, the sample size calculation using the G-Power analysis yields a total sample size of n = 13. In this table we list the age of participating patients, the time the distalizers have been worn, the size of distalizers used and the failure rate.
We add
Table 2 for descriptive purpose. We see an age span between 11-49 years of age, a range of distalizers between 18 and 27mm. The short distalizers are those bonded from premolar to molar, the longer ones (
) bonded from canine to molar. The time needed to achieve a molar Class I was on average 14.6 weeks, with a total failure rate, left and right distalizer per patient combined, of 94%. On average, we see a breakage of one distalizer, left or right, per patient. The failures were breakage of the arm at the canine pad.
The results are shown using overlay of the STL files before and after treatment [
1,
35].
The overlay results of the 3D scans show the distalisation and derotation
Figure 11.
A high correlation was found between the derotation values of the left upper molar and the right upper molar (
= 0.713;
p = 0.002) (
Figure 13C). There is no correlation between the right upper molar distalization and the left upper molar distalization values (
= 0.214;
p = 0.426) (
Figure 13B). There was a high correlation found between the right upper canine distalization values and the right upper molar distalization values (
= 0.789;
p = <0.001) (
Figure 12C), even though no correlation could be found between distalization values of the left upper canine and the left upper molar (
= 0.417;
p = 0.108) (
Figure 12D). No correlation could be found between the displacement value of the upper left molar and it’s derotation angle (
= 0.139;
p = 0.608) (
Figure 12B), and a high correlation could be found between the displacement of the upper right molar and it’s derotation angle (
= 0.765;
p = <0.001) (
Figure 12A). There is a high correlation between the right upper canine distalization and the left upper canine distalization values (
= 0.630;
p = 0.009) (
Figure 13A).
Intraclass correlation coefficients (ICC) showed good interindividual and intraindividual agreement (interindividual: mean ICC: 0.95, range: 0.93 - 0.98; intraindividual: mean ICC: 0.99, range: 0.990-0.993)
Table 4.
Descriptive statistics of the distal tooth displacement of upper canines (mm), upper first molars (mm), and the derotation angle of the upper first molars (°).
Table 4.
Descriptive statistics of the distal tooth displacement of upper canines (mm), upper first molars (mm), and the derotation angle of the upper first molars (°).
|
n |
Mean |
SD 1
|
Minimum |
Maximum |
Upper canine displacement |
32 |
3.157 |
1.453 |
0.88 |
7.06 |
Upper molar displacement |
32 |
2.465 |
1.336 |
0.82 |
7.06 |
Upper molar derotation angle |
32 |
6.098 |
4.250 |
0.58 |
16.08 |
Author Contributions
Conceptualization, S.B. ; methodology, S.B.; software, S.B. and S.T.; validation, S.B.and S.T.; formal analysis, S.B. and S.T..; investigation, S.B.; resources, S.B.; data curation, S.B. and S.T.; writing—original draft preparation, S.B..; writing—review and editing, S.B.; visualization, S.B. and S.T.; supervision, S.B.; project administration, S.B.. Both authors have read and agreed to the published version of the manuscript.”
Figure 1.
First a cylinder is swept over a preformed spline to form the arm, the cylinder is then cut away. Lastly a flattened sphere is formed as part of the hinge.
Figure 1.
First a cylinder is swept over a preformed spline to form the arm, the cylinder is then cut away. Lastly a flattened sphere is formed as part of the hinge.
Figure 2.
A hemi-sphere is added to form the canine or premolar attachment
Figure 2.
A hemi-sphere is added to form the canine or premolar attachment
Figure 3.
(A). Another hemisphere is being added and the elastic attachment is being formed, a cut-out is made with a round edged box. (B). The upper edge of the thus resulting hook is rounded off.
Figure 3.
(A). Another hemisphere is being added and the elastic attachment is being formed, a cut-out is made with a round edged box. (B). The upper edge of the thus resulting hook is rounded off.
Figure 4.
Flattening the side of the hemisphere and preparing the cutout.
Figure 4.
Flattening the side of the hemisphere and preparing the cutout.
Figure 5.
(A). With the arm 1.1 time enlarged prepare the cutout in the molar pad. (B). Make a cutout to allow the rotation of the arm in the molar pad.
Figure 5.
(A). With the arm 1.1 time enlarged prepare the cutout in the molar pad. (B). Make a cutout to allow the rotation of the arm in the molar pad.
Figure 6.
(A). The finished molar pad. (B). The finished distalizer.
Figure 6.
(A). The finished molar pad. (B). The finished distalizer.
Figure 7.
The distalizers on the print plate; the distalizers in the washer; the distalizers after post cure.
Figure 7.
The distalizers on the print plate; the distalizers in the washer; the distalizers after post cure.
Figure 8.
Motions in sizes 16-29mm, after curing.
Figure 8.
Motions in sizes 16-29mm, after curing.
Figure 9.
3D printed cyclic distalizer tester.
Figure 9.
3D printed cyclic distalizer tester.
Figure 10.
3D printed strain tester.
Figure 10.
3D printed strain tester.
Figure 11.
The overlays of the 16 patients.
Figure 11.
The overlays of the 16 patients.
Figure 12.
(A) Fit plot of right upper molar derotation and distalization; (B) Fit plot of left upper molar derotation and distalization; (C) Fit plot of right upper canine and molar distalization; (D) Fit plot of left upper canine and molar distalization
Figure 12.
(A) Fit plot of right upper molar derotation and distalization; (B) Fit plot of left upper molar derotation and distalization; (C) Fit plot of right upper canine and molar distalization; (D) Fit plot of left upper canine and molar distalization
Figure 13.
(A) Fit plot of the upper canine distalization; (B) Fit plot of the upper molar distalization; (C) Fit plot of the upper molar derotation
Figure 13.
(A) Fit plot of the upper canine distalization; (B) Fit plot of the upper molar distalization; (C) Fit plot of the upper molar derotation
Table 1.
Results of clinical evaluation.
Table 1.
Results of clinical evaluation.
ID |
Age in Decimals |
Time Worn |
Size Right |
Size Left |
Failure |
Failure |
# |
[years] |
[weeks] |
[mm] |
[mm] |
# |
# |
1 |
14.95 |
16.71 |
25 |
25 |
0 |
0 |
2 |
14.95 |
16.71 |
25 |
25 |
1 |
0 |
3 |
12.62 |
11.57 |
25 |
25 |
0 |
0 |
4 |
15.41 |
17.71 |
26 |
27 |
1 |
0 |
5 |
11.74 |
17.57 |
26 |
26 |
0 |
1 |
6 |
14.28 |
9.14 |
24 |
25 |
0 |
0 |
7 |
17.91 |
13.57 |
24 |
24 |
0 |
0 |
8 |
49.34 |
14.00 |
26 |
26 |
1 |
1 |
9 |
12.94 |
11.29 |
19 |
19 |
1 |
1 |
10 |
14.92 |
17.14 |
27 |
27 |
1 |
1 |
11 |
14.11 |
10.00 |
26 |
26 |
0 |
1 |
12 |
11.89 |
24.86 |
26 |
26 |
1 |
0 |
13 |
15.41 |
10.29 |
25 |
25 |
1 |
1 |
14 |
14.06 |
19.57 |
24 |
25 |
0 |
0 |
15 |
13.07 |
9.86 |
18 |
18 |
0 |
0 |
16 |
15.34 |
13.29 |
24 |
24 |
0 |
1 |
Table 2.
Descriptive analysis.
Table 2.
Descriptive analysis.
Variable |
Statistics or Category |
Values |
]2*Age [years] |
mean ± SD 1
|
16.43 ± 8.91 |
|
median (range) |
14.60 (11.74 - 49.34) |
Size [mm] |
median (range) |
25.00 (18.00 - 27.00) |
]2*Time worn [weeks] |
mean ± SD |
14.58 ± 4.31 |
|
median (range) |
13.79 (9.14 - 24.86) |
Total failures 2
|
mean |
0.94 |
Table 3.
Descriptive statistics of the distal tooth displacement, left and right, of the upper canines (mm), upper first molars (mm), and the derotation angle of the upper first molars (°).
Table 3.
Descriptive statistics of the distal tooth displacement, left and right, of the upper canines (mm), upper first molars (mm), and the derotation angle of the upper first molars (°).
|
Mean |
SD 1
|
Minimum |
Maximum |
Left upper canine displacement |
3.054 |
1.476 |
0.88 |
7.06 |
Right upper canine displacement |
3.620 |
1.471 |
1.24 |
6.63 |
Left upper molar displacement |
2.597 |
1.565 |
0.82 |
7.06 |
Right upper molar displacement |
2.333 |
1.097 |
0.83 |
4.21 |
Left upper molar derotation angle |
5.711 |
4.488 |
0.58 |
15.98 |
Right upper molar derotation angle |
6.484 |
4.108 |
0.63 |
16.08 |