To evaluate the residual surface roughness of 5 common diamond-coated interproximal reduction (IPR) systems after consecutive in vitro applications in relation to system, diamond grain size, and instrument thickness.
Methods
IPR was performed on 80 extracted human incisors using motor-driven strips and discs under predefined conditions. The IPR auxiliaries were applied at 5 consecutive sessions of 20 s on intact interproximal surfaces, and the surface profile (Ra, Rz, Rmax) was analyzed at baseline and after each session with an optical profilometer.
Results
No overall significant difference in the roughness values was found between systems (P = 0.07 for Ra, P = 0.33 for Rz, and P = 0.48 for Rmax). There was a significant average decrease of Ra, Rz, and Rmax for all systems for every unit increase in time by −0.171 μm (P < 0.001), −3.297 (P ≤ 0.001), and −2.788 μm (P = 0.001), respectively. Ra, Rz, and Rmax values increased significantly, i.e., by 0.194 μm (P = 0.003), 5.890 μm (P = 0.001), and 5.319 μm (P = 0.010) as instrument thickness increased by one unit. No significant reductions in Ra, Rz, and Rmax were observed across grain sizes (−0.008 μm [P > 0.05], −0.244 μm [P > 0.05], and −0.179 μm [P > 0.05], respectively). There was no evidence of interaction between system and time as the P values for Ra, Rz, and Rmax were 0.88, 0.51, and 0.70, respectively.
Conclusions
All IPR materials presented significant gradual decrease of surface roughness after repeated applications. There were no significant roughness changes among auxiliaries of different grain sizes. Thinner auxiliaries showed significantly more roughness reduction, possibly requiring more frequent replacement than thick auxiliaries in clinical practice.
Introduction
Space gaining procedures, e.g., tooth extractions, arch expansion, and reshaping of interproximal enamel surfaces (i.e., interproximal reduction [IPR]) are commonly applied in clinical orthodontics. Since the original introduction of IPR [1], several authors [2‐6] have described in detail IPR indications and protocols for handheld or handpiece-mounted enamel cutting instruments. Overall, IPR has been used to address arch length discrepancies, to enhance anterior esthetics and interocclusal relationships, and to improve long-term stability of the treatment outcome [7].
The residual enamel roughness [8‐10], and especially, the increased susceptibility to caries in vitro [11‐13] initially discouraged clinicians from performing IPR in everyday practice. This perception has been drastically changed in recent years with the best available evidence indicating that IPR does not increase the incidence of caries on treated teeth [14]. Moreover, regardless of the stripping method used (i.e., abrasive strips, tungsten carbide burs or oscillating perforated diamond discs), finishing with Sof-Lex polishing discs can yield smoother surfaces than intact enamel [15].
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While most of the research focused on post-IPR enamel effects, very little has been published so far on the wear of IPR materials after multiple uses [16]. Such information may have direct clinical implications since the particle size of the abrasive determines the amount of enamel reduction as well as the necessary time for polishing [17]. Lione et al. [16] demonstrated by means of tribological testing a 60% decrease in the abrasive capacity of motor-driven strips after 5 min of in vitro use, whereas at the same time almost complete detachment of diamond abrasive grains was observed by scanning electron microscope in three patients receiving IPR on mandibular incisors.
Given the growing acceptance of IPR as a minimally invasive procedure by dentists and orthodontists [18], and the widespread use of aligner treatment in combination with IPR [19], it would be interesting from a clinical point of view to investigate the surface changes on contemporary IPR materials over time. Thus, the aims of this study are to assess the roughness changes of 5 popular diamond-coated IPR systems after consecutive in vitro applications in relation to system, diamond grain size, and instrument thickness. The null hypothesis is that there is no difference in the outcome between any of the parameters.
Materials and methods
Eighty extracted human permanent incisors with macroscopically intact interproximal surfaces, free of caries and restorations were collected from the undergraduate clinic of the Department of Preventive, Restorative, and Pediatric Dentistry, Dental School/Medical Faculty, University of Bern, Bern, Switzerland. Before extraction, patients had been informed about the use of the teeth for research purposes and verbal consent had been obtained. After extraction, the teeth were pooled. The local ethics committee categorizes pooled teeth as an “irreversibly anonymized biobank” and thus, no previous ethical approval was needed. The incisors to be used were cleaned under tap water with a scaler to remove debris and then stored in 2% chloramine solution in a refrigerator (4 °C) until needed. The incisors were then mounted in cylindrical stainless steel molds with self-curing acrylic resin (Paladur, Heraeus Kulzer, Hanau, Germany). After curing of the acrylic resin, the stainless steel molds were removed and the embedded incisors were stored in the refrigerator at 100% humidity. For the IPR procedures, the embedded incisors were then randomly allocated to 16 groups of 5 teeth each.
IPR procedures
Five IPR systems, namely 16 instruments, were considered for the purposes of the study: DiaStrip (DentaSonic, Cham, Switzerland), Intensiv Ortho-Strips System (Intensiv SA, Montagnola, Switzerland), G5-ProLign (SDC Switzerland SA, Bioggio, Switzerland), Galaxy IPR Diamond Discs (Ortho Technology®, Lutz, FL, USA), and OS Discs (Komet USA, Rock Hill, SC, USA). The technical characteristics of IPR auxiliaries are summarized in Table 1.
Table 1
Technical details of the interproximal reduction (IPR) instruments tested in the study
Tab. 1
Technische Details der getesteten IPR(Interproximal-Reduktion)-Systeme
System
Manufacturer
Instrument coding
Thickness (mm)
Particle size (μm)
Handpiece
Manufacturer
DentaSonic Diastrip
Alpin Orthodontics, Lucerne, Switzerland
DS-25
0.15
25
DentaSonic water cooling HP
Alpin Orthodontics, Lucerne, Switzerland
DS-40
0.20
40
DS-60
0.30
60
Ortho-Strips System
Intensiv SA, Montagnola, Switzerland
OS-25
0.15
25
Intensiv Swingle Reciprocating Contra Angle (WG-69 A)
W&H, Bűrnmoos, Austria
OS-40
0.20
40
OS-60
0.25
60
SDC-G5-Prostrip
SDC Switzerland SA, Bioggio, Switzerland
SDC-15
0.15
15
Ti-Max X55
NSK-Nakanishi Inc., Kanuma, Japan
SDC-20
0.20
30
SDC-30
0.30
40
Galaxy IPR Diamond Discs
Ortho Technology®, Lutz, FL, USA
OT-11
0.19
64
KaVo GENTLE-power LUX 10LP Straight 1:2
KaVo Dental, Charlotte, NC, USA
OT-13
0.19
64
OT-55
0.20
46
OT-56
0.20
46
OS Segment Discs
Komet USA, Rock Hill, SC, USA
OS-10
0.20
57
Komet OS 31
W&H, Bürnmoos, Austria
OS-20
0.20
25
OS-18
0.18
49
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IPR was carried out by the same operator (first author) according to the manufacturers’ recommendations. For Galaxy IPR Diamond Discs, the straight handpiece was operated at 5000 rpm; for the rest of the IPR systems, the contra-angle handpieces were operated at 40,000 rpm with water cooling. The tested auxiliaries underwent five consecutive IPR sessions on intact proximal surfaces. To reproduce the average clinical treatment time, IPR sessions were set at 20 s [20, 21], reaching 100 s in total use for each auxiliary. After completion of IPR procedures, all systems were cleaned thoroughly with distilled water.
Surface roughness evaluation
The surfaces of IPR auxiliaries were analyzed with an optical profilometer (FRT MicroProf® 100, equipped with a H0 sensor, Fries Research & Technology, Bergisch Gladbach, Germany). Linear traces were recorded at a pixel density of 1000/mm. Due to the different forms of the instruments, different total lengths of the traces were obtained. For the DiaStrip system, the Intensiv Ortho-Strips system, and the G5-ProLign system, the whole abrasive part could be measured. The resulting trace lengths were 13 mm (DiaStrip and Intensiv Ortho-Strips) and 17 mm, respectively (G5-ProLign). The sector-shaped OS Discs were measured at the outer edges of the discs, where trace lengths of 5 mm could be obtained. For the disc shaped Galaxy IPR Diamond Discs, traces were measured radially from the outer edges toward the center. For three of the discs, namely OT–11, OT–13, and OT–55, radial traces through the whole abrasive part could be obtained in that way. The trace lengths were 2.7 mm (OT–11), 2.5 mm (OT–13), and 5.2 mm (OT–55). For the fourth disc, OT–56 containing a perforated surface, not the whole abrasive surface could be measured, as there was no radial linear path through it. We nevertheless obtained 3.7 mm long traces for this disc type. The average surface roughness (Ra; in μm), the maximum roughness depth (Rmax; in μm), and the arithmetic mean height of the surface profile (Rz; in μm) where then determined for all the traces measured with a special software (Mark III, Fries Research & Technology GmbH, Bergisch-Gladbach, Germany). Profilometric measurements were performed at baseline, i.e., before initiating IPR (T0), and after each session, i.e., at 20, 40, 60, 80, and 100 s (T1–T5) by a second examiner (third author), blinded to the experimental groups.
Statistical analysis
Random effects linear regression models were fitted using Ra, Rz, and Rmax as the dependent variables respectively and system, grain size, thickness, and time. Interactions between system and time were also assessed. The level of statistical significance was set at 5%. Statistical analysis was conducted with the Stata Statistical Software (Release 15, StataCorp LLC, College Station, TX, USA).
Results
The surface roughness values (Ra, Rz, Rmax) obtained by the optical profilometer are presented in Table 2. Surface roughness decreased with time across IPR system, thickness, and grain-size groups. No overall significance of system was found using likelihood ratio tests (P = 0.07 for Ra, P = 0.33 for Rz, and P = 0.48 for Rmax).
Table 2
Surface roughness measurements (Ra, Rz, Rmax) at T0–T5 provided by the optical profilometer
Tab. 2
Oberflächenrauigkeit (Ra, Rz, Rmax) bei T0-T5, bestimmt mittels optischer Profilometrie
Surface roughness
T0
T1
T2
T3
T4
T5
IPR instrument
Ra
Rz
Rmax
Ra
Rz
Rmax
Ra
Rz
Rmax
Ra
Rz
Rmax
Ra
Rz
Rmax
Ra
Rz
Rmax
DS-25
1.996
50.811
63.073
1.761
46.8
51.61
1.627
43.895
49.101
1.523
39.546
40.175
1.433
42.672
48.643
1.468
40.757
41.319
DS-40
2.706
77.553
80.898
2.272
60.146
71.697
2.318
53.886
55.318
2.069
54.128
64.354
1.868
49.4
55.217
1.841
52.39
60.024
DS-60
3.579
92.398
96.5
2.256
52.47
55.611
2.265
55.871
63.219
2.275
67.198
76.742
2.001
47.094
49.367
1.815
51.222
63.311
IS-25
2.123
46.556
49.907
1.265
36.031
42.811
1.094
24.967
35.24
1.002
21.14
27.393
0.958
21.296
28.162
0.886
18.024
31.953
IS-40
2.423
52.262
58.385
1.797
44.448
49.165
1.534
36.608
41.795
1.443
32.385
37.767
1.474
35.042
36.412
1.449
36.393
39.497
IS-60
3.639
77.174
77.914
2.82
70.49
75.396
3.115
64.874
72.997
2.866
60.143
57.158
2.821
70.393
84.167
2.638
57.729
64.94
SDC-15
1.469
37.944
41.337
1.179
28.534
37.355
0.939
20.882
28.813
0.866
24.548
37.364
0.798
18.176
35.35
0.964
29.739
39.506
SDC-20
2.046
49.054
46.547
1.351
32.965
41.713
1.26
31.506
36.009
1.14
39.034
47.517
1.089
27.566
30.763
1.083
32.601
43.837
SDC-30
2.962
88.329
102.899
2.394
60.385
60.518
2.106
59.55
60.958
1.859
63.778
68.31
1.774
57.301
67.678
1.74
57.731
74.252
OT-11
3.194
56.316
66.149
2.922
78.925
92.196
1.912
44.553
57.881
1.741
30.466
33.528
1.411
22.911
27.009
1.459
30.462
35.047
OT-13
2.609
37.305
39.579
2.551
40.367
43.287
2.064
36.415
40.861
2.087
42.491
55.831
2.517
88.871
126.988
2.205
36.562
39.991
OT-55
2.579
57.198
61.315
2.45
59.692
65.81
2.693
48.448
57.414
2.295
52.165
55.602
2.304
45.331
49.129
2.295
54.133
55.107
OT-56
3.108
53.304
57.259
3.045
57.001
59.035
2.669
48.473
57.826
2.65
56.407
62.212
2.787
61.525
68.337
2.75
47.704
50.539
OS-10
1.815
28.514
33.391
1.327
16.332
18.311
1.205
16.762
18.659
1.093
14.958
15.885
1.102
20.974
24.574
1.085
16.099
19.309
OS-20
2.994
61.357
64.217
2.548
59.923
55.016
2.538
61.091
69.381
2.284
55.479
60.335
2.162
54.391
72.695
2.105
53.419
61.681
OS-18
2.817
77.06
92.416
1.637
49.912
58.769
1.523
37.215
46.107
1.237
29.93
38.545
1.509
38.25
46.107
1.37
29.133
31.678
DS DiaStrip, IS Intensiv Ortho-Strips System, SDC G5-ProLign, OT Galaxy IPR Diamond Discs, OS OS Discs
There was a significant average decrease of Ra, Rz, and Rmax for all systems for every unit increase in time by −0.171 μm (95% confidence interval [CI]: −0.203, −0.139; P < 0.001), −3.297 (95% CI: −4.493, −2.100; P ≤ 0.001), and −2.788 μm (95% CI: −4.422, −1.154; P = 0.001), respectively (Table 3). Ra, Rz, and Rmax values increased significantly, i.e., by 0.194 μm (95% CI: 0.068, 0.321; P = 0.003), 5.890 μm (95% CI: 2.282, 9.497, P = 0.001), and 5.319 μm (95% CI: 1.258, 9.379; P = 0.010) as instrument thickness increased by one unit (Table 3). There was no significant average reduction of roughness values across grain sizes, viz. −0.008 μm (95% CI: −0.025, 0.008; P > 0.05), −0.244 μm (95% CI: −0.709, 0.221; P > 0.05), −0.179 μm (95% CI: −0.695, 0.337; P > 0.05) (Table 3).
Table 3
Coefficients, associated confidence intervals (95% CIs), and P-values from the random effects linear models for Ra, Rz, Rmax by system, thickness, grain size group, and time
Tab. 3
Koeffizienten, assoziierte Konfidenzintervalle (95%-KIs) und p-Werte der linearen Random-Effekt-Modelle für Ra, Rz, Rmax nach System, Dicke, Korngröße und Anwendungen
Ra
Rz
Rmax
Systema
Coefficient
P-Value
95% CI
Systemb
Coefficient
P-Value
95% CI
Systemc
Coefficient
P-Value
95% CI
DS
0.181
0.505
−0.350, 0.712
DS
11.852
0.144
−2.851, 26.554
DS
12.263
0.141
−4.060, 28.587
IS
0.175
0.516
−0.353, 0.703
IS
5.022
0.500
−9.587, 19.632
IS
5.045
0.542
−11.175, 21.264
SDC
−0.491
0.109
−1.091, 0.109
SDC
−3.543
0.676
−20.153, 13.608
SDC
−0.428
0.964
−18.871, 18.014
OT
0.695
0.009*
0.172, 1.219
OT
11.195
0.130
−3.290, 25.680
OT
11.776
0.151
−4.305, 27.857
OS
Reference
OS
Reference
OS
Reference
Grain size
−0.008
0.323
−0.025, 0.008
Grain size
−0.244
0.304
−0.709, 0.221
Grain size
−0.179
0.496
−0.695, 0.337
Thickness
0.194
0.003*
0.068, 0.321
Thickness
5.890
0.001*
2.282, 9.497
Thickness
5.319
0.010*
1.258, 9.379
Time
−0.171
0.000*
−0.203, −0.139
Time
−3.297
0.000*
−4.493, −2.100
Time
−2.788
0.001*
−4.422, −1.154
DS DiaStrip, IS Intensiv Ortho-Strips System, SDC G5-ProLign, OT Galaxy IPR Diamond Discs, OS OS Discs
aOverall significance of system for Ra, P = 0.07
bOverall significance of system for Rz, P = 0.33
cOverall significance of system for Rmax, P = 0.48
*Value statistically significant
There was no evidence of interaction between system and time as the likelihood ratio tests P values for Ra, Rz, and Rmax were 0.88, 0.51, and 0.70, respectively, and thus the interactions terms were dropped from the model. Roughness reduction by time, was comparable among systems (Fig. 1).
Fig. 1
Roughness changes of interproximal reduction (IPR) instruments in relation to time (a Ra; b Rz; c Rmax). DS DiaStrip, IS Intensiv Ortho-Strips System, SDC G5-ProLign, OT Galaxy IPR Diamond Discs, OS OS Discs
Abb. 1
Veränderung der Oberflächenrauigkeit der IPR(Interproximal-Reduktion)-Systeme in Abhängigkeit der Anzahl Anwendungen (a Ra; b Rz; c Rmax). DS DiaStrip, IS Intensiv Ortho-Strips System, SDC G5-ProLign, OT Galaxy IPR Diamond Discs, OS OS Discs
×
Discussion
As the popularity of IPR is increasing in nonextraction orthodontic treatment with fixed appliances and clear thermoplastic aligners, it is worthwhile to thoroughly explore the mechanical behavior of IPR systems. To the best of our knowledge, this is the first study designed to investigate the surface roughness changes in an extended list of commonly used handpiece-driven IPR instruments.
The lack of overall significance in roughness changes between systems indicates that no system was found superior to others in withstanding abrasive loss. All tested materials exhibited a significant reduction in surface roughness with time, which was comparable for all IPR systems. Given instrument surfaces were cleaned before each profilometric evaluation, it may be expected that in clinical conditions the decrease in roughness might be more rapid since besides detachment of diamond granules, increasing accumulation of tooth material on the instrument surface during the repeated applications might take place [16]. In addition, in daily practice, IPR is performed between adjacent teeth. In case proper contacts and mechanical access are not provided, forcing the stripping auxiliary into tight contact points and application of a heavy load by the clinician, will result in instrument deformation and a more rapid loss of abrasive power [16].
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Thicker IPR auxiliaries showed significantly less abrasive wear compared to auxiliaries with thinner stripping segments. This finding implies that regardless of IPR system, thinner stripping instruments may require more frequent replacement when used in vivo. As other investigators stated, instrument thickness may influence the instrument deflection and achieved enamel reduction. The thicker or the more solid the IPR instrument, the more efficient the distribution of the applied force to the enamel surface [22].
Surface roughness of IPR systems was quantified in the present study by profilometry, a broadly used method for measuring the surface profile of dental materials [23‐25]. Nevertheless, profilometry has been criticized for inducing sample damage and its inability to measure overall surface roughness due to scanning a single line in a preselected area [26, 27]. By using a noncontact optical profilometer, we avoided any potential sample damage. Although the profilometer used would allow measurement of the roughness parameters for whole surfaces, the different kinds of perforations of the auxiliaries made it impossible to measure surfaces in a standardized way for all the auxiliaries. Therefore, we decided to rather measure traces of maximal lengths across the cross-sections of the abrasive parts of the auxiliaries. Furthermore, the optical profilometer provides an extremely high vertical resolution (<10 nm) and a set of roughness values that permits statistical analysis [28].
It is well-accepted that the amount of enamel reduction is influenced by operator- or technique-related aspects such as exerted pressure, hardness, and particle size of the abrasive, IPR duration, and tooth-related aspects such as enamel hardness [17]. As there is no data in the literature about the optimal applied force [22], to ensure standardization of the experimental IPR technique, enamel preparation was carried out by a single clinician within a predefined period, strictly following manufacturers’ instructions for use.
Certain caveats need to be acknowledged when translating our study findings into clinical practice. The sample teeth were mounted in acrylic resin, and therefore, it may be presumed that no physiologic tooth movement during IPR was simulated. Alternative embedment in silicone, like in past studies [22, 29], has been criticized since silicone may fatigue faster that biological tissues. Possible loosening of the teeth in the silicone base could lead to insufficient resistance to the mechanical movement of the auxiliary, and eventually insufficient loading by the clinician during IPR [29]. Furthermore, during or after IPR in vivo, stretching of periodontal fibers might occur consequent to the initial aligning, causing tooth movement and underestimation of the stripping outcome [29]. Unlike clinical conditions, IPR in this in vitro investigation was carried out on individual teeth without the need for opening up the interproximal space. This was chosen deliberately to facilitate access to interproximal areas and direct study of surface roughness changes of IPR instruments after multiple applications.
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Future studies should aim to evaluate the efficiency of powered IPR systems in vivo as well as user friendliness and patient comfort [22]. It would be useful to couple the abrasive wear of IPR auxiliaries with the actual amount of the stripped enamel, and to assess patient perception during IPR procedures with different systems. In this way, valuable recommendations can be made to clinicians about the lifecycle and frequency of replacement of IPR instruments to maximize treatment efficiency and patient comfort.
Conclusions
No system was found superior to others in withstanding abrasive wear. All tested powered stripping materials presented a significant decrease of surface roughness after repeated in vitro use. The grain size of the stripping segment did not have a significant effect on the observed roughness changes. Significantly less abrasive wear was observed in thicker auxiliaries, implying longer potential clinical use compared to thin IPR auxiliaries.
Acknowledgements
This work was supported by Research Grant of the European Orthodontic Society awarded to the first author.
Compliance with ethical guidelines
Conflict of interest
C. Livas, T. Baumann, S. Flury and N. Pandis declare that they have no competing interests.
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Ethical standards
The local ethics committee (Kantonale Ethikkommission, Bern, Switzerland; reference number: Req - 2016-00332) categorizes pooled teeth as an “irreversibly anonymized biobank” and thus, no previous ethical approval was needed.
Open Access This article is distributed under the terms of the Creative Commons Attribution 4.0 International License (http://creativecommons.org/licenses/by/4.0/), which permits unrestricted use, distribution, and reproduction in any medium, provided you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license, and indicate if changes were made.
