Introduction
Interproximal reduction (IPR) removes enamel and leaves grooves and furrows on the tooth surface, which may increase the risk of caries. The aims of this study were to assess the nanotopography of enamel surfaces produced by the most commonly used IPR instruments and to evaluate the effect of polishing after IPR.
Methods
Enamel slabs were cut from the interproximal surfaces of healthy premolars and then treated with diamond burs, strips, or discs, or Sof-Lex polishing discs (3M ESPE, St Paul, Minn). All samples were cleaned by sonication in distilled water. The control group had no IPR performed and was subjected only to cleaning by sonication. The enamel surfaces were assessed using atomic force microscopy.
Results
The IPR instruments all produced surfaces rougher than the control sample; however, the samples that received polishing with Sof-Lex discs after enamel reduction were smoother than untreated enamel ( P <0.05 for all comparisons). The larger grit medium diamond burs and medium strips generated rougher enamel surfaces than their smaller grit counterparts: fine diamond burs and fine strips ( P <0.001). The difference in roughness generated by mesh and curved disks was not statistically significant ( P = 0.122), nor was the difference caused by fine strips and mesh discs ( P = 0.811) or by fine strips and curved discs ( P = 0.076) (surface roughness values for medium bur, 702 ± 134 nm; medium strip, 501 ± 115 nm; mesh disc, 307 ± 107 nm; fine bur, 407 ± 95 nm; fine strip, 318 ± 50 nm; curved disc, 224 ± 65 nm). The smoothest surfaces were created by use of the entire series of Sof-Lex polishing discs after the enamel reduction (surface roughness, 37 ± 14 nm), and these surfaces were significantly smoother than the control surfaces (surface roughness, 149 ± 39 nm; P = 0.017).
Conclusions
Different IPR instruments produced enamel surfaces with varied nanotopography and different degrees of roughness. Enamel surfaces treated with diamond-coated burs were the roughest, followed by diamond-coated strips and diamond coated discs. Polishing with Sof-Lex polishing discs after IPR reduced the enamel surface roughness, and this surface was even smoother than untreated enamel.
Highlights
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Enamel nanotopography after interproximal reduction (IPR) was evaluated by atomic force microscopy.
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IPR instruments produced surfaces with different nanotopography and roughness.
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Medium diamond burs generated the roughest enamel surfaces.
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Sof-Lex disc polishing after IPR made surfaces even smoother than untreated enamel.
Interproximal reduction (IPR), also known as enamel reduction, interdental stripping, air rotor stripping, slenderizing, or reproximation, involves removal of enamel from the mesial or distal surfaces of the teeth. It is commonly used to create space or to correct tooth size discrepancies during orthodontic treatments with fixed and removable appliances and may be used in both the anterior or posterior regions of the mouth. A recent study reported that most orthodontists (66%) routinely performed IPR in their practices. By reducing the width of enamel at the interproximal surfaces, IPR may be effective in improving dental alignment and for enhancing postorthodontic stability, particularly in the mandibular anterior region. In addition, IPR can reshape and improve anterior dental esthetics, for example by removing the black triangles that may become evident after alignment of crowded segments.
IPR, however, inevitably alters the tooth enamel, changing the enamel surface morphology and contour. Numerous qualitative studies have shown that removal of this outer enamel leaves many grooves and furrows on the surfaces of the teeth. Using scanning electron microscopy (SEM), grooved and roughened enamel surfaces have been observed on the interproximal enamel of both deciduous and permanent teeth. These grooves and furrows form “hills and valleys,” regularly or irregularly distributed, over the entire treated area.
The SEM studies, however, provide only a subjective measure of surface roughness. There are only a few quantitative studies of enamel after IPR, and they have mainly measured surface roughness ( R a ). It has been found that IPR increased the surface roughness, regardless of the instruments used. This roughness may increase the susceptibility of stripped enamel to bacterial adhesion and biofilm formation, which is then shielded from the mechanical clearance of salivary flow, brushing, or flossing, and thereby may promote demineralization and the buildup of plaque and calculus. Numerous studies have established that various dental materials with rough surfaces promote bacterial adhesion: eg, composite resin, porcelain, cobalt-chromium alloy, and dental implants. However, other studies have found that IPR did not lead to an increased caries risk. Whether IPR actually increases the susceptibility of the stripped enamel to caries is still a matter of debate. This may be because roughness is only 1 parameter of surface topography (detailed surface features) that influences bacterial adhesion, or it may be because the changes in the enamel surface are not significant enough to progress to a clinical event. Other topographic features of enamel surface after IPR are still poorly understood. A comprehensive investigation of surface shape and features of enamel after IPR is essential to understand the relationship between IPR and bacterial adhesion.
The aims of this study were to investigate the nanotopography of enamel surfaces produced by the most commonly used IPR instruments and to evaluate the effect on surfaces of polishing after IPR.
Material and methods
Sixty-four premolars, removed from patients at the University of Otago School of Dentistry in Dunedin, New Zealand, for orthodontic purposes, were collected using the following exclusion criteria: staining, demineralization, decay, fluorosis, enamel cracks, defects, or restorations. Ethical approval for the study was obtained from the University of Otago Ethics Committee (reference number 13/105).
The extracted teeth were immediately cleaned and disinfected using 70% ethanol and stored at 4°C in sterile distilled water for less than 1 week, as described previously, before the experiments. Enamel blocks measuring 3.5 mm (height) × 3.5 mm (width) × 2 mm (depth) were cut from the interproximal surfaces of the teeth. The 2-mm depth was measured from the highest point of the outer enamel toward the dentin. The blocks were cut using a straight, cylindrical, coarse diamond bur (FG 842 012; Hager & Meisinger, Neuss, Germany) with special care taken not to damage the outer enamel in any way, and then randomly allocated to 1 of 7 IPR instrument groups or the control group (n = 8 per group).
For the enamel surface preparation, the 7 IPR instruments that are most commonly used in orthodontic clinics were used in the study ( Table I ), including diamond burs, diamond strips, diamond discs, and Sof-Lex polishing discs (3M ESPE, St Paul, Minn). There was also a control group that was not subjected to any IPR procedures.
| IPR instrument | Model | Manufacturer | Grit | Hand piece |
|---|---|---|---|---|
| Burs | ||||
| Medium | Safe-tipped medium diamond | Dentsply, York, Pa | 100-120 μm | High speed (400,000 rpm) with water cooling |
| Fine | Safe-tipped fine diamond | Dentsply, York, Pa | 50 μm | High speed (400,000 rpm) with water cooling |
| Strips | ||||
| Medium | SS-Med interproximal strip-W | Dentsply, York, Pa | 100-120 μm | N/A |
| Fine | SS-Fine interproximal strip-W | Dentsply, York, Pa | 50 μm | N/A |
| Discs | ||||
| Mesh disc | Flexview mesh disc | Dentsply, York, Pa | 100-120 μm | Slow speed (5000 rpm) |
| Curved disc | Flexview curved disc | Dentsply, York, Pa | 50 μm | Slow speed (5000 rpm) |
| Polishing | ||||
| Sof-Lex series | Sof-Lex system kit | 3M ESPE, St Paul, Minn | Variable | Slow speed (5000 rpm) |
| None | ||||
| Control | N/A | N/A | N/A | N/A |
A total of 64 enamel slabs were used in the experiments (n = 8 per group, including the control group). All enamel stripping was carried out according to the manufacturers’ instructions for each instrument and performed by 1 investigator (L.M.). For all groups, the sample was held along the axial walls in mosquito forceps while the IPR instrument was used on the outer enamel surface. For the burs, the hand pieces were run at 400,000 rpm with water cooling; for the discs, the hand pieces were run at 5000 rpm. For the strips, the sample was held in the mosquito forceps and pushed back and forth along the strip horizontally.
Each IPR instrument (ie, bur, strip, and disc) was used for 1 enamel sample only and then replaced. To ensure equal reduction of all teeth, an enamel reduction of 0.2 mm, measured by vernier calipers, was performed on each enamel surface. For the polishing group, the coarse Sof-Lex disc was used until enamel reduction of 0.2 mm had been achieved and then the medium, fine, and extra fine Sof-Lex discs were used sequentially for 20 seconds each to polish the reduced surface (ie, 1 minute of polishing in total).
After completion of IPR, the samples were placed individually in 100 mL of distilled water and cleaned by sonication (Elmasonic S-30; Elma Schmidbauer, Singen, Germany) for 2 minutes. The enamel samples in the control group were only cleaned with sonication for 2 minutes without any IPR performed.
The surface nanotopography of the prepared enamel samples was assessed using atomic force microscopy (AFM) (Nanosurf Naio, Liestal, Switzerland), in contact mode with an ACLA probe (Applied NanoStructures, Mountain View, Calif) at 190 kHz. All enamel slabs from each group were assessed and imaged at 3 randomly selected areas (50 × 50 μm). Surface plots were made to obtain a 3-dimensional perspective of the surface from which the average surface roughness values for peak height, valley depth, and peak-valley height were calculated for that area ( Table II ). Each of the 3 areas contributed to an overall average calculation to give an overall surface roughness, peak height, valley depth, and peak-valley height value for that specific enamel sample. A line along the y-axis of each 50 × 50 μm section was randomly selected, and measurements were plotted to produce a 2-dimensional profile (graph) of the surface through that section.
| Parameter | Description |
|---|---|
| Roughness (Ra) | Average distance from the roughness profile |
| Root mean square | Quadratic mean, the square root of the mean of the squares of the samples |
| Peak height | Maximum z-value where z is a function of x and y coordinates |
| Valley depth | Minimum z-value where z is a function of x and y coordinates |
| Peak-valley height | Difference between peak height and valley depth |
∗ All measurements were relative to the center plane of the profile.
Statistical analysis
Statistical analysis was performed using SPSS software for Mac (version 19.0; IBM, Armonk, NY). The data are presented as means ± standard deviations and compared using 1-way analysis of variance. The threshold for type I error was set at 0.05. The Bonferroni correction was used for multiple testing.
Results
For the qualitative analysis of enamel surfaces after IPR, enamel surfaces became progressively smoother when IPR instruments were changed from burs to strips to discs to polishers ( Figs 1 and 2 ). The surface prepared with the larger-grit diamond bur had the highest and sharpest peaks and troughs. The surfaces prepared with the large-grit diamond strips also had sharp peaks and troughs, although the peaks and troughs were smaller. Enamel surfaces prepared with the mesh disc had even smaller peaks and troughs and also had linear scratches evident across the sample. No enamel rod-type structures could be identified in any of the enamel samples. The enamel samples polished by the Sof-Lex series after IPR had a relatively smooth surface with some small linear scratches evident, but the surfaces appeared smoother than the untreated (control) samples, which had small peaks and troughs across them.
For the quantitative analysis of enamel surfaces after IPR, the different IPR instruments produced different degrees of enamel surface roughness ( Figs 1-3 ). Overall, the diamond burs produced the roughest surfaces, followed by diamond strips and discs compared with the control enamel ( P <0.001) ( Table III ). Use of the Sof-Lex polishing series after IPR created the smoothest surfaces, which were even smoother than the untreated control samples ( P = 0.017). The larger-grit medium diamond burs and medium strips produced rougher surfaces than their smaller-grit counterparts—fine diamond burs and fine strips; both, P <0.001. There was no statistically significant difference in roughness generated by mesh and curved disks, P = 0.122, nor between roughness caused by fine strips and mesh disks ( P = 0.811) or between fine strips and curved discs ( P = 0.076) ( Table IV ).
| IPR instrument | Surface roughness (mean ± SD, nm) | P value (relative to control) |
|---|---|---|
| Medium bur | 702 ± 134 | <0.001 |
| Medium strip | 501 ± 115 | <0.001 |
| Fine bur | 407 ± 95 | <0.001 |
| Fine strip | 318 ± 50 | <0.001 |
| Mesh disc | 307 ± 107 | <0.001 |
| Curved disc | 224 ± 65 | 0.036 |
| None (control) | 149 ± 39 | – |
| Sof-Lex polishing | 37 ± 14 | 0.017 |
| Instrument | Burs | Strips | Discs | None | |||
|---|---|---|---|---|---|---|---|
| Medium | Fine | Medium | Fine | Mesh | Curved | Control | |
| Burs | |||||||
| Medium | – | – | – | – | – | – | – |
| Fine | <0.001 | – | – | – | – | – | – |
| Strips | |||||||
| Medium | <0.001 | 0.033 | – | – | – | – | – |
| Fine | <0.001 | 0.045 | <0.001 | – | – | – | – |
| Discs | |||||||
| Mesh | <0.001 | 0.026 | <0.001 | 0.811 ∗ | – | – | – |
| Curved | <0.001 | <0.001 | <0.001 | 0.076 ∗ | 0.122 ∗ | – | – |
| None | |||||||
| Control | <0.001 | <0.001 | <0.001 | <0.001 | <0.001 | 0.036 | – |
| Polishing | |||||||
| Sof-Lex series | <0.001 | <0.001 | <0.001 | <0.001 | <0.001 | <0.001 | 0.017 |
The roughest surface was created by the medium diamond bur ( R a , 702 ± 134 nm) and was significantly ( P <0.001) rougher than the surface treated with the medium diamond strip ( R a , 501 ± 115 nm) and the mesh disc ( R a , 307 ± 107 nm) ( Table III ). In the group of smaller grit diamond instruments, the roughest surface was created by the fine diamond bur ( R a , 407 ± 95 nm), followed by the fine diamond strip ( R a , 318 ± 50 nm), and curved disc ( R a , 224 ± 65 nm) ( Table III ) ( P <0.05 for all comparisons; Table IV ). Differences between all groups were statistically significant, except for the differences between the surfaces prepared with discs when compared with each other and with the surfaces prepared with the fine diamond strip ( Table IV ).
The highest peak, deepest valley, and largest peak-valley height were recorded on the samples prepared by the diamond burs ( Tables V and VI ; Fig 2 ), followed by those prepared with the strips and discs. The lowest readings were recorded on the samples polished after IPR with the Sof-Lex polishing series, which had values even lower than the untreated control samples.
