In this in-vitro study, we aimed to investigate the predictability of the expected amount of stripping using 3 common stripping devices on premolars.
One hundred eighty extracted premolars were mounted and aligned in silicone. Tooth mobility was tested with Periotest (Medizintechnik Gulden, Modautal, Germany) (8.3 ± 2.8 units). The selected methods for interproximal enamel reduction were hand-pulled strips (Horico, Hapf Ringleb & Company, Berlin, Germany), oscillating segmental disks (O-drive-OD 30; KaVo Dental, Biberach, Germany), and motor-driven abrasive strips (Orthofile; SDC Switzerland, Lugano-Grancia, Switzerland). With each device, the operator intended to strip 0.1, 0.2, 0.3, or 0.4 mm on the mesial side of 15 teeth. The teeth were scanned before and after stripping with a 3-dimensional laser scanner. Superposition and measurement of stripped enamel on the most mesial point of the tooth were conducted with Viewbox software (dHal Software, Kifissia, Greece). The Wilcoxon signed rank test and the Kruskal-Wallis test were applied; statistical significance was set at alpha ≤0.05.
Large variations between the intended and the actual amounts of stripped enamel, and between stripping procedures, were observed. Significant differences were found at 0.1 mm of intended stripping ( P ≤0.05) for the hand-pulled method and at 0.4 mm of intended stripping ( P ≤0.001 to P = 0.05) for all methods. For all scenarios of enamel reduction, the actual amount of stripping was less than the predetermined and expected amount of stripping. The Kruskal-Wallis analysis showed no significant differences between the 3 methods.
There were variations in the stripped amounts of enamel, and the stripping technique did not appear to be a significant predictor of the actual amount of enamel reduction. In most cases, actual stripping was less than the intended amount of enamel reduction.
Grinding interproximal tooth surfaces to reduce tooth size is a common procedure in orthodontics. The indications for interproximal enamel reduction are lack of space, Bolton tooth-size discrepancy, correction of morphologic anomalies, tooth reshaping, and reduction of interdental gingival papilla retraction.
Interproximal enamel reduction is also known as interdental stripping, enamel approximation, or slenderizing. Several procedures are used in daily orthodontics to perform precise interdental stripping as part of the treatment plan. Reduction of enamel can be achieved with hand-held or motor-driven abrasive strips but also with disks or burs mounted on a hand piece.
There are different guidelines regarding the optimal amount of enamel reduction. Fillion recommended reduction maximums of 0.3 mm for maxillary incisors, 0.2 mm for mandibular incisors, and 0.6 mm for premolars and molars. Sheridan and Ledoux postulated that a gain of 0.4 mm of space by enamel reduction per proximal surface of premolars and molars is possible, and Stroud et al claimed that up to 0.6 mm of enamel reduction is attainable. As a rule of thumb, various authors consider a reduction of the original enamel by 50% to be acceptable.
Long-term results of interproximal enamel reduction showed no iatrogenic damage—eg, dental caries, gingival problems, or increased alveolar bone loss. Profilometry and scanning electron microscopy were used in studies to prove that a treated enamel surface after reduction and polishing can be similar to or smoother than untreated enamel. Zhong et al showed that it is possible to reduce interproximal enamel in a reasonable time (2.2 minutes per surface) with a good or very good outcome in 90% of patients, and with enamel surfaces after stripping smoother than untreated enamel.
If the treatment plan calls for stripping, it is important to be able to reduce the enamel by the exact amount required.
Although many studies are focusing on the surface irregularities that could remain after grinding and polishing, only 1 study was identified that presents a quantitative evaluation of stripped enamel. Therefore, the aim of this study was to investigate in vitro on premolars the actual stripping and the intended stripping of 3 commonly used stripping devices. The null hypothesis was that there is no difference between the intended and the actual amounts of enamel reduction. Additionally, the differences between the 3 stripping methods regarding stripping predictability were assessed.
Material and methods
One hundred eighty teeth were randomly divided into 3 groups according to 3 commonly used stripping methods. In each of the 3 stripping method groups, enamel reductions were set at 0.1, 0.2, 0.3, or 0.4 mm on 1 side; therefore, 15 premolars were allocated for each stripping level subgroup. Ethical approval by the University of Geneva (number 09-129) was obtained to collect extracted premolars from patients who had extraction therapy at the Department of Orthodontics. The patients were given an information sheet describing the study and asked to sign a consent form.
The premolars were stored in 3% thymol solution. They were then aligned and mounted in silicone (Curadent Protesil; Zeta Dental, Riazzino, Switzerland) to simulate the mobility of the natural periodontium. To prevent loosening of the teeth, they were fixed in the silicone base with super glue at the apex of the root. The mobility of the teeth was verified on 30 teeth with Periotest (Medizintechnik Gulden, Modautal, Germany) to make sure that the situation in the mouth was simulated as close as possible. The achieved values (8.3 ± 2.8 units) were within the range of standard values published by Schulte (−2 to +11 for premolars).
Three methods of interproximal enamel reduction were selected for comparison: group A was treated with traditional hand-pulled strips (45-μm grits, 0.09-mm thickness; Horico, Hopf Ringleb & Company, Berlin, Germany). Group B had oscillating segmented disks (30- to 40-μm grits, 0.13-mm thickness, O-drive OD 30; KaVo Dental, Biberach, Germany; OS 1 FH disk Komet; Gebr. Brasseler GmbH & Company KG, Lemgo, Germany). In group C, the enamel was reduced by motor-driven abrasive strips (40-μm grits, 0.3-mm thickness, Orthofile; SDC Switzerland, Lugano-Grancia, Switzerland).
Danesh et al analyzed the amount of reduced enamel by polishing after interproximal reduction; it was shown to be between 0 and 0.02 mm. Since this is a relatively small amount, which does not seem clinically significant, we focused only on grinding and left out the polishing.
Before and after stripping, the teeth were scanned by a 3-dimensional (3D) laser scanner (R-250; 3Shape, Copenhagen, Denmark) after they were sprayed with a minimal layer of Pico Scan Spray (Picodent, Wipperfürth, Germany) to prevent reflections in the scan process. The final STL files (Surface Tesselation Language) were imported into Viewbox software (dHAL Software, Kifissia, Greece), where the 3D objects were superimposed. Then a plane was defined at the long axis of the tooth, and the software measured the distance from the sagittal plane to the farthest points mesially and distally on the tooth.
A digital caliper (Schieblehre digital 59112; Fino, Bad Bocklet, Germany) was used to verify the 3D measuring procedure by measuring the mesiodistal premolar widths.
During the stripping process, the amount of enamel reduction was controlled with a gauge, as proposed by several authors.
All measurements and superimpositions were performed by the same operator (A.M.J.). In order to test the 3D measuring procedure, 30 tooth widths were measured by digital caliper (mean, 7.50 ± 0.36 mm) and by the 3D software (mean, 7.49 ± 0.35 mm). No significant difference was found between these 2 measuring procedures. A high correlation was found between these methods (R = 0.99, P ≤0.001), in accordance with the study of Alcan and Ceylanoğlu.
Three-dimensional superimpositions and measurements were repeated for 30 teeth after 10 days. A high coefficient of reliability was found (R = 0.99, P ≤0.001). The random error of the method was calculated with Dahlberg’s formula (Se = 0.0183 mm).