Roughness Analysis on Composite Materials (Microfilled, Nanofilled and Silorane) After Different Finishing and Polishing Procedures

Materials and Finishing/Polishing Procedures Tested in the Study

The roughness parameters have been investigated for the following materials:

• ESTHETX HD: Photopolymerizable micro-hybrid composite, radiopaque, utilized in the restoration of the anterior and posterior zones.
• CERAMX MONO: Photopolymerizable nano-hybrid composite, used in direct and indirect restorations of anterior and posterior zones, for splinting and tooth stub restoration.
• FILTEK SILORANE: Photopolymerizable silorane composite, used in the restoration of the posterior zone.

A large variety of finishing and polishing tools are currently available on the market. Some of these have been investigated in the present study.

• Tungsten Carbide Multi-blade Milling Cutters: These devices are classified according to the number of blades (from 8 to 30). The cutting tool includes an entry and exit angle and to correctly work, must advance in the counter-rotation sense;
• Diamond Milling Cutters: These cutters are produced in different shapes and dimensions and with different types of grain (the dimensions of which range in the interval 8 - 50 μm). They are usually utilized in sequence; the cutter with the higher granulometry (50 - 30 μm) is used to rough-hew and mold the restoration. The final finishing is then performed with a lower granulometry (8 - 15 μm) cutter. The shape of the cutter changes according to the part to finish; flame-shaped milling cutters are usually utilized for vestibular zones, olive-shaped milling cutters for the palatal ones.
• Abrasive Wheels: These are flexible, decreasing granulometry wheels for finishing and polishing, with different gauges and therefore, different rigidity. Each granulometry is associated with a different color. They can be used singularly or in combination with diamond or multi-blade milling cutters. The most common discs are made of a paper frame sprinkled with abrasive (oxide alloy) of different grain. The discs have a small eyelet to be click-inserted to the spindle mounted on the hand-piece. The finishing and polishing will be performed in three or four operative steps. The wheels used in the present study were Sof-lex (3M, St. Paul, MN, USA). The system includes wheels with four types of oxide alloy grain, from rough to superfine.
• Silicon Rubber Tips: These are flexible rubber tips composed of a silicon mold to which carbide silicon, oxide alloy or diamond abrasive particles are added. These devices are strictly used for the final finishing and polishing; they are available on the market in different colors, shapes and grain dimensions (15 - 40 μm). They are mounted on the micromotor and used under large amounts of water spray. In addition to the different grain tips used in the multi-step procedure, the one-step silicone rubber tip was recently introduced on the market. Once the finishing is terminated, the operator utilizes the one-step silicone rubber to obtain a mirror polishing.
• Polishing Pastes: These are diamond or oxide alloy pastes with different grains. They have to be dry-applied on the composite one or more times with discs, plastic tips or goat-hair brushes.

The main features of the adopted procedures, the producer and batch number (of the items utilized in the experiments) are listed in Table 1.

Preparation of Samples

For each tested material 52 samples were realized, using thermoplastic resin molds. The molds were built by using the following items/devices:

• Nails: The nail head (with a diameter of 8 mm and a thickness of 2 mm) was used as a model for the molds;
• Extrastrong and Soft Chalk (Elite Dental Stones, Zhermack Spa, Badia Polesine (RO), Italy);
• Thermoplastic Resin Discs with a 125.0 mm diameter and a 3.0 mm thickness (Imprelon, Scheu Dental GmbH, Iserlohn, Germany)
• Infrared Heater: Biostar (Scheu Dental GmbH, Iserlohn, Germany).

The following procedure was adopted.

1. With the impression chalk, a prism was realized in which the nails were inserted (Fig. 1(a));
2. Thermoplastic disks were heated with the Biostar system at the temperature of 220° C for 90 seconds thus losing their initial consistency (Fig. 1(b));
3. The discs were inserted into the model at a pressure of 4.1 atm so as to reproduce the shape of the nail head (Fig. 1(c));
4. After being shaped and once the cooling was completed, the discs were finally removed (Fig. 1(d));
5. The discs were finally cut with a milling cutter in many different squares (Fig. 1(d-e)) the surface of which was then polished (Fig. 1(f)).

The composite materials were hence put in the molds with a putty knife and covered with a Mylar strip and a microscope slide (Fig. 2(a)). The strip was utilized so as to avoid the formation of an oxygen inhibition layer, whereas through the slide a load of about 20 N was applied on the composite surface thus enabling the consolidation and the discharge of material’s excess.

The samples, together with the strip and the slide, were radiated for 40 seconds (according to the Manufacturer’s prescriptions), by a halogen lamp (Degulux soft-star, Degussa-Hülls, Hanau, Germany) placed perpendicularly with respect to the sample surface and working at an intensity greater than 600 mW/cm² (Fig. 2(b)). After the polymerization was completed, the samples were kept for 24 hours at the temperature of 37°C and 100 % of humidity so as to complete the process and then dried.

Finishing and Polishing of the Samples

After examining the samples to check any possible inner voids, the samples were finished and polished by adopting the following procedures:

• Procedure A: (control group): A control group including 4 samples was collected for each of the tested composite materials. All the samples of the three control groups were neither finished nor polished after polymerization.
• Procedure B: 16 samples for each composite were treated with this procedure. The finishing procedure was carried out for thirty seconds with medium Sof-Lex (3M, St. Paul, MN, USA) discs. The discs, according to the Producer’s guidelines, were mounted on the micromotor together with the spindle. The discs were then used with uniform and regular movements from the restoration towards the margin (and not vice-versa) avoiding forward-backward movements on the composite/mold margin which could have resulted in the formation of a white line and hence of a discontinuous region. The whole procedure was operated in the absence of water. At the end of the finishing process, the samples were washed and dried with an airflow. Twelve samples (four for each composite) were not polished (Group B1). The remaining samples (Group B2) were polished as follows:

- 12 samples (4 for each composite) were polished for 30 seconds with fine and superfine Sof-lex discs always inserted to the spindle and mounted on the hand-piece (Group B2I).

- 12 samples (4 for each composite) were polished for 30 seconds with a silicone rubber tip. The rubber tip was used with slightly circular movements at a speed of 6000 rpm, under an adequate refrigeration obtained with a 50 ml/min water flow. A special care was taken in minimizing the pressure exerted with the disc on the polished surface (Group B2II);

- 12 samples (4 for each composite) were polished with the following procedure:

- Superficial degreasing with 95° alcohol;

- Application of Experience seal coat (DEI Italia S.r.l., Mercallo (VA), Italy) on the surface with circular movements with a microbrush gently rubbing it until the solvent evaporated;

- Removal, after 3 minutes, of the surplus with a new microbrush and photopolymerization for 90 seconds with a halogen lamp (Degulux soft-star).

- Dry-application of Experience polish paste (DEI Italia S.r.l.) with soft goat-hair wheel until the complete removal of any residual seal coat fast;

- Dry-polishing with cotton mop and Experience Polish Paste (Group B2III).

Procedure C: 16 samples for each composite were treated with this procedure. The samples were finished with two flame-shaped diamond milling cutters: firstly, a 50 μm (red ring) rough-grain, then a 30 μm (yellow ring) extrafine grain milling cutter was utilized. Each milling cutter operated for thirty seconds. The cutters were mounted on the turbine and used under abundant irrigation. After the finishing, the samples were washed and dried with an airflow. Twelve samples (four for each composite) were not polished (Group C1). The remaining samples (Group C2) were classified and polished according to the scheme utilized for Procedure B and summarized in Table 2. For instance, Group C2I includes 12 samples (4 for each composite) polished for 30 seconds with fine and extrafine-grain Sof-lex discs inserted into the spindle and mounted on the hand-piece. Analogously, Group C2II includes 12 samples polished for 30 seconds with a silicone rubber tip, while Group C2III includes 12 samples submitted to the same treatment as that utilized for samples of Group B2III (Table 2).

Procedure D: 16 samples for each composite were treated with this procedure. The samples were finished with two tungsten carbide multi-blade milling cutters Q series (yellow-blue) and UF series (white) for 30 seconds for each milling cutter. They were mounted on the turbine and used under plentiful irrigation. After the finishing was completed, the samples were washed and dried with airflow. Twelve samples (four samples for each composite) were not polished (Group D1). The remaining samples (Group D2) were polished according to the scheme shown in Table 2.

In order to reduce the variability of the preparation procedures, finishing and polishing were performed by the same operator. At the end of these phases the samples were washed with distilled water and left drying for 24 hours before the roughness analysis.

Roughness Analysis

Any surface, even if worked with great accuracy, when observed with a microscope will reveal roughness due to grooves and crests determining local distancing, more or less extended, of the real surface from the one conceived in the design. Roughness (or absolute roughness) is a property of the object surface, it consists of geometric micro-imperfections either originally included in the material of

which the surface is made or due to the manufacturing process. These imperfections usually take the form of grooves or scratches varying in shape, depth and direction. In order to assess the roughness of a surface, a profilometer is used. The measuring procedure is performed in the recording of the profile of the surface along a specific measuring path (or scanning path). This profile is then analyzed defining a numerical parameter which is the roughness value. A crucial phase of the roughness measurement process is the filtering procedure which allows the assessment of the sole quality of the surface to be separated from the effects that the geometrical errors have on the measured profile. In the present study, after the finishing and polishing were performed, the superficial roughness of the samples was measured with the profilometer Mahr MarSurfGD25 (Mahr Inc., Gottingen, Germany) (Fig. 3). The measuring conditions, according to ISO (International Standards Organization) 4287 Standard were:

• Value of the profilometric resolution: 0.01 μm
• Transverse length (TL): 5.6 mm (n=5)
• Sample length (SL): 2.5 μm
• Vertical band width: ±250 μm
• Scanning Rate Vt: 0.50 mm/s
• Number of acquired points: 11200

The assessed parameter was Ra, i.e. the average arithmetic roughness. It is the most common parameter and represents the average absolute value of the deviations of the detected surface from the “average value” of the profile (technical surface) (ISO 4287). Also in this case, all the roughness measurements were carried out by the same operator.

Statistical Analysis

The statistical analysis was aimed at identifying, by two-way analysis of variance (ANOVA), the factors that affect the quality of the restoration surface. A complete factorial plan was drawn up (Fig. 4), assessing two factors: (i) material; (ii) procedure. The null hypothesis H_o was that factors (i) and (ii) do not affect the roughness values. H_o was assumed to hold true for p-values ≥ 0.05 (95 % confidence interval). For factor (i) three levels were fixed: ESTHETX HD, CERAMX MONO and FILTEK SILORANE; for factor (ii) four levels: procedure A, B, C and D. A total of 156 roughness values were submitted to statistical analysis: 12 values for Procedure A 4 (replications) × 3 (number of tested composite materials); 48 values for Procedure B 4 (replications) × 4 (number of sub-groups, i.e. B1, B2I, B2II, B2III) × 3 (number of tested composite materials); 48 values for Procedure C 4 (replications) × 4 (number of sub-groups, i.e. C1, C2I, C2II, C2III) × 3 (number of tested composite materials); 48 values for Procedure D 4 (replications) × 4 (number of sub-groups, i.e. D1, D2I, D2II, D2III) × 3 (number of tested composite materials).

A non-parametric test (analysis of variance on ranks with Tukey post-hoc test) was utilized to compare the roughness values obtained with the different procedures and materials.