Laboratory methods for evaluating the wear of denture teeth and their correlation with clinical results

Abstract

Objective

To correlate different laboratory wear simulation protocols for three denture tooth materials with clinical wear results of the same materials.

Methods

Three denture tooth materials were evaluated for which clinical wear data of posterior denture teeth were available: DCL (double cross-linked PMMA with organic fillers; Ivoclar Vivadent), experimental material EM (double cross-linked PMMA with organic fillers; Ivoclar Vivadent), and NFC (PMMA with inorganic nanofillers, Candulor). The clinical data on the three denture tooth materials (10 subjects for each material) came from clinical studies conducted at three different locations. The investigators sent the impressions to one center where they were analyzed with the same methodology and by the same operator. Four different wear simulation protocols were evaluated in a chewing simulator (Willytec) with integrated thermocycling (5 °C/55 °C) and 8 specimens for each group: (A) molar denture tooth against premolar denture tooth with 1 mm sliding, lifting, 5 kg load; (B) standardized conical ceramic stylus (Ø2.3 mm); (C) standardized ball-shaped ceramic stylus (Ø6 mm); (D) standardized conical stylus (Ø2.3 mm) cut with a special bur from the denture tooth material to be tested. For the protocols B, C and D, the stylus slid under a load of 3 kg for 3 mm on the flat specimen without lifting. All the tests were run for 100,000 chewing cycles. The maximum vertical wear of the material and stylus was quantified on replicas of improved white stone with the etkon es1 scanner and the match 3-D software.

Results

The ranking of the materials according to mean clinical vertical wear after 1 year was as follows: DCL = EM > NFC. The ranking of the materials according to the mean vertical wear was as follows (ANOVA post hoc Tukey B, p < 0.05): group A: NFC > DCL = EM; group B: NFC > DCL > EM; group C: NFC > DCL = EM; group D: DCL = EM > NFC.

Significance

Only the results of the experimental setup with standardized antagonists of the same denture tooth material against flat specimens were similar to the clinical wear results with a comparable relative difference in mean vertical wear between the materials. When evaluating denture teeth for wear in the laboratory, a protocol should be applied that matches the clinical wear results.

Introduction

Wear of denture teeth is still regarded as a clinical problem, as excessive wear can be observed clinically after only a short period of time, however, not in all patients . Age and gender are potential risk factors with men showing more wear than women and younger patients more than older ones. There are some peculiarities with regard to patients who wear complete dentures. Edentulous patients show increased wear rates compared to dentate patients, as the receptors of the periodontal ligament, which control the chewing forces, are missing; the occlusal forces are regulated only by the masticatory muscles . Edentulous patients have a higher chewing frequency than dentate patients . In dentate patients, males have higher chewing forces than female patients but such a difference has not been detected in edentulous patients .

In recent years, new denture tooth materials have been introduced into the market, which claim to be more wear-resistant. Wear resistance has been primarily investigated in vitro with wear simulators. Different wear devices and methods are available that use different wear simulation concepts. Some methods (Minnesota, Alabama, OHSU, Zurich) are specified by the ISO Technical Specification on two-body and three-body wear . According to the FDA guidelines for laboratory test devices, most of the simulators available are not designed to be effectively used for the purpose mentioned . The methods in question were originally developed to test the suitability of composite resins for the direct restoration of posterior teeth. The different methods follow different wear concepts and approaches and therefore vary widely with regard to the main wear-influencing factors, such as load, abrasive medium, number of cycles, force actuator, shape and material of the stylus. The results cannot be directly compared as was shown by the blind round robin test on different dental materials (composite resins, ceramic, amalgam) examined with 5 different methods (ACTA, Alabama, Ivoclar, Munich, OHSU, Zurich) . The variability of the test results varied tremendously between the methods.

The same methods that are used to test resin composites for wear have been used to test denture tooth materials, without analyzing the suitability of the method for denture teeth. Some of these methods use metal , ceramic , denture teeth or human teeth as the antagonist material; some use an abrasive medium which can be artificial (e.g. PMMA particles) or natural (e.g. millet or poppy seeds) . Another approach is to assess the wear resistance by subjecting the materials to a toothbrushing device or to carbide abrasive paper . As far as the wear of denture teeth is concerned, the results are contradictory. Furthermore, they do not correlate to clinical data. For instance, the results of the nano-filled composite tooth NFC in some simulations indicate a higher wear resistance compared to a denture tooth material that is based on PMMA . However, the same authors have recently published data that showed a high wear rate of the NFC material in vitro .

It would be ideal if each denture material could be evaluated clinically with valid wear-quantifying methods. However, due to the technical difficulty of accurately measuring 3-dimensional (3D) changes, there are few studies that have analyzed the wear of artificial teeth in vivo .

The objective of the present study was to develop a laboratory wear method for denture tooth materials whose results correlate with clinical wear data. For this purpose three different denture tooth materials were selected for which clinical wear data are available. The three materials have different chemical compositions ( Table 1 ). One material (NFC) contains silica fillers, while the other two materials (DCL, EM) contain prepolymer fillers. The clinical wear data derived from clinical studies that were conducted at three centers. Four different experimental methods were applied:

  • 1.

    Molar tooth against premolar.

  • 2.

    Flat specimen against standardized conical ceramic stylus.

  • 3.

    Flat specimen against standardized ball-like ceramic stylus.

  • 4.

    Flat specimen against standardized conical stylus made of the same denture tooth material.

Table 1
Denture tooth materials and composition.
Denture tooth Manufacturer Composition
NFC Candulor UDMA, DMA, silanized silica fillers, UDMA/PMMA-prepolymer
DCL Ivoclar Vivadent PMMA, MMA, DMA, PMMA-prepolymer
EM Ivoclar Vivadent PMMA, MMA, DMA, PMMA-prepolymer

Two null hypotheses were formulated:

  • 1.

    There is no significant difference between the three materials with regard to clinical wear.

  • 2.

    None of the laboratory wear methods will match the clinical results on wear.

Materials and methods

Clinical wear data on denture teeth

For the clinical wear data on denture teeth, cast replicas from clinical trials that investigated the three denture tooth materials NFC, DCL and EM were analyzed. The study with the NFC material was carried out at the University of Innsbruck, Austria, that with the materials DCL and EM at the University of Buffalo, USA. The compositions of the three materials are listed in Table 1 . All three denture tooth materials were evaluated with the same study protocol:

  • Complete maxillary and mandibular dentures.

  • Impression of the posterior teeth with a customized tray and silicone impression material (Virtual heavy and light body, Ivoclar Vivadent) after the denture adaptation phase (baseline) and after 12 months of clinical service.

  • Pouring of impressions with improved dental stone (Fuji Super Hard Rock, white, GC Corp., Japan).

  • Quantification of wear with an etkon es1 laser scanner (see below).

  • Occlusal mapping of the attrition zones of posterior teeth (each first molar and first and second premolar).

  • Average of maximum vertical loss of the attrition zones of molars and premolars per subject. Statistical unit is the subject.

  • Wear quantification was carried out by the same investigator who applied the same methodology and technique.

The wear data for each subject were averaged out so that the statistical unit was the subject. A detailed description of the wear quantification process is published elsewhere .

For each material, 12-month wear data of 10 subjects were available for analysis. For the DCL group, the mean age of the subjects was 69 (±11) years, for the EM group 62 (±12) years and for the NFC group 69 (±8) years. There were 6 male and 4 female subjects in the DCL group and 7 male and 3 female subjects in the other two groups.

Laboratory wear methods

The three denture tooth materials, for which clinical data after 12 months of service were available, were subjected to different wear simulation approaches. For the simulated wear, a commercially available simulator called Willytec (SD Mechatronik, Feldkirchen-Westerham, Germany) was used. This chewing simulator operates with dead weights that are mounted on bars, which are lowered with a stepper motor. The weight can be varied between 1 and 11 kg. Additionally, a lateral movement, which is also driven by the stepper motor, can be integrated into the wear method. Both the vertical and horizontal axes are computer-controlled. The chewing simulator comprises eight chambers so that eight specimens can be tested at the same time. Simultaneous flooding and evacuation of each chamber with water at different temperatures (thermocycling, 5 °C/55 °C) is available.

Experiment 1: Denture tooth against denture tooth

For this experiment the same tooth shape and size was selected for all three denture tooth materials (Ortholingual, Ivoclar Vivadent). To obtain a more or less standardized sliding path, maxillary denture tooth molars were slid against maxillary premolars, whereby the buccal cusp of the premolar slid 1 mm on the buccal cusp of the molar ( Fig. 1 ). Less favorable sliding paths and tooth contacts would have been produced, if one had opted for a molar/molar occlusion simulation. To obtain a three-point contact in centric occlusion, the denture teeth were fixed in an occludator. The contact points were checked with black articulation foil (Hanel, Germany).

Fig. 1
Maxillary molar tooth (lower) opposed to maxillary premolar (upper) (Experiment 1, group A).

The specimens were subjected to the following wear simulation protocol (group A):

  • 100,000 chewing cycles, 5 kg weight, 1 mm lateral movement with lifting of the antagonist, simultaneous thermocycling (5 °C/55 °C, 105 s per temperature phase).

Experiment 2: Flat specimen against ceramic antagonist

Flat specimens ( n = 8) were cut from the bulk of the denture tooth material and luted into SEM holders. Before the specimens were tested, they were kept dry at a temperature of 37 °C for 24 h. Next, the specimens were polished with 600 grit SiC, 1200 grit SiC and 2500 grit SiC grit by means of a polishing device (Phoenix 4000, Buehler GmbH, Düsseldorf, Germany). The antagonists were made of pressed IPS Empress leucite-reinforced ceramic (Ivoclar Vivadent) with two different geometries ( Fig. 2 ):

  • Conical shaped antagonist with a radius of 0.6 mm at a height of 200 μm from the cuspal tip to the base (1.18 mm at a height of 600 μm) (group B).

  • Ball-like antagonist with a radius of 6 mm (in relation to the entire sphere) (group C).

Fig. 2
Ceramic stylus: (left) conical and (right) ball-shaped stylus (Experiment 2).

To ensure standardization, wax patterns of both stylus shapes were produced by a specialized company (Eichenberger AG, Rheinach, Switzerland).

The antagonists were glazed twice at a temperature of 870 °C and luted to aluminum SEM holders with resin cement (Dual Cement, Ivoclar Vivadent). They were polymerized with light for 40 s with an Astralis 5 curing light (650 mW/cm 2 ). They were additionally cured for 10 min in a polymerization device (Spectramat, Ivoclar Vivadent).

The specimens were subjected to the following wear simulation protocol:

  • 100,000 chewing cycles, 3 kg weight, 3 mm lateral movement without lifting of the antagonist, simultaneous thermocycling (5 °C/55 °C, 105 s per temperature phase).

Experiment 3: Flat specimen against antagonist of the same denture tooth material

Flat specimens were prepared in the same way as in the previously described test. The antagonists were drilled out of bulk denture tooth material with a diamond-coated bur (Komet, Germany) to obtain the same shape as that of the conical ceramic antagonists described in the previous section ( Fig. 3 ). For this purpose blocks were prepared in a cylindrical shape. The material was prepared with the antagonist bur with a straight dental handpiece (KaVo, Germany) at slow speed (2000 r/min) and water cooling. After the preparation of the shape, the antagonists were polished with Ivoclar polishing paste (Ivoclar Vivadent) and a polishing wheel.

Fig. 3
Bur (left) to cut standardized antagonists (right) from denture tooth materials (Experiment 3).

The specimens were subjected to the following wear simulation protocol:

  • 100,000 chewing cycles, 3 kg weight, 3 mm lateral movement without lifting of the antagonist, simultaneous thermocycling (5°/55 °C, 105 sec per temperature phase) (group D) ( Fig. 4 ).

    Fig. 4
    Stylus sliding over the flat specimen (Experiment 3, group D).

Quantification of wear

After completing the wear generating procedure, impressions of the material were made using a low viscosity vinyl polysiloxane material (Virtual light, Ivoclar Vivadent). After 4 h, replicas of the impressions were fabricated with white improved dental stone (Type IV, Fuji Superhard Rock, GC Corporation, Japan) using a vacuum, vibrator and two bars of pressure.

The plaster replicas were analyzed by means of a commercially available laser scanning device (etkon es1, Straumann CADCAM, Gräfelfing, Germany) and the appropriate match-3D software. The measuring principle is explained in detail elsewhere . For the quantification of the material loss of the molars and premolars, baseline and follow-up scans were superimposed by referencing the scans and matching the objects with the match-3D procedure until a standard deviation of less than 15 μm was obtained (8000 iterations, minimum 1200 points per matching procedure) ( Fig. 5 ). On the flat specimens, the area around the wear facet was used as the reference for the quantification of material loss. The maximum vertical material (and antagonist loss) (99% quantile) was automatically calculated by the software.

Fig. 5
Scans and differential wear picture of a specimen pair of Experiment 1 (group A). (Left) scan of molar and premolar before and (right) the same specimens after wear simulation. The red areas indicate vertical loss of material.

SEM analysis

From each experiment, two specimens were selected for SEM analysis. The specimens were sputter coated (BAL-TEC SCD 500, Leica Microsystems, Switzerland) and subsequently analyzed with the SEM VP DSM (Zeiss, Germany). The flat specimens were examined at ×25 and ×90 magnification and the molar and premolar teeth as well as the ceramic antagonists at varying magnifications (×40–×200), according to the region of interest.

Statistical analysis

As both the in vivo wear and in vitro wear showed not normal distribution, the data were log-transformed to achieve quasi normality. An analysis of variance (ANOVA) with post hoc Tukey B was carried out ( p < 0.05) to evaluate whether the wear data of a material were significantly different from that of another material. To determine whether gender was equally distributed between the three test groups of the clinical trials, a cross-table was created and assessed with the chi-square test ( p < 0.05).

Materials and methods

Clinical wear data on denture teeth

For the clinical wear data on denture teeth, cast replicas from clinical trials that investigated the three denture tooth materials NFC, DCL and EM were analyzed. The study with the NFC material was carried out at the University of Innsbruck, Austria, that with the materials DCL and EM at the University of Buffalo, USA. The compositions of the three materials are listed in Table 1 . All three denture tooth materials were evaluated with the same study protocol:

  • Complete maxillary and mandibular dentures.

  • Impression of the posterior teeth with a customized tray and silicone impression material (Virtual heavy and light body, Ivoclar Vivadent) after the denture adaptation phase (baseline) and after 12 months of clinical service.

  • Pouring of impressions with improved dental stone (Fuji Super Hard Rock, white, GC Corp., Japan).

  • Quantification of wear with an etkon es1 laser scanner (see below).

  • Occlusal mapping of the attrition zones of posterior teeth (each first molar and first and second premolar).

  • Average of maximum vertical loss of the attrition zones of molars and premolars per subject. Statistical unit is the subject.

  • Wear quantification was carried out by the same investigator who applied the same methodology and technique.

The wear data for each subject were averaged out so that the statistical unit was the subject. A detailed description of the wear quantification process is published elsewhere .

For each material, 12-month wear data of 10 subjects were available for analysis. For the DCL group, the mean age of the subjects was 69 (±11) years, for the EM group 62 (±12) years and for the NFC group 69 (±8) years. There were 6 male and 4 female subjects in the DCL group and 7 male and 3 female subjects in the other two groups.

Laboratory wear methods

The three denture tooth materials, for which clinical data after 12 months of service were available, were subjected to different wear simulation approaches. For the simulated wear, a commercially available simulator called Willytec (SD Mechatronik, Feldkirchen-Westerham, Germany) was used. This chewing simulator operates with dead weights that are mounted on bars, which are lowered with a stepper motor. The weight can be varied between 1 and 11 kg. Additionally, a lateral movement, which is also driven by the stepper motor, can be integrated into the wear method. Both the vertical and horizontal axes are computer-controlled. The chewing simulator comprises eight chambers so that eight specimens can be tested at the same time. Simultaneous flooding and evacuation of each chamber with water at different temperatures (thermocycling, 5 °C/55 °C) is available.

Experiment 1: Denture tooth against denture tooth

For this experiment the same tooth shape and size was selected for all three denture tooth materials (Ortholingual, Ivoclar Vivadent). To obtain a more or less standardized sliding path, maxillary denture tooth molars were slid against maxillary premolars, whereby the buccal cusp of the premolar slid 1 mm on the buccal cusp of the molar ( Fig. 1 ). Less favorable sliding paths and tooth contacts would have been produced, if one had opted for a molar/molar occlusion simulation. To obtain a three-point contact in centric occlusion, the denture teeth were fixed in an occludator. The contact points were checked with black articulation foil (Hanel, Germany).

Fig. 1
Maxillary molar tooth (lower) opposed to maxillary premolar (upper) (Experiment 1, group A).

The specimens were subjected to the following wear simulation protocol (group A):

  • 100,000 chewing cycles, 5 kg weight, 1 mm lateral movement with lifting of the antagonist, simultaneous thermocycling (5 °C/55 °C, 105 s per temperature phase).

Experiment 2: Flat specimen against ceramic antagonist

Flat specimens ( n = 8) were cut from the bulk of the denture tooth material and luted into SEM holders. Before the specimens were tested, they were kept dry at a temperature of 37 °C for 24 h. Next, the specimens were polished with 600 grit SiC, 1200 grit SiC and 2500 grit SiC grit by means of a polishing device (Phoenix 4000, Buehler GmbH, Düsseldorf, Germany). The antagonists were made of pressed IPS Empress leucite-reinforced ceramic (Ivoclar Vivadent) with two different geometries ( Fig. 2 ):

  • Conical shaped antagonist with a radius of 0.6 mm at a height of 200 μm from the cuspal tip to the base (1.18 mm at a height of 600 μm) (group B).

  • Ball-like antagonist with a radius of 6 mm (in relation to the entire sphere) (group C).

Fig. 2
Ceramic stylus: (left) conical and (right) ball-shaped stylus (Experiment 2).

To ensure standardization, wax patterns of both stylus shapes were produced by a specialized company (Eichenberger AG, Rheinach, Switzerland).

The antagonists were glazed twice at a temperature of 870 °C and luted to aluminum SEM holders with resin cement (Dual Cement, Ivoclar Vivadent). They were polymerized with light for 40 s with an Astralis 5 curing light (650 mW/cm 2 ). They were additionally cured for 10 min in a polymerization device (Spectramat, Ivoclar Vivadent).

The specimens were subjected to the following wear simulation protocol:

  • 100,000 chewing cycles, 3 kg weight, 3 mm lateral movement without lifting of the antagonist, simultaneous thermocycling (5 °C/55 °C, 105 s per temperature phase).

Experiment 3: Flat specimen against antagonist of the same denture tooth material

Flat specimens were prepared in the same way as in the previously described test. The antagonists were drilled out of bulk denture tooth material with a diamond-coated bur (Komet, Germany) to obtain the same shape as that of the conical ceramic antagonists described in the previous section ( Fig. 3 ). For this purpose blocks were prepared in a cylindrical shape. The material was prepared with the antagonist bur with a straight dental handpiece (KaVo, Germany) at slow speed (2000 r/min) and water cooling. After the preparation of the shape, the antagonists were polished with Ivoclar polishing paste (Ivoclar Vivadent) and a polishing wheel.

Nov 28, 2017 | Posted by in Dental Materials | Comments Off on Laboratory methods for evaluating the wear of denture teeth and their correlation with clinical results
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