INTRODUCTION

A huge number of dental restorative materials are available today for prosthetic purposes. The ideal restorative should resemble as close as possible the tooth hard tissues that are to be replaced. Among material properties, wear behavior seems of crucial importance, as over time either a reduced wear resistance or an exaggerated abrasiveness may severely jeopardize the esthetic and functional outcome of extensive occlusal rehabilitations, especially when treating patients with parafunctions.

Several studies have analyzed the in vitro wear resistance of restorative materials opposing either human enamel antagonists or dedicated artificial abraders.^1-4 The abrasiveness of gold-based alloys, resin composites, feldspathic porcelains, glass ceramics, and polycrystalline zirconia-based materials toward tooth hard tissues has also been the subject of extensive investigation.^5-9

In previous research, dental gold-based alloys showed wear characteristics very similar to human enamel.^1,2 Nevertheless, in spite of their excellent marginal accuracy^10,11 and uncontested mechanical/tribological properties,¹² an increasing demand for better esthetics persuaded clinicians to withdraw full-gold restorations in favor of alternative tooth-colored materials.

Dental ceramics exhibit superior optical properties, excellent color stability, and proven biocompatibility.^13-15 Their clinical reliability has also increased^16-20 following the latest advances in adhesive dentistry^21-25 and the recent introduction of strengthened and enhanced ceramic systems.²⁶ Ceramic materials are wear resistant,^3,4 but they may damage the opposing enamel.^27-30 The general belief that human enamel might be subject to accelerated wear when opposed by traditional porcelain-fused-to-metal crowns⁵ was further confirmed in vivo in 2011 by Silva and others.⁶ Contradictorily, in a similar in vivo study by Etman and others,⁷ metal-ceramic crowns produced the least tooth wear in comparison to polycrystalline-alumina copings veneered with feldspathic porcelain and to hot-pressed high-leucite glass ceramics. A recent review indicated that some all-ceramic crowns are as wear friendly as metal-ceramic crowns.⁸ The author of the same review failed to find a strong association between tooth wear against ceramics and any specific causal agent,⁸ including the material hardness or its chemical composition, thus underlying the compelling need for additional studies on this specific research topic. The most recent in vitro studies reported for some new all-ceramic systems an abrasiveness very close to that of human enamel⁹ as well as a wear resistance similar to that of traditional gold alloys.²

In a direct comparison between properties, such as flexural strength, hardness, or optical behavior, ceramic/glass-ceramic materials are generally superior to dental composites.³¹ Nevertheless, thanks to continuous innovations in filler composition and particle size/morphology, current micro-/nanohybrid composites definitely show proper esthetic/mechanical features for successful use in all areas of the mouth.^32,33 Additionally, the increasing appeal of composite resins is warranted by their ease of use, the possibility of an easy and invisible intraoral repair of minor defects induced by function, and the opportunity to choose a direct or an indirect approach.³¹ Those characteristics are extremely attractive, as minimally invasive solutions today seem to be preferred in every branch of dentistry.^34-36 Composites are traditionally considered more wear friendly than dental ceramics. In general, resin-based materials produce lower enamel antagonist wear than ceramic-based ones, both in the manually polymerized and in the CAD/CAM versions.³⁷ In a recent in vitro study, resin composite antagonists led to the lowest wear on the opposing enamel, being significantly reduced compared to the enamel wear recorded against lithium disilicate glass-ceramic abraders.³⁸

Moreover, innovative and enhanced resin composites have been recently introduced, showing promising in vitro wear resistance values, statistically similar to those of human enamel and gold-based alloys.¹

Notwithstanding the great efforts that have been made toward investigating the wear properties of prosthetic materials opposing human enamel or dedicated artificial abraders, little is currently known about the in vitro wear behavior of a specific dental restorative material opposing itself.

The likelihood of restorative materials opposing themselves in routine clinical practice appears rather strong. The full-mouth rehabilitation of completely edentulous patients using traditional or implant-supported prostheses likely leads to an artificial occlusion with a prosthetic material on the upper jaw opposing the same prosthetic material on the lower jaw.³⁹ A similar condition may also occur when partially edentulous patients are treated to replace or restore missing or damaged antagonistic teeth. Today, the esthetic and functional rehabilitation of severely eroded or abraded patients is preferably accomplished with additive ceramic/composite-based restorations, which are almost simultaneously bonded to all teeth, finally leaving each newly restored tooth in functional contact with a similarly restored antagonistic tooth.⁴⁰ A full-mouth prosthetic approach has been described to be the best treatment option in the less frequent cases of amelogenesis imperfecta.^41,42 Even when only a single tooth requires a full-coverage restoration, it is not uncommon to ascertain that the antagonist had already been restored by employing the same dental material.

To our knowledge, only one in vitro study has investigated the two- and three-body wear between resin composites used both as samples and as antagonistic abraders.⁴³ Yet for the reasons outlined above, such information seems particularly important both when planning extensive full-mouth rehabilitations and when selecting the appropriate material to restore one or more teeth that oppose already restored antagonistic teeth.

On these bases, the purpose of the present in vitro study was to assess the two-body wear of a type 3 gold alloy, an yttria-stabilized zirconia polycrystalline ceramic, a heat-pressed feldspathic porcelain, a lithium disilicate glass ceramic (milled and heat pressed), and three different heat-cured resin composites opposing standardized antagonistic cusps made out of the same restorative materials. Each sample was subjected to 120,000 mastication simulation cycles. The null hypothesis tested was that no difference could be detected in the wear resistance among the materials under investigation.

METHODS AND MATERIALS

A complete list of the materials tested in the present study, together with some data about their composition, is given in Table 1.

Table 1: Summary of the Materials Used in the Experimental Groups. Technical Data Were Provided by the Respective Manufacturers.

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Antagonist Fabrication

An artificial stainless-steel cusp having a slight conical shape and a 2-mm-diameter round tip (Figure 1a,b) was used as a template to produce eight sets of 10 standard antagonists (n=10) employing each one of the eight restorative materials under investigation.

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For IPS e.max CAD and Katana Zirconia ML (Figure 1c), the template steel cusp was scanned in order to guide the CAM of the antagonists.

For Ceram.X Universal, Enamel Plus Function (Figure 1g), and Enamel Plus HRi, a polyvinyl-siloxane impression of the template steel cusp was taken. The so-achieved cusp-shaped silicon mold was used to produce standard resin-based composite antagonists by applying and individually light curing three 2-mm-thick composite layers. After manufacturing, resin composite cusps were heat cured as already described for composite cylindrical specimens.

The same cusp-shaped silicon mold employed for composite antagonists was also used to produce wax replicas of the template steel cusp. The wax replicas were then invested in order to manufacture standard IPS e.max Press (Figure 1i), Cerabien ZR Press, and Aurocast8 (Figure 1e) antagonists following the lost-wax technique and the respective manufacturer instructions.

Wear Testing

After manufacturing, all specimens and cusps were stored for 24 hours at 37°C and then subjected to a two-body wear test in a dual-axis chewing simulator (CS-4.2, SD Mechatronik GmbH, Feldkirchen-Westerham, Germany) according to the methodology described by D'Arcangelo and others.¹ In brief, autopolymerizing acrylic resin was used to secure the cusps on the antagonist holders and to fix each specimen inside the sample chamber.

Each specimen was loaded against a standard cusp made out of the same restorative material at 1.6 Hz for a total of 120,000 chewing cycles. The masticatory cycle in this study consisted of three phases: contact with a vertical force of 5 kg, horizontal sliding of 0.7 mm, and separation of the specimen and its antagonistic cusp. The chewing simulation parameters used are summarized in Table 2.

Table 2 Configuration of Parameters Set for Wear Method.

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Data Analysis

After wear testing, a three-dimensional surface analysis of all specimens was performed with a CAD/CAM contact scanner (Dental Scanner, Renishaw Inc, West Dundee, IL, USA): the sample vertical wear (mm) and its volumetric loss (mm³) were then calculated.¹ Moreover, the difference between the pretest and posttest height of each antagonistic cusp was measured and assumed as the antagonist vertical wear (mm). The vertical wear of each cusp was also used to calculate the volume of the spherical cap corresponding to the antagonist volumetric loss (mm³), according to the following formula:

where h is the spherical cap height, corresponding to the antagonist vertical wear (mm), and R is the spherical cap radius, equal to 1 mm for each antagonist because of the standardized manufacturing process.

The total vertical wear (mm) for each sample/antagonist pair was finally calculated as the sum of each sample vertical wear and the corresponding antagonist vertical wear. Similarly, the total volumetric loss (mm³) was calculated as the sum of the sample volumetric loss and the corresponding antagonist volumetric loss.

Means (and standard deviations) for total vertical wear and total volumetric loss were calculated for the eight materials under investigation. Mean values were then compared using one-way analysis of variance (ANOVA) tests and Tukey honestly significant difference tests (α=0.05).

RESULTS

Tables 3 and 4 show the total vertical wear and total volumetric loss mean values recorded for each test material after 120,000 mastication simulation cycles against antagonistic cusps made out of the same restorative material. The contribution of mean antagonist wear and mean sample wear to the ultimate calculation of the total wear is also given. Data are presented as box plots in Figures 2 and 3, sorting groups from the least to the greatest wear on the category axis. The one-way ANOVA tests showed that the mean value differences observed for the total vertical wear (F=26.995; p<0.001) and for the total volumetric loss (F=38.957; p<0.001) were statistically significant.

Table 3: Mean values (and standard deviations [SD]) for the sample vertical wear (mm), antagonist vertical wear (mm), and total vertical wear (mm) achieved in the experimental groups (n=10). Total vertical wear mean values were compared using a one-way analysis of variance test. Same letters indicate no statistically significant differences.

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Table 4: Mean values (and standard deviations [SD]) for the sample volumetric loss (mm³), antagonist volumetric loss (mm³), and total volumetric loss (mm³) achieved in the experimental groups (n=10). Total volumetric loss mean values were compared using a one-way analysis of variance test. Same letters indicate no statistically significant differences.

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The least total vertical wear and total volumetric loss mean values were recorded on zirconia samples opposing zirconia cusps, with a statistically significant difference compared to the total wear of the gold alloy facing gold alloy cusps (p=0.044 for vertical wear; p=0.033 for volumetric loss). Compared to the gold alloy, slightly increased but not significantly different total mean wear values were registered on heat-cured Enamel Plus Function (p=0.671 for vertical wear; p=1.000 for volumetric loss), heat-cured Ceram.X (p=0.311 for vertical wear; p=0.627 for volumetric loss), and Cerabien ZR Press (p=0.217 for vertical wear; p=0.770 for volumetric loss). The use of e.max CAD led to significantly increased total volumetric loss mean values compared to the gold alloy (p=0.005), while no statistically significant differences were recorded between the same two materials in terms of total vertical wear (p=0.074). The use of heat-cured Enamel Plus HRi and e.max Press was associated with the highest total vertical wear mean values and significantly increased compared to the vertical wear observed in all the other experimental groups but with no statistically significant difference between one another (p=0.775).

Representative SEM images of the wear facets observed on some abraded samples are shown in Figure 4. No wear or shallow depressions were observed on Zirconia (Figure 4a). A slight increase in wear facet dimension was observed on gold alloy (Figure 4b) and on Enamel Plus Function (Figure 4c). The worn surfaces on the heat-cured resin composite appeared smooth, and their borders were clear. Worn ceramic specimens (Figure 4d) showed instead a fairly coarse surface.

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DISCUSSION

The null hypothesis tested in the present study had to be rejected. Significant differences were observed in the wear behavior among the restorative materials under investigation. In an experimental model where every material was tested against an antagonistic cusp made out of the same material, the highest total vertical wear and volumetric loss values were recorded on the heat-pressed lithium disilicate (e.max Press) and on a particular heat-cured nanohybrid composite (Enamel Plus HRi, shade UE2), specifically commercialized by the manufacturer as an esthetic material for anterior restorations.

Sample/antagonist pairs made out of Katana Zirconia ML showed the least total vertical wear and volumetric loss mean values, confirming the high wear resistance exhibited by zirconia-based polycrystalline ceramics in previous investigations.⁴⁴

Two innovative resin-based composites were also tested in the present study. After 120,000 chewing simulation cycles against antagonists made out of the same material, the heat-cured Enamel Plus Function and the heat-cured Ceram.X Universal showed an extremely promising wear behavior, very similar to that of the gold-based alloy, in terms of both vertical wear and volumetric loss. Enamel Plus Function was recently introduced by the manufacturer as a clinical alternative to Enamel Plus HRi for posterior teeth, with the goal of increasing mechanical properties and improving long-term outcomes when used on load-bearing occlusal surfaces.¹ It has been formulated by putting the greatest effort toward optimizing the bond between filler particles and the resin matrix,¹ which might explain the positive wear resistance observed. Ceram.X Universal, on the other hand, is based on a proprietary filler technology called SphereTEC and contains granulated spherical submicron glass fillers. According to the manufacturer, this new filler technology, in combination with an optimized resin matrix, improves the esthetics and polishability and provides high fracture toughness. Even the enhanced wear properties recorded in the present study for this new resin-based composite might be somehow correlated with its unique chemical composition.

The total vertical wear mean values for the milled lithium disilicate ceramic (e.max CAD) and for the heat-pressed feldspathic porcelain (Cerabien ZR Press) were also not statistically different from the gold alloy. Nevertheless, the wear behavior of ceramics should not be considered similar to that of metal or composite resin. To some extent, metal and composite resins wear through a mechanism involving plastic deformation and adhesion, while ceramics wear through microfractures.^38,45

The wear properties of the gold alloy are likely due to its inherently increased ductility, provided by metal bonds in its structure, compared to the brittle property of ceramics.⁴⁶ The SEM pictures of worn gold alloy samples suggest the occurrence of a plastic deformation over the wear process (Figure 4b). On the other hand, the SEM photographs of worn ceramic specimens (Figure 4d) demonstrated a coarse surface and flaws probably generated by exfoliated hard debris. Even if crack formations of ceramic could not be seen in the SEM observation, fatigue wear might have occurred due to repetitive loading on the brittle ceramic surface. The heat-cured Enamel Plus Function (Figure 4c) displayed a relatively uniform wear surface: the lack of evident irregularities and images of filler dislodgement/protrusion might be correlated to the optimum bond between filler particles and resin matrix.

For decades, the use of metal or gold on the occlusal surfaces has been considered a valid solution in all cases where the prosthetic occlusion was in contact with natural enamel, resin composite, porcelain, or a combination of such materials,⁴⁷ causing minimal wear to the antagonist⁴⁸ and little interference with the patient occlusal balance.³ In recent in vitro studies, a type 3 gold alloy exhibited the same wear rates of human enamel.^1,2 As a consequence, dental materials that closely resemble the gold alloy in their wear behavior should probably be considered the most physiological substitutes for lost tooth hard tissues.

Excessive wear or exaggerated abrasiveness, on the other hand, should be avoided, as it may lead to unacceptable restoration and/or antagonist damage, with possible alterations of the functional path of masticatory movements. When anterior teeth are involved, both esthetics and anterior guidance function are impaired, finally leading to increased stresses on the masticatory system and possible temporomandibular joint dysfunctions.^49-51

Many studies have attempted to relate the wear resistance and/or the abrasiveness of dental materials to specific material properties, such as surface topography, fracture toughness, or hardness.^52-54

According to Fischer and others,⁵⁵ for most materials, metal in particular, the wear resistance can indeed be considered directly proportional to the hardness. However, for the abrasion caused by most ceramics, hardness and wear are probably not strictly associated with each other.^56-58 The wear caused by ceramics appears more related to surface roughness and fracture toughness^55,59,60 and should be conveniently considered as a multifactorial condition.⁶¹

Unlike the case of ceramics, composites produce wear on their antagonist through hard filler protruding from the abraded resin matrix, and the hardness is thought to be a reliable predictor of their abrasiveness.^53,54

According to the general knowledge about wear between two contacting materials, a softer material is abraded more easily than an opposing harder one.⁵⁴ However, in the present study, each tested material was also used to manufacture the respective antagonistic abrader in order to mimic in vitro the common clinical situation of two opposing restorations made out of the same dental material. Thus, in this study, every sample was tested against an antagonistic abrader showing exactly the same mechanical properties. Furthermore, the total wear (sample wear plus antagonist wear) was calculated and assumed as the parameter under investigation. In a similar experimental scenario, hardness is maybe less correlated with total wear because, even assuming that a harder material would easily abrade its antagonist, probably it is also less likely to be worn out compared to a softer one and vice versa. Interestingly, even though the manufacturer reports the same Vickers hardness value for both the heat-pressed and the milled versions of lithium disilicate (5800 MPa), in this study a statistically significant difference was detected in the wear properties of e.max Press and e.max CAD. This finding confirmed that the wear behavior of a brittle substrate (like ceramic) is perhaps different from that of a composite, and, consequently, the use of hardness as a wear predictor for all the materials tested did not seem an appropriate solution.

In the present study, both pressed and milled ceramic materials were subjected to a standardized surface polishing procedure before testing. Although in a few studies no statistically significant differences were recorded between the wear properties of polished and glazed lithium disilicate ceramics, a trend toward higher antagonist wear was noted for the glazed ones.^62,63 Moreover, several studies have reported that glazed zirconia may lead to increased opposing enamel wear compared to polished zirconia.^62,64-71

As a consequence, assuming that wear-friendly dental materials should always be preferred, it appeared more clinically relevant to deepen the wear properties of a polished ceramic instead of focusing on a glazed one. Furthermore, the wear resistance and the abrasiveness of a glazed ceramic could be influenced by the specific ceramic glaze applied rather than representing an intrinsic property of the material itself.

As a general rule, well-conducted randomized controlled clinical trials should be considered the best method to evaluate the quality of dental materials. However, they are costly, time consuming, and hard to standardize. Therefore, in vitro research still remains an indispensable step for initial screening of material properties, and dynamic tests appear to be extremely valuable in predicting the clinical performance of biomaterials subjected to the cyclic solicitations generated by physiological movements.^28,72,73

CONCLUSIONS

Within the limitations of an in vitro model designed to test the two-body wear resistance of a dental restorative material opposing an antagonist made out of the same material, the following conclusions could be drawn:

• 1

  Among the esthetic and adhesive materials investigated, two heat-cured resin composites (Enamel Plus Function and Ceram.X Universal), a heat-pressed feldspathic porcelain (Cerabien ZR Press), and a milled lithium disilicate glass ceramic (IPS e.max CAD) showed a vertical wear statistically similar to the traditional type 3 gold alloy.

• 2

  Total vertical wear and total volumetric loss observed on the monolithic zirconia (Katana Zirconia ML) were significantly reduced compared to the gold alloy and to all the other tested materials.

• 3

  Total vertical wear and total volumetric loss recorded on the heat-pressed lithium disilicate glass ceramic (IPS e.max Press) were significantly increased compared to what was observed for the milled lithium disilicate glass ceramic (IPS e.max CAD).