Wear of Modern Denture Teeth—A Systematic Review

The purpose of this systematic review was to identify the different methods used to measure artificial teeth wear and to determine which denture teeth (or which combination of tooth types) have better wear resistance. The focused patient, intervention, comparison, and outcome (PICO) question for this review was “which available denture teeth or combination of teeth have higher wear resistance?” The method of testing and measuring the artificial teeth wear was also evaluated as a secondary outcome. We searched OVID Medline, PubMed and SCOPUS using the following terms (MeSH words) with any synonyms and closed terms: “wear”, “denture teeth”, “denture tooth”, or “artificial tooth”, “tooth wear”, or combination of “denture tooth wear”. Database searches were limited to the English language and studies published between years 1997 and 2021. Further hand searches were carried out of studies identified from the bibliographies of relevant articles. The electronic data base search identified 293 articles of which 213 were eliminated after removing duplicates and screening the titles of the articles. A further 31 articles were eliminated upon application of the exclusion criterion and full text reading because they were either not in the English language or were review articles. Only 41 articles met the inclusion criterion; along with addition of one hand search article, a total of 42 articles were included in the review. The studies showed that rate of denture teeth wear is influenced by factors such as the material and composition of the artificial teeth, the denture tooth antagonist, the tooth being replaced, patient’s age and sex, the type of removable prosthesis and the amount of the superficial layer removed during occlusal adjustments. The testing parameters were inconsistent across all studies. In conclusion, artificial teeth of same material should be used as antagonists where possible, and the superficial outer layer of the teeth must be preserved as much as possible during occlusal adjustments to enhance wear resistance. Denture teeth should be selected taking into consideration the tooth being replaced, the age and sex of the patient, and the type of prosthesis. Incorporation of nano fillers into acrylic resin teeth does not increase wear resistance; therefore, there is no evidence to favour the use of nano-filled composite teeth over micro-filled or conventional acrylic resin teeth.


Introduction
Wear resistance is an important physical property of artificial teeth used in removable prosthodontics. It is also an important determinant of the longivity of dentures because it determines the ability of the prosthesis to maintain the established occlusal relationship [1]. In addition to aesthetics and speech, patient expectations of their new dentures are also high in respect of masticatory efficiency, which can be compromised if denture teeth wear occurs [2].
Denture teeth wear over time due to forces of mastication during functional and parafunctional movement is inevitable [3,4]. Bruxism and clenching habits have been stated to cause wear of composite restorations through what is known as a two-body tribochemical/biomechanical wear, and this process is referred to clinically as abrasive wear [5,6]. Turssi et al. further say that wear is a function of this tribological system, and the variable movements and loadings that can occur and the aggressive chemical, thermal, and biological components involved make the intraoral tribology of dental composites highly complex [5]. Surface loss is usually observed more on posterior teeth than on the anterior teeth [3]. Teeth wear is of great concern to both patients and clinicians because the altered teeth morphology can result in loss of occlusal vertical dimension, compromised masticatory efficiency, fatigue of the muscles of mastication and patient discomfort, as well as affecting aesthetics [7]. Ultimately dentures are considered inadequate when teeth wear compromises their stability and retention, as it plays a crucial role in occlusal stability on the preservation of crestal ridges. Extensive wear can lead to iatrogenic resorptions and flabby tissues [8]. Excessive wear of denture teeth has also even been reported to affect the quality of life of dentures wearers [3].
Wear resistance is an essential property of denture teeth, since this maintains the patients vertical dimension and masticatory efficiency [9,10]. Porcelain denture teeth were initially used for denture construction until the introduction of acrylic resin teeth in the 1940s, and then composite teeth in the 1980s [11,12]. Acrylic resin teeth are made of polymethyl methacrylate resin (PMMA). To enhance their wear resistance, the double crosslinked (DCL) and interpenetrating network teeth (IPN) are now available with cross linking agents having been added to the chemical structure [13]. Composite layered teeth were initially introduced as micro-filled composites in an effort to improve the wear resistance of artificial teeth; this was later refined by changing the filler particles to nanocomposite in order to improve the materials' polishability [11]. By contrast, the Stober et al., 2010, study reported that acrylic resin teeth (DCL and PMMA) had higher wear resistance than human enamel [14]. It has also been reported that opposing teeth combinations of acrylic to acrylic showed less wear than feldspathic ceramic to ceramic [15]. Recent advances in digital dentistry have also seen introduction of digital dentures, whereby teeth are milled or printed from resin and bonded to the denture base [16]. However, there is a dearth of published data on the wear properties of this modern denture tooth material.
Several in vivo and in vitro studies of wear resistance of acrylic resin, composite and ceramic denture teeth have been conducted using the same teeth or zirconia and human teeth as antagonists [12,13,15]. However, there is no clear guideline in terms of which type of tooth has superior wear resistance properties and which combination of teeth better preserves the teeth morphology over time. Studies have used several methods to measure teeth wear either in vitro, such as the Ivoclar Vivadent method (IVOCLAR) [17], the Zurich method (ZURICH) [18], the Oregon Health Sciences University Oral Wear Simulator (OHSU) [19], the Munich Method (MUNICH) Oslo model [20], and the ACTA method (ACTA) [21], or in vivo, but there is a standard guideline as to which testing equipment or method is better or accurate [22].
Thus, the aims of this study are to: (1) systematically review the different methods used to measure artificial teeth wear and determine which methods are valid; and (2) determine which denture teeth or combination of type of teeth produces better wear resistance outcome.

Search Strategy
The focused patient, intervention, comparison, and outcome (PICO) question for this review was "which available denture teeth or combination of teeth have higher wear resistance?" The method of testing and measuring the artificial teeth wear was also evaluated as a secondary outcome. We searched OVID Medline, PubMed and SCOPUS using the following terms (MeSH words) with any synonyms and closed terms: "wear", "denture teeth", "denture tooth", or "artificial tooth", "tooth wear", or combination of "denture tooth wear". Database searches were limited to the English language and studies published between years 1997 and 2021. Further hand searches were carried out of studies identified from the bibliographies of relevant studies. The studies were then included or excluded from the total sample by following the inclusion and exclusion criteria (Table 1).

Selection of Studies
Two independent reviewers (V.M., J.C.) initially reviewed the titles and abstracts of the selected studies. After applying the inclusion and exclusion criteria, full texts were selected for both reading and final selection. All differences in choices between the two authors were analysed by two further reviewers (A.P., M.T.T.), and agreement was established through discussion as per the Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA) guidelines.

Data Extraction and Method of Analysis
The data were independently extracted by the reviewers from all the studies included using data extraction tables. These were double-checked and any questions arising during the screening and the data extraction stage were discussed until agreement was reached by the reviewers. The information extracted from the selected studies for analysis were author(s), years of publication, materials used for the study, testing methods (load applied, sliding distance, number of cycles, and frequency) and testing equipment, evaluation methods, and any further analysis of the worn tooth structure.

Risk of Bias
The assessment of risk of bias for all included studies used the Consolidated Standards of Reporting Trials (CONSORT) guidelines [23]. The assessment was done at the study level, with each rated as either "unclear", "low risk" or "high risk".

Assessment of Risk of Bias
Most studies showed low risk of bias for outcome and reporting bias; however, it was unclear for allocation concealment and blinding of outcome assessment. Half the studies showed high risk of bias for randomising sequence generation; however, this may be due to the need to select the teeth for dentures in both in vivo and in vitro studies ( Figure 2).

Assessment of Risk of Bias
Most studies showed low risk of bias for outcome and reporting bias; however, it was unclear for allocation concealment and blinding of outcome assessment. Half the studies showed high risk of bias for randomising sequence generation; however, this may be due to the need to select the teeth for dentures in both in vivo and in vitro studies ( Figure 2).

PMMA Denture Teeth
A total of 27 studies evaluated the wear of artificial acrylic teeth (PMMA) ( Table 3). The wear rate for PMMA denture teeth has been reported to be significantly lower against the same antagonist and human enamel [13,34]. A study also reported that acrylic resin causes minimal damage to opposing human enamel; the study did not record any measurable wear of the enamel [14]. Several studies have indicated that the composition of PMMA influences the wear resistance; the addition of filler particles such as silica and titanium dioxide improve the wear resistance of PMMA [4,9,

PMMA Denture Teeth
A total of 27 studies evaluated the wear of artificial acrylic teeth (PMMA) ( Table 3). The wear rate for PMMA denture teeth has been reported to be significantly lower against the same antagonist and human enamel [13,34]. A study also reported that acrylic resin causes minimal damage to opposing human enamel; the study did not record any measurable wear of the enamel [14]. Several studies have indicated that the composition of PMMA influences the wear resistance; the addition of filler particles such as silica and titanium dioxide improve the wear resistance of PMMA [4,9,10,27,40,41,43,47,48,[50][51][52][53]. By contrast, the findings of Stober et al. (2006) and  did not indicate any connection between the wear resistance and the chemical composition of the samples [13,49]. Removal of the superficial layer of certain artificial teeth during occlusal adjustment has also been shown to increase rate of wear [24,50]. Studies have also shown that wear was tooth dependent and the wear rate increased from incisors to molars [28,30]. Gender is another factor influencing the rate of artificial teeth wear with males demonstrating greater wear than females [28,29]. Wear rates of artificial teeth have also been reported to decrease with increasing age of the wearers [30]. A study has also shown that complete denture teeth exhibit lower wear rates than removable partial dentures [29]. Table 3. Main findings of the selected studies on denture teeth wear (PMMA).

Type of Study Teeth Tested Antagonist Outcome
Abe et al., [34] in vitro Semi-crown posteriors and Duracross posteriors (Nissin Dental Products, Kyoto, Japan)

Human enamel
Height loss after 20 × 10 4 sliding times with enamel antagonist for Semi-crown posteriors (acrylic resin) was 160 µm and for Duracross posteriors (high strength resin) was~300 µm. High strength resin tends to possess fragile characteristics contrary to its mechanical improvement in comparison with acrylic resin.
Ghazal et al., [13] in vitro Experimental resin teeth (Ivoclar Vivadent, Schaan, Liechtenstein) 1st premolar teeth of same tested denture material and caries free human teeth The experimental teeth recorded total vertical loss of 98 ± 6 µm and total volume loss of 0.035 ± 0.009 mm 3 against the same denture teeth cusps antagonist. Against the human enamel, a total vertical loss of 249 ± 48 µm and total volume loss of 0.093 ± 0.021 mm 3 was recorded.
Ghazal et al., The experimental resin teeth recorded total vertical loss of 24 ± 3 µm and total volume loss of 0.011 ± 0.001 mm 3 . The Polystar selection resin teeth recorded total vertical loss of 34 ± 8 µm and total volume loss of 0.019 ± 0.004 mm 3 , which was higher than the experimental resin. The vertical loss of opposing experimental teeth was 36 ± 5 µm, which was less than the opposing Polystar teeth that recorded 38 ± 8 µm of substance loss.  Silicon nitride ceramic ball The mean vertical loss for Premium 8 teeth was 1.03 ± 0.036 µm and mean volume loss was 0.939 ± 0.055 (mm 3 × 10 − 3 ); mean vertical loss for Yamahachi PX teeth were 0.698 ± 0.032 µm and mean volume loss was 0.650 ± 0.0062 (mm 3 × 10 − 3 ); mean vertical loss for Shenglian/SJ teeth was 12.1 0.0086 µm and mean volume loss was 16.4 0.48 (mm 3 × 10 − 3 ).

Enamel abraders
The amount of wear of the Kenson (0.006 ± 0.002 mm) and the Classic (0.005 ± 0.003 mm) resin denture teeth was much higher than the composite resin teeth used in the study-DB Plus (0.003 ± 0.002 mm) and MLI (0.003 ± 0.002 mm) at 5000 cycles. Similar trends were also found with these two acrylic resin teeth after 10,000 cycles. Ilangkumaran et al., (2014) [11] in vitro Acry Plus (Ruthinium Group, Badia Polesine, Italy) 600 grit sandpaper The mean wear value of Acry plus teeth was 627.80 µm and was greater compared to nano-composite teeth used in the study.
Jooste et al.,  Complete dentures Mean wear of teeth recorded after 36 months for males were: SR-Vivodent Orthotyp-PE-272 µm for maxillary dentures and 192 µm for mandibular dentures; Trueblend SLM-266 µm for maxillary dentures and 218 µm for mandibular dentures; Bioform IPN-225 µm for maxillary dentures and 202 µm for mandibular dentures. For Females mean wear recorded were: SR-Vivodent Orthotyp-PE-260 µm for maxillary dentures and 190 µm for mandibular dentures; Trueblend SLM-278 µm for maxillary dentures and 207 µm for mandibular dentures; Bioform IPN-172 µm for maxillary dentures and 114 µm for mandibular dentures. Average teeth wear for maxillary dentures were 243 µm, and for mandibular dentures, it was 181 µm. Greater wear was reported among male denture wearers compare to female denture wearers. Greater wear was also reported for premolars and molars than for canines. Complete dentures of same teeth Mean vertical loss recorded after 12 months of denture wear for the experimental resin teeth was 0.060 mm for maxillary dentures and 0.049 mm for mandibular dentures. Wear was tooth dependent (increasing from incisors to molars). These differences diminished once wear rates were adjusted for occlusal area, there was significantly more vertical wear for the posterior teeth than for the anterior teeth in the maxilla.
Sheety and Shenoy (2010) [47] 3 ). There is no correlation between materials properties such as hardness and wear resistance. In selecting artificial denture teeth, not only type, but also composition, and sub-enamel layer properties should be considered as well. Occlusal adjustment prior to, and after, denture delivery will inevitably expose this layer into occlusion with the opposing dentition; natural or artificial. Suzuki (2004) [10] in The results showed that the wear coefficient values for the superficial layers of Artiplus IPN specimens were significantly lower than those of the SR Vivodent and Magister specimens. This indicates that better wear performance can be expected from Artiplus just before complete wear of the superficial layer.
Nevertheless, once the superficial layer is worn away, all the tooth brands studied will likely perform similarly.

Double Cross-Linked Acrylic (DCL) Denture Teeth
Only four studies evaluated DCL denture teeth ( Table 4). The study by Heintze et al., (2012), showed that wear of double cross link acrylic was influenced by the filler particles; DCL with nano-fillers recorded the lowest wear rate against the same denture teeth. However, the study further reported highest wear value for a flat DCL specimen against a conical ceramic antagonist, and interestingly, the lowest wear value for a flat specimen of DCL against a ball-like ceramic antagonist [54]. Wear of DCL denture teeth is also tooth dependent; teeth wear increased from incisors to molars [31,34]. Similar to PMMA, a consistent trend of decreasing wear with increasing age has also been reported for DCL teeth [26,34]. A study showed females denture wearers recorded less wear than men (Heintze et al., 2015). This study further reported a lower rate of wear among patients with good fit dentures and for smokers [26]. The study by Schmid-Schwap et al., (2009), showed greater wear of teeth on the maxillary dentures than mandibular dentures [31]. Table 4. Main findings of the selected studies on denture teeth wear (DCL).
Complete dentures with same denture teeth and also splitmouth and two-arm design Analysis for wear measurements were conducted according to patient-and therapy-related factors. Denture teeth wear was not measured and analysed for individual types of denture teeth. A consistent trend recorded was decreasing wear associated with increasing age. Females recorded less wear than men. Less wear also reported among patients with well fitted dentures and a modest trend towards less wear for smokers.

Composite Resin
A total of 12 studies evaluated composite resin denture teeth ( Table 5). The vertical loss of composite resin has been reported to be lower than cross-linked and conventional acrylic resin artificial teeth [39,40]. A study by Stober et al., (2010), showed that incorporation of nano-fillers result in more wear compared to composite teeth with traditional micro fillers, while the study by Suzuki et al., (1990), reported that micro-filed composites were harder than other artificial teeth [10,48]. A study by Zeng et al., (2005), showed that the wear resistance was related to the Vickers hardness of the material; teeth wear increased as the hardness reduced [52]. This study further reported that the composites with 68% and 47% organic filler content had higher microhardness and showed better wear resistance than composite with 42% filler content [53]. However, the study by Suwannaroop et al., (2011), did not show any correlation between the material properties and wear resistance, and hence concluded that hardness cannot provide a forecast of teeth material wear resistance [50]. The investigators further stated that when selecting teeth, in-addition to composition, sub-enamel layer properties should also be considered [50].

Silicon nitride ceramic ball
Huge Kaijing showed the lowest mean vertical loss (0.6360 ± 0.011 µm) and lowest mean volume loss (0.507 ± 0.021 mm 3 × 10 − 3 ) when compared to conventional acrylic resin. This value was similar to highly cross-linking polymer teeth used in the study. Hirano et al., (1998) [40] in vitro DB Plus and MLI (Myerson, a Division of Austenal Inc. Chicago, USA) Enamel abraders Least wear values were observed with the DB Plus and MLI composite denture teeth, which showed similar wear rates: DB Plus (mean = 0.003, SD = 0.002 mm) and MLI (mean = 0.003, SD = 0.002 mm) composite teeth at 5000 cycles. In addition, similar trends were found with these teeth after 10,000 cycles: DB Plus (mean = 0.004, SD = 0.002 mm) and MLI (mean = 0.004, SD = 0.002 mm) denture teeth. No antagonist, complete denture, partial dentures, natural teeth, crowns, fixed partial denture Mean vertical loss for entire occlusal surface at 6 months was 8 µm, at 12 months was 18 µm, and at 24 months was 40 µm.
The mean vertical loss of denture teeth against no antagonist was 17 µm, against natural teeth was 36 µm, against metal crown was 304 µm, against denture teeth was 39 µm, and against resin veneering material was 15 µm. Complete denture, partial dentures, natural teeth, crowns, fixed partial denture After 24 months, overall wear of the complete occlusal surface of denture teeth was 91 µm, and maximum wear of the occlusal contact areas was 329 µm. Overall wear of the occlusal surface of the denture teeth against no antagonist was 22.7 µm, against complete dentures with same teeth was 58.9 µm, against partial dentures was 67.6 µm, and against natural teeth was 115.8 µm.

Mandibular teeth
The total volume loss recorded for Endura was 0.26 ± 0.07 (mm 3 ), for Duradent was 0.87 ± 0.03 (mm 3 ), and for Duracross was 0.22 ± 0.08 (mm 3 ). The total height loss recorded for Endura was 0.18 ± 0.04 mm, for Duradent was 0.24 ± 0.03mm, and for Duracross was 0.11 ± 0.04 mm. Total volume loss and total height loss are highest in Duradent, followed by Endura, and lowest in Duracross. Greater teeth wear was recorded on the maxillary teeth compared to mandibular teeth.
Type of antagonist material has also been reported to affect the wear resistance of composite teeth; using the same composite antagonist produces better results [36]. Several studies have shown that ceramic and partial denture antagonist result in greater wear of composite teeth [31,32,38,43]. However, a study evaluating wear behaviour of hybrid composite resin artificial teeth found that the hybrid composite against a hybrid composite resin antagonist resulted in lower wear depth, while also wider worn surfaces on the lower specimen than enamel and composite resin antagonist [43]. Irrespective of the type of antagonist, clinically relevant vertical loss of composite denture teeth occurs after 24 months [32].

Nanocomposite
A total of 12 studies evaluated nanocomposite denture teeth (Table 6). Nano-filled composite teeth has been reported to have greater wear resistance than conventional acrylic resin teeth [9,10,47]. Ghazal and Kern (2010) showed that nano-filled composite causes less wear of the opposing enamel of human teeth than ceramic [14]. Studies have reported that the wear depth of nano-filled composite was not statistically different from those of the micro-filled composite and cross-linked acrylic denture teeth [10,46]. On the contrary to this, the Stober et al., (2010), study showed that incorporation of nanofillers in composite resin teeth resulted in greater wear values compared to traditional microfillers [14]. However, the study by , showed no statistical differences in the total vertical substance loss between the combination's nano-filled composite resin against enamel antagonist and feldspathic ceramic against enamel antagonist [9]. The surface roughness of antagonist also influences the wear resistance of nano-filled composite; the wear increases with an increase in surface roughness [36]. Increase in loading force also increases the vertical loss of nano-filled composite teeth [14]. Nano-filled composite shows a high initial wear resistance; however, long-term wear resistance needs improvement to be comparable to natural tooth enamel [9]. The , study showed that zirconia and alumina antagonists are better antagonist options against nano-filled composite than felspathic ceramic [9]. Table 6. Main findings of the selected studies on denture teeth wear (nanocomposite).

Zirconia ceramic balls
Mean wear of condyloform against antagonist surface roughness of 0.24 µm was 33 µm, with a surface roughness of 0.75 µm was 42 µm, and with a surface roughness of 2.75 µm was 65 µm. The wear value of nanofiled composite increased with the increase in antagonist surface roughness.   [15] in vitro Condyloform II NFC (Candulor AG, Wangen, Switzerland) 6 mm zirconia ball Vertical loss of condyloform teeth under 20 N loading was 29 µm, under 49 N loading was 33 µm, and under 78 N loading was 37 µm. The vertical loss of nano-filled composite increases with loading force. The influence of the loading force on the amount of wear of composite resin was greater than that of human enamel.

Manidbular human first premolars
The vertical loss for Condyloform II was 105 ± 32 µm and volume loss was 0.038 ± 0.0130 µm, which was statistically more wear than ceramic teeth. Compared to ceramic teeth, the vertical loss of opposing human enamel was very low (11 ± 6 µm).
Ghazal et al., [9] in vitro Condyloform II NFC (Candulor AG, Wangen, Switzerland) Steatite ceramic ball The mean vertical loss of Condyloform after 120,000 cycles was 50 ± 14 µm, after 240,000 cycles was 65 ± 12 µm, after 480,000 was 84 ± 20 µm, after 840,000 cycles was 102 ± 27 µm, and after 1,200,000 cycles was 117 ± 30 µm. The nano-composite resin teeth of this study showed a high initial wear resistance. However, long-term wear resistance of the composite teeth still needs improvement to be comparable with that of natural tooth enamel.
Ghazal et al., [13] in vitro Condyloform II NFC (Candulor AG, Wangen, Switzerland) 1st premolar teeth of same tested denture material and caries free human teeth Total vertical loss of condyloform teeth against condylofom antagonist was 35 ± 9 µm and against human enamel was 169 ± 18 µm. The combination of nano-filled composite resin-nano-filled composite resin teeth showed comparable wear values with those of feldspathic ceramic-feldspathic ceramic teeth. The combinations of nano-filled composite resin-enamel and feldspathic ceramic-enamel showed statistically no differences in the total vertical substance loss. However, the wear of the antagonistic human enamel caused by nano-filled composite resin teeth was significantly less than that caused by feldspathic ceramic teeth, but more compared to acrylic resin teeth.
Ghazal et al.,  [10] in vitro Veracia (Shofu, Kyoto, Japan) Stainless steel stylus Veraia recorded wear depth of 90.5 ± 10.2 µm. The nanocomposite denture tooth used in this study possesses superior surface hardness and wear resistance compared to the conventional acrylic denture tooth, and its wear depth was not statistically different from those of the micro-filled composite and cross-linked acrylic denture teeth.

Porcelain
A total of eight studies evaluated porcelain teeth (Table 7). A study by Abe et al., (2001), showed ceramic and enamel to be the worst antagonist combination for dental restorations; the study reported almost ten times more tooth surface loss of the opposing enamel [34]. Similar findings were also reported by Ghazal and Kern, (2010), which showed more wear of antagonistic enamel than that of feldspathic ceramic teeth [14]. In contrast, the study by , showed that ceramic denture teeth demonstrated a two-body wear similar to human enamel [9]. A lower incidence of TMD (temporomandibular disorders) among complete denture wears with porcelain teeth was reported than those with acrylic resin teeth; this is associated to the high wear resistance of porcelain [3]. A study by Wang et al., (2010), which evaluated the effect of pH changes on tooth surface loss of ceramics, found that there was no association with tooth surface loss and pH changes; however, the presence of abrasive particles increased the tooth surface loss [51]. The , study showed that zirconia-and alumina-based antagonists are better antagonists against ceramic than steatite due to their better wear resistance against ceramic [1]. A further study showed that although porcelain had the highest hardness value than the other artificial teeth tested, it showed the largest wear value against hybrid composite as the upper opposing specimen [43]. The total height loss after 20 × 10 4 sliding times of enamel was 380 µm and for porcelain was~40 µm. The surface roughness values after the wear test for porcelain was 3.7 µm and for enamel was 3.6 µm. The greatest height loss among the test pairs was that of the porcelain-enamel pair in the study. The poorest combination is porcelain-enamel.
Arafa, (2016) [3] in vivo Porcelain (dent supply, Bensheim, Germany) Resins and porcelain teeth on dentures Clinical wear of porcelain denture teeth ranged from 0.022 mm to 0.061 mm across the 6-month to 24-month period. There was a higher incidence of TMD among patients wearing complete dentures with artificial teeth made of acrylic resin in comparison with those wearing complete dentures with teeth made of porcelain.

Mandibular human first premolars
The vertical loss of opposing enamel recorded after 300,000 cycles was 148 ± 16 µm, and for Bonartiic was 21 ± 6 µm. Feldspathic ceramic teeth showed significantly higher wear resistance than composite or acrylic resin teeth.   [9] in vitro Bonartic CT (Candulor, Opfikon, Switzerland) Steatite ceramic ball After 1,200,000 chewing cycles, the mean vertical substance loss and volume loss for the ceramic teeth was 36 µm and 0.029 mm 3 . Ceramic denture teeth demonstrated a two-body wear similar to human enamel.
Ghazal et al.,  [52] in vitro Y-TZP (Yittria-TetragonalZirconiaPolycrystalline) (manufacturer/supplier not stated) The total mass loss of Y-TZP material after 20 hrs of exposure decreased with the increase in pH of the aqueous solution with both presence of erosive particles (pH 2.0-0.587 kg −2 to pH of 9.5-0.0347 kg −2) . Without the presence of the erosive particles, the total mass loss also reduced slightly (pH 2.0-0.007 kg −2 to pH 9.5-0.006 kg −2 ). The total mass loss remained 0.004 kg −2 after exposure to solution with no particles at a pH of 2.0 and at a pH of 9.5.

3D-Printed Denture Teeth
Only one study was found which evaluated 3D-printed teeth ( Table 7). The study reported that three-dimensional printing appeared to be useful for the fabrication of denture teeth with a sufficient level of wear resistance [16]. The results of this study showed that the wear resistance of 3D-printed teeth is comparable to conventional acrylic resin teeth. However, fine cracks were seen on the SEM images against metal antagonist.

Discussion
The rate of denture teeth wear is influenced by various factors such as the material and composition of the artificial teeth, the denture tooth antagonist, the tooth being replaced, patient's age and sex, the type of removable prosthesis, and the amount of superficial layer removed during occlusal adjustments. From the eleven in vivo studies, there seems to be a correlation between volumetric loss and surface area of denture teeth. However, findings are inconsistent across all studies due to differing experimental conditions, including type of testing and load applied.
The wear behaviour of the artificial teeth examined differed with regards to their respective compositions. No consistent conclusions have been drawn from studies evaluating the wear resistance of the available denture teeth. In general, higher wear was observed in PMMA teeth without filler particles than PMMA teeth modified with fillers. A study found an association between wear resistance and the Vickers hardness of artificial teeth materials; rate of teeth wear increased as the hardness reduced [10]. In contrast, Suwannaroop et al., (2011), did not find any correlation between the material properties and wear resistance of denture teeth [50]. Composite and nanocomposite teeth have a greater wear resistance compared to PMMA and DCL teeth [39,46]. However, the wear resistance of PMMA teeth can be improved with incorporation of filler particles, such as silica and titanium dioxide [38,41,53]. Similarly, addition of nano-fillers to DCL acrylic resin also improves the wear behaviour of the teeth [53]. The wear property of denture teeth is associated with the micro-structure of the material. PMMA has a linear microstructure, while modified resin teeth with the addition of cross-linking agents have a cross-linked structure. The ideal cross-linking structure of modified resins improves the mechanical properties of denture teeth [10].
Moreover, a better outcome can be achieved in removable prosthodontics by selecting denture teeth according to the type of antagonist. In complete denture fabrication, the same denture teeth should be used for both arches where possible. The study by Heintze et al., (2012), reported the lowest wear rate against the same denture teeth [53]. Similarly, the wear rate of PMMA denture teeth has been reported to be significantly lower against PMMA denture teeth and a human enamel antagonist [12,13]. Ceramic with an enamel antagonist is not a good combination as it results in high rates of wear of the enamel [14,34]. Nanofilled composite as an antagonist against human enamel is a better alternative compared to feldspathic ceramic as it causes less wear of enamel [1]. However, composite teeth with nano-fillers have also been reported to result in more wear compared to teeth with traditional micro-fillers. Significant wear of composite teeth after 24 months has been reported as inevitable, irrespective of the type of antagonist [32].
Wear resistance is gender and tooth dependent, with higher vertical wear demonstrated in male subjects and posterior teeth [28]. Similarly, the study by Stober et al., (2014), also showed maximum vertical loss of the occlusal contact areas of premolars and molars [33]. Age is also an influencing factor for denture teeth wear; wear rates decrease with increasing age of denture wearers [30]. A possible reason for this could be that as individual's age, the type and quantity of their food intake changes along with a decrease in the efficiency of the muscles of mastication.
Material composition of surface layer influences wear resistance. Therefore, the surface layer of artificial teeth should not be polished off or removed during occlusal adjustment to maintain good wear resistance. When compared with complete dentures, teeth on partial dentures exhibit a higher rate of wear [29]. This could be due to the different material of the antagonist, which in most cases are either enamel or ceramic.
The methodology of conducting experiments was inconsistent across all studies and therefore difficult to determine which method is more reliable to measure artificial teeth wear. Various wear testing machines or self-made wear machines, and different testing parameters for wear testing were adopted. This included occlusal wear in vivo, pin-onblock, pin-on-disc, two-body wear testing machine, three-body wear testing machine, and masticatory simulators. Among the methods to measure artificial teeth wear, the masticatory simulator was the most common type of testing, with 49 N load applied for 300,000 cycles. Thermocycling of 5 • C to 55 • C, varying between 30,000 and 60,000 cycles, is often the ageing protocol of choice by researchers.
The limitation of this study was that comparative evaluation of two studies could not be done. From the data search, only one study evaluating 3D-printed teeth was found, and this study's evaluation antagonist was limited to zirconia and metal antagonists with 2-body wear. On the other hand, only one study evaluated the effect of pH changes on tooth surface loss. There are several types of teeth available from the market, and some of them are marketed as products with improved mechanical properties. The results of this systematic review may assist clinicians in selecting denture teeth from the perspective of wear resistance.

Conclusions
Based on the findings of this systematic review, the following conclusions were drawn:

1.
Artificial teeth should be selectively chosen for dentures taking into consideration the type of antagonist. The same denture teeth should be used as antagonists where possible for the best outcomes.

2.
During occlusal adjustment, the superficial layer should be preserved as much as possible to enhance wear resistance.

3.
The included studies demonstrated that the incorporation of nano-fillers into acrylic resin teeth does not increase wear resistance. Therefore, there is no evidence to favour the use of nano-filled composite teeth over micro-filled or conventional acrylic resin teeth.

4.
The studies reported that wear resistance was related to the tooth, age and sex of patients and the type of prosthesis; therefore, selecting teeth with higher wear resistance for replacement of posterior teeth, partial dentures and for male patients can produce a better outcome.

Conflicts of Interest:
The authors declare no conflict of interest.