/home/opencode/cpanel/stomaeduj_hacked/uploads/2018/04/Article_5_4_1.pdf DENTAL MATERIALS Original Article INFLUENCE OF CHEWING LOAD ON WEAR RATE OF POLYMETHYL METHACRYLATE DOUBLE CROSS-LINKED DENTURE TEETH IN VITRO Jean-François Roulet1a* , Abdullah Al-Naser2b, William Martin3c, Nader Abdulhameed1d, Chiayi Shen1e 1 Department of Restorative Dental Sciences, College of Dentistry, University of Florida, Gainesville, FL, USA 2 Department of Restorative Dental Sciences, Graduate Prosthodontics, College of Dentistry, University of Florida, Gainesville, FL, USA 3 Center for Implant Dentistry, College of Dentistry, University of Florida, Gainesville, FL, USA a DDS, PhD b DDS, MS c DMD, MS, FACP d BDS, MS e PhD ABSTRACT DOI: 10.25241/stomaeduj.2018.5(4).art.1 Purpose of the Study: Compare occlusal wear of PMMA DCL denture teeth under two different OPEN ACCESS This is an Open Access article under the CC BY-NC loads in vitro. 4.0 license. Materials & Methods: Sixteen mandibular second premolars (SR Orthoplane DCL) with a flat Peer-Reviewed Article occlusal surface (specimens) were worn by sixteen maxillary second premolar-antagonists Citation: Roulet J-F, Al-Naser A, Martin W, (Ortholingual DCL). These teeth were subjected in a chewing simulator (CS-4, SD Mechatronik) up Abdulhameed N, Shen C. Influence of chew- to 240,000 loading cylces at 19.6N (LL ≈ full denture) and 68.6N (HL ≈ implant-overdenture) and ing load on wear rate of polymethyl meth- acrylate double cross-linked denture teeth in TC (2,222 x 5 °C - 55 °C). Replicas of mandibular teeth were obtained at 0, 10,000; 20,000; 40,000; up vitro. Stoma Edu J. 2018;5(4):210-219 to 240,000 cycles with polyvinyl-siloxane impressions and dental stone. Antagonist-replicas were Academic Editor: David Christopher made at baseline and at 240,000 cycles. The volumetric wear was determined with Geomagic Watts, BSc, PhD, DSc, Professor, The University of Manchester, Manchester, United Kingdom after scanning replicas with a laser scanner. Linear regressions and ANOVA were used for statistical analysis. Received: November 10, 2018 Revised: November 20, 2018 Results: The wear rate of the HL-specimens was significantly higher than that of the LL-group (p Acccepted:November 30, 2018 < 0.0001). The LL-wear rate became linear after 60,000 cycles and was calculated to be 0.182 x 10-6 Published: December 04, 2018 mm3/stroke. The HL-wear rate was linear from 20,000 to 140,000 cycles and was 1.056 x 10-6 mm3/ *Corresponding author: Professor stroke, then up to 240,000 cycles 0.656 x 10-6 mm3/stroke. At 240,000 cycles the HL-group showed Jean-François Roulet, DDS, PhD, Prof hc, Director of Center for Dental Biomaterials significantly higher antagonist-wear (p < 0.0001). The antagonists in both groups demonstrated Department of Restorative Dental Sciences, College of Dentistry, University of Florida, higher wear than their opposing specimens (p < 0.08). 1395 Center Drive, Room D9-6, PO Box Conclusions: HL generated significantly higher wear of both the specimens and the antagonists. 100415, Gainesville FL 32610-0415, USA Tel: +1 352 273 5850; Fax: +1 352 846 The antagonists showed higher wear than the specimens. As a clinical consequence one may 1643, e-mail: jroulet@dental.ufl.edu expect more wear of denture teeth in implant supported overdentures than in full dentures. Copyright: © 2018 the Editorial Council Keywords: wear, denture, PMMA, cross linked, in vitro. for the Stomatology Edu Journal. 1. Introduction lost [8]; projections for 2050 predict the number of Estimates show that in the US the adult population in edentulous people in the US at 8.6 million [6]. Life need of one or two complete dentures will increase expectancy will continue to increase due to advances from 35.4 million adults in the year 2000 to 37.9 million in the medical fields. adults in 2020 [1]. Despite the fact that prevention is For over 100 years, complete maxillary and able to avoid tooth loss [2,3] these numbers are very mandibular dentures have been the traditional high. There are basically two reasons for this. First, the standard of care for edentulous patients [9], in which population demographics have changed dramatically. patients still perceive improved treatment success in Based on US census statistics, there are substantial terms of increased prosthesis retention and stability in trends observed for the time period 1991 – 2020. The this treatment method [10]. However, many patients total adult population will increase significantly from have limitations with stability and retention; over 187 million to 245 million; adults aged 55 to 74 years 50% of mandibular prostheses had such problems will increase by 86% from 39,280,000 to 73,099,000 [11]. Edentulous patients have prosthodontic and and senior adults 75 years and older will increase by physiologic limitations. With respect to physiology, 61%, from 13,489,000 to 21,835,000 [4,5]. Second, the significant amount of mechanoreception is population older than 55 did not profit much from compromised after teeth loss due to absence of the the benefits of prevention [1], and finally the social periodontal ligament, which contains sensory fibers. structure [6,7] combined with health care insurance It leads to abnormal changes in magnitude, precision, being not mandatory has favored tooth extractions and direction of occlusal load application [12]. Awad vs restorative dentistry. Thus, the aging population et al [13] reported that incorporating new dentures will bring with it an increase in the number of teeth may result in improvements of overall satisfaction 210 Stoma Edu J. 2018;5(4): 210-219. http://www.stomaeduj.com INFLUENCE OF CHEWING LOAD ON WEAR RATE OF POLYMETHYL METHACRYLATE DOUBLE CROSS-LINKED DENTURE TEETH IN VITRO reported by patients, regarding aesthetics, comfort, Original Article Table 1. Ingredients of PMMA DCL denture teeth [60]. and speech. On the other hand, for some patients, the incorporation of new dentures may not improve Ingredients Weight % their function [13]. This confirms that there is a wide Polymethyl methacrylate 33 - 35 variety in the ability of edentulous patients to tolerate complete dentures [13,14]. Among elderly denture Dimethylacrylate 5-7 patients, 25 % experienced pain when chewing and 41 % needed more time for chewing. This can be Cross-linked PMMA 59 explained with age related physiological changes, decreased motor control of the tongue, decreased UDMA/PMMA fillers 0 biting force, and medication induced xerostomia Pigments < 0.5 [15]. People wearing full dentures have less than 20% of the masticatory performance of those with Initiators & stabilizers < 0.5 a natural dentition [16,17]. If a few teeth, preferably canines, are left and usable in the mandible, teeth to be approximately 20 Newton [32], another study supported overdentures can be incorporated, which reported a mean chewing force of nearly 70 Newton means that complete dentures are supported by in the mandibular implant-retained overdentures both edentulous ridges and the retained natural [33], indicating an overall increase of load on denture roots. This solution shows increased retention, teeth within the studies’ limitations. stability, and comfort for the patients with increased Wear of acrylic resin teeth was reported in the quality of life [18]. With the introduction of titanium literature. Wear is a phenomenon that occurs when implants that osseointegrate in the 1970s [19-21] the two surfaces undergo a slipping movement under way was open for implant supported overdentures an applied load [34]. It is a complex, multifactorial with higher probability of success in the mandible process [35]. Abrasive wear occurs in natural and when overdentures are supported by implants rather artificial teeth, it is the removal of material by the than tooth roots [22]. In the year 2002, the McGill act of rubbing, cutting, or scraping [36]. Two-body consensus statement on overdentures declared abrasive wear occurs between denture teeth [37,38], that “Mandibular two-implant overdentures as first which is friction between two surfaces without an choice standard of care for edentulous patients [9] abrasive agent or medium present. The attritional are based on an overwhelming evidence” [8]. This wear resistance of restorative materials limits the decision has been supported and the superiority service time of the restorations [39]. of implant supported overdentures has been Nowadays, many edentulous patients are treated confirmed with systematic reviews [23,24]. using removable complete dentures with PMMA Removable complete dentures consist of DCL denture teeth as a common choice, because denture bases, which contain mostly polymethyl of their better wear resistance [40]. In addition, methacrylate [25]. Their designs are patient-specific, there is an increased trend in patient acceptance made during the prostheses processing in dental of implant-retained overdentures. With a potential laboratories. Denture teeth are the other component increase in motor control after implant treatment, of complete dentures, in general, there are three it might be beneficial to know if the difference in different materials used to fabricate teeth by dental chewing force will accelerate the wear of this type manufacturers. The ceramic type was first introduced of popular artificial teeth. Therefore, the objective of in Europe in 1789, its use is limited nowadays due to this in vitro study was to compare the occlusal wear difficulty in adjustment and potential fracture from rate of the modern PMMA DCL denture teeth under the denture base [26], although they have favorable two different loads. A low load simulated a mean esthetics and wear resistance. Acrylic resin denture chewing force in conventional complete dentures teeth were introduced in the 1940s, and they and a high load simulated a mean chewing force in contain mostly polymethyl methacrylate (PMMA). implant-retained overdentures. They are more frequently used than ceramic teeth Null hypotheses: in removable prosthodontics [27], due to some - The in vitro wear of DCL PMMA denture teeth is advantages such as excellent fracture toughness, independent of the chewing load; easy occlusal adjustment and high bond strength - The in vitro wear of DCL PMMA denture teeth to the denture base [28]. Their previous generations increases linearly with the number of chewing cycles. showed problematic wear resistance [29]. There are four subgroups under PMMA teeth; a) conventional unfilled, b) inorganically filled, c) highly cross- 2. Materials and Methods linked, and d) double cross-linked (DCL), which has DCL PMMA denture teeth (Ivoclar Vivadent, Schaan, improved mechanical and physical properties [30]. Liechtenstein) were selected. Sixteen mandibular Composite resin denture teeth were introduced in second premolars (SR Orthoplane DCL ML6, REF the 1980s. It is claimed that they have more favorable 565849, LOT UP0794) were used as specimens and esthetics and wear resistance [30]. sixteen maxillary second premolars (Ortholingual Some studies showed an increase in motor control DCL LU6, REF 565736, LOT UP2255) were used as and perception in removable implant-retained antagonists. The composition of these denture prostheses [31]. In patients with complete dentures, teeth is shown on Table 1. These teeth were placed a mean chewing force was reported in one study Stomatology Edu Journal 211 INFLUENCE OF CHEWING LOAD ON WEAR RATE OF POLYMETHYL METHACRYLATE DOUBLE CROSS-LINKED DENTURE TEETH IN VITRO Original Article Figure 1. Use of a polyvinyl siloxane putty jig to achieve consistent (a) Impression making of an antagonist tooth (left) and a specimen tooth relationship between the antagonist teeth to their holders. Note the (right). mechanical retention on the antagonist holder to obtain a rotation lock. (b) A stone replica of a specimen tooth. (a) A mounted antagonist tooth with its palatal cusp in contact with the center of a flat occlusal surface on a specimen tooth. (c) A stone replica of an antagonist tooth. Figure 3. Impression making of specimens and antagonists (a, b, c). (b) A specimen tooth was stabilized onto its antagonist tooth using a in a chewing simulator (CS-4, SD Mechatronik, dental stick wax. Feldkirchen-Westerham, Germany) at two different loads: 1. 19.6 Newton (low-load group): 8 specimens and 8 antagonists 2. 68.6 Newton (high-load group): 8 specimens and 8 antagonists The maxillary premolars (antagonists) were mounted with cold-curing acrylic resin (Pro Base Cold, Ivoclar Vivadent) onto antagonist metal holders, modified with a rotary instrument to achieve extra retention, air-abraded with aluminum oxide particles and conditioned with Monobond Plus (Ivoclar Vivadent). A polyvinyl siloxane putty jig (Virtual® XD Putty, Ivoclar Vivadent, Amherst, NY) was made after mounting the (c) Lowering the mouting jig arm to position specimen teeth into cold- first antagonist in order to get consistent relationship curing acrylic resin. between the antagonist teeth to their holders (Fig. 1). The antagonists were slightly tipped to position their Figure 2. Mounting of specimen teeth (a, b, c). palatal cusps at the highest point to be in contact with the specimens, their buccal cusps were shortened to 212 Stoma Edu J. 2018;5(4): 210-219. http://www.stomaeduj.com INFLUENCE OF CHEWING LOAD ON WEAR RATE OF POLYMETHYL METHACRYLATE DOUBLE CROSS-LINKED DENTURE TEETH IN VITRO be in level with the embedding medium. A mounting Original Article jig system was used to conveniently achieve specific occlusal relationship between those prepared antagonists and their opposing specimens outside the chewing simulator machine. It was aimed to have each palatal cusp in contact with the center of the monoplane mandibular premolar’s buccal cusp. Every mandibular premolar was stabilized using a small amount of dental sticky wax to a mounted antagonist in the defined position, and then the assembly was lowered into an individualized specimen holder, (a) A 3-D image of a specimen tooth showing a wear facet on the mono- loaded with cold-curing acrylic resin for embedding plane cusp. (Fig. 2). These metal holders had irregular designs, in order to prevent positional errors. When the setting became fully completed, a polyvinyl siloxane putty jig was made to securely support the assembly’s upper and lower components, which contained a pair of denture teeth, and then they were moved into the chewing simulator. The occlusal relationship in each pair of teeth was verified before having definitive positioning inside the machine. The specimens were mounted randomly into the 8 chambers of the chewing simulator and stressed mechanically and thermally at 1.2 Hz with horizontal (b) Selection of the facet and isolating the non-worn surfaces (in red), movement of 0.7 mm. Mechanical stresses were low- which were deleted subsequently. load (19.6 N) and high-load (68.6 N). Thermal stresses were introduced by cycling between 5 °C and 55 °C, dwell time was 30 s at each temperature with 15 s changing times for a total of 90 s per cycle. For 240,000 chewing strokes, there were 2,222 thermal cycles. Impressions of mandibular teeth were obtained before starting the experiment and after 10,000; 20,000; 40,000; 60;000; 80,000; 100,000; 120,000, 140,000, 160,000, 180,000, 200,000, 220,000, and 240,000 chewing strokes using polyvinyl siloxane impression materials (Virtual® XD Extra-Light Body and Virtual® (c) Close-up view: the facet area with its non-worn periphery after delet- XD Heavy Body, Ivoclar Vivadent). Impressions of ing the surrounding surfaces. antagonist teeth were made at baseline and after the end of the experiment only. The following impression technique was used: one step heavy body/wash using small plastic containers (plastic bottle caps) as impressions trays (Fig. 3). The impressions were poured using type IV dental stone (Silky-Rock, Whip Mix, Louisville, U.S.) after being boxed (Fig. 3). (d) Lateral view: the space within the facet was filled in creating a flat sur- 2.1. Quantification of wear face above in level with the non-worn periphery; it resembled the surface before starting wear simulation. Stone models of both specimens and antagonists Figure 4. Quantification of wear in the specimen teeth (a-d). were scanned using a laser scanner (Laserscanner LAS-20, SD Mechatronik). The amount of wear was delete all data that did not have any changes in the calculated using a 3-D software (Geomagic Control, final wear image inferior to the worn cusp’s borders. 3Dsystems, Rock Hill, SC, USA). With respect to the After isolating the superimposed worn and non- specimens, the periphery of each worn area (facet) worn cusp tips the non-worn periphery was used on the flat buccal cusp surface was selected first to as a reference to fill the space inside the cusp tip, be used as a reference. Then all the surrounding data volumetric measurements were taken in each model. (non-worn surfaces) were deleted, the depression The last step was to calculate the difference between inside the facet was filled using the “fill command”, them to yield in the material loss in cubic millimeter. generating a flat surface coronally at the same level with the non-worn borders, the last step was to 2.2. Evaluation of wear pattern volumetrically measure the space within the worn Surface details of the wear areas of both selected area, which represented the amount material loss in specimens and antagonists were inspected after cubic millimeter (Fig. 4). As for the antagonists, 3-D completion of chewing rounds using digital images of both the baseline and the final wear were microscopy (VH-1000 series, KEYENCE, Itasca, IL, U.S.) superimposed using four reference points. Circular at 100x magnification, in order to evaluate the wear trimming was done for the superimposed models to pattern. Stomatology Edu Journal 213 INFLUENCE OF CHEWING LOAD ON WEAR RATE OF POLYMETHYL METHACRYLATE DOUBLE CROSS-LINKED DENTURE TEETH IN VITRO 2.3. Statistical analysis Original Article Linear regression (SAS 9.4) of the amount of wear in volume against the number of chewing cycles was used to determine the wear rate of each specimen tested. The statistical difference between the mean wear rates of the two loading groups was determined by linear regression, ANOVA, and Tukey test. The statistical differences of the mean of total volume of wear of antagonists and the specimens as influenced by the loading were determined with the T-tests. 3. Results The experimental data show that there is a linear relationship between the cumulative wear volume Figure 5. Mean wear volume as function of loading cycles. The high load and the number of cycles (R2 > 95% for all specimens), group exhibits two segments (1 & 2) linear relationship and the straight line excluding the first reading of the high-load group and represents the best fit curve of the linear regression. (p < 0.0001). the first two readings of the low-load group (Fig. 5). Table 2. ANOVA and Tukey test for both segments of high load wear In addition, the high-load group appears to have two and the low load wear of the specimens. Dependent Variable: wear rate. segments with distinct linear relation intersecting Sum of Mean Source DF F Value Pr > F at 140,000 cycles. ANOVA and Tukey test showed Squares Square significant differences (p < 0.0001) between the two Model 2 3.05751734 1.52875867 116.84 <.0001 Error 21 0.27476608 0.01308410 segments (68.6N-1 and 68.6N-2) of the high load Corrected group and between the high load group and the low Total 23 3.33228342 load group (Fig. 5, Tab. 2). For the high load group two R-Square Coeff Var Root CYCLE separate wear rates were calculated: from 20,000 to MSE Mean 140,000 cycles it was 1.056 x 10-6 mm3/stroke, then up 0.917544 18.10987 0.114386 0.631621 Mean to 240,000 cycles 0.656 x 10-6 mm3/stroke. The wear Source DF Anova SS Square F Value Pr > F rate of the low load group was 0.182 x 10-6 mm3/stroke LOAD 2 3.05751734 1.52875867 116.84 <.0001 (Tab. 2). Tukey Grouping Mean N LOAD (N) The wear of the antagonists at the end of the A 1.05603 8 68.6 experiment (240.000 cycles) is shown in Fig. 6. As B 0.65607 8 68.6 for the sample teeth, there is a significant difference C 0.18277 8 19.6 between the high load and the low load group. Comparing the sample wear with the antagonist wear at 240.000 cycles T-tests revealed statistical differences (p < 0.001) between low-load group and either segment of high-load group, and between the two segments of the high-load group (Tab. 3). The microscopic images showed comparable wear patterns among the two groups’ specimens, unlike that of the antagonists, which showed coarse and irregular surface texture in the low-load group as compared to the high-load group (Figs. 7 & 8). Figure 6. Wear of the antagonists in both groups (Mean ± SD), p < 0.001. Table 3. Total wear volume (mean ± SD; in mm3) at the end of the 4. Discussion experiment. Different capital superscript letters show significant PMMA denture teeth have been used more frequently differences for columns and different low letters show significant than other types due to favorable procedural and differences for rows (p < 0.001). chemical properties [41]. Greater significance is added Loading cycle Type of surface 19.6 N 68.6 N in studies when a commonly used material is selected Antagonist 0.065 ± 0.004Aa 0.298 ± 0.015Ab for testing. The occlusal surface wear is a result of the Specimen 0.063 ± 0.002Ba 0.259 ± 0.090Bb combination of impact wear and sliding wear during process of mastication [42,43]. Progressive denture of nearly 70 N with overdentures [33]. We assume that teeth wear results in insufficient posterior teeth these values may reflect most clinical subjects, and support and consequently may lead to changes in the can be used in chewing simulation studies. vertical and horizontal yaw relations and may cause A large number of chewing simulators have been functional and aesthetical impairments [44,45]. In used to determine in vitro wear of acrylic resin denture order to assess the wear of a dental material such as teeth [46-49]. In the present study the denture teeth denture teeth, it is advised to use impact and sliding were subjected to two-body wear, as it was reported as both occur on teeth surfaces during mastication. in the literature [50-52]; it simulates the type of wear Denture teeth are used for edentulous patients, who that occurs in full dentures with bilaterally balanced have a reported mean chewing force of nearly 20 N occlusion [51]. With respect to the selection of wearing full dentures [32], and a mean chewing force chewing cycles number, a wear simulation study 214 Stoma Edu J. 2018;5(4): 210-219. http://www.stomaeduj.com INFLUENCE OF CHEWING LOAD ON WEAR RATE OF POLYMETHYL METHACRYLATE DOUBLE CROSS-LINKED DENTURE TEETH IN VITRO Original Article (a) A specimen tooth after final wear in the low-load group. (b) A facet on a specimen tooth in the low-load group (close-up view). (c) A specimen tooth after final wear in the high-load group. (d) A facet on a specimen tooth in the high-load group (close-up view). Figure 7. Microscopic digital images of specimen teeth in both groups a, b, c & d). of IPN PMMA denture teeth had wear results after was considered accurate and reliable in other studies 200,000 chewing cycles [53] comparable to 1-year [59-61]. Furthermore, we had experienced distortions clinic follow-up results with the same material in two of the flat surface from wear facets scanned directly in vivo studies [54,55]. Thus, Coffey et al [53] assumed from ceramic discs [62]. The interpretation was that that the amount of wear occurring at 200,000 cycles the laser beam, slightly entering a translucent surface would correspond to approximately 1 year of clinical was interacting with the material differently at an function. Regarding the selection of fluid, distilled edge than on a regular surface. Therefore, for the water was proved as a suitable intermediate medium, present study the replica technique was preferred, it did not have a significant difference with respect to where this phenomenon cannot happen with a stone mechanical properties of enamel and denture-base surface. materials when compared to human saliva [56,57]. The results of the present study showed significant The selection of both specimens and antagonists differences in the wear rate between the low-load denture teeth from the same material was done to and the high-load groups for the specimens (Fig. 5) simulate clinical conditions. The antagonist’s material and the antagonists as well (Fig. 6). Mean wear rate substantially influenced the wear rates in two-body of the high-load group specimens was 5.8 times wear [58]. In the present study, denture teeth with greater before 140,000 cycles and 3.6 times greater flat occlusal surfaces were used to simplify the wear after that turning point than the other group. As for analysis process. Measuring and interpreting the their antagonists’ mean wear rate, it was also greater wear facet on a flat surface is more predictable than by factor of 4.6. The linear regression analysis of the determining the wear on a complex occlusal surface specimens wear rate showed an overall straight- with possibly more than one wear facet. Furthermore, line data in both groups, indicating approximately it allowed comparisons with other studies using consistent relationship between number of cycles a similar approach [51]. When specimens with flat and amount of wear. No wear was detected in the surfaces were exposed to antagonists from the same low-load group until 10,000 chewing strokes, which material, wear results were close to that of denture might either show the lower limit of wear detection teeth’s clinical wear [59]. Type IV low expansion of this method or demonstrate favorable short-term dental stone replicas of denture teeth were produced wear resistance. after making polyvinyl siloxane impressions to There are some possible explanations for the shift investigate wear using a laser scanner, this method in the high-load group’s specimens wear rate after Stomatology Edu Journal 215 INFLUENCE OF CHEWING LOAD ON WEAR RATE OF POLYMETHYL METHACRYLATE DOUBLE CROSS-LINKED DENTURE TEETH IN VITRO Original Article (a) An antagonist tooth after final wear in the low-load group. (b) Worn palatal cusp tip after final wear in the low-load group (close-up view). (c) An antagonist tooth after final wear in the high-load group. (d) Worn palatal cusp tip after final wear in the low-load group (close-up view). Figure 8. Microscopic digital images of antagonist teeth in both groups (a, b, c & d). 140,000 chewing strokes. Upon removal of fillers in aluminum oxide antagonists and the greater chewing the specimens due to the fatigue process of the filler/ load they used (40 N) [51]. A 1-year clinical study matrix during lateral movement, the antagonists showed a comparable result to the high-load group of became rough, causing accelerated wear [59]. The the present study after 240,000 strokes, assuming the enamel layer’s ingredients of the PMMA denture average number of annual chewing strokes is close teeth might have different mechanical properties to the suggested rate [65]. The results of the present than that of dentin layer [25], it is possible that study also support those from an in vitro study, which dentin layers of specimens and/or antagonists were had steatite balls as antagonists, the researchers used exposed at around 140,000 strokes. The influence of 49 N chewing load [52], and their reported values the antagonist shape on wear rate of their opposing were in between those of our low-load group and surfaces was reported, a ball-shape stylus generated high-load group at both 120,000 and 240,000 strokes. significantly less wear on PMMA denture teeth than Wear resistance is an important physical property of a conical ceramic stylus as it created less fatigue removable denture teeth [48,66]. Clinical problems stress [59,63,64]. In the present study, more flattened were detected such as loss of vertical dimension, loss antagonist’s cusps appeared in the high-load group. of masticatory efficiency, faulty teeth relationship that We recommend using high-resolution microscopy to could affect patients and dental practice [11,44,67]. assess the ultrastructure of the specimens after each The previous generations of PMMA denture teeth chewing round, which might help to detect possible had poor wear performance, as was detected in an in differences in wear pattern. The difference may be vitro three-body wear assessment for non-DCL PMMA due to the measuring technique as well. In the high teeth [29]. On the contrary, PMMA DCL denture teeth load group some of the wear facets were extended showed higher in vitro wear resistance than the slightly beyond the flat surface with the result that conventional type [30,68]. With respect to the clinical some of the volume was missing due to the fact that wear assessment, patient-related factors should the flat surface was used as a reference. be considered, since there are differences among The specimens’ wear results were compared to individuals in muscle activity, duration of dentures previously reported data. One in vitro study showed wear, presence or absence of para-functional significantly greater wear rate after 100,000 strokes habits and abrasiveness of food [65]. According to [51] as compared to the low-load group in the present a clinical study, higher wear was detected in the study, it might be caused by the abrasive nature of implant-retained overdentures [33], this finding is in 216 Stoma Edu J. 2018;5(4):210-219. http://www.stomaeduj.com INFLUENCE OF CHEWING LOAD ON WEAR RATE OF POLYMETHYL METHACRYLATE DOUBLE CROSS-LINKED DENTURE TEETH IN VITRO agreement with the present study, since the high- 1996. 116th edition. Washington DCUS. 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Dr. Roulet is a member of many professional organizations, has won numerous awards, and holds four patents. He is editor of Prophylaxe Impuls and Stomatology Edu Journal. His areas of interest include minimally invasive dentistry, dental materials (ie, composites and ceramics), adhesive dentistry, esthetic dentistry, and application concepts in preventive dentistry. Questions 1. Which teeth were worn? qa. Ceramic; qb. Double crosslinked PMMA; qc. PMMA; qd. Composite. 2. Which loads were used? qa. 50 and 100 N; qb. 20 and 80 N; qc. 19.6 and 68.6 N; qd. 20 and 50 N. 3. How many load cycles were performed? qa. 120,000; qb. 240,000; qc. 500,000; qd. 2,222. 4. Which result is correct? qa. The high load group showed significantly higher wear; qb. The samples showed more wear than the antagonists; qc. The antagonists showed equal wear in both load groups; qd. There were no statistically significant differences in wear of the samples. Stomatology Edu Journal 219