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Wear of resin denture teeth in partial removable dental prostheses
G Model JPOR 558 No. of Pages 5
journal of prosthodontic research xxx (2019) xxx–xxx
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Journal of Prosthodontic Research journal homepage: www.elsevier.com/locate/jpor
Original article
Wear of resin denture teeth in partial removable dental prostheses Thomas Stobera,* , Justo Lorenzo Bermejob , Stefan Ruesa , Peter Rammelsberga a b
Department of Prosthodontics, Heidelberg University Hospital, Heidelberg, Germany Institute of Medical Biometry and Informatics, Heidelberg University Hospital, Heidelberg, Germany
A R T I C L E I N F O
A B S T R A C T
Article history: Received 18 September 2018 Received in revised form 12 March 2019 Accepted 2 April 2019 Available online xxx
Purpose: To investigate occlusal wear of resin denture teeth in partial removable dental prostheses worn by partially edentulous patients. Methods: Thirty patients with partial removable dental prostheses were included in the study. Thirty-two patients with complete dentures served as a reference group. Occlusal wear after two years was evaluated indirectly using gypsum casts and a three-dimensional laser-scanner device. Overall wear of complete occlusal surfaces and maximum wear of occlusal contact areas were measured. Patient and prosthesis data were analyzed using univariate and multiple linear mixed models. Results: Overall wear of denture teeth in partial removable dental prostheses was 91 (SD 85) mm, and maximum wear of occlusal contact areas was 329 (SD 204) mm (means and standard deviations). Average and maximum wear values for teeth in complete dentures were both lower than those for teeth in partial removable dental prostheses. However, differences between wear of different types of denture did not reach statistical significance after adjustment for gender, type of tooth, dental status of the opposing jaw, and antagonist material. Statistical analysis revealed that wear was greater for denture teeth occluding with ceramic crowns and/or fixed partial dentures as antagonists. Conclusions: Resin denture teeth in partial removable and complete dental prostheses are subjected to clinically important occlusal wear that might destabilize occlusion and cause further problems. Patientrelated factors and dental status affect wear behavior and should be taken into consideration when treating patients with removable dentures. © 2019 Japan Prosthodontic Society. Published by Elsevier Ltd. All rights reserved.
Keywords: Wear Occlusal wear Denture teeth Removable dental prosthesis Complete dental prosthesis
1. Introduction Removable dental prostheses remain a successful treatment option for partially edentulous patients, in particular for treatment of free-end saddles [1,2]. Stable occlusion is the foundation of longterm clinical success for all prosthetic treatments. In this respect, the wear resistance of denture teeth significantly affects the longterm success of partial and complete removable dental prostheses. For this reason, numerous in-vitro [3–9] and in-vivo [10–15] studies have been conducted on the wear behavior of denture teeth. Furthermore, many in-vitro studies have used different wear simulators to examine the wear of composite materials used for posterior restorations [16–19]. However, laboratory investigations of wear behavior usually have shortcomings. Contemporary chewing simulators employ only one or two of several possible wear mechanisms, and there is limited evidence of a correlation
* Corresponding author at: Department of Prosthodontics, Heidelberg University Hospital, Im Neuenheimer Feld 400, D-69120 Heidelberg, Germany. E-mail address: [email protected] (T. Stober).
between simulator wear data and clinical wear results [20,21]. It is therefore useful to study wear behavior in clinical settings. Recent clinical investigations of the wear of resin denture teeth have recorded acceptable occlusal stability with a vertical loss of approximately 150 mm in occlusal stress-bearing areas after one year [13–15]. However, this means that wear of resin denture teeth is clearly greater than that of enamel ones, for which annual wear of 32–51 mm [22] and 60–68 mm [23] has been recorded. Interestingly, nearly all clinical studies of the wear behavior of denture teeth have been conducted for patients wearing complete dentures. Few clinical data are available concerning the wear behavior of denture teeth in partial removable dental prostheses. Only one study (with a small number of patients) has been published on this subject [12]. In their pilot study, Ohlmann et al. calculated mean wear of 22.8 mm for teeth in partial removable dental prostheses after six months. However, this calculation was for the entire occlusal surface, not just the occlusal contact area. Furthermore, the authors found indications that gender, antagonist material, and type of denture significantly affect the wear behavior of resin denture teeth [12].
https://doi.org/10.1016/j.jpor.2019.04.004 1883-1958/ © 2019 Japan Prosthodontic Society. Published by Elsevier Ltd. All rights reserved.
Please cite this article in press as: T. Stober, et al., Wear of resin denture teeth in partial removable dental prostheses, J Prosthodont Res (2019), https://doi.org/10.1016/j.jpor.2019.04.004
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T. Stober et al. / journal of prosthodontic research xxx (2019) xxx–xxx Table 1. Characteristics of study participants and their dental prostheses.
Number of patients Gender (female %) Mean age of patients (years) Jaw (maxilla %) Kennedy classification (%)
a
Partial dental prosthesis (study group)
Complete dental prosthesis (reference)
30 40.0 68.8 (SD 6.9) 53.3 Kennedy class I: 73.3 Kennedy class II: 20.0 Kennedy class III: 6.7
32 65.6 65.2 (SD 11.6) 96.9a n.a. n.a. n.a.
If participants were edentulous in both jaws, only the upper denture was included in the study.
The main objective of our study was to investigate occlusal wear of resin denture teeth in partial removable dental prostheses and to compare these results with those for complete dentures. The null hypothesis was that wear behavior would not be affected by denture type. We also evaluated the effects of patient-related factors and the dental status of the opposing jaw on this wear. 2. Materials and methods Thirty patients receiving new partial removable dental prostheses with double-crown attachments were recruited for this study. Double-crown-retained removable dental prostheses have proved a successful treatment option for partially edentulous patients [24,25]. All study participants had to sign a consent form. The local university review board approved the study protocol (ethical approval no. 084/2003). The mean age of participants was 68.8 years (SD 6.9, range 47–83), and 40% were female. Thirty-two patients with new complete dentures who were edentulous in one or both jaws were recruited as a reference group. These participants had a mean age of 65.2 years (SD 11.6, range 44– 86), and 65.6% were female. Sixteen patients received a complete denture in the maxilla only, one patient in the mandible only, and 15 patients in both the maxilla and mandible, depending on their dental status. To reduce patient-related effects for patients receiving two dentures, only the denture in the maxilla was analyzed. Further characteristics of the participants and their dental prostheses are summarized in Table 1. The dentures were fabricated according to standard prostheticdenture treatment protocols. For patients receiving complete dentures in both jaws, dentures were adjusted in centric occlusion and in accordance with the principle of bilateral balanced occlusion. Denture teeth in both groups were made from poly (methyl methacrylate) with 14% inorganic fillers (Vitapan; VITA Zahnfabrik, Bad Säckingen, Germany). Removal of pressure spots and/or occlusal adjustments was performed within four weeks of incorporation. Occlusal wear was evaluated indirectly using impressions and gypsum casts of the dentures. Impressions made from vinyl polysiloxane using the dual-viscosity technique (Flexitime Putty and Flexitime Correct Flow; Kulzer, Germany) were prepared four weeks (baseline — t0) and 24 months (t1) after incorporation. Before taking impressions, all denture teeth were examined for calculus, stains, or foreign debris and, if necessary, cleaned using an ultrasonic cleaning system. The casts were made from gypsum type-IV dental stone (GC Fujirock EP Pearl White; Leuven, Belgium). The extent of wear was determined using a laser-scanner device (Willytec Laserscan 3D; SD Mechatronik, Feldkirchen-Westerham, Germany). The main steps of this now commonly used method, first described by Mehl et al. [26], are briefly summarized below. The occlusal surfaces of the posterior denture teeth were digitized indirectly using a laser scanner on gypsum casts. All premolars and molars were scanned separately. The default settings for the scanning process were adjusted as follows: The
number of light lines was increased from 400 to 700 and the step distance was 30 mm. The data records were then analyzed for wear-induced surface changes using the surface-analysis software Match 3D, Version 1.6 (Willytec; SD Mechatronik). The extent of wear was calculated by superimposing the baseline (t0) and followup (t1) scans using reference-free 3D superimposition [26]. To prevent wear zones or cast artifacts from impairing superimposition, a threshold value of 30 mm was defined. The superimposition process of the two corresponding scans was automatic and was accepted if the standard deviations were less than 20 mm. The entire occlusal surface of each premolar and molar denture tooth was analyzed, whereby the surface-analysis software was used to calculate overall wear of the entire occlusal surface and maximum wear of the occlusal contact area. The 2% quantile was used to eliminate outliers. Statistical analysis was conducted using SAS version 9.4 (SAS Institute Inc.; Cary, NC, USA). Univariate and multiple linear mixed models were used to evaluate possible effects of the following variables on the extent of wear: denture type, gender, age, tooth type, jaw, dental status of the opposing jaw, and antagonist material. The study participant was included as a random effect to account for the correlation of interpatient data. Probability values smaller than 0.05 were considered statistically significant. Assuming standard deviations equal to 85 mm for the overall wear of denture teeth and 204 mm for the maximum wear of occlusal contact areas, the study was able to detect true differences of 24 mm for overall wear and 56 mm for maximum wear with 80% statistical power (n = 303 teeth in complete and n = 157 teeth in partial dental prosthesis, unpaired t-test). 3. Results Results were obtained for 30 participants wearing partial removable dental prostheses (157 denture teeth). After 24 months, overall wear of the complete occlusal surface of denture teeth was 91 (85) mm, and maximum wear of the occlusal contact areas was 329 (204) mm (mean values with standard deviations). In the reference group of 32 edentulous participants with complete dentures (303 denture teeth), overall wear was 64 (73) mm and maximum wear was 255 (171) mm. Statistical analysis conducted using a univariate linear mixed model revealed that occlusal wear of partial dentures was significantly greater than that of complete dentures (Table 2: overall wear data; Table 3: maximum wear data). Because wear was analyzed by means of a multiple linear mixed model, however, the significant effect of denture type was lost. Several patient-related factors (gender, age, tooth type, jaw, dental status of the opposing jaw, antagonist material) were also included in the statistical analysis. The results for these are shown in Tables 2 and 3. Use of the univariate linear mixed model revealed statistically significant effects on the extent of wear for gender, tooth type, dental status of the opposing jaw, and antagonist material. Occlusal wear values of denture teeth with a natural tooth, crown, or fixed partial denture antagonist were greater than
Please cite this article in press as: T. Stober, et al., Wear of resin denture teeth in partial removable dental prostheses, J Prosthodont Res (2019), https://doi.org/10.1016/j.jpor.2019.04.004
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Table 2. Overall wear of complete occlusal surfaces of the denture teeth. Results from statistical analysis using univariate and multiple linear mixed models (including patient as a random effect). Overall wear
Level
n
Mean
Complete Partial Female Male 65 years >65 years Maxilla Mandible Premolar Molar No antagonist Complete denture Partial denture Natural tooth a/o crown a/o FDP No antagonist Enamel Metal alloy Ceramic Resin denture tooth
303 157 249 211 198 262 287 173 255 205 7 215 144 94 7 42 15 33 363
63.5 90.6 59.7 88.0 62.3 80.6 71.2 75.2 78.8 65.1 22.7 58.9 67.6 115.8 22.7 100.5 96.6 144.5 63.0
Variable Type of denture Gender Age Jaw Type of tooth Dental status of opposing jaw
Antagonist material
Univariate linear mixed model Difference Ref. 27.1 Ref. 25.4 Ref. 13.2 Ref. 3.1 Ref. 14.6
95%
CI
Multiple linear mixed model p-Value
0.04
Ref. 7.9 Ref. 18.9 Ref. 6.1 Ref. 7.5 Ref. 10.3
12.2
42.0
0.0004
11.2
39.6
0.0005
1.5
27.9
0.08
11.5
17.7
0.68
28.8
0.4
Difference
95%
CI
p-Value
33.3
0.54
34.3
0.02
9.0
21.3
0.43
9.1
24.1
0.37
24.4
3.8
0.15
17.5 3.6
Ref. 8.7 56.9
7.2 38.6
24.6 75.1
0.28 0.000000002
Ref. 17.2 44.5
3.3 11.7
37.7 100.7
0.10 0.12
Ref. 3.9 44.0 37.5
48.1 9.8 61.5
40.3 78.2 13.6
0.86 0.01 0.002
Ref. 7.4 44.4 9.4
37.8 9.2 49.8
52.5 79.5 68.7
0.75 0.01 0.75
Table 3. Maximum wear of occlusal contact areas of the denture teeth. Results from statistical analysis using univariate and multiple linear mixed models (including patient as a random effect). Maximum wear
Level
n
Mean
Variable Type of denture Gender Age Jaw Type of tooth Dental status of opposing jaw
Antagonist material
Univariate linear mixed model Difference
Complete Partial Female Male 65 years >65 years Maxilla Mandible Premolar Molar No antagonist Complete denture Partial denture Natural tooth a/o crown a/o FDP No antagonist Enamel Metal alloy Ceramic Resin denture tooth
303 157 249 211 198 262 287 173 255 205 7 215 144 94
254.6 329.0 240.6 326.4 251.0 301.9 277.4 284.2 282.9 276.3 154.4 246.6 261.9 393.2
7 42 15 33 363
154.4 369.7 324.9 465.1 253.3
Ref. 74.4 Ref. 78.5 Ref. 38.2 Ref. 4.6 Ref. 8.8
95%
CI
Multiple linear mixed model p-Value
Difference
39.0
109.8
0.00004
45.0
112.0
0.000005
3.2
73.2
0.03
30.3
39.4
0.80
42.7
25.2
0.61
Ref. 37.7 Ref. 55.4 Ref. 19.8 Ref. 13.0 Ref. 3.7
Ref. 15.2 146.6
22.3 103.4
52.8 189.7
0.43 0.00000000007
Ref. 41.3 86.4
Ref. 44.8 95.4 116.4
148.8 14.9 172.8
59.3 175.8 60.0
0.40 0.02 0.00006
Ref. 27.7 103.9 19.2
that of denture teeth occluding with removable partial or complete dentures. Antagonist material also had a significant effect on wear, which was greatest for denture teeth occluding with ceramic crowns and/or fixed partial dentures. Furthermore, multivariate analysis (multiple linear mixed model) was conducted to detect any possible associations or interactions between variables. In contrast to the univariate analysis, the multivariate analysis showed that only gender and antagonist material significantly affected wear. In particular, wear differences by the dental status of opposing jaw estimated using multiple linear regression (overall 44.5 mm and maximum 86.4 mm for “Natural tooth a/o crown a/o FDP” vs. “Complete denture”) did not reach statistical significance, probably due to the confounding effects of gender and antagonist material. 4. Discussion The main result of our study is that occlusal wear of denture teeth in partial dental prostheses was statistically comparable to
95%
CI
p-Value
105.1
0.27
91.5
0.003
15.8
55.4
0.28
25.9
52.0
0.51
29.3
36.8
0.82
9.8 47.9
92.4 220.7
0.11 0.21
133.8 21.4 158.6
78.4 186.5 120.2
0.61 0.01 0.79
29.7 19.3
that of denture teeth in complete dental prostheses, thus confirming the null hypothesis. We also proved that antagonist material significantly affects the extent of wear. Nonetheless, because sample sizes for some categories were small (e.g., only 15 metal-alloy crown antagonists were included), some results should be interpreted with caution. The wear recorded for complete dental prostheses after two years (overall wear: 64 mm; wear in occlusal contact areas: 255 mm) are in agreement with the results of previous clinical studies using the same research method. Schmid-Schwap et al. observed vertical loss of 121–221 mm for posterior denture teeth in occlusal contact areas after one year [13]. Heintze et al. recorded wear of approximately 200 mm for complete dental prostheses after two years of clinical service [15]. In both studies, the denture teeth investigated primarily contained poly(methyl methacrylate) with cross-linked organic filler particles [13,15]. Our own previous study—which used the same denture teeth as this study (a matrix of poly(methyl methacrylate) and 14% inorganic fillers)—suggested maximum occlusal wear of 226 mm in occlusal contact areas after two years [14].
Please cite this article in press as: T. Stober, et al., Wear of resin denture teeth in partial removable dental prostheses, J Prosthodont Res (2019), https://doi.org/10.1016/j.jpor.2019.04.004
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Regarding the wear behavior of partial removable dental prostheses, only one clinical study (with a small number of subjects) is available for comparison with our findings. Ohlmann et al. calculated overall wear of 22.8 mm for the complete occlusal surface area of partial removable dental prostheses after six months [12]. Assuming a linear increase, this is equivalent to occlusal wear of approximately 91 mm after two years—identical to the results presented here. It should be noted that the results here relate to partial removable dental prostheses with double-crown attachments. The use of different anchoring elements, such as clasps, might produce different results. Nearly all clinical wear studies, including ours, have found that occlusal wear of denture teeth varies greatly between patients [10– 15,27]. One possible explanation for this is that wear processes in the oral cavity result from a complex interaction of mechanical forces (e.g., bruxism) and several other factors (e.g., erosion, corrosion processes, and nutritional habits). The effect of patientrelated factors on the extent of wear is, however, disputed. With regard to gender, our study indicates that wear is greater for males than females. This is in agreement with the results of Ohlmann et al. [12], who also studied occlusal wear for a group of patients wearing partial and complete dental prostheses. However, no study of patient cohorts wearing only complete dental prostheses has found a significant effect for gender on occlusal wear [11,13,14]. Only Heintze et al. have reported that wear tends to be greater for men [27]. One possible explanation is that an effect of gender on occlusal forces has not been found for edentulous subjects [28]. Among dentate subjects, in contrast, significantly greater bite forces have been proven for men [29]. Furthermore, it has been shown that the bite forces of complete-denture wearers are lower than those of wearers of partial removable dental prostheses [30]. Another point of discussion is the effect of age on the extent of occlusal wear. Our study found no significant difference between subjects younger than 65 years old and those older than 65 years old. Previous studies have found that wear tends to decrease as a person’s age increases; lower occlusal forces were attributed to a reduction in muscle activity as a result of aging [13,27]. In agreement with previous results, a noticeable effect on occlusal wear was found for dental status and antagonist material [12,14]. Our data clearly show that wear of denture teeth increases by a factor of 1.5 (wear in occlusal contact areas) to 2 (overall wear of complete occlusal surface) when natural teeth and/or crowns and/or fixed dental prostheses are in occlusion, compared with complete dentures. Wear is further increased by ceramic antagonist occlusal surfaces. These findings (increased wear of antagonistic natural teeth and/or crowns and/or fixed dental prostheses) can also be attributed to varying bite forces. Bite forces of subjects with partial and/or complete denture prostheses are much smaller than those of subjects with natural dentition [30]. A lack of periodontal receptors among edentulous subjects might also result in less wear. In this context it should be noted that the surface structure of antagonists—in particular roughness—strongly affects wear behavior [31,32]. It is usually recognized that ceramic restorations should be polished thoroughly to minimize their abrasive effect. The use of diamond-impregnated polishers and diamond polishing paste as a final polish are recommended for this purpose. The effect on wear of tooth type (premolar vs. molar) remains unclear. Our results are indicative of slightly higher overall wear rate for premolars (14.6 mm, univariate p = 0.04), but the difference did not reach statistical significance in the multiple linear mixed model (p = 0.15). Other studies have reported that volumetric wear depends on tooth type, although differences decreased substantially after results were adjusted for tooth surface [13].
Finally, the clinical relevance of our results must be addressed. The following ‘physiological’ enamel–enamel-generated wear of natural premolars and molars after two years have been reported: 54–91 mm [22], 75–115 mm [23], and 40–80 mm [33]. In comparison, our study suggests wear rate is three to four times greater for denture teeth (255–329 mm) than for natural ones, which might be clinically relevant. The clinical relevance of the reported differences between the wear of denture teeth in partial removable dental prostheses and that in complete dental prostheses is open to question, however. Nevertheless, the wear resistance of denture teeth should be as high as possible because a partial removable dental prosthesis should restore chewing function, esthetics, and speech for many years to come. It should also prevent migration of the remaining teeth. To prevent the destabilization of occlusion, the wear of denture teeth should, ideally, be similar to the ‘physiological’ wear of natural teeth. The suitability of the method used to measure wear in our study has been verified in previous clinical studies [12–14]. It is regarded as a valid method [34] with high reproducibility and accuracy of 10–15 mm [26]. Nonetheless, it is important to note that the algorithm of the underlying matching process tends to underestimate actual wear, because both positive and negative values are considered when calculating differences. Changes in the dimensions of impressions, expansion of stone gypsum replicas during setting, and scanner accuracy are further weaknesses of the method. All these factors are inherent errors and might contribute to slight differences between wear values when compared with other studies. Notable positive aspects of the study are the prospective study design with a control group and the 24-month follow-up period after denture placement. Weaknesses are the small number of participants and small sample sizes of some subgroups; because this reduced the power of statistical analysis, some significant factors might not have been detected. 5. Conclusion Wear of dentures measured in this study after two years of clinical use should be regarded as clinically important because such wear might destabilize occlusion, thus resulting in other problems. Wear tends to be greater for partially edentulous patients wearing removable dental prostheses than for patients wearing complete dentures. However, the differences did not reach statistical significance. Within the limitations of this study, it can be concluded that the antagonist material affects occlusal wear of denture teeth. This should be taken into account when treating patients with removable dentures. Declaration of interest Financial support was used to compensate patients for their additional effort attending follow-ups. The authors have no other financially beneficial arrangements with Kulzer and/or Wieland Dental. Acknowledgments We thank Heraeus Kulzer (now Kulzer), Germany and Wieland Dental, Germany for financial support. We also thank Hazel Davies, copy editor, for English-language revision, and Clemens Schmitt for assistance with measurement of wear. References [1] Celebi c A, Knezovi c-Zlatari c D. A comparison of patient’s satisfaction between complete and partial removable denture wearers. J Dent 2003;31:445–51.
Please cite this article in press as: T. Stober, et al., Wear of resin denture teeth in partial removable dental prostheses, J Prosthodont Res (2019), https://doi.org/10.1016/j.jpor.2019.04.004
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journal of prosthodontic research xxx (2019) xxx–xxx
Contents lists available at ScienceDirect
Journal of Prosthodontic Research journal homepage: www.elsevier.com/locate/jpor
Original article
Wear of resin denture teeth in partial removable dental prostheses Thomas Stobera,* , Justo Lorenzo Bermejob , Stefan Ruesa , Peter Rammelsberga a b
Department of Prosthodontics, Heidelberg University Hospital, Heidelberg, Germany Institute of Medical Biometry and Informatics, Heidelberg University Hospital, Heidelberg, Germany
A R T I C L E I N F O
A B S T R A C T
Article history: Received 18 September 2018 Received in revised form 12 March 2019 Accepted 2 April 2019 Available online xxx
Purpose: To investigate occlusal wear of resin denture teeth in partial removable dental prostheses worn by partially edentulous patients. Methods: Thirty patients with partial removable dental prostheses were included in the study. Thirty-two patients with complete dentures served as a reference group. Occlusal wear after two years was evaluated indirectly using gypsum casts and a three-dimensional laser-scanner device. Overall wear of complete occlusal surfaces and maximum wear of occlusal contact areas were measured. Patient and prosthesis data were analyzed using univariate and multiple linear mixed models. Results: Overall wear of denture teeth in partial removable dental prostheses was 91 (SD 85) mm, and maximum wear of occlusal contact areas was 329 (SD 204) mm (means and standard deviations). Average and maximum wear values for teeth in complete dentures were both lower than those for teeth in partial removable dental prostheses. However, differences between wear of different types of denture did not reach statistical significance after adjustment for gender, type of tooth, dental status of the opposing jaw, and antagonist material. Statistical analysis revealed that wear was greater for denture teeth occluding with ceramic crowns and/or fixed partial dentures as antagonists. Conclusions: Resin denture teeth in partial removable and complete dental prostheses are subjected to clinically important occlusal wear that might destabilize occlusion and cause further problems. Patientrelated factors and dental status affect wear behavior and should be taken into consideration when treating patients with removable dentures. © 2019 Japan Prosthodontic Society. Published by Elsevier Ltd. All rights reserved.
Keywords: Wear Occlusal wear Denture teeth Removable dental prosthesis Complete dental prosthesis
1. Introduction Removable dental prostheses remain a successful treatment option for partially edentulous patients, in particular for treatment of free-end saddles [1,2]. Stable occlusion is the foundation of longterm clinical success for all prosthetic treatments. In this respect, the wear resistance of denture teeth significantly affects the longterm success of partial and complete removable dental prostheses. For this reason, numerous in-vitro [3–9] and in-vivo [10–15] studies have been conducted on the wear behavior of denture teeth. Furthermore, many in-vitro studies have used different wear simulators to examine the wear of composite materials used for posterior restorations [16–19]. However, laboratory investigations of wear behavior usually have shortcomings. Contemporary chewing simulators employ only one or two of several possible wear mechanisms, and there is limited evidence of a correlation
* Corresponding author at: Department of Prosthodontics, Heidelberg University Hospital, Im Neuenheimer Feld 400, D-69120 Heidelberg, Germany. E-mail address: [email protected] (T. Stober).
between simulator wear data and clinical wear results [20,21]. It is therefore useful to study wear behavior in clinical settings. Recent clinical investigations of the wear of resin denture teeth have recorded acceptable occlusal stability with a vertical loss of approximately 150 mm in occlusal stress-bearing areas after one year [13–15]. However, this means that wear of resin denture teeth is clearly greater than that of enamel ones, for which annual wear of 32–51 mm [22] and 60–68 mm [23] has been recorded. Interestingly, nearly all clinical studies of the wear behavior of denture teeth have been conducted for patients wearing complete dentures. Few clinical data are available concerning the wear behavior of denture teeth in partial removable dental prostheses. Only one study (with a small number of patients) has been published on this subject [12]. In their pilot study, Ohlmann et al. calculated mean wear of 22.8 mm for teeth in partial removable dental prostheses after six months. However, this calculation was for the entire occlusal surface, not just the occlusal contact area. Furthermore, the authors found indications that gender, antagonist material, and type of denture significantly affect the wear behavior of resin denture teeth [12].
https://doi.org/10.1016/j.jpor.2019.04.004 1883-1958/ © 2019 Japan Prosthodontic Society. Published by Elsevier Ltd. All rights reserved.
Please cite this article in press as: T. Stober, et al., Wear of resin denture teeth in partial removable dental prostheses, J Prosthodont Res (2019), https://doi.org/10.1016/j.jpor.2019.04.004
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T. Stober et al. / journal of prosthodontic research xxx (2019) xxx–xxx Table 1. Characteristics of study participants and their dental prostheses.
Number of patients Gender (female %) Mean age of patients (years) Jaw (maxilla %) Kennedy classification (%)
a
Partial dental prosthesis (study group)
Complete dental prosthesis (reference)
30 40.0 68.8 (SD 6.9) 53.3 Kennedy class I: 73.3 Kennedy class II: 20.0 Kennedy class III: 6.7
32 65.6 65.2 (SD 11.6) 96.9a n.a. n.a. n.a.
If participants were edentulous in both jaws, only the upper denture was included in the study.
The main objective of our study was to investigate occlusal wear of resin denture teeth in partial removable dental prostheses and to compare these results with those for complete dentures. The null hypothesis was that wear behavior would not be affected by denture type. We also evaluated the effects of patient-related factors and the dental status of the opposing jaw on this wear. 2. Materials and methods Thirty patients receiving new partial removable dental prostheses with double-crown attachments were recruited for this study. Double-crown-retained removable dental prostheses have proved a successful treatment option for partially edentulous patients [24,25]. All study participants had to sign a consent form. The local university review board approved the study protocol (ethical approval no. 084/2003). The mean age of participants was 68.8 years (SD 6.9, range 47–83), and 40% were female. Thirty-two patients with new complete dentures who were edentulous in one or both jaws were recruited as a reference group. These participants had a mean age of 65.2 years (SD 11.6, range 44– 86), and 65.6% were female. Sixteen patients received a complete denture in the maxilla only, one patient in the mandible only, and 15 patients in both the maxilla and mandible, depending on their dental status. To reduce patient-related effects for patients receiving two dentures, only the denture in the maxilla was analyzed. Further characteristics of the participants and their dental prostheses are summarized in Table 1. The dentures were fabricated according to standard prostheticdenture treatment protocols. For patients receiving complete dentures in both jaws, dentures were adjusted in centric occlusion and in accordance with the principle of bilateral balanced occlusion. Denture teeth in both groups were made from poly (methyl methacrylate) with 14% inorganic fillers (Vitapan; VITA Zahnfabrik, Bad Säckingen, Germany). Removal of pressure spots and/or occlusal adjustments was performed within four weeks of incorporation. Occlusal wear was evaluated indirectly using impressions and gypsum casts of the dentures. Impressions made from vinyl polysiloxane using the dual-viscosity technique (Flexitime Putty and Flexitime Correct Flow; Kulzer, Germany) were prepared four weeks (baseline — t0) and 24 months (t1) after incorporation. Before taking impressions, all denture teeth were examined for calculus, stains, or foreign debris and, if necessary, cleaned using an ultrasonic cleaning system. The casts were made from gypsum type-IV dental stone (GC Fujirock EP Pearl White; Leuven, Belgium). The extent of wear was determined using a laser-scanner device (Willytec Laserscan 3D; SD Mechatronik, Feldkirchen-Westerham, Germany). The main steps of this now commonly used method, first described by Mehl et al. [26], are briefly summarized below. The occlusal surfaces of the posterior denture teeth were digitized indirectly using a laser scanner on gypsum casts. All premolars and molars were scanned separately. The default settings for the scanning process were adjusted as follows: The
number of light lines was increased from 400 to 700 and the step distance was 30 mm. The data records were then analyzed for wear-induced surface changes using the surface-analysis software Match 3D, Version 1.6 (Willytec; SD Mechatronik). The extent of wear was calculated by superimposing the baseline (t0) and followup (t1) scans using reference-free 3D superimposition [26]. To prevent wear zones or cast artifacts from impairing superimposition, a threshold value of 30 mm was defined. The superimposition process of the two corresponding scans was automatic and was accepted if the standard deviations were less than 20 mm. The entire occlusal surface of each premolar and molar denture tooth was analyzed, whereby the surface-analysis software was used to calculate overall wear of the entire occlusal surface and maximum wear of the occlusal contact area. The 2% quantile was used to eliminate outliers. Statistical analysis was conducted using SAS version 9.4 (SAS Institute Inc.; Cary, NC, USA). Univariate and multiple linear mixed models were used to evaluate possible effects of the following variables on the extent of wear: denture type, gender, age, tooth type, jaw, dental status of the opposing jaw, and antagonist material. The study participant was included as a random effect to account for the correlation of interpatient data. Probability values smaller than 0.05 were considered statistically significant. Assuming standard deviations equal to 85 mm for the overall wear of denture teeth and 204 mm for the maximum wear of occlusal contact areas, the study was able to detect true differences of 24 mm for overall wear and 56 mm for maximum wear with 80% statistical power (n = 303 teeth in complete and n = 157 teeth in partial dental prosthesis, unpaired t-test). 3. Results Results were obtained for 30 participants wearing partial removable dental prostheses (157 denture teeth). After 24 months, overall wear of the complete occlusal surface of denture teeth was 91 (85) mm, and maximum wear of the occlusal contact areas was 329 (204) mm (mean values with standard deviations). In the reference group of 32 edentulous participants with complete dentures (303 denture teeth), overall wear was 64 (73) mm and maximum wear was 255 (171) mm. Statistical analysis conducted using a univariate linear mixed model revealed that occlusal wear of partial dentures was significantly greater than that of complete dentures (Table 2: overall wear data; Table 3: maximum wear data). Because wear was analyzed by means of a multiple linear mixed model, however, the significant effect of denture type was lost. Several patient-related factors (gender, age, tooth type, jaw, dental status of the opposing jaw, antagonist material) were also included in the statistical analysis. The results for these are shown in Tables 2 and 3. Use of the univariate linear mixed model revealed statistically significant effects on the extent of wear for gender, tooth type, dental status of the opposing jaw, and antagonist material. Occlusal wear values of denture teeth with a natural tooth, crown, or fixed partial denture antagonist were greater than
Please cite this article in press as: T. Stober, et al., Wear of resin denture teeth in partial removable dental prostheses, J Prosthodont Res (2019), https://doi.org/10.1016/j.jpor.2019.04.004
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Table 2. Overall wear of complete occlusal surfaces of the denture teeth. Results from statistical analysis using univariate and multiple linear mixed models (including patient as a random effect). Overall wear
Level
n
Mean
Complete Partial Female Male 65 years >65 years Maxilla Mandible Premolar Molar No antagonist Complete denture Partial denture Natural tooth a/o crown a/o FDP No antagonist Enamel Metal alloy Ceramic Resin denture tooth
303 157 249 211 198 262 287 173 255 205 7 215 144 94 7 42 15 33 363
63.5 90.6 59.7 88.0 62.3 80.6 71.2 75.2 78.8 65.1 22.7 58.9 67.6 115.8 22.7 100.5 96.6 144.5 63.0
Variable Type of denture Gender Age Jaw Type of tooth Dental status of opposing jaw
Antagonist material
Univariate linear mixed model Difference Ref. 27.1 Ref. 25.4 Ref. 13.2 Ref. 3.1 Ref. 14.6
95%
CI
Multiple linear mixed model p-Value
0.04
Ref. 7.9 Ref. 18.9 Ref. 6.1 Ref. 7.5 Ref. 10.3
12.2
42.0
0.0004
11.2
39.6
0.0005
1.5
27.9
0.08
11.5
17.7
0.68
28.8
0.4
Difference
95%
CI
p-Value
33.3
0.54
34.3
0.02
9.0
21.3
0.43
9.1
24.1
0.37
24.4
3.8
0.15
17.5 3.6
Ref. 8.7 56.9
7.2 38.6
24.6 75.1
0.28 0.000000002
Ref. 17.2 44.5
3.3 11.7
37.7 100.7
0.10 0.12
Ref. 3.9 44.0 37.5
48.1 9.8 61.5
40.3 78.2 13.6
0.86 0.01 0.002
Ref. 7.4 44.4 9.4
37.8 9.2 49.8
52.5 79.5 68.7
0.75 0.01 0.75
Table 3. Maximum wear of occlusal contact areas of the denture teeth. Results from statistical analysis using univariate and multiple linear mixed models (including patient as a random effect). Maximum wear
Level
n
Mean
Variable Type of denture Gender Age Jaw Type of tooth Dental status of opposing jaw
Antagonist material
Univariate linear mixed model Difference
Complete Partial Female Male 65 years >65 years Maxilla Mandible Premolar Molar No antagonist Complete denture Partial denture Natural tooth a/o crown a/o FDP No antagonist Enamel Metal alloy Ceramic Resin denture tooth
303 157 249 211 198 262 287 173 255 205 7 215 144 94
254.6 329.0 240.6 326.4 251.0 301.9 277.4 284.2 282.9 276.3 154.4 246.6 261.9 393.2
7 42 15 33 363
154.4 369.7 324.9 465.1 253.3
Ref. 74.4 Ref. 78.5 Ref. 38.2 Ref. 4.6 Ref. 8.8
95%
CI
Multiple linear mixed model p-Value
Difference
39.0
109.8
0.00004
45.0
112.0
0.000005
3.2
73.2
0.03
30.3
39.4
0.80
42.7
25.2
0.61
Ref. 37.7 Ref. 55.4 Ref. 19.8 Ref. 13.0 Ref. 3.7
Ref. 15.2 146.6
22.3 103.4
52.8 189.7
0.43 0.00000000007
Ref. 41.3 86.4
Ref. 44.8 95.4 116.4
148.8 14.9 172.8
59.3 175.8 60.0
0.40 0.02 0.00006
Ref. 27.7 103.9 19.2
that of denture teeth occluding with removable partial or complete dentures. Antagonist material also had a significant effect on wear, which was greatest for denture teeth occluding with ceramic crowns and/or fixed partial dentures. Furthermore, multivariate analysis (multiple linear mixed model) was conducted to detect any possible associations or interactions between variables. In contrast to the univariate analysis, the multivariate analysis showed that only gender and antagonist material significantly affected wear. In particular, wear differences by the dental status of opposing jaw estimated using multiple linear regression (overall 44.5 mm and maximum 86.4 mm for “Natural tooth a/o crown a/o FDP” vs. “Complete denture”) did not reach statistical significance, probably due to the confounding effects of gender and antagonist material. 4. Discussion The main result of our study is that occlusal wear of denture teeth in partial dental prostheses was statistically comparable to
95%
CI
p-Value
105.1
0.27
91.5
0.003
15.8
55.4
0.28
25.9
52.0
0.51
29.3
36.8
0.82
9.8 47.9
92.4 220.7
0.11 0.21
133.8 21.4 158.6
78.4 186.5 120.2
0.61 0.01 0.79
29.7 19.3
that of denture teeth in complete dental prostheses, thus confirming the null hypothesis. We also proved that antagonist material significantly affects the extent of wear. Nonetheless, because sample sizes for some categories were small (e.g., only 15 metal-alloy crown antagonists were included), some results should be interpreted with caution. The wear recorded for complete dental prostheses after two years (overall wear: 64 mm; wear in occlusal contact areas: 255 mm) are in agreement with the results of previous clinical studies using the same research method. Schmid-Schwap et al. observed vertical loss of 121–221 mm for posterior denture teeth in occlusal contact areas after one year [13]. Heintze et al. recorded wear of approximately 200 mm for complete dental prostheses after two years of clinical service [15]. In both studies, the denture teeth investigated primarily contained poly(methyl methacrylate) with cross-linked organic filler particles [13,15]. Our own previous study—which used the same denture teeth as this study (a matrix of poly(methyl methacrylate) and 14% inorganic fillers)—suggested maximum occlusal wear of 226 mm in occlusal contact areas after two years [14].
Please cite this article in press as: T. Stober, et al., Wear of resin denture teeth in partial removable dental prostheses, J Prosthodont Res (2019), https://doi.org/10.1016/j.jpor.2019.04.004
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Regarding the wear behavior of partial removable dental prostheses, only one clinical study (with a small number of subjects) is available for comparison with our findings. Ohlmann et al. calculated overall wear of 22.8 mm for the complete occlusal surface area of partial removable dental prostheses after six months [12]. Assuming a linear increase, this is equivalent to occlusal wear of approximately 91 mm after two years—identical to the results presented here. It should be noted that the results here relate to partial removable dental prostheses with double-crown attachments. The use of different anchoring elements, such as clasps, might produce different results. Nearly all clinical wear studies, including ours, have found that occlusal wear of denture teeth varies greatly between patients [10– 15,27]. One possible explanation for this is that wear processes in the oral cavity result from a complex interaction of mechanical forces (e.g., bruxism) and several other factors (e.g., erosion, corrosion processes, and nutritional habits). The effect of patientrelated factors on the extent of wear is, however, disputed. With regard to gender, our study indicates that wear is greater for males than females. This is in agreement with the results of Ohlmann et al. [12], who also studied occlusal wear for a group of patients wearing partial and complete dental prostheses. However, no study of patient cohorts wearing only complete dental prostheses has found a significant effect for gender on occlusal wear [11,13,14]. Only Heintze et al. have reported that wear tends to be greater for men [27]. One possible explanation is that an effect of gender on occlusal forces has not been found for edentulous subjects [28]. Among dentate subjects, in contrast, significantly greater bite forces have been proven for men [29]. Furthermore, it has been shown that the bite forces of complete-denture wearers are lower than those of wearers of partial removable dental prostheses [30]. Another point of discussion is the effect of age on the extent of occlusal wear. Our study found no significant difference between subjects younger than 65 years old and those older than 65 years old. Previous studies have found that wear tends to decrease as a person’s age increases; lower occlusal forces were attributed to a reduction in muscle activity as a result of aging [13,27]. In agreement with previous results, a noticeable effect on occlusal wear was found for dental status and antagonist material [12,14]. Our data clearly show that wear of denture teeth increases by a factor of 1.5 (wear in occlusal contact areas) to 2 (overall wear of complete occlusal surface) when natural teeth and/or crowns and/or fixed dental prostheses are in occlusion, compared with complete dentures. Wear is further increased by ceramic antagonist occlusal surfaces. These findings (increased wear of antagonistic natural teeth and/or crowns and/or fixed dental prostheses) can also be attributed to varying bite forces. Bite forces of subjects with partial and/or complete denture prostheses are much smaller than those of subjects with natural dentition [30]. A lack of periodontal receptors among edentulous subjects might also result in less wear. In this context it should be noted that the surface structure of antagonists—in particular roughness—strongly affects wear behavior [31,32]. It is usually recognized that ceramic restorations should be polished thoroughly to minimize their abrasive effect. The use of diamond-impregnated polishers and diamond polishing paste as a final polish are recommended for this purpose. The effect on wear of tooth type (premolar vs. molar) remains unclear. Our results are indicative of slightly higher overall wear rate for premolars (14.6 mm, univariate p = 0.04), but the difference did not reach statistical significance in the multiple linear mixed model (p = 0.15). Other studies have reported that volumetric wear depends on tooth type, although differences decreased substantially after results were adjusted for tooth surface [13].
Finally, the clinical relevance of our results must be addressed. The following ‘physiological’ enamel–enamel-generated wear of natural premolars and molars after two years have been reported: 54–91 mm [22], 75–115 mm [23], and 40–80 mm [33]. In comparison, our study suggests wear rate is three to four times greater for denture teeth (255–329 mm) than for natural ones, which might be clinically relevant. The clinical relevance of the reported differences between the wear of denture teeth in partial removable dental prostheses and that in complete dental prostheses is open to question, however. Nevertheless, the wear resistance of denture teeth should be as high as possible because a partial removable dental prosthesis should restore chewing function, esthetics, and speech for many years to come. It should also prevent migration of the remaining teeth. To prevent the destabilization of occlusion, the wear of denture teeth should, ideally, be similar to the ‘physiological’ wear of natural teeth. The suitability of the method used to measure wear in our study has been verified in previous clinical studies [12–14]. It is regarded as a valid method [34] with high reproducibility and accuracy of 10–15 mm [26]. Nonetheless, it is important to note that the algorithm of the underlying matching process tends to underestimate actual wear, because both positive and negative values are considered when calculating differences. Changes in the dimensions of impressions, expansion of stone gypsum replicas during setting, and scanner accuracy are further weaknesses of the method. All these factors are inherent errors and might contribute to slight differences between wear values when compared with other studies. Notable positive aspects of the study are the prospective study design with a control group and the 24-month follow-up period after denture placement. Weaknesses are the small number of participants and small sample sizes of some subgroups; because this reduced the power of statistical analysis, some significant factors might not have been detected. 5. Conclusion Wear of dentures measured in this study after two years of clinical use should be regarded as clinically important because such wear might destabilize occlusion, thus resulting in other problems. Wear tends to be greater for partially edentulous patients wearing removable dental prostheses than for patients wearing complete dentures. However, the differences did not reach statistical significance. Within the limitations of this study, it can be concluded that the antagonist material affects occlusal wear of denture teeth. This should be taken into account when treating patients with removable dentures. Declaration of interest Financial support was used to compensate patients for their additional effort attending follow-ups. The authors have no other financially beneficial arrangements with Kulzer and/or Wieland Dental. Acknowledgments We thank Heraeus Kulzer (now Kulzer), Germany and Wieland Dental, Germany for financial support. We also thank Hazel Davies, copy editor, for English-language revision, and Clemens Schmitt for assistance with measurement of wear. References [1] Celebi c A, Knezovi c-Zlatari c D. A comparison of patient’s satisfaction between complete and partial removable denture wearers. J Dent 2003;31:445–51.
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Please cite this article in press as: T. Stober, et al., Wear of resin denture teeth in partial removable dental prostheses, J Prosthodont Res (2019), https://doi.org/10.1016/j.jpor.2019.04.004