Thomas Stober

Enamel wear caused by monolithic zirconia crowns after 6 months of clinical use

The purpose of this study was to evaluate enamel wear caused by monolithic zirconia crowns and to compare this with enamel wear caused by contralateral natural antagonists. Twenty monolithic zirconia crowns were placed in 20 patients requiring full molar crowns. For measurement of wear, impressions of both jaws were made at baseline after crown cementation and at 6-month follow-up. Mean and maximum wear of the occlusal contact areas of the crowns, of their natural antagonists and of the two contralateral natural antagonists were measured by the use of plaster replicas and 3D laser scanning methods. Wear differences were investigated by the use of two-sided paired Students t-tests and by linear regression analysis. Mean vertical loss (maximum vertical loss in parentheses) was 10 (43) μm for the zirconia crowns, 33 (112) μm for the opposing enamel, 10 (58) μm for the contralateral teeth and 10 (46) μm for the contralateral antagonists. Both mean and maximum enamel wear were significantly different between the antagonists of the zirconia crowns and the contralateral antagonists. Gender and activity of the masseter muscle at night (bruxism) were identified as possible confounders which significantly affected wear. Under clinical conditions, monolithic zirconia crowns seem to be associated with more wear of opposed enamel than are natural teeth. With regard to wear behaviour, clinical application of monolithic zirconia crowns is justifiable because the amount of antagonistic enamel wear after 6xa0months is comparable with, or even lower than, that caused by other ceramic materials in previous studies.

Accuracy of the newly formulated vinyl siloxanether elastomeric impression material

STATEMENT OF PROBLEMnA newly formulated vinyl siloxanether elastomeric impression material is available, but there is little knowledge of its accuracy in relation to existing materials.nnnPURPOSEnThe purpose of this in vitro study was to assess the accuracy of disinfected vinyl siloxanether impressions and compare the accuracy to a common vinyl polysiloxane and a polyether impression system.nnnMATERIAL AND METHODSnImpressions were made from a modified dentoform master model containing a simulated crown preparation. Dimensional changes (mm) between the master model and working casts (type IV gypsum) were assessed. The following were evaluated: vinyl polysiloxane in a 1-step, dual-viscosity technique (VPS Dual), polyether as monophase material (PE Mono), and vinyl siloxanether in a 1-step, dual-viscosity (VSE Dual), and monophase technique (VSE Mono). Measurements of the master model and working casts, including anteroposterior (AP) and cross-arch (XA) dimensions, were made with a measuring microscope. The simulated crown preparation was measured in mesiodistal (MDG, MDO), buccolingual (BLG, BLO), and occlusogingival dimensions (OGL, OGB). Disinfection involved immersion for 10 minutes in potassium peroxomonosulfate, sodium benzoate, tartaric acid solution, or no disinfection (control) (n=8). A multivariate GLM statistical approach (MANOVA) was used to analyze the data (alpha=.05). Pearsons correlation test was used for related dimensions.nnnRESULTSnThe AP and XA dimensions of working casts were larger than the master for the disinfected condition and control. Whether disinfected or not, the working dies were shorter in height (OGB, OGL), larger in the buccolingual dimension (BLO, BLG), somewhat larger in the MDO dimension, and somewhat smaller in the MDG dimension compared to the prepared tooth of the master model, resulting in an irregular or oval profile. There were significant differences among the impression systems for each dimension except AP. Differences between the disinfected and nondisinfected conditions were significant (P=.03) with respect to dimensions of the gypsum working cast, but not for dimensions of the working die (P=.97). In general, differences relative to the master were small and of minor clinical significance considering marginal gaps of crowns smaller than 150-100 mum are considered clinically acceptable.nnnCONCLUSIONSnVSE monophase impressions and VSE dual-viscosity impressions demonstrated acceptable accuracy for clinical use with immersion disinfection, since the results for VSE were comparable to the results for PE and VPS materials, and the differences as compared to the master model were small.

Three-body wear of resin denture teeth with and without nanofillers

STATEMENT OF PROBLEMnThe wear behavior of newly developed denture teeth with nanofillers may be different from teeth with other chemical formulations.nnnPURPOSEnThe purpose of this study was to examine the 3-body wear resistance of 11 different commercially available resin denture teeth.nnnMATERIAL AND METHODSnThe materials tested were conventional (SR Orthotyp PE, Orthognath) and cross-linked acrylic resin teeth without inorganic fillers (Premium 8, SR Postaris DCL, Trubyte Portrait, Artiplus), composite resin teeth with inorganic fillers (SR Orthosit PE, Vitapan), and composite resin teeth (experimental materials) with inorganic nanofillers (NC Veracia Posterior, e-Ha, Mondial). Human enamel and a ceramic denture tooth (Lumin Vacuum) were used as reference materials. The 3-body wear test was performed in a wear machine developed by the Academic Center for Dentistry Amsterdam (ACTA), with millet suspension acting as an abrasive medium (n=10, test load: 15 N, slip rate: 20%, number of cycles: 100,000). Wear was determined with the aid of a profilometer. Data were analyzed with the Kruskal-Wallis test and Mann-Whitney U test using the closed testing approach (significance level for familywise error rate, alpha=.05).nnnRESULTSnNone of the acrylic and composite resin materials tested in this study demonstrated the 3-body wear resistance of ceramic teeth or human enamel. Teeth with inorganic fillers demonstrated significantly lower wear values than conventional or cross-linked acrylic resin teeth without fillers. Composite resin teeth with traditional fillers showed significantly lower wear than composite resin teeth with nanofillers.nnnCONCLUSIONSnDenture teeth with and without inorganic fillers differed significantly with regard to the degree of wear generated in the ACTA wear simulator. The incorporation of nanofillers did not improve the wear resistance compared to teeth with traditional fillers.

Clinical assessment of enamel wear caused by monolithic zirconia crowns

The purpose of this study was to measure enamel wear caused by antagonistic monolithic zirconia crowns and to compare this with enamel wear caused by contralateral natural antagonists. Twenty monolithic zirconia full molar crowns were placed in 20 patients. Patients with high activity of the masseter muscle at night (bruxism) were excluded. For analysis of wear, vinylpolysiloxane impressions were prepared after crown incorporation and at 6-, 12-, and 24-month follow-up. Wear of the occlusal contact areas of the crowns, of their natural antagonists, and of two contralateral natural antagonists (control teeth) was measured by use of plaster replicas and a 3D laser-scanning device. Differences of wear between the zirconia crown antagonists and the control teeth were investigated by means of two-sided paired Students t-tests and linear regression analysis. After 2 years, mean vertical loss was 46 μm for enamel opposed to zirconia, 19-26 μm for contralateral control teeth and 14 μm for zirconia crowns. Maximum vertical loss was 151 μm for enamel opposed to zirconia, 75-115 μm for control teeth and 60 μm for zirconia crowns. Statistical analysis revealed significant differences between wear of enamel by zirconia-opposed teeth and by control teeth. Gender, which significantly affected wear, was identified as a possible confounder. Monolithic zirconia crowns generated more wear of opposed enamel than did natural teeth. Because of the greater wear caused by other dental ceramics, the use of monolithic zirconia crowns may be justified.

A comparison of two digital techniques for the fabrication of complete removable dental prostheses: A pilot clinical study

STATEMENT OF PROBLEMnThe introduction of digital techniques might improve the quality and cost-effectiveness of treatment with complete removable dental prostheses (CDs).nnnPURPOSEnThe purpose of this pilot clinical trial was to study and compare the clinical feasibility, complications during fabrication, and quality of 2 types of digitally designed CDs.nnnMATERIAL AND METHODSnFive participants were recruited into this preliminary clinical trial. For each participant, 2 pairs of digital CDs were designed. Prosthesis bases were fabricated by using identical data, either by milling from polymethyl methacrylate blanks or by injection molding. The treatment involved 4 clinical appointments. Polyvinyl siloxane impressions were made with custom trays and were subsequently digitalized. After evaluating esthetics and function with trial dentures, the CD bases were fabricated. To evaluate the workflow and quality of the prostheses, the clinical outcome was measured on 6-point scales ranging from poor (grade 6) to excellent (grade 1). For both prosthesis types, the following aspects were examined: fit, retention, esthetics, phonetics, maxillomandibular relation, and occlusion.nnnRESULTSnBoth types of digital CDs could be fabricated without major complications. Only a few minor complications occurred during the fabrication process, predominantly esthetic issues. No pronounced difference was found between the prostheses concerning functional aspects. The definitive esthetic outcome was rated as very good.nnnCONCLUSIONSnThe CDs fabricated using digital technology met the clinical requirements. However, more research is needed to confirm the results of this investigation.

Comparability of clinical wear measurements by optical 3D laser scanning in two different centers

OBJECTIVEnThe purpose of this study was to compare the use of different variables to measure the clinical wear of two denture tooth materials in two analysis centers.nnnMETHODSnTwelve edentulous patients were provided with full dentures. Two different denture tooth materials (experimental material and control) were placed randomly in accordance with the split-mouth design. For wear measurements, impressions were made after an adjustment phase of 1-2 weeks and after 6, 12, 18, and 24 months. The occlusal wear of the posterior denture teeth of 11 subjects was assessed in two study centers by use of plaster replicas and 3D laser-scanning methods. In both centers sequential scans of the occlusal surfaces were digitized and superimposed. Wear was described by use of four different variables. Statistical analysis was performed after log-transformation of the wear data by use of the Pearson and Lin correlation and by use of a mixed linear model.nnnRESULTSnMean occlusal vertical wear of the denture teeth after 24 months was between 120μm and 212μm, depending on wear variable and material. For three of the four variables, wear of the experimental material was statistically significantly less than that of the control. Comparison of the two study centers, however, revealed correlation of the wear variables was only moderate whereas strong correlation was observed among the different wear variables evaluated by each center.nnnSIGNIFICANCEnModerate correlation was observed for clinical wear measurements by optical 3D laser scanning in two different study centers. For the two denture tooth materials, wear measurements limited to the attrition zones led to the same qualitative assessment.

Effect of impact velocity and specimen stiffness on contact forces in a weight-controlled chewing simulator.

OBJECTIVESnChewing simulators are used for preclinical evaluation of newly developed dental restorative materials. To guarantee the independence of test conditions, contact forces during chewing simulation should be independent of the specimen. Because of its mode of operation, i.e., impact of an antagonist, this requirement is not met for a widely used chewing simulator (Willytec/SD Mechatronik, Feldkirchen-Westerham, Germany). This study was therefore intended to clarify the extent to which specimen stiffness affects maximum contact force at different impact velocities. Possible differences between the forces in the eight test chambers were also of interest.nnnMETHODSnFrom each of five dental materials differing in Youngs modulus, eight cylindrical disks were manufactured and embedded in specimen holders. Alumina spheres were used as antagonists. During chewing simulations with different impact velocities and dental materials, vertical acceleration was recorded and contact forces were estimated on the basis of these measurements.nnnRESULTSnSpecimen stiffness and impact velocity had a substantial effect on maximum contact force. The force overshoot relative to the static load ranged from 4% for small specimen stiffness and low impact velocity to values greater than 200% for high specimen stiffness and high impact velocity. Large differences between the chambers were also detected.nnnSIGNIFICANCEnWeight-controlled chewing simulations should be performed either with a low impact velocity or with a spring-damper system (placed between mass and specimen) which efficiently reduces the effects of contact force variation. Influence of specimen stiffness on contact forces must be considered at data interpretation.

Improvement of oral health-related quality-of-life by use of different kinds of double-crown-retained removable partial dentures

Abstract Objective. To verify the hypotheses that treatment with double-crown-retained removable partial dentures (RPDs) improves oral health-related quality-of life (OHRQoL) over a 36-month period and that the performance of RPDs retained by use of electroplated double crowns (EP-RPDs) was different to that of RPDs retained by use of cast double crowns (C-RPDs). Materials and methods. Fifty-four patients (mean age = 64 years, 63% men) were recruited and randomly assigned to C-RPD or EP-RPD. OHRQoL was assessed pre-treatment, post-treatment and 6, 12, 24 and 36 months after insertion, by use of the oral health impact profile (OHIP). An unweighted total score was calculated (OHIP-SUM). A two-level hierarchical model was used for statistical analysis. First-level units were the measurements on the six occasions; second-level units were the patients. Results. Improvement of OHRQoL was observed in both groups after treatment (t = 7.27, p < 0.001). Whereas a treatment–material interaction indicated that treatment with EP-RPDs resulted in greater immediate improvement of OHRQoL, a time–material interaction indicated that long-term improvement was greater for C-RPDs. Conclusions. Treatment with EP-RPDs and C-RPDs improved OHRQoL initially. Over a period of 36 months the effect was significant. The treatment is, therefore, a promising therapeutic option. The cast conical design seems to have advantages with regard to long-term OHRQoL.

Clinical comparison of metal ceramic resin-bonded fixed dental prostheses with a conventional and a mixed retainer design.

STATEMENT OF PROBLEMnLittle is known about the clinical performance of fixed dental prostheses that rigidly connect adhesive retainers and crowns.nnnPURPOSEnThe purpose of this retrospective analysis was to assess and compare the clinical performance of resin-bonded fixed dental prostheses with a conventional and a mixed retainer design.nnnMATERIAL AND METHODSnIncluded as participants were all patients of the hospital since 2004 with regular clinical follow-up who were treated with 3-unit or 4-unit metal ceramic resin-bonded fixed dental prostheses with a conventional 2-adhesive retainer design (conventional group) or with a mixed type combining an adhesive retainer with a crown or a partial crown retainer (mixed group). The analysis included 84 participants with a total of 57 (64%) prostheses in the conventional group and 32 (36%) prostheses in the mixed group. Treatment and data collection were standardized. Cumulative survival without failure (defined as a restoration in need of replacement), chipping, and debonding were estimated by the Kaplan-Meier method and compared for the groups (log-rank test) (α=.05).nnnRESULTSnDuring a mean observation period of 4.1 years (SD, 2.5 years; minimum, 0.4 years; maximum, 9.4 years), 12 complications occurred in 10 restorations. Defects of the ceramic veneer (n=6) were observed most frequently. The estimated 5-year failure-free survival rate was 97.4% (standard error, .025) in the conventional group and 95% (standard error, .049) in the mixed group (log-rank, P=.32). The 5-year cumulative survival rate without chipping was 90.7% (standard error, .064) for the conventional group and 93.8% (standard error, .061) for the mixed group (log-rank, P=.44). The 5-year cumulative survival rate without debonding was 95.1% (standard error, .034) for the conventional group and 91.5% (standard error, .058) for the mixed group (log-rank, P=.54).nnnCONCLUSIONSnLimited by a small sample size and a short follow-up period, the findings of this retrospective analysis indicatexa0that the clinical performance, in terms of survival and the incidence of complications, is comparable for conventional resin-bonded fixed dental prostheses and those that rigidly connect an adhesive wing and a partial or complete crown. Acceptablexa05-year survival and complication rates imply that the medium-term prognosis for resin-bonded restorations withxa0axa0retentive preparation design is comparable with that for conventional fixed dental prostheses.

Retention behavior of double-crown attachments with zirconia primary and secondary crowns

OBJECTIVEnTo investigate whether adequate retention of zirconia conical crown (CC) attachments can be achieved, and to investigate their long-term retention.nnnMETHODSnSixteen individual zirconia primary crowns were produced with convergence angles of 1° or 2° (eight of each). After determination of the convergence angles of the primary crowns, monolithic zirconia secondary crowns were manufactured. To evaluate the retention behavior of all-zirconia CC, the crowns were fitted with forces from F=12.5-100 N. Force magnitudes during the loosening process (L) were then measured. L/F ratios were recorded and the coefficient of friction (μ0) was calculated. Long-term retention was tested with up to 50,000 cycles of denture integration at a speed of 30 mms(-1) and a fitting force magnitude of 53 N.nnnRESULTSnEven when primary crowns were manufactured with the utmost care, the real convergence angles were greater than the nominal angles of the standardized burs (1° and 2°) by 0.28° (SD 0.11°). Without mechanical aging, mean L/F was 0.632 (SD 0.038) for 1° samples and 0.526 (SD 0.022) for 2° samples, indicative of high retentive forces of up to 63.2% of the fitting forces. When all the test results were used, best-fitting curves gave μ0=0.117 for new samples and μ0=0.126 for samples after 50,000 integration cycles.nnnSIGNIFICANCEnWhen the correct milling and sintering parameters are chosen, the retention behavior of zirconia CCs is adequate and stable. This innovative type of attachment is appealing because of the beneficial properties of zirconia and the efficient CAD/CAM-based manufacture.