Total Occlusal Convergence Angles Research Articles

Assessment of Convergence Angle of Tooth Preparations for Complete Crowns Among Dental Students: Typodont vs Simulator.

Introduction The technology-enhanced learning and simulation-based learning are critically important pedagogic tools. They allow students to perfect their preclinical training by improving their skills and their manual dexterity while facilitating the acquisition of the know-how necessary for reproduction more realistically and faithfully of the behaviors required for a better dental practice. Retention is one of the mechanical fundamental principles of preparation of cemented fixed prostheses. It depends on several factors including the convergence of the axial walls. The undercut must be sparing in the reduction of tissue volume to obtain a low degree of convergence and a sufficient height of the preparation to comply with the retention and stabilization requirements of the prosthetic element. A draft value of 6° was recommended initially, but a range extending up to 16° has been accepted according to Weed et al. and Dodge et al., as being clinically achievable while providing good retention. Are students able to reproduce, in preclinical, total occlusal convergence (TOC) angles recommended on typodont and simulator? Objective The evaluation of the TOC of the preparations made on typodont and simulator by the students in the 3rd year of the Faculty of Dental Medicine of Casablanca (FDMC). Material and Methods. A total of 140 dental preparations for cast crowns and metal-ceramics made by thirty-five 3rd year FDMC students were scanned by using the IDENTICA HYBRID optical scanner. The STL files were read by the 3D-TOOL-FREE software, two images were extracted for each preparation using the screen capture tool, and the two mediodistal (MD) and buccolingual angles (BL) were measured by the MB-RULER software. The statistical data were analyzed using the SPSS, software and comparisons were made by Student's t-test. Results An overall average of 11.99° ± 4.48 was recorded for the preparations on the typodont with 11.40° ± 5.09 in the MD direction and 12.58° ± 4.74 in the BL direction. Concerning the simulator preparations, we recorded an overall average of 11.31° ± 4.16 with 10.81° ± 4.29 in the MD direction and 11.80° ± 5.44 in the BL direction. No significant difference was observed when comparing the preparations made on the typodont and the simulator. A percentage of 68.6% and 74.3% of the preparations made on the typodont and the simulators respectively fall within an acceptable range of 6 to 16°. Discussion. The TOC values achieved by the majority of students correspond to the recommended values which are 6 to 16° on average. The results of the simulator preparations are similar to the results of Marghalani for dental students at King Abdulaziz University, and Tiu et al., at the University of Otago in New Zealand. Conclusion This study highlighted the difference between what is taught in dental schools, which is theoretically possible, and the academic results of actual practice. The generally recommended 6° tapers have proven difficult to achieve for many dental students. A margin of 6° to 16° of TOC angle is clinically achievable and allows sufficient retention. We can retain that most of the sample of our study had values that fall within the recommended range.

Comparative Evaluation of the Reproductive Trueness of Zirconia Crowns Fabricated Using Additive Manufacturing and Conventional Milling.

To evaluate the occlusal reproductive trueness of zirconia crowns fabricated using additive manufacturing (AM) and to compare the surface roughness of crowns fabricated using AM and conventional milling (CM). Crowns were manufactured using AM and CM on abutments with total occlusal convergence angles of 16 and 20 degrees. The surface roughness of the AM crowns was less than that of the CM crowns. The differences in reproduction of the occlusal morphology of the abutment crown were greater at 16 degrees than at 20 degrees. AM could be an effective method for manufacturing zirconia crowns.

Assessment of Total Occlusal Convergence for Crown Preparation: A Comparative Study

Aim: The aim of the study was to assess the total occlusal convergence guessing by sight and ruler methods and compare with the snapshots for accuracy. Materials and methods: The methods used for total occlusal convergence (TOC) preparation were assessed among students, interns, and dental specialists after die preparation and compared with the snapshots for accuracy. Results: In all, 47% of the specialist’s observation achieved the recommended TOC angle, while 29.25% was obtained by undergraduates. Conclusion: In conclusion, specialist’s observation was better in both ruler and sight guessing methods compared to dental students and interns.

Adequately Prepared? A Study Using an Innovative Computer Application to Measure Clinical Crown Convergence Angles Achieved by Students at a UK Dental School.

This study examined the total occlusal convergence angles created for full coverage crown preparations by students at a UK dental school. Working casts of 82 clinical crown preparations were scanned using a 3D scanner. Stereolithographic files were uploaded to Preppr, a crown preparation analysis application. Mean bucco-lingual convergence angle were 19.6° (+/-11.7) and mesial-distally 17.8° (+/-11.1). Smallest bucco-lingual convergence angles were achieved for canine teeth with the largest on molar teeth. The smallest mesio-distal values were on canine teeth with the largest on molar teeth. Ideal total convergence angles (4-14°) were achieved in 23% of bucco-lingual preparations and 33% of mesio-distal preparation. Results for clinically acceptable angles (10-20°) were 30% and 40% respectively. There were no statistically significant differences between tooth types for mean bucco-lingual values. (p=0.623), mesio-distal mean values were statistically different by tooth type (p=0.003). Mean values for mandibular molars were significantly higher than for maxillary incisors (p=0.001) and mandibular molars had significantly higher values than maxillary canines (p=0.045). Results in this study were comparable to those of other students and qualified clinicians, with a minority of preparation achieving ideal values.

Effect of Low Power Magnification on Total Occlusal Convergence Angles in Crown Preparation

Effect of Low Power Magnification on Total Occlusal Convergence Angles in Crown Preparation

Fracture Resistance of Computer-Aided Design/Computer-Assisted Manufactured Glass-Ceramic Incisor Crowns Fabricated on Reverse-Tapered Preparations.

To evaluate the fracture resistance of computer-aided design/computer-assisted manufactured (CAD/CAM) glass-ceramic incisor crowns fabricated on reverse-tapered preparations. Resin abutments with known total occlusal convergence angles (-8, 0, and 12 degrees; n = 20) were scanned with the TRIOS scanner, and glass-ceramic crowns were fabricated with KaVo Arctica, stored in water for either 1 or 30 days, and submitted to fracture test. The fracture resistance of crowns made on reverse-tapered preparations was not significantly different (P = .471) from crowns fabricated over abutments with 0- or 12-degree tapers, irrespective of the water storage period. The fracture resistance of anterior glass-ceramic crowns fabricated over reverse-tapered preparations was similar to that of crowns made on abutments with 0- or 12-degree tapers.

Convergence angles of all-ceramic full crown preparations performed in Dubai private practice.

BackgroundThis study aimed to determine the degree of taper and total occlusal convergence angles (TOC) for all-ceramic bonded crown preparations carried out by private practitioners in Dubai, UAE.Material and MethodsA convenience sample of all-ceramic crown preparations carried out by private dental practitioners were scanned (Carestream CS 3500) from casts and the digital images assessed. The degree of taper was measured on the axial walls of each crown preparation and the bucco-lingual and mesio-distal convergence angles subsequently calculated.ResultsA total of 154 dentists prepared a total of 206 crown preparations (72 anterior, 134 posterior). The mean convergence angles mesio-distally for all preparations was 24.6° (sd 11.8º), and for the bucco-lingual it was 32.6° (sd 15.3°). The mean TOC was 28.6°. In anterior preparations, the mean bucco-lingual convergence angle was 38.8° (sd 12.2°) compared to 29.3° (sd 15.5°) for posterior preparations (p<0.001). Mean mesio-distal convergence anteriorly was 20.6° (sd 10.18°) compared to 26.7° (sd 12.16°) posteriorly (p<0.001). Distal and buccal taper were significantly greater on posterior teeth (<0.001) compared to anteriors whereas lingual taper was greater on anterior teeth (p<0.001). Mesial taper was not different. Premolars had significantly lower convergence values compared to other teeth.ConclusionsBucco-lingual and mesio-distal convergence angles significantly exceeded the clinically acceptable convergence angle of between 10° and 22°. Greater axial taper is recommended for resin bonded all-ceramic crowns but reliance on adhesion in such preparations rather than parallelism may reduce retention and have increased biologic cost to pulp health. Key words:All-ceramic crown preparations, convergence angles, axial taper.

Marginal and Internal Fit of CAD/CAM Crowns Fabricated Over Reverse Tapered Preparations.

Intraoral digital scanning can accurately record single abutment tooth preparations despite their geometry, and the algorithms of the CAD software can be set to manage different abutment forms. The purpose of this in vitro study was to evaluate the marginal and internal fit of CAD/CAM zirconia crowns fabricated over conventional and reverse-tapered preparations. Crown preparations with known total occlusal convergence (TOC) angles (-8°, -4°, 0°, 8°, 12°, 16°, and 22°) were digitally created from a maxillary left central incisor and printed in acrylic resin. Next, casts were scanned with a TRIOS intraoral scanner, and crowns were designed with KaVo multiCAD software using default parameters (50 μm cement space) on abutments with positive TOC angles, whereas reverse-tapered abutments (negative TOC angles) were digitally blocked out at 0° and had an extra mesiodistal gap set to 50 μm. Then, zirconia crowns were fabricated, and their marginal and internal discrepancies were recorded with the silicone replica technique. All replicas were examined under a stereomicroscope at 50× magnification. Collected data were analyzed with one-way ANOVA and post hoc Tukey test for marginal fit. For the axial and incisal fit, measured values did not follow a normal distribution; therefore, the Kruskal-Wallis and the Dunn/Bonferroni multiple comparison tests were applied (p = 0.05). The mean marginal fit of -8° crowns (58.2 ± 6.0 μm) was statistically different (p < 0.0001) from all the remaining crowns (range 42.1-47.3 μm). Also, the internal fit was statistically significant when comparing crowns fabricated over abutments with positive and negative TOC angles (p < 0.0001). The largest median axial discrepancies were found in the -8° (165.5 μm) and -4° (130.8 μm) groups; however, when evaluating the incisal fit, they showed the smallest discrepancies (67.3 and 81.8 μm, respectively). Under the conditions of this study, the marginal and internal fit of zirconia crowns fabricated over inverse-tapered preparations is within clinically accepted values.

Influence of abutment tooth geometry on the accuracy of conventional and digital methods of obtaining dental impressions

Statement of problemDirect (intraoral) and indirect (desktop) digital scanning can record abutment tooth preparations despite their geometry. However, little peer-reviewed information is available regarding the influence of abutment tooth geometry on the accuracy of digital methods of obtaining dental impressions. PurposeThe purpose of this in vitro study was to evaluate the influence of abutment tooth geometry on the accuracy of conventional and digital methods of obtaining dental impressions in terms of trueness and precision. Material and methodsCrown preparations with known total occlusal convergence (TOC) angles (−8, −6, −4, 0, 4, 8, 12, 16, and 22 degrees) were digitally created from a maxillary left central incisor and printed in acrylic resin. Each of these 9 reference models was scanned with a highly accurate reference scanner and saved in standard tessellation language (STL) format. Then, 5 conventional polyvinyl siloxane (PVS) impressions were made from each reference model, which was poured with Type IV dental stone scanned using both the reference scanner (group PVS) and the desktop scanner and exported as STL files. Additionally, direct digital impressions (intraoral group) of the reference models were made, and the STL files were exported. The STL files from the impressions obtained were compared with the original geometry of the reference model (trueness) and within each test group (precision). Data were analyzed using 2-way ANOVA with the post hoc least significant difference test (α=.05). ResultsOverall trueness values were 19.1 μm (intraoral scanner group), 23.5 μm (desktop group), and 26.2 μm (PVS group), whereas overall precision values were 11.9 μm (intraoral), 18.0 μm (PVS), and 20.7 μm (desktop). Simple main effects analysis showed that impressions made with the intraoral scanner were significantly more accurate than those of the PVS and desktop groups when the TOC angle was less than 8 degrees (P<.05). Also, a statistically significant interaction was found between the effects of the type of impression and the TOC angle on the precision of single-tooth dental impressions (F=2.43, P=.002). Visual analysis revealed that the intraoral scanner group showed a homogeneous deviation pattern across all TOC angles tested, whereas scans from the PVS and desktop scanner groups showed marked local deviations when undercuts (negative angles) were present. ConclusionsConventional dental impressions alone or those further digitized with an extraoral digital scanner cannot reliably reproduce abutment tooth preparations when the TOC angle is close to 0 degrees. In contrast, digital impressions made with intraoral scanning can accurately record abutment tooth preparations independently of their geometry.

Convergence angles and margin widths of tooth preparations by New Zealand dental students

Statement of problemFundamental crown preparation principles are taught and are highly regarded in dental education. Whether tooth preparations made by dental students match these principles is unclear. PurposeThe purpose of this retrospective study was to report the total occlusal convergence (TOC) and margin widths of crown preparations clinically prepared by New Zealand predoctoral dental students between 2013 and mid-2015. Material and methodsA total of 371 stereolithography files of tooth preparations for metal ceramic crowns prepared by predoctoral dental students were extracted from the Technical Services Laboratory database at the University of Otago. The files were put through the preparation measuring software Preppr, with outputs being TOC angles in faciolingual and mesiodistal cross sections and margin widths in facial, lingual, distal, and mesial aspects. Means, standard deviations, 95% confidence intervals, and distributions using box and whisker graphs were calculated and presented. ResultsThe majority of TOC angles fell within an acceptable range of 10 to 20 degrees; however, the angles ranged from undercuts (<0 degrees) to >60 degrees. The majority of margin widths were between 0.5 and 1 mm, while the maximum was approximately 2 mm and the minimum was 0 mm. ConclusionsPredoctoral dental students in New Zealand are able to produce literature-recommended TOC angles and margin widths for metal ceramic crowns; however, further attention and training are needed for excessive tooth preparations, mainly in the form of large TOC angles.

Custom total occlusal convergence angle sticker fabrication

This article describes a method of fabricating a custom total occlusal convergence angle sticker with photo editing software and label stickers. The custom total occlusal convergence angle sticker can help clinicians achieve an accurate degree of taper during axial wall reduction of tooth preparation.

Reporting numeric values of complete crowns. Part 1: Clinical preparation parameters

Statement of problemAn implemented objective measuring system for measuring clinical tooth preparations does not exist. PurposeThe purpose of this study was to compare clinically achieved tooth preparations for ceramic crowns by general dentists with the recommended values in the literature with an objective measuring method. Material and methodsTwo hundred thirty-six stone dies prepared for anterior and posterior complete ceramic crown restorations (IPS e.max Press; Ivoclar Vivadent) were collected from dental laboratories. The dies were scanned and analyzed using the coordinate geometry method. Cross-sectioned images were captured, and the average total occlusal convergence angle, margin width, and abutment height for each preparation was measured and presented with associated 95% confidence intervals. ResultsThe average total occlusal convergence angles for each tooth type was above the recommended values reported in the literature. The average margin widths (0.40 to 0.83 mm) were below the minimum recommended values (1 to 1.5 mm). The tallest preparations were maxillary canines (5.25 mm), while the shortest preparations were mandibular molars (1.87 mm). ConclusionsComplete crown preparations produced in general practice do not achieve the recommended values found in the literature. However, these recommended values are not based on clinical trials, and the effects of observed shortfalls on the clinical longevity of these restorations are not predictable.

Total Occlusal Convergence and Margin Design in Relation to Survival of Glass-Ceramic Crowns: A Review

This review aims to report on preparation geometry parameters and their relation to crown failures in complete glass-ceramic clinical studies. A review of the literature was conducted in MEDLINE/PubMed with an additional manual search. The articles were screened through an inclusion and exclusion criteria and information regarding the Total Occlusal Convergence (TOC) angle and margin design was extracted along with its survivability. Nineteen articles were found, three were retrospective studies and 16 were prospective. Three articles specified TOC and 16 articles specified their margin design. Eighty failures were reported but lack of information between core and margin design and failures meant the relationship was inconclusive. There were a limited number of long-term clinical studies assessing glass-ceramic complete crowns. These studies provided insufficient information regarding the TOC angles of the prepared crowns and its potential effect on the survivability of glass-ceramic crowns. Further studies need to be conducted to determine how the TOC and margin design can affect the survival rates of glass-ceramic complete crowns.

Influence of convergence angle of tooth preparation on the fracture resistance of Y-TZP-based all-ceramic restorations

ObjectiveTo investigate the influence of the convergence angle of tooth preparation on the fracture load of Y-TZP-based ceramic (YZ – Vita YZ) substructure (SB) veneered with a feldspathic porcelain (VM9 – Vita VM9). MethodsFinite element stress analysis (FEA) was performed to examine the stress distribution of the system. Eighty YZ SB were fabricated using a CAD–CAM system and divided into four groups (n=20), according to the total occlusal convergence (TOC) angle: G6 – 6° TOC; G12 – 12° TOC; G20 – 20° TOC; and G20MOD – 20° TOC with modified SB. All SB were veneered with VM9, cemented in a fiber reinforced epoxy resin die, and loaded to failure. Half of the specimens from each group (n=10) were cyclic fatigued (106 cycles) before testing. Failure analysis was performed to determine the fracture origin. Data were statistically analyzed using Anova and Tukey's tests (α=0.05). ResultsThe greatest mean load to fracture value was found for the G20MOD, which was predicted by the FEA. Cyclic fatigue did not significantly affect the load of fracture. Catastrophic failure originating from the internal occlusal surface of the SB was the predominant failure mode, except for G20MOD. SignificanceThe YZ–VM9 restorations resisted greater compression load than the usual physiological occlusal load, regardless of the TOC angle of preparations. Yet, the G20MOD design produced the best performance among the experimental conditions evaluated.

The ability of dental students and faculty to estimate the total occlusal convergence of prepared teeth

Total occlusal convergence (TOC) is an important aspect of tooth preparation, but the accuracy of its visual estimation has not been determined. The purpose of this study was to determine which TOC angles are considered ideal, maximal, and minimal, and also to determine the accuracy of visually estimating TOC. Thirteen dentoform teeth were prepared, for complete gold crown restorations, with TOC angles ranging between -3 and 30. Thirty students and 30 faculty members visually identified the teeth they believed had ideal, maximal, and minimal convergence and then estimated the angle of TOC for each tooth. Data were analyzed with a 2-sample binomial test and the 2-sample t test (alpha=.05), as well as the general linear model (the mixed procedure). Teeth selected had TOC angles of 9-12 degrees, 21-30 degrees, and 3-12 degrees for ideal, maximal acceptability, and minimal acceptability, respectively. Students and faculty exhibited similar visual estimation capabilities. Greater accuracy of visual estimation of TOC angles occurred with teeth judged to have minimal or ideal convergence, as opposed to those with maximally acceptable angles, indicating estimation accuracy decreased as convergence increased. More than half of the estimates were within 5 degrees of the measured TOC. However, 11 ideal, 14 maximal, and 5 minimal estimates were judged inaccurately by 11 to 20 degrees, and 5 estimates were inaccurate by more than 20 degrees. There were more underestimates than overestimates. Most visual estimates were accurate, but several were inaccurate by more than 10 degrees. The most common inaccuracies were underestimated TOC. Students and faculty were not significantly different in their ability to accurately estimate TOC. Students and faculty were less accurate when estimating large, as opposed to small, TOC angles.

Marginal accuracy and fracture strength of ceromer/fiber-reinforced composite crowns: Effect of variations in preparation design

Statement of Problem. Targis/Vectris restorations provide excellent esthetics and clinical success; however, the relationship of their marginal accuracy and fracture strength to the tooth preparation design requires further investigation. Purpose. The aim of this study was to evaluate the effect of variations in tooth preparation design on the marginal accuracy before and after cementation and on the fracture strength of the ceromer/fiber reinforced composite crown. Material and Methods. Three metal dies with varying total occlusal convergence angles (6 degrees, 10 degrees, 15 degrees) were prepared. A total of 30 (10 for each angle) Targis/Vectris crowns were fabricated. The restorations were evaluated at 48 points on the entire circumferential margin with a stereomicroscope measuring in micrometers for margin adaptation before and after cementation. The specimens then were compressively loaded to failure in a universal testing machine. Marginal adaptation was analyzed with Kruskal-Wallis test and post-hoc Dunnett test (α=0.05). The fracture strength was analyzed with analysis of variance and the Scheffe adjustment at the 95% significance level. Fracture surfaces of the crowns were examined with a scanning electron microscope to determine the mode of fracture. Results. The smallest marginal gap was recorded in angled crowns with a 6-degree convergence (47 μm mean). The marginal gap of most (95.6%) of the crowns was within a clinically acceptable level (established as ≤100 μm). The mean marginal gap increased significantly after cementation, with the largest increase in the 6 degree group (76 μm mean). The 6-degree angled crowns had a significantly higher fracture strength (1543 N) than the more convergent crowns (1366 N) (P<.001). Mean fracture strength of all the crowns was 1390 N. Scanning electron microscope observation showed that all crowns exhibited a 2-phase fracture pattern: a crack and chipping of the Targis layer followed by adhesive failure. Conclusion. Within the limitations of this experimental design, decreasing the axial convergence angle of the ceromer/fiber-reinforced composite crowns diminished their marginal gap and increased their fracture strength. (J Prosthet Dent 2002;88:388-95.)