Accuracy of contacts calculated from 3D images of occlusal surfaces.

OBJECTIVES: Compare occlusal contacts calculated from 3D virtual models created from clinical records to contacts identified clinically using shimstock and transillumination. METHODS: Upper and lower full arch alginate impressions and vinyl polysiloxane centric interocclusal records were made of 12 subjects. Stone casts made from the alginate impressions and the interocclusal records were optically scanned. Three-dimensional virtual models of the dental arches and interocclusal records were constructed using the Virtual Dental Patient Software. Contacts calculated from the virtual interocclusal records and from the aligned upper and lower virtual arch models were compared to those identified clinically using 0.01mm shimstock and transillumination of the interocclusal record. Virtual contacts and transillumination contacts were compared by anatomical region and by contacting tooth pairs to shimstock contacts. Because there is no accepted standard for identifying occlusal contacts, methods were compared in pairs with one labeled "standard" and the second labeled "test". Accuracy was defined as the number of contacts and non-contacts of the "test" that were in agreement with the "standard" divided by the total number of contacts and non-contacts of the "standard". RESULTS: Accuracy of occlusal contacts calculated from virtual interocclusal records and aligned virtual casts compared to transillumination were: 0.87+/-0.05 and 0.84+/-0.06 by region and 0.95+/-0.07 and 0.95+/-0.05 by tooth, respectively. Comparisons with shimstock were: 0.85+/-0.15 (record), 0.84+/-0.14 (casts), and 81+/-17 (transillumination). CONCLUSIONS: The virtual record, aligned virtual arches, and transillumination methods of identifying contacts are equivalent, and show better agreement with each other than with the shimstock method.

Changes in compaction stress distributions in roots resulting from canal preparation.

AIM: To examine if canal enlargement with instruments of controlled taper leads to more uniform stress distributions within a root, thereby reducing fracture susceptibility. METHODOLOGY: Finite element models of a mandibular incisor were constructed with round and oval canal profiles, based on measurements from extracted teeth. The shapes of rotary nickel-titanium instruments (ProTaper F1, F2, and F3 and ProFile size 30, 0.04 taper and size 30, 0.06 taper; Dentsply Maillefer) were superimposed on the canals. Equivalent stresses and circumferential stresses in the root were calculated for a compaction load. RESULTS: The highest stresses were found at the canal wall. Round canals showed lower uniform distributions, whilst oval canals showed uneven distributions with high concentrations at the buccal and lingual canal extensions and greater stresses in the coronal and middle thirds than in the apical third. Preparation of round canals introduced only small circumferential stress increases in the apical half; preparation of oval canals produced substantial reductions where the canal was enlarged to a smooth round shape. Even where fins were not completely eliminated, the maximum stresses were still reduced by up to 15%. External distal and mesial surfaces of roots with oval canals showed moderate stress concentrations that were minimally affected by preparations, whilst stress concentrations emerged on roots with round canals when preparation sizes increased. CONCLUSIONS: The potential for reducing fracture susceptibility exists as a result of round canal profiles achieved and smooth canal taper. Even when fins were not contacted by the instrument, stresses within the root were lower and more evenly distributed than before preparation.

The frictional coefficients and associated wear resistance of novel low-shrink resin-based composites.

OBJECTIVES: Frictional forces play a major role in the oral wear process of dental resin-based composites (RBCs) and it would be of interest to consider how the energy from friction is dissipated at the material surface. Consequently, the micromechanical wear properties of conventional methacrylate compared with novel oxirane RBCs were assessed. METHOD: The frictional coefficient (mu), volume loss and Vickers hardness number (VHN) of oxirane (EXL596 and H1) and methacrylate RBCs (Z100 and Filtek Z250) were evaluated. Archard's wear equation was implemented to obtain the wear coefficient (K) and expressed as a 'fraction of friction' (K/micro) to indicate the dissipation of frictional energy that resulted in wear. Scanning electron microscopy (SEM) was used to qualitatively asses the wear facets of each RBC following 50000-cycles. RESULTS: The mean frictional coefficients observed between the oxirane and methacrylate RBCs were not significantly different (P > 0.05). However, the volume loss of EXL596 and H1 (5.9 +/- 0.4 and 4.7 +/- 0.3 x 10(-2) mm(3)) was significantly increased compared with Z100 and Filtek Z250 (1.7 +/- 0.2 and 2.3 +/- 0.3 x 10(-2) mm(3)). The VHN of EXL596 and H1 was either significantly greater (P = 0.021) or similar (P = 0.089) to Filtek Z250, respectively. An increase in K/micro was reported for EXL596 and H1 (34.7 +/- 4.1 and 22.8+ /- 2.4 x 10(-4)) compared with Z100 and Filtek Z250 (8.50 +/- 0.7 x 10(-4) and 8.62 +/- 1.0 x 10(-4)) (P < 0.05). SEM images of the oxirane RBCs exhibited increased surface fatigue and delamination of the surface layers compared with the methacrylate RBC specimens following 50,000-cycles. CONCLUSION: The significant decrease in wear resistance of the oxirane compared with methacrylate RBCs was unexpected since frictional coefficients and/or surface hardness were statistically similar. The decreased wear resistance of EXL596 and H1 compared with Z100 and Filtek Z250 was further explained by the increase in K/micro from wear theory and the associated increase in surface fatigue identified from SEM. The simplistic testing procedure combined with SEM utilized in the current investigation provided a greater insight into the wear mechanism by considering the effect of frictional energy at the specimen surface which may benefit the development of improved wear resistance for experimental RBC materials.

Factors influencing optical 3D scanning of vinyl polysiloxane impression materials.

PURPOSE: Future growth in dental practice lies in digital imaging enhancing many chairside procedures and functions. This revolution requires the fast, accurate, and 3D digitizing of clinical records. One such clinical record is the chairside impression. This study investigated how surface angle and surface roughness affect the digitizing of vinyl polysiloxane impression materials. MATERIALS AND METHODS: Seventeen vinyl polysiloxane impression materials were digitized with a white light optical digitizing system. Each sample was digitized at 3 different angles: 0 degrees, 22.5 degrees, and 45 degrees, and 2 digitizer camera f-stops. The digitized images were rendered on a computer monitor using custom software developed under NIH/NIDCR grant DE12225. All the 3D images were rotated to the 0 degrees position, cropped using Corel Photo-Paint 8 (Corel Corp, Ottawa, Ontario, Canada), then saved in the TIFF file format. The impression material area that was successfully digitized was calculated as a percentage of the total sample area, using Optimas 5.22 image processing software (Media Cybernetics, LP, Silver Spring, MD). The dependent variable was a Performance Value calculated for each material by averaging the percentage of area that digitized over the 3 angles. New samples with smooth and rough surfaces were made using the 7 impression materials with the largest Performance Values. These samples were tested as before, but with the additional angle of 60 degrees. Silky-Rock die stone (Whip Mix Corp, Louisville, KY) was used as a control. RESULTS: The Performance Values for the 17 impression materials ranged from 0% to 100%. The Performance Values for the 7 best materials were equivalent to the control at f/11 out to a surface angle of 45 degrees; however, only Examix impression material (GC America Inc, Alsip, IL) was equivalent to the control at f/11/\16. At the 60 degrees surface angle with f/11/\16, the Performance Values were 0% for all the impression materials, whereas that for the control was 90%. The difference in the Performance Values for the smooth and rough surface textures was 7%, which was not significant. CONCLUSIONS: The digitizing performance of vinyl polysiloxane impression materials is highly material and surface angle-dependent and is significantly lower than the die stone control when angles to 60 degrees are included. It is affected to a lesser extent by surface texture.

Nondestructive, in vitro quantification of crown margins.

STATEMENT OF PROBLEM: It is important that artificial crowns fit the prepared tooth accurately, as marginal deficiencies are predisposed to plaque accumulation and lead to increased risk of periodontal disease. Various methods of evaluation for marginal fit are described in the literature, but most approaches are limited by destructive methods of assessment and/or small points of measurement. PURPOSE: This study compared, in vitro, the marginal fit of 4 types of complete crowns on human premolar teeth with the use of nondestructive profilometry. This method determined whether fit was influenced by type of crown or surface morphology of the tooth, namely, grooved or ungrooved surfaces. MATERIAL AND METHODS: Four groups of specimens were prepared for complete crowns: group BA, bonding alloy with chamfer finish line; group G, gold alloy with chamfer finish line; group PC, porcelain with a chamfer finish line; and group PS, porcelain with a shoulder finish line. Two profiles of grooved mesial and ungrooved distal surfaces of the teeth were performed: (1) teeth prepared for each type of crown and (2) teeth with crowns seated but not cemented. Marginal fit (absolute marginal discrepancy) from the finish line edge of the tooth preparations to crown edges (CE) and leading edges (LE) of crowns were measured. RESULTS: A 2-way analysis of variance for crown type and tooth surface morphology revealed significant differences between crown types for all measurement parameters, except vertical LE. The effect of surface morphology was not significant, except for vertical LE (P<.05). For all parameters, except vertical LE, the ranking of marginal fit discrepancies from greatest to least was as follows: group PC, G, BA, and PS. For vertical LE distances, the ranking was PS, BA, G, and PC (P<.05). CONCLUSION: Profilometry was used as a nondestructive, accurate method of evaluating the absolute marginal fit of different types of crowns. Marginal fits varied continuously around the circumference of each crown and made clinical assessment of fit accuracy subjective and arduous.

Correlation of noncarious cervical lesion size and occlusal wear in a single adult over a 14-year time span.

STATEMENT OF PROBLEM: Noncarious cervical lesions are described as having a multifactorial cause, with occlusal trauma and toothbrush abrasion frequently mentioned as major factors. Finite element modeling studies have demonstrated a relocalization of occlusal stresses to the cervical area due to flexure of the crown. This may cause microcracking, especially under tensile stresses, that will lead to a loss of enamel and dentin in the cervical region. Clinical confirmation of an occlusal cause for noncarious cervical lesions has been difficult to obtain. PURPOSE: This study investigated whether occlusal wear was correlated with an increase in the size of noncarious cervical lesions. MATERIAL AND METHODS: Loss of contour at occlusal and cervical sites on 3 teeth of a single individual was measured using digital and visualization techniques at 3 time intervals over a 14-year time span. The 1983 baseline casts and 1991, 1994, and 1997 clinical impressions of a single adult patient with existing noncarious cervical lesions were replicated in epoxy. Surfaces of all replicas were digitized with a contact digitizing system. Sequential digitized surfaces were fit together and analyzed using AnSur-NT surface analysis software. Clinical losses of surface contour by volume and depth of the left mandibular first molar and first and second premolars were recorded. RESULTS: Nine measurements of cervical volume loss (range 0.9 to 11.5 mm(3)) and 9 corresponding measurements of occlusal volume loss (range 0.39 to 7.79 mm(3)) were made. The correlation between occlusal and cervical volume loss was strong (r(2)=0.98) and significant (P<.0001). CONCLUSION: For the single adult patient in this study, there was a direct correlation between occlusal wear and the growth of noncarious cervical lesions.

Wear of enamel and veneering ceramics after laboratory and chairside finishing procedures.

PURPOSE: This in vitro study compared the wear of enamel against 3 types of ceramics with high esthetic potential (designed for layering techniques): feldspathic porcelain (Creation), aluminous porcelain (Vitadur alpha), and low-fusing glass (Duceram-LFC). Laboratory finishing (glazing/polishing) and chairside polishing with a Dialite kit were simulated to compare their respective effects on wear. METHODS: Tooth-material specimen pairs were placed in an artificial mouth using closed-loop servohydraulics. Constant masticatory parameters (13.5 N occlusal force, 0.62 mm lateral excursion; 0.23 second cuspal contact time) were maintained for 300, 000 cycles at a rate of 4 Hz. The occlusal surface of each pair was mapped and digitally recorded before and after each masticatory test. Quantitative changes were measured in terms of depth and volume of wear. Quantitative wear characteristics were assessed by SEM. RESULTS: Significant differences were observed (2-factor ANOVA, P <.05). Duceram-LFC generated increased volume loss of enamel (0.197 mm(3)) compared with Creation (0.135 mm(3)) and Vitadur alpha (0.153 mm(3)). Creation exhibited the lowest ceramic wear and lowest combined volume loss (0.260 mm(3); the sum of the data for enamel and the opposing material) compared with Duceram-LFC (0.363 mm(3)) and Vitadur alpha (0.333 mm(3)). The most significant differences among materials were observed in volume loss, not in depth of wear. For all 3 ceramic systems, qualitative SEM evaluation revealed an abrasive type of wear. Wear characteristics of chairside polished specimens were similar to those of laboratory finished specimens (glazed and polished). CONCLUSION: Duceram-LFC was the most abrasive ceramic for the antagonistic tooth. Creation ceramic was the least abrasive material and most resistant to wear. Defects, brittleness, and the possibly insufficient toughness of LFC may explain its increased abrasiveness. Laboratory and chairside finishing procedures generated similar results.

Comparison of two measurement techniques for clinical wear.

OBJECTIVES: Clinical wear of restorations is generally evaluated by marginal integrity over time. In this study, both a subjective and an objective method for wear assessment are compared, and the relative advantages and disadvantages of each are considered. METHODS: A surface evaluation technique for quantitative measurement of wear, as developed at the University of Minnesota (UMN) has been compared to the commonly employed method of wear assessment used by Leinfelder (LF). Measurements were made by comparing suitable casts of restored teeth before and after clinical function. Semi-quantitative wear assessment was studied in comparison with detailed quantitative information about the topology of the pre- and post-wear occlusal surfaces obtained from stylus profilometry and processed by imaging techniques. Fourteen model sets of baseline, 2, 3 and 5 year old Class II composite restorations from a Danish clinical trial were evaluated using both techniques. RESULTS: In general, after 5 years of function, the digitizing method (UMN) generated wear values that were twice as high, indicating that general wear of restorative materials is underestimated by the LF-method. CONCLUSIONS: The comparison clearly revealed the advantages and limitations of each technique. Evaluation of wear by means of the LF technique provides global semi-quantitative data on restoration margins relative to enamel, underestimating general wear of the restorative material. Advanced 3-D measuring techniques such as the UMN method provide extensive quantitative data regarding wear patterns of the entire occlusal surface, i.e. restoration and enamel. Such a highly accurate technique is capable of differentiation between wear behavior of restorative materials early on in clinical studies. In addition, through its digital alignment procedure, the UMN method provides data on accuracy of the replication process used in clinical studies.

Retention of adhesive cement on the tooth surface after crown cementation.

STATEMENT OF PROBLEM: Adhesive cements increase crown retention, but it is unknown if traces of cement remain undetected on the tooth surface after clinical removal of excess cement, which could exacerbate plaque retention. PURPOSE: This study measured the surface area, volume, mean depth, and maximum depth of a resin composite and a compomer luting cement left adherent on the tooth surface after removal of excess cement, as judged clinically. METHODS AND MATERIAL: Four groups of specimens (n = 48) were prepared for full coverage crowns: group AC bonding alloy with chamfer finish line, group G gold alloy with chamfer finish line, group PC porcelain with a chamfer finish line, and group PS porcelain with a shoulder finish line. Two profiles of the mesial and distal surfaces of the teeth were carried out: (1) tooth with crown seated but not cemented and (2) tooth with the crown cemented in place. Two cements and 2 methods of cement removal were studied. RESULTS: A 4-way analysis of variance for cement, crown type, method of removal, and tooth surface morphology showed that significantly greater volumes and mean depth, but not surface areas, of resin composite cement remained adherent than compomer cement (P<.05). Among crown types, significant differences were found for cement volume (group G>AC, G>PC, G>PS), cement surface area (group AC>PC, G>PC, G>PS), and maximum cement depth (group G>AC). There was no significant difference between the 2 methods of cement removal. Significantly larger surface areas and maximum depths of cement were retained on the anatomically grooved mesial surface of the maxillary first premolars than on the ungrooved distal surface. CONCLUSION: Subclinical cement retention occurred after crown cementation, which was influenced by cement, crown type, and tooth surface morphology but not method of cement removal.

Dimensional changes during veneering procedures on discoloured teeth.

OBJECTIVES: The aim of the study was to determine the dimensional changes of teeth when veneering procedures are involved. METHODS: Fifteen discoloured central incisors were selected from a group of 180 veneer restorations (VRs) composed of three different types of materials (direct resin composite, indirect resin composite and porcelain). Impressions and dies were made before treatment, after preparation and after placement and finishing the restoration. Contact stylus profilometry and subsequent analysis of the 3-D surface images provided quantitative data of the differences between the various treatment phases. RESULTS: The preparation reduction was the most for indirect resin composite VRs and the least for direct resin composite VRs. All veneer restorations showed nearly the same thickness and dimensional change after treatment, except one, which resulted in an additional increase of volume of the tooth. CONCLUSION: From the results of this study it is concluded that the dimensions of a discoloured tooth treated with a veneer restoration unintentionally increased, resulting in overcontour.

Variation in tooth wear in young adults over a two-year period.

STATEMENT OF PROBLEM: Although all the processes of loss of hard tissue are important, attrition on the occlusal surfaces commands our attention. PURPOSE OF STUDY: The enamel wear rate of 18 young adults over 2 consecutive years was measured independently by volume loss and mean depth loss. Any significant differences in tooth wear resulting from gender and a clinical diagnosis of bruxism were identified. MATERIAL AND METHODS: A strict protocol for dental impressions provided epoxy models, which were digitized with a null point contact stylus. AnSur software provided a complete morphologic description of changes in the wear facets. RESULTS: The mean loss for all teeth measured was 0.04 mm3 by volume and 10.7 microns by depth for the first year. CONCLUSIONS: These numbers were approximately doubled at 2 years of cumulative wear.

Evaluation of factors affecting the accuracy of impressions using quantitative surface analysis.

Impression material goes from a plastic to an elastic state during setting. Movement of the impression and excessive seating pressure during this transition can cause distortion in the impressions. The purpose of this study is to determine if the impression distortion is related to movement during setting or to distortion of the putty phase in the two-step impressioning technique. A master model of a maxillary quadrant of teeth was impressed using four different procedures: 1) one-step technique without movement (1S-NM); 2) one-step technique with movement (1S-M); 3) two-step technique without movement (2S-NM); and 4) two-step technique with movement (2S-M). An artificial oral environment and surface analysis technique of the Minnesota Dental Research Center for Biomaterials and Biomechanics were used to produce the impressions and measure their accuracy. A digitized image of the first premolar of the master model was aligned with a digitized image of the first premolar of each epoxy model using AnSur. The root mean squared difference (RMS) between the aligned images is a measure of the distortion. The corresponding RMS values for the different methods were: 1S-NM = 23.7 +/- 9.21; 1S-M = 20.4 +/- 3.9; 2S-NM = 20.5 +/- 7.7; 2S-M = 21.3 +/- 4.4. Statistical analysis using a two-way analysis of variance showed no difference at the 0.05 level of significance. Pairwise comparison using the Tukey method showed that neither technique (one-step vs two-step) nor movement is a significant factor. These results showed that low seating pressure will not cause any greater distortions in the two-step impression technique than in the one-step technique, and minor movement during the setting of the impression material will no cause distortion.

Home-use tooth bleaching agents: an in vitro study on quantitative effects on enamel, dentin, and cementum.

Studies on home-use bleaching agents containing carbamide or hydrogen peroxide demonstrate minimal topographic alteration and insignificant organic change to tooth material. This in vitro study evaluated the effects of a three-step commercial home-use bleaching agent on extracted human incisors over time. Each tooth was digitized by baseline and sequential profilometry and analyzed using computer software. Statistically significant volume loss was evident in cementum and dentin after simulations of 4 and 8 weeks of use.

Wear rates of artificial denture teeth opposed by natural dentition.

The wear rate of polymeric or composite-based artificial denture teeth on fixed, removable and implant prostheses is important in the maintenance of cuspal form, masticatory efficiency and occlusal stability. A servo-hydraulic based artificial mouth in which an occlusion was established between artificial denture teeth, and the palatal cusp of a human maxillary third molar was established and used to assess the wear resistance of 5 artificial denture tooth materials. Volumetric wear was determined for Dentsply Trublend SLM, Dentsply IPN, Vita Vitapan, Ivoclar Orthotyp and Ivoclar Orthosit denture teeth. Scanning electron microscopy was also done to assess the textural character of the wear facets of each denture tooth sample. The lowest volume loss was observed for Trublend SLM followed by IPN, Orthotyp, Vitapan and Orthosit. Textural assessment of the materials was consistent with the objective wear data.

The wear of enamel opposing shaded ceramic restorative materials: an in vitro study.

The wear rate of intact human enamel opposed by Olympia porcelain gold, Dicor, Ceramco porcelain, and externally shaded Dicor and Ceramco was investigated with an artificial oral environment. The enamel-material couples were subjected to 300,000 masticatory cycles at a maximal occlusal force of 13.4 N while they were continuously bathed with 37 degrees C deionized water. Both the enamel and material surfaces were analyzed by use of a three-dimensional surface monitoring computer program, AnSur, to record the removal of the material and the maximal loss of vertical height. The enamel opposing the externally shaded materials abraded two to five times more than that opposing the unshaded materials and 10 to 15 times more than enamel opposing gold. The wear rates for enamel opposing the gold and unshaded Dicor were similar both in the removal of material and in the loss in vertical height.

Measurement of preventive resin restorations using computer profilometry.

The amount of extracoronal material placed when a tooth is restored with a preventive resin restoration (PRR) was quantitatively measured together with the projected area of each cavity preparation. Thirty-three patients (mean age 11 yrs, 6 mos) requiring a PRR on a lower first permanent molar were selected. Impressions of the teeth were made with polyvinylsiloxane before and after cavity preparation and then restored with P-50 and Concise sealant. Following occlusal adjustment, a third impression was taken. Replicas were digitized using computer profilometry. Computer images of occlusal surfaces were superimposed using a "goodness of fit" method. The volume of extracoronal material placed on all molar teeth (n = 33) was X = 6.52 +/- 2.9 mm3. The mean depth applied was X = 222.52 +/- 83.09 mu and the mean area of material applied over the entire tooth surface was X = 27.13 +/- 5.72 mm2. The cavity preparations occupied, on average 19.02 percent of the occlusal surface.

Fissure sealant wear at 30 months: new evaluation criteria.

The volume, depth and area of fissure sealant wear at 30 months is reported. Eighteen premolar teeth were included in the study. The teeth were sealed (Concise, White Sealant, 3M Dental Products Div., St Paul, MN, USA) and an impression taken, yielding a baseline record. Subsequent impressions were taken at 6 months and 30 months. Epoxy replicas were made from all impressions and the occlusal surface of each replica was digitized using a displacement stylus and programmable retrieval system. A goodness-of-fit routine was used to compare the digitized replicas and quantify wear. At 30 months, there was a mean sealant volume loss of 0.43 +/- 0.24 mm3 (mean of the maximum depth loss = 221.8 +/- 115.1 microns; mean area loss = 0.62 +/- 0.15 mm2) for all teeth. A new parameter, the Occlusal Stability Ratio (OSR), is discussed which describes the relationship between sealant area and sealant volume.

Characterization of two small-particle composite resins.

The in vitro occlusal wear of two recently introduced, small-particle composite resins was evaluated. Twelve disks of each material were opposed by maxillary third molars and subjected to 300,000 masticatory cycles in a servohydraulic model of the human masticatory apparatus. Before and after profiles of the surfaces of both the disks and the teeth were mapped and digitally recorded on magnetic disks. P-50 disks showed significantly (P less than .05) less volume loss than did Herculite disks. Enamel that opposed P-50 disks showed significantly (P less than .001) more wear than did enamel that opposed Herculite disks.