Technical note: Artificial Resynthesis Technology for the experimental formation of dental microwear textures.

Dental microwear formation on the posterior dentition is largely attributed to an organism's diet. However, some have suggested that dietary and environmental abrasives contribute more to the formation process than food, calling into question the applicability of dental microwear to the reconstruction of diet in the fossil record. Creating microwear under controlled conditions would benefit this debate, but requires accurately replicating the oral environment. This study tests the applicability of Artificial Resynthesis Technology (ART 5) to create microwear textures while mitigating the challenges of past research. ART 5 is a simulator that replicates the chewing cycle, responds to changes in food texture, and simulates the actions of the oral cavity. Surgically extracted, occluding pairs of third molars (n = 2 pairs) were used in two chewing experiments: one with dried beef and another with sand added to the dried beef. High-resolution molds were taken at 0, 50, 100, 2500, and 5000 simulated chewing cycles, which equates to approximately 1 week of chewing. Preliminary results show that ART 5 produces microwear textures. Meat alone may produce enamel prism rod exposure at 5000 cycles, although attrition cannot be ruled out. Meat with sand accelerates the wear formation process, with enamel prism rods quickly obliterated and "pit-and-scratch" microwear forming at approximately 2500 cycles. Future work with ART 5 will incorporate a more thorough experimental protocol with improved controls, pH of the simulated oral environment, and grit measurements; however, these results indicate the potential of ART 5 in untangling the complex variables of dental microwear formation.

Use of a continual sweep motion to compare air polishing devices, powders and exposure time on unexposed root cementum.

Low abrasive air polishing powders are a viable method for subgingival biofilm removal. This in vitro study evaluated the effects of air polishing using a standard tip on cementum following clinically recommended protocols. Forty-eight teeth were randomly divided into eight groups with six teeth per group. Teeth were treated using either a Hu-Friedy EMS or DENTSPLY Cavitron® air polishing device. One of three glycine powders (Air-flow 25 µm, Clinpro 45 µm, Clinpro+TCP 45 µm) or a sodium bicarbonate powder (NaHCO3  85 µm) was sprayed on cementum using a clinically relevant sweeping motion. Volume and depth of cementum removed after 5 and 90 s exposures were calculated. Surface texture was evaluated using SEMs taken following the last exposure. After 5 s exposures, neither unit nor powder had a substantial effect on volume loss or defect depth. After 90 s exposures, differences between powders existed only for the DENTSPLY unit (p < 0.0001). Pairwise comparisons for this unit revealed mean volume loss and maximum defect depth were greater for NaHCO3 85 µm than the glycine powders (p < 0.0001). The 90 s exposure produced greater mean volume loss and defect depth for all powders (p < 0.0001). SEM images revealed dentinal tubule exposure with all powders; however, exposed tubules were larger and more prevalent for NaHCO3 85 µm. Root surface loss was similar for glycine powders evaluated in this study. Differences in powder performance between units may be related to tip apertures and spray patterns. Additional research is needed to determine if cementum loss is greater than what occurs with conventional biofilm removal methods, such as curets and ultrasonic scalers.

Predicting first molar width using virtual models of dental arches.

In dentistry, large regions of dentition often are restored with minimal information about the original anatomy. The ability to predict missing anatomy from existing anatomy would aid such restorations. This study investigated the relationship between first molar mesial-distal width and arch shape using newly defined reference points and three-dimensional (3D) digital methods. Full-mouth dental stone casts from 167 dental students were scanned and rendered as 3D virtual models. Maxillary and mandibular arch lengths and widths and first molar mesial-distal widths were measured on the virtual models using new definitions incorporating virtual planes. A linear mixed model of the first molar width regressed on the other measurements was done. Intraobserver reproducibility was evaluated by means of intraclass correlation (ICC) and standard deviation of measurement error (SDME). All measured distances were averaged as a combined group and as gender groups. The correlation coefficients between the maxillary and mandibular first molar widths were over 0.70 (P < 0.01). Intraobserver error was small. ICCs were over 0.92 and SDMEs were from 0.11 to 0.21 mm. Arch dimensions and first molar widths were defined and measured. Regression equations were calculated for predicting first molar width. The prediction of first molar width using arch dimension is essential for virtual designing of missing first molars. This approach also provides reliable reference point definitions for the virtual dental model which was impossible with traditional measurement methods. Therefore, this study would be helpful for understanding the 3D anatomy of dental arch and fabrication of automatic prosthodontic restorations.

Comparison of the finite helical axis and the rectangular coordinate system in representing orthodontic tooth movement.

In orthodontics, tooth movement is typically described using the rectangular coordinate system (XYZ); however, this system has several disadvantages when performing biomechanical analyses. An alternative method is the finite helical axis (FHA) system, which describes movement as a rotation about and a translation along a single axis located in space. The purpose of this study was to examine differences between the FHA and the XYZ systems in analyzing orthodontic tooth movement. Maxillary canine retraction was done using sliding mechanics or a retraction spring with midpalatal orthodontic implants used as measuring references. Tooth movement calculated with the FHA was compared with the corresponding movement in the rectangular coordinate system weekly over a 2-month interval in eight patients. The FHA showed that sliding mechanics controlled rotation of the canine better than the retraction spring (Ricketts retractor), and that the Ricketts retractor controlled tipping better. Changes in the FHA direction and position vectors with time showed that the biomechanical forces are not uniform during the treatment period. In both mechanics, the FHA provided a simple biomechanical model for canine retraction.

Intra-oral restorative materials wear: rethinking the current approaches: how to measure wear.

OBJECTIVE: To determine what wear parameter(s) have clinical relevance and what factors are important for accurate measurement of these parameters in vivo and in vitro. METHOD: Describe biomechanical factors affecting mastication and the mechanics of wear. Investigate how they impact the wear of teeth and restorative materials. Based on this information, define the advantages and disadvantages of using volume, depth, and area parameters to quantify wear. Describe direct and indirect methods of measuring wear and point out advantages and disadvantages of each. RESULTS: The preferred parameter for quantifying wear is volume. It is independent of occlusal factors and is a measure of work done. If material and environmental factors remain constant, volume loss is linear with time. Depth and area have limited clinical value because of their dependence on occlusal factors; plus, they are not linear with time. When measuring wear the material of interest and the opposing material must be considered; especially if the opposing material is enamel. Wear is best measured by comparing sequential 3D images. Measuring systems should be calibrated with their error reported using sigma values rather means and standard deviations. The quality of the alignment of the sequential images should be included in the error analysis. Cost and availability of 3D imaging systems has severely limited their use in clinical studies. SIGNIFICANCE: Wear is an important consequence of occlusal interactions. If not controlled, wear could lead to poor masticatory function with a concomitant reduction in quality of life and possible deterioration of systemic health.

An intra-oral hydraulic system for controlled loading of dental implants.

This study reports a method for controlling loads on an in vivo dental implant and its application for the investigation of early loading versus delayed loading of dental implants. The method was developed for the purpose of studying an ongoing hypothesis that amounts to bone loss around dental implants are related to mechanical-mediated adaptation of the alveolar bone. Using a customized intra-oral hydraulic system, the daily loading over a dental implant has been completed and recorded for six Sinclair swine. Each pig had a 5-month duration implant loading. During the experiments (loading), no analgesic treatment was supplied. The mean of the in vivo daily loadings was confirmed through an in vitro bench test after each animal was euthanized. Variations of the averaged loading input among the six animals were smaller than 10%. Preliminary data produced by the model suggests that cervical bone loss is less for early loading than for delayed loading. The current system is expected to provide a useful load control model for the study of alveolar bone adaptation around dental implants in relation to various loadings.

Tooth deformation patterns in molars after composite restoration.

OBJECTIVE: Residual stresses from polymerization shrinkage in composite restorations deform a tooth. This may cause debonding, enamel crack propagation, and post-operative sensitivity. Deformation due to shrinkage has been measured previously at a few discrete points. The purpose of this study was to analyze cuspal deformation pattern of the occlusal portion of molars for various cavity types and sizes after restoration with a light-initiated composite. METHODS: Five extracted human molars were successively prepared as Class I, Class II OM, large Class II OM, and large Class II MOD. The cavities were filled with a light-curing composite using a dentin adhesive system. The occlusal portion of the unrestored cavity and the restoration were digitized with a profilometer. The digitized data of the unrestored and restored tooth were used to calculate the cuspal contour change with Cumulus software. Deformation was visualized as a color contour map. RESULTS: Cuspal deformation showed up in the contour map as a reduction of buccal and lingual contour perpendicular to the surface. Large Class II MODs exhibited the highest cuspal deformation, followed by large OM restorations. Cuspal deformations in Class I and small Class II OM restorations were not significantly different. SIGNIFICANCE: When a composite restoration was cured, the surrounding tooth deformed due to polymerization shrinkage. Cavity type and size affected how much cusps moved inward as a result of polymerization shrinkage. This study quantified and visualized the pattern of cuspal deformation.

Residual shrinkage stress distributions in molars after composite restoration.

OBJECTIVE: Experimental measurements on various restoration configurations have shown that restored teeth deform under the influence of polymerization shrinkage, but actual residual stresses could not be determined. The purpose of this study was to calculate and validate shrinkage stresses associated with the reported tooth deformations. METHODS: Three different restoration configurations were applied in a finite element model of a molar. The composite properties were based on experimentally determined composite behavior during polymerization. The occlusal deformation pattern and the residual stress states of the tooth, restoration, and tooth-restoration interface were calculated using a polymerization model based on the post-gel shrinkage concept. Reported strain gauge measurements and occlusal deformation patterns were used for validation. RESULTS: The shrinkage stresses depended on the configuration and size of the restorations. The tooth's resistance against polymerization shrinkage diminished with loss of dental hard tissue. Larger restorations resulted in lower stress levels in the restoration and tooth-restoration interface, but increased stresses in the tooth. The maximum stress values found for different configurations were not decisively different. SIGNIFICANCE: The validated model indicated that shrinkage stress cannot be based on composite properties or restoration configuration alone, but has to be approached as a distributed pattern that depends on the location and on the properties of tooth and restoration, geometry, constraints, and restoration procedures. Tooth deformation was indicative of stresses in the tooth rather than in the restoration or across the tooth-restoration interface.

Relationships between tissue properties and operational parameters of a dental handpiece during simulated cavity preparation.

A preliminary study was conducted on the development of an intelligent dental handpiece with functionality to detect subtle changes in mechanical properties of tooth tissue during milling. Such equipment would be able to adopt changes in cutting parameters and make real-time measurements to avoid tooth tissue damage caused by overexertion and overextension of the cutting tool. A modified dental handpiece, instrumented with strain gauges, microphone, displacement sensor, and air pressure sensor, was mounted to a linear movement table and used to mill three to four cavities in >50 bovine teeth. Extracted sound frequency and density were analyzed along with force, air pressure, and displacement for correlations and trends. Experimental results showed a high correlation (coefficient close to 0.7) between the feed force, the rotational frequency, and the averaged gray scale. These results could form the basis of a feedback control system to improve the safety of dental cutting procedures. This article is written in memory of Dr Hongyan Sun, who passed away in 2011 at a young age of 37.

Quantitative analysis of tooth surface loss associated with gastroesophageal reflux disease: a longitudinal clinical study.

BACKGROUND: Acid regurgitation resulting from gastroesophageal reflux disease (GERD) causes dissolution of tooth structure. The authors conducted a longitudinal clinical study to measure tooth surface loss associated with GERD. METHODS: The authors made replicas of dental impressions obtained from 12 participants with GERD and six control participants at baseline and six months. Using an optical scanner, they digitized the tooth surfaces of these replicas. They then analyzed the volume of tooth surface loss and characterized it as noncontact erosion or erosion/attrition. RESULTS: Mean (standard deviation) volume loss per tooth in participants with GERD (0.18 [0.12] cubic millimeter) was significantly higher than that in control participants (0.06 [0.03] mm(3); t test; P < .013). Nine participants with GERD exhibited tooth surface loss with characteristics of erosion (noncontact erosion in three participants, erosion/attrition in eight participants). CONCLUSIONS: Tooth surface loss in participants with GERD was significantly greater than that in control participants. The pattern of surface loss was characteristic of erosion in noncontact areas and around contact areas. CLINICAL IMPLICATIONS: Anterior and posterior teeth of participants with GERD were affected by erosive tooth wear. In addition, the amount of erosive tooth wear on occlusal surfaces was twice as high when there was evidence of attrition.

Wear of a dental composite in an artificial oral environment: A clinical correlation.

The study objective was to correlate wear between an in vitro method for simulating wear and in vivo wear of a posterior dental composite. Ten subjects (12 restorations) were selected from a five-year clinical study (University of North Carolina, School of Dentistry) that assessed wear of SureFil composite (Caulk, Dentsply). Subject casts were digitized and changes in volume and mean depth with time were calculated from the 3D digital models for contact and contact-free wear. SureFil composite disks were mounted in the University of Minnesota's Artificial Oral Environment, opposed by natural enamel, subjected to mandibular-like movements for 150 K, 300 K, 600 K, 1.2 M, and 1.5 M cycles, and loaded with peak forces of 13 N (n = 7) or 30 N (n = 3). Wear rates were calculated as the slope of the linear regressions fitting the wear data. Data were analyzed using one-way ANOVAs and post hoc t-tests where appropriate (p = 0.05). Clinical restorations included contact wear on seven restorations and contact-free wear on all restorations. Contact-free wear was less than contact wear (p < 0.01). SureFil clinical wear rates were 0.012 mm/year (mean depth) and 0.023 mm(3)/year (volume). Clinical restorations expanded slightly during the first year. Using a conversion rate of one year equals 3 × 10(5) cycles, there were no significant differences between the clinical and simulated data except depths at Year 5 and 13 N volume at Year 4. The 30 N simulation reproduced the clinical data if contact-free wear was taken into account. Good agreement between simulated and clinical wear implies that in vitro simulation can screen new composite formulations.

Can hygroscopic expansion compensate polymerization shrinkage? Part I. Deformation of restored teeth.

OBJECTIVE: Restorative materials exhibit contraction and expansion due to polymerization and water absorption. Each process deforms and stresses a restored tooth structure in opposite direction. This study evaluated the tooth deformations during these competing processes. METHODS: Large MOD cavities were prepared in 10 extracted molars. Five were restored with a hydrophobic resin composite (Filtek Supreme, 3M ESPE) and the others with a hydrophilic resin-modified glass-ionomer (Ketac Nano, 3M ESPE). The restored molars and two unrestored controls were stored in water for 24 weeks. The molars were digitized with an optical scanner at baseline, after preparation, restoration, and at 1, 2, 4, 8, 16, and 24 weeks water immersion. The digitized buccal, lingual, and restoration surfaces were analyzed to determine their deformation patterns. The results were statistically analyzed using ANOVA followed by Student-Newman-Keuls post hoc tests (p=0.05). RESULTS: The buccal and lingual tooth surfaces moved 13-14 µm inward after restoration. After water immersion, cuspal deformation in the resin composite group gradually decreased, reversing the shrinkage deformation within four weeks. The immersed resin-modified glass ionomer group reversed shrinkage deformation within one week, and continued to expand further to 28 µm after 24 weeks. Cuspal deformations after water immersion were significantly different with the two restoratives. Restoration surfaces also expanded after water immersion, while the control teeth showed no significant deformation. SIGNIFICANCE: Polymerization shrinkage deformation was compensated by hygroscopic expansion within 4 weeks in teeth restored with a hydrophobic resin composite, while a hydrophilic restorative over-compensated polymerization shrinkage within 1 week causing tooth expansion.

Accuracy of two impression techniques with angulated implants.

STATEMENT OF PROBLEM: Accurate recording of implant locations is required so that definitive restorations are properly supported and do not place additional stress on the implants. Angulated implants may result in inaccurate impressions, and the impression technique may affect the accuracy of the definitive cast. PURPOSE: The purpose of this study was to determine the effect the combined interaction of impression technique, implant angulation, and implant number has on the accuracy of implant definitive casts. MATERIAL AND METHODS: One definitive stone cast was fabricated for each of 6 experimental groups and 1 control group. All 7 definitive casts had 3 implants arranged in a triangular pattern creating a plane. In the 6 experimental groups, the center implant was perpendicular to the plane of the cast while the outer implants had 5, 10, or 15 degrees convergence towards or divergence away from the center implant. The control definitive cast had all 3 implants parallel to each another and perpendicular to the plane of the cast. Five open tray and 5 closed tray addition silicone impressions were made of each definitive cast. Impressions were poured with type IV dental stone, and a fine tip measuring stylus was used to record multiple axis (X-Y-Z) coordinates on the top surface of the implant hex and on the cast base. Computer software was used to align the data sets and vector calculations determined the difference in degrees between the implant angles in the definitive cast and the duplicate casts. Statistical analysis used repeated-measures ANOVA (alpha=.05) with post-hoc tests of significant interactions. RESULTS: The angle errors for the closed and open tray impression techniques did not differ significantly (P=.22). Implant angulations and implant numbers differed in average angle errors but not in any easily interpreted pattern (P<.001). The combined interaction of impression technique, implant angulation, and implant number had no effect on the accuracy of the duplicate casts compared to the definitive casts (P=.19). CONCLUSIONS: The average angle errors for the closed and open tray impression techniques did not differ significantly. There was no interpretable pattern of average angle errors in terms of implant angulation and implant number. The magnitude of distortion was similar for all combinations of impression technique, implant angulation, and implant number.

Factors affecting enamel and ceramic wear: a literature review.

Enamel wear by ceramics may adversely affect maintenance of the vertical dimension of occlusion and can increase the potential for thermal sensitivity. In this article, factors related to the abrasion of enamel by dental ceramics are critically reviewed. Concepts of physical, microstructural, chemical, and surface characteristics of dental ceramics on wear are presented based on research published since 1950. A PubMed search for key words (wear of enamel and ceramic) was supplemented with a hand search to identify relevant peer-reviewed articles published in English. Based on the literature, it can be concluded that material factors, their proper handling, and control of the patient's intrinsic risk factors related to wear are critically important to the reduction of enamel wear by dental ceramics.

Comparing maximum intercuspal contacts of virtual dental patients and mounted dental casts.

STATEMENT OF PROBLEM: Quantitative measures of occlusal contacts are of paramount importance in the study of chewing dysfunction. A tool is needed to identify and quantify occlusal parameters without occlusal interference caused by the technique of analysis. PURPOSE: This laboratory simulation study compared occlusal contacts constructed from 3-dimensional images of dental casts and interocclusal records with contacts found by use of conventional methods. MATERIALS AND METHODS: Dental casts of 10 completely dentate adults were mounted in a semi-adjustable Denar articulator. Maximum intercuspal contacts were marked on the casts using red film. Intercuspal records made with an experimental vinyl polysiloxane impression material recorded maximum intercuspation. Three-dimensional virtual models of the casts and interocclusal records were made using custom software and an optical scanner. Contacts were calculated between virtual casts aligned manually (CM), aligned with interocclusal records scanned seated on the mandibular casts (C1) or scanned independently (C2), and directly from virtual interocclusal records (IR). Sensitivity and specificity calculations used the marked contacts as the standard. Contact parameters were compared between method pairs. Statistical comparisons used analysis of variance and the Tukey-Kramer post hoc test (P=<.05). RESULTS: Sensitivities (range 0.76-0.89) did not differ significantly among the 4 methods (P=.14); however, specificities (range 0.89-0.98) were significantly lower for IR (P=.0001). Contact parameters of methods CM, C1, and C2 differed significantly from those of method IR (P<.02). The ranking based on method pair comparisons was C2/C1 > CM/C1 = CM/C2 > C2/IR > CM/IR > C1/IR, where ">" means "closer than." CONCLUSIONS: Within the limits of this study, occlusal contacts calculated from aligned virtual casts accurately reproduce articulator contacts.

Laser digitization of casts to determine the effect of tray selection and cast formation technique on accuracy.

UNLABELLED: STATEMENT OF PROBLEM; Several studies have attempted to determine the ideal combination of dental materials and laboratory techniques to produce the most accurate dental cast. Most have made use of 2-dimensional manual measuring devices, which neglect to account for the dimensional changes that exist along a 3-dimensional surface. PURPOSE: The purpose of this study was to determine the effect of impression tray selection and cast formation techniques on the dimensional change of a dental cast with the use of new, 3-dimensional optical digitizing technology. MATERIAL AND METHODS: Multiple impressions of a machined steel die that resembled a dental arch were made with custom and stock impression trays and vinyl polysiloxane impression material. The impressions were poured in type V artificial dental stone and allowed to set with the tray inverted or noninverted. The steel master die and stone casts were digitized with the Steinbichler Comet 100 Optical Digitizer, which was developed at the Minnesota Dental Research Center for Biomaterials and Biomechanics. Three-dimensional images of the stone casts were aligned to the 3-dimensional image of the master die and analyzed with AnSur-NT software. Multiple measurements of the master die and stone casts were analyzed to determine the accuracy of the 3-dimensional technology and of the impression and cast fabrication techniques. Planar distances between the center of each crown preparation were measured, as were crown heights. Data were analyzed with analysis of variance (P<.05), and root mean square error values were determined. RESULTS: Casts were compared with a total of 45 significance tests, of which only 4 yielded P<.05. There was no pattern to these results, which suggests that they were false-positive findings. CONCLUSION: Results obtained with the use of new optical digitizing technology indicated that neither impression tray type nor cast formation technique affected the accuracy of final casts.

Clinical evaluation of a new resilient denture liner. Part 1: Compliance and color evaluation.

PURPOSE: A new experimental resilient denture liner (MPDS-SL; Lai Laboratories, Burnsville, MN) and Molloplast-B (Buffalo Dental Manufacturing, Syosset, NY) were clinically evaluated for compliance and color change over a 1-year period. MATERIALS AND METHODS: In this crossover study, each of 20 patients had 2 dentures fabricated with long-term, silicone-based resilient liners, 1 denture with Molloplast-B and the other with MPDS-SL. Each denture was used for 6 months, during which time each patient kept a journal detailing his or her use and cleaning regimen. The 2 materials were assessed for compliance and color at the beginning of the study and again after 3 months and 6 months of use. Compliance was determined by applying a 3-lb force to the surface of the material following a square-wave pattern, using a closed-loop servohydraulic testing system. The force and position values were recorded using a storage oscilloscope. Compliance was measured at 3 locations on each denture and analyzed using data-acquisition software. Images of the dentures were captured using a zoom stereomicroscope with a charge-coupled video camera and image analysis software. The color was measured at 3 locations on each denture; RGB and L* a* b* were calculated. RESULTS: Compliance increased from baseline to 3 months and from 3 months to 6 months for almost all locations on both materials. Molloplast-B and MPDS-SL differed in average change in compliance at 6 months; the average change in compliance from baseline to 6 months was 453 (standard error, 46) for Molloplast-B and 284 (standard error, 46) for MPDS-SL (p = 0.019). For both materials, color changed significantly from baseline to 3 months and from baseline to 6 months (p < 0.01). MPDS-SL changed significantly less than Molloplast-B from baseline to 6 months for R (p = 0.039), G (p = 0.037), B (p = 0.005), and L* (p = 0.042). CONCLUSION: For both materials, compliance increased over 6 months of wear. The color change for MPDS-SL was considerably less significant than that for Molloplast-B.