Ronald D. Woody

In vitro investigation of the wear of human enamel by dental porcelain

STATEMENT OF PROBLEM Metal ceramic systems are advocated when both esthetics and strength are required. A major drawback to many porcelains is their wear and destruction of opposing natural dentition. PURPOSE This study evaluated the wear of enamel opposing 1 traditional and 2 recently introduced low-fusing feldspathic dental porcelains. MATERIAL AND METHODS Six blocks of Ceramco II, Finesse, and Omega 900 feldspathic porcelain materials were fabricated and fired according to manufacturer recommendations. Porcelain blocks were polished through 0.25 microm diamond polishing paste. Thirty-six enamel specimens were obtained and milled to a 2 mm (+0.5 mm) diameter. Enamel specimens were attached to an offset cam motor operating at 60 Hz. Enamel specimens were in constant contact with the stationary porcelain blocks under a load of 600 g and traversed a distance of 8 mm. Diameter and height of enamel specimens were measured at times of 0, 6, 12, 24, and 48 hours and subsequent determination of enamel volume loss recorded. Profilometric surface roughness of the ceramic blocks was also measured for the corresponding times. RESULTS Statistical analysis revealed that both Finesse and Omega 900 feldspathic porcelains caused significantly less enamel volume loss when compared with Ceramco II porcelain at all time intervals. Surface roughness revealed no consistent significant differences among porcelains. CONCLUSION Both Finesse and Omega 900 porcelains were not as destructive to human tooth structure as Ceramco II porcelain. These results suggest an advantage of the new, lower-fusing porcelains in conditions where natural dentition wear is a concern.

Effects of interfacial variables on ceramic adherence to cast and machined commercially pure titanium

STATEMENT OF THE PROBLEM Titanium-ceramic bonding is less optimal than conventional metal-ceramic bonding, due to excessive oxidation of titanium during porcelain firing. PURPOSE This in vitro study evaluated the effects of porcelain firing atmosphere and gold sputter coating on titanium surfaces on porcelain bonding to machined and as-cast titanium substrates. Material and methods Eight groups of ASTM grade 2 commercially pure (CP) titanium specimens (13 mm x13 mm x1 mm) were prepared (n=10). A conventional Au-Pd-In metal-ceramic alloy (Orion) and an ultra low-fusing porcelain (Finesse) served as the control (n=10). Forty machined titanium specimens were prepared from 1.00-mm thick titanium sheets with a diamond band saw. Forty titanium specimens were produced in a centrifugal dental titanium casting machine. All titanium specimens were airborne particle abraded with 110-microm alumina particles, whereas the control specimens were airborne particle abraded with 50-microm alumina particles. Forty titanium specimens (20 specimens each of as-cast and machined titanium) were randomly selected for gold sputter coating before ceramic firing. An ultra low-fusing porcelain (Vita Titankeramik) was fused on the central 6-mm diameter circular area on each titanium specimen. Porcelain firing environments for the titanium specimens consisted of vacuum and a reduced argon atmosphere. Porcelain was debonded by a biaxial flexure, constant strain test at a cross-head speed of 0.25 mm/min. Specimens were analyzed by standardized SEM/EDS analysis 3 times throughout the study to determine the silicon atomic percentage (Si at %): (1) after airborne particle abrasion, before porcelain application; (2) after the application of the first layer of porcelain; and (3) after the fracture of porcelain from the metal substrate. The titanium-ceramic adhesion was characterized by determining the area fraction of adherent porcelain (AFAP). Results were analyzed by analysis of variance and the Student-Newman-Keuls test (alpha=.05). RESULTS Statistical analysis showed a significant difference in the AFAP values among all the groups. AFAP value of the control group was significantly higher (135.35 +/- 23.68) than those of the experimental groups (P<.001). For the machined titanium, AFAP value of gold sputter-coated/argon group (91.38 +/- 7.93) was significantly higher than the rest of the groups (P<.001). For the as-cast titanium fired in vacuum, significantly lower AFAP values (P<.001) were found in the gold sputter-coated group (50.2 +/- 11.26 vs 66.15 +/- 10.41). AFAP values between the argon groups with or without the gold coating were not significantly different (P=.303); however, both argon groups (93.83 +/- 4.65 and 98.09 +/- 6.35) showed significantly higher AFAP values compared with the vacuum groups (P<.001). CONCLUSION Firing porcelain in a reduced argon atmosphere significantly improved titanium-ceramic bonding for machined and as-cast titanium. The sputter-coated gold layer on titanium provided improved titanium-ceramic bonding only when combined with firing porcelain in reduced argon atmosphere. When porcelain was fired in vacuum in the presence of the gold layer, the titanium-ceramic bonding was weakened in as-cast titanium and was not affected in machined titanium. Conventional noble metal-ceramic bonding was superior to titanium-ceramic bonding regardless of the interfacial variables examined in this study.

Apparent Cytotoxicity of Base Metal Casting Alloys

The cytotoxic potential of four alloy materials was tested; three were Ni-Cr based alloys and one was a Fe-Cr based alloy. Adverse cellular changes were not found in tissue culture studies of castings of the alloys. Cultures containing Ni-Cr powders did show prominent zones of lysis and cell alteration. The cytologic potential of products obtained from surfaces of casting alloys may offer means for estimating long-term clinical toxicity.

Review of the pH of hemostatic agents used in tissue displacement

Newer hemostatic agents such as the tetrahydrozolines and oxymetazolines have a more acceptable pH and should be kinder to tooth structure and soft tissue than the conventional solutions. Although additional study is needed, it would seem prudent to be cautious in using low pH hemostatic agents and avoid the exposure to sensitive intraoral tissues-particularly delicate tissues--or tooth preparation close to the dental pulp.

Effects of bur abrasive particle size and abutment composition on preparation of ceramic implant abutments

STATEMENT OF PROBLEM Amid increasing use of preparable ceramic implant abutments, there is a lack of quantitative data to show which abrasive particle size of diamond bur yields the fastest reduction and provides the smoothest surface. PURPOSE The research aim was to determine the effects of diamond bur abrasive particle size and abutment material composition on preparation efficiency, prepared surface roughness, and surface deterioration of diamond burs. MATERIAL AND METHODS Fifteen alumina (Cera Base) and 15 zirconia (ZiReal) implant abutments were each machined using a high-speed hand piece with a diamond bur having 1 of 3 abrasive particle sizes (150, 100, or 30 microm) (n=5). Control abutments (n=5) were analyzed without machining. Abutments were weighed before starting and between machining cycles. Three profilometry measurements (root mean square surface roughness) were made for each abutment. Scanning electron micrographs were made of each bur. Lost abrasive particles were then counted on each micrograph through a randomly placed template. Two-way analysis of variance (alpha=0.05) was used to test for significant effects. RESULTS Bur abrasive particle size and ceramic type had a significant interactive effect on the amount of material removed (P<.001). Super coarse (150 microm) burs yielded the roughest surfaces for each abutment material (P<.001), and prepared alumina surfaces were rougher than zirconia surfaces (P<.001). Super coarse burs showed the highest proportion of lost particles (P<.001). Abutment composition did not significantly affect bur wear. CONCLUSION Super coarse burs yielded the most efficient material removal for alumina abutments. All abrasive particle sizes removed a similar amount of material from zirconia abutments. Fine-grained alumina abutments experienced greater material removal and rougher prepared surfaces compared with zirconia abutments. Material was removed by an intergranular fracture mechanism for alumina abutments, in contrast to transgranular fracture for zirconia abutments.

Marginal gap of crowns made with a phosphate-bonded investment and accelerated casting method

STATEMENT OF PROBLEM Numerous materials and methods have been used for complete crown fabrication. Conventional investing and casting procedures for phosphate-bonded investments require a 2- to 4-hour process before completion. Accelerated laboratory techniques have been used, but may not result in castings with equal marginal accuracy. PURPOSE This study measured the marginal gap and determined the clinical acceptability of single castings invested in a phosphate-bonded investment with the use of conventional and accelerated methods. MATERIAL AND METHODS Forty-four individual stone casts were poured from impressions made from a master die. Conventional and accelerated methods of investing and casting were followed in the fabrication of 44 single-unit castings. Twenty-two casts were used in each of the 2 groups. Each casting and its respective stone die were examined with a microscope at 4 predetermined sites. Perpendicular and 25-degree tilted measurements of marginal gap were documented for each. Evidence of marginal gap was then evaluated by t test. RESULTS Measurements recorded on the perpendicular and on a 25-degree tilt showed no statistically significant difference between conventional and accelerated groups. All gap measurements except one were within the range of clinical acceptability. The measurements revealed that conventional and accelerated perpendicular gap means were 13.2 and 13.6 microm, respectively, and the average tilted gap means were 31.6 and 32.2 microm, respectively. CONCLUSIONS A phosphate-bonded investment (Ceramigold) selected for an accelerated casting technique produced single castings within 30 minutes with marginal gaps comparable to those found that used conventional methods.

Failure Modes with Point Loading of Three Commercially Available Denture Teeth

PURPOSE A common problem associated with implant-supported prostheses is the fracture of denture teeth. This study was designed to compare the fracture modes of three denture teeth by compressive load at a 30 degrees off-axis angle. MATERIAL AND METHODS Three denture teeth (Vident Duostat, Ivoclar Vivadent, and Dentsply Trubyte) processed to two denture base processing systems [injection-molded (IM) SR-Ivocap system and compression-molded (CM) denture base resin] were evaluated. Each specimen was processed to a metal framework. Ultimate failure strength of each system when point loaded at a 30 degrees off-axis angle was recorded, along with a visual inspection of each specimen. RESULTS The average load fracture for each group was (in N): Vident CM 1106.97 +/- 223.20, Vident IM 1168.18 +/- 322.52, Dentsply CM 1098.08 +/- 286.32, Dentsply IM 1023.80 +/- 282.45, Ivoclar CM 1616.98 +/- 204.87, and Ivoclar IM 1373.54 +/- 282.58. There was a significant difference between the groups and the Ivoclar CM group. The Ivoclar CM group had the highest average load force, and the Dentsply IM group had the lowest average load force. On average, the teeth within the groups fractured at a higher compression force than the average maximum occlusal force in natural dentition. Dentsply and Vident denture teeth fractured more horizontally, and the Ivoclar denture teeth fractured more vertically within the groups. There was no significant difference among the groups between the IM and CM processing methods. CONCLUSIONS In the present in vitro study, all specimens were able to withstand 30 degrees off-axis loading with the exception of one specimen. With these results, this would indicate that these denture teeth are able to withstand normal occlusal forces.

A soft tissue simulated cast for implant prosthesis

A master cast that simulates the mucosa around implant fixtures and abutments facilitates implant prosthesis fabrication. It is helpful if this simulation is not altered during laboratory procedures. A technique is described for a master cast that includes a removable mucosal component. This technique can be applied when either implant or abutment analogs are positioned in the cast.

Evaluation of the examination given by the American Board of Prosthodontics

Abstract The authors would like to express their appreciation to those who participated in this evaluation of the examination given by the American Board of Prosthodontics. The various phases and sections of the examination were evaluated and discussed. Although it is impossible to state each individual opinion, this evaluation was representative and reflects the views stated by most candidates of the 1970–1974 examination period. A variety of suggestions were made for improving the current status and relevance of the Board examination. However, revisions should be made for progressive improvement and not for change alone, which in itself could create a new group of problems and inconsistencies. Most candidates questioned agree that some modifications of the Board examination are desired, but they also feel that the present high standards must be maintained. Desire was expressed for an increased interchange of ideas among the Board, the Diplomates, and the candidates. Candidates are seeking additional information regarding parameters and grading procedures used during the examination, and more definitive explanations of why unsuccessful candidates did not meet the standards of the Board are needed. Diplomates desire additional enlightenment on activities of the Board and the examination itself, with greater input into the election of examiners. Establishing communication creates an atmosphere that provides the opportunity for meeting problems and understanding them as they surface. Exchanging views may produce some type of agreement on delineating the major problems involved, if not uniformity of opinion. In many cases, compromise may be the highest level of agreement that can be achieved. Thus, it is recommended that the American Board of Prosthodontics investigate various modalities to increase its avenue of communication with both candidates and Diplomates. The American Board of Prosthodontics has an outstanding history of dedication and service to the specialty of prosthodontics. The Board should continue to maintain a dynamic and open-minded view in its philosophies, with sufficient flexibility to accept thoughtful and disciplined change.