A Preliminary Study on the Accuracy of Cast Metal Removable Partial Denture Frameworks Produced from Wax, Printed, and Milled Patterns

PURPOSE: To compare the accuracy of two digital workflows for producing resin patterns to be cast into metal frameworks compared to an identical framework manufactured conventionally from a wax pattern. MATERIALS AND METHODS: Nine casts were duplicated from a maxillary master cast of a partially edentulous arch. Their accuracy was determined by measuring the same points in two and three dimensions using a reflex microscope, which was also used to measure all frameworks to an accuracy of 4 µm. The same design was used throughout. Three casts were used to make a framework conventionally from an invested wax pattern. Six casts were scanned, and a digital pattern created. Three patterns were milled from a resin block, and three were 3D printed with resin. Then each pattern was cast. RESULTS: The sample size precluded direct statistical conclusions, but no significant differences were found. Duplicate models showed minimal differences compared to the master cast. All patterns and all frameworks showed some level of difference compared to the master cast, but no differences were greater than those reported in the literature as being clinically acceptable. The maximum overall discrepancy between the cast frameworks was 0.64 mm, and at the rest seats was 0.262 mm. CONCLUSION: Within the limitations of this study, given the very small actual differences both within and between the groups of the three different workflows, the use of digitally produced resin patterns prior to their being cast as metal frameworks is both feasible and well within the accepted limits for clinical acceptability.

A Preliminary Study on the Accuracy of Cast Metal Removable Partial Denture Frameworks Produced from Wax, Printed, and Milled Patterns.

PURPOSE: To compare the accuracy of two digital workflows for producing resin patterns to be cast into metal frameworks compared to an identical framework manufactured conventionally from a wax pattern. MATERIALS AND METHODS: Nine casts were duplicated from a maxillary master cast of a partially edentulous arch. Their accuracy was determined by measuring the same points in two and three dimensions using a reflex microscope, which was also used to measure all frameworks to an accuracy of 4 µm. The same design was used throughout. Three casts were used to make a framework conventionally from an invested wax pattern. Six casts were scanned, and a digital pattern created. Three patterns were milled from a resin block, and three were 3D printed with resin. Then each pattern was cast. RESULTS: The sample size precluded direct statistical conclusions, but no significant differences were found. Duplicate models showed minimal differences compared to the master cast. All patterns and all frameworks showed some level of difference compared to the master cast, but no differences were greater than those reported in the literature as being clinically acceptable. The maximum overall discrepancy between the cast frameworks was 0.64 mm, and at the rest seats was 0.262 mm. CONCLUSION: Within the limitations of this study, given the very small actual differences both within and between the groups of the three different workflows, the use of digitally produced resin patterns prior to their being cast as metal frameworks is both feasible and well within the accepted limits for clinical acceptability.

A comparison of different methods and materials for establishing maximal intercuspal position: A clinical study.

STATEMENT OF PROBLEM: Which method or material used to record the intercuspal position yields the best accuracy of location of the maxillary and mandibular casts is unclear. PURPOSE: The purpose of this clinical study was to determine the most reliable method of recording a patient's maximal intercuspal position by comparing 2 common methods with 2 popular registration materials. MATERIAL AND METHODS: Complete-arch impressions were made of both jaws with a polyvinyl siloxane impression material in a metal stock tray followed by 4 interocclusal registrations for each of the 17 participants. Two registration techniques were used. More than 1 record was obtained in the first technique, as the participant had to close through a wax sheet or a polyvinyl siloxane material. In the second technique, a polyvinyl siloxane material was injected from the buccal aspect between occluded tooth surfaces. Casts were made from Type 4 stone and vertical measurements of the casts were carried out by using digital vernier calipers, accurate to 100 µm. Hand articulation of the casts was used as the control. Consistency of measurement was assessed by the intraclass correlation coefficient, and comparisons were made by using repeated-measures regression analysis. RESULTS: Statistical analysis showed significant discrepancies when the participant closed through both wax and polyvinyl siloxane material (P<.001). No statistically significant differences to the control group were present when polyvinyl siloxane was injected laterally after closure. CONCLUSIONS: Hand articulation was the most accurate method of reproducing the maximal intercuspal position in a completely dentate individual with horizontal and vertical occlusal stability and was therefore considered the control. Recording the position after the participant had closed by using a polyvinyl siloxane material was the most accurate.

Guidelines for the choice of circumferential wrought wire and cast clasp arms for removable partial dentures.

PURPOSE: The aim of this study was to establish guidelines for the selection of cast and wrought-wire clasps for removable partial dentures (RPDs) that would be appropriate for clinically encountered undercuts and facial curvatures. METHODS: Randomly selected discarded casts were collected and 30 premolars and 30 molars were surveyed, sectioned to a line representing the clasp and scanned using a flatbed scanner. The average clasp curvature and length for each group was determined and a three-dimensional model printed, to which wrought wire clasps of 0.9- and 1.0-mm diameter were adapted. Standard wax clasp patterns were adapted and cast in a stellite alloy. Each clasp was deformed beyond its proportional limit; and the forces exerted at that limit and at deflections of 0.25 mm, 0.5 mm, and 0.75 mm were measured, and a safety limit was calculated that would ensure elastic deformation at the required undercuts. RESULTS: A table was produced with guidelines for those clasps that would provide the highest retentive force within the proposed safety limit. The highest forces were provided by cast clasps in a 0.25-mm undercut. Wrought round wire of 1-mm diameter provided the next highest retentive forces, in a 0.25-mm undercut for premolar clasps arms and 0.5-mm for molar clasps. CONCLUSIONS: The results provide valid guidelines for the use of combinations of clasp material and undercut that would exert the maximum retentive force without deformation for both short (premolar) as well as long (molar) clasps, for wrought and cast clasps.

Quantitative Analysis of Selected Microorganisms Present at Various Sites in a Prosthetics Clinic and Dental Laboratory during Complete Denture Fabrication.

Background: Contamination with oral commensals and pathogenic microorganisms, and cross contamination between clinic and laboratory can occur. The amount of contamination has not been determined. Methods: Samples from different clinical and laboratory stages before and after disinfection (17 sites, 10 samples per stage) were collected. Laboratory surfaces and equipment were swabbed for 10 days (11 sites). Swabs were cultured for total mixed flora, Streptococci, Lactobacilli, Staphylococcus aureus, aerobic Gram-negative bacteria (AGNB) and Candida. Knowledge of infection control among staff and students was assessed. Results: Clinic: In total, 30-40% of the samples overall were contaminated with mixed flora and Streptococci of >100 cfu/swab; >100 cfu of AGNB and Candida were present on 6% and 1% of samples; 2% contained <100 cfu of S. aureus. Laboratory: In total, 17-48% of the samples overall were contaminated with mixed flora and Streptococci of >100 cfu/swab; >100 cfu of AGNB were present on 11% of samples; none contained >100 cfu of Candida. Disinfection significantly reduced the level of all organisms. Knowledge of infection control was sufficient, but compliance was poor. Conclusion: Although the count of mixed flora was high, potential pathogens such as S. aureus and Candida were low. In immunocompromised patients, this can become a problem.

Metamerism of Three Different Pigments for Facial Prostheses and a Method to Improve Shade Evaluation.

PURPOSE: To assess the illuminant metamerism of three different pigments used for maxillofacial prostheses under three different illumination sources and to standardize subjective shade-matching and observation protocols. MATERIALS AND METHODS: Nine silicone shade tabs were constructed for three volunteers who had light, medium, and dark skin tones using make-up, oil paint, or silicone pigments (three shade tabs per volunteer). A shade recipe was recorded during session one, and further tabs were constructed by following the recipe at two subsequent sessions. Using an observation protocol, expert examiners rated the shade match under three different illuminants. RESULTS: The intra-rater and inter-rater reliability revealed acceptable reliability (ICC > 0.6). Two-way analysis of variance revealed significant differences (P < .05) for pigments and illuminants and for the interaction of pigments with illuminant on the raters' scores. CONCLUSION: Illuminant metamerism affected the appearance of the silicone shade tabs. Oil paint with fluorescent or incandescent illumination had the worst effect. It was found possible to construct a consistent shade guide and to recommend an observation protocol, which should be repeatable if similarly experienced examiners are used.

Viability of Candida albicans in Denture Base Resin After Disinfection: A Preliminary Study.

PURPOSE: To quantify the depth of penetration of Candida albicans (C albicans) into a denture base resin and to investigate its viability after disinfection. MATERIALS AND METHODS: Heat-polymerizing polymethyl methacrylate plates were contaminated with C albicans, then washed, dried, and prepared for scanning electron microscopy (SEM). Vertical surfaces were cut, and the depth of any penetration was measured. For viability after disinfection, plates were contaminated, subjected to one of two disinfection techniques, and subcultures were taken to determine viability. RESULTS: The results showed that at 7, 14, and 21 days after exposure, the mean depth of penetration of C albicans was 33.9, 96.9, and 97.0 µm, respectively. The depth of penetration was time dependent, with the deepest being 631 µm at 21 days. Daily subcultures for 10 days revealed that penetrated cells remained viable and were not affected by the disinfectant. CONCLUSION: To the present authors' knowledge, this is the first time that the viability of denture base-penetrated C albicans has been shown, even after disinfection. The implication of these results is that denture resin is likely to be penetrated, especially in patients with chronic or recurrent denture stomatitis. If the C albicans remain viable, then the infected denture-fitting surfaces may have to be replaced by removing at least a 1-mm layer of contaminated resin.

The Effect of Too Much Caring: A Preliminary Study.

PURPOSE: To determine the presence of burnout and psychiatric morbidity among prosthodontic residents and prosthodontists working in a maxillofacial prosthetic clinic and to compare these outcomes between those treating cancer patients and those not treating cancer patients. MATERIALS AND METHODS: Two questionnaires were used: the Maslach Burnout Inventory (MBI) and the General Health Questionnaire (GHQ-12). MBI assesses the three components of burnout (emotional exhaustion, depersonalization, and personal accomplishment), and the GHQ-12 screens for nonpsychotic psychiatric morbidity. Six residents and four specialists involved in managing head and neck cancer patients were referred to as cancer clinicians to distinguish them from the specialists not involved in managing cancer patients (noncancer clinicians), who served as controls for other stressors within this work environment. RESULTS: Twelve (86%) questionnaires were returned. There were no statistically significant (P = .89) differences between the cancer and noncancer clinicians from the GHQ-12 results; however, the cancer clinicians showed increased levels of exhaustion and depersonalization. These differences were not statistically significant (P = .36 and P = .41, respectively), but the effect sizes (Cohen's d) were moderate. For personal accomplishment both groups scored in the moderate burnout range, with the noncancer clinicians scoring higher. CONCLUSION: There is no doubt that the clinicians participating in this preliminary study were under some stress, and a moderate effect size indicated that cancer clinicians experience this to a greater extent.

Guide-Plane Retention in Designing Removable Partial Dentures.

PURPOSE: To compare the influence of abutment teeth guide planes and guiding surfaces on retention of a removable partial denture (RPD). MATERIALS AND METHODS: Extracted teeth embedded into a maxillary cast in the first premolar and second molar positions simulated two bounded saddles. Acrylic resin RPDs were made with no guide planes, then with guide planes, then with guiding surfaces added to directly contact the guide planes. The maximum loads on removal from the cast were recorded. RESULTS: There was a significant increase in retention force of 1.6 times when only guide planes were present and of 10.2 times when guiding surfaces intimately contacted the guide planes. CONCLUSION: The retention of acrylic resin RPDs can be substantially increased by making their guiding surfaces intimately contact the guide planes of the teeth.

A Randomized Controlled Trial of Mastication with Complete Dentures Made by a Conventional or an Abbreviated Technique.

PURPOSE: The aim of this randomized clinical trial was to test the hypothesis that there are no statistically significant differences after 3 and 6 months in masticatory performance or chewing ability of people with new complete dentures made by an abbreviated or a conventional technique. MATERIALS AND METHODS: The trial included 20 edentulous participants at a dental school in Brazil assigned randomly to receive dentures made by either a conventional technique involving six clinical sessions or by an abbreviated technique involving three clinical sessions. At baseline with old dentures and at 3 and 6 months with new dentures, masticatory performance was measured by counting the number of chewing strokes and the time before participants had an urge to swallow and by calculating the medium particle size of a silicone material after 20 chewing strokes and at the urge to swallow. On each occasion, the participants recorded on visual analog scales their ability to chew five food textures. Statistical significance (P ≤ .05) of changes in masticatory performance and chewing ability during the trial were analyzed with generalized estimating equations. RESULTS: Both techniques improved masticatory performance between baseline and 6 months and the ability to bite and chew all foods apart from hard apples. CONCLUSION: There were no significant differences in masticatory performance or chewing ability after 6 months between complete dentures made by a conventional or an abbreviated technique.

The three-dimensional casting distortion of five implant-supported frameworks.

The aim of this study was to assess three-dimensional distortion in cast full-arch, screw-retained titanium implant frameworks. A conventional commercial laboratory one-piece casting was used implementing the lost-wax technique. Five wax patterns were fabricated on a die-stone cast poured from a plaster impression of a five-implant brass analog. A reflex microscope was used to determine the three-dimensional casting error. Significant differences were found in distortion between wax patterns and castings, which, given the need to keep within 150 microm of misfit for passivity, were larger than the wax frameworks by between 416 and 477 microm. The greatest distortion occurred at the terminal implant abutments and in the vertical dimensions, but the distortion was inconsistent, indicating its three-dimensional nature. It is doubtful whether any conventionally cast titanium framework can be made to the degree of accuracy required to fit passively on its abutments because of the multiple variables inherent in this process.

Fracture resistance of acrylic resin and metal-reinforced acrylic resin distal extension cantilevers of fixed implant-supported prostheses.

The fracture resistance of acrylic resin and metal-reinforced acrylic resin fixed implant-supported prosthesis cantilever arms to linear vertical compressive forces was determined. Ten superstructures of each type were secured on 5 evenly distributed implants. Force was applied to the cantilever arms 15 mm distal to the distal-most implant, and 2 tests were conducted on each sample. In each sample, fracture of the acrylic resin was noted at the distal-most implant; however, in the nonreinforced samples, the acrylic resin fractured off completely, leaving the implants unaffected. There was no significant difference between the mean fracture value of the nonreinforced samples (679 N) and reinforced samples (628 N).

Guidelines for a minimum acceptable protocol for the construction of complete dentures.

PURPOSE: To investigate the feasibility of obtaining expert consensus on the prosthodontic principles to be followed when constructing complete dentures, so that any modifications to materials and methods would not sacrifice those principles--a philosophy known as "appropriatech." These principles would then comprise a Minimum Acceptable Protocol (MAP) for complete dentures. MATERIALS AND METHODS: A Delphi survey technique was used that requested yes/no answers to a variety of statements describing the different stages in the construction of complete dentures. Respondents could also provide comments on any aspect of the questionnaire. The statements were then modified in light of the responses and comments received, and recirculated. Three rounds of questionnaires were used, and only statements achieving a 90% or greater consensus were included in the MAP. The respondents were randomly selected by country from the 2004 membership e-mail list of the International College of Prosthodontists. RESULTS: Forty-one respondents answered the first questionnaire, 39 the second, and 36 the third. The 75 statements in the first questionnaire were gradually reduced as consensus was reached, and eventually 18 statements remained with 90% or greater agreement. CONCLUSION: Even though expert opinion is regarded as the lowest level of evidence, there are no other methods available to derive such a protocol, and the Delphi technique was useful in obtaining the consensus. This MAP could now be used to help assess clinical techniques that attempt to reduce time and costs while producing a quality service-in other words, which will conform to the philosophy of appropriatech.

Applications of rapid prototyping technology in maxillofacial prosthetics.

PURPOSE: The purpose of this study was to compare the accuracy, required time, and potential advantages of rapid prototyping technology with traditional methods in the manufacture of wax patterns for two facial prostheses. MATERIALS AND METHODS: Two clinical situations were investigated: the production of an auricular prosthesis and the duplication of an existing maxillary prosthesis, using a conventional and a rapid prototyping method for each. Conventional wax patterns were created from impressions taken of a patient's remaining ear and an oral prosthesis. For the rapid prototyping method, a cast of the ear and the original maxillary prosthesis were scanned, and rapid prototyping was used to construct the wax patterns. For the auricular prosthesis, both patterns were refined clinically and then flasked and processed in silicone using routine procedures. Twenty-six independent observers evaluated these patterns by comparing them to the cast of the patient's remaining ear. For the duplication procedure, both wax patterns were scanned and compared to scans of the original prosthesis by generating color error maps to highlight volumetric changes. RESULTS: There was a significant difference in opinions for the two auricular prostheses with regard to shape and esthetic appeal, where the hand-carved prosthesis was found to be of poorer quality. The color error maps showed higher errors with the conventional duplication process compared with the rapid prototyping method. CONCLUSION: The main advantage of rapid prototyping is the ability to produce physical models using digital methods instead of traditional impression techniques. The disadvantage of equipment costs could be overcome by establishing a centralized service.