Fracture resistance of cingulum rest seats in CAD-CAM tooth-colored crowns for removable partial denture abutments.

STATEMENT OF PROBLEM: The prevalent use of computer-aided design and computer-aided manufacturing (CAD-CAM) for tooth-colored ceramic materials has led to several case reports and retrospective clinical studies of surveyed crowns used to support removable partial dentures. How the specific contour of a cingulum rest seat affects the fracture resistance of these CAD-CAM tooth-colored materials is unknown. PURPOSE: The purpose of this in vitro study was to compare the fracture resistance of monolithic CAD-CAM tooth-colored mandibular canine-surveyed ceramic crowns with cingulum rest seats of different designs. MATERIAL AND METHODS: Five groups (n=24/group) of CAD-CAM tooth-colored crowns were milled from the same standard tessellation language (STL) file: group EM, lithium disilicate-based material (IPS e.max CAD CEREC blocks); group SM, zirconia-based material (NexxZr T); group LP, zirconia-based material (Lava Plus High Translucency); group ZC, zirconia-based material (ZirCAD LT); and group MZ, composite resin (MZ100 CEREC blocks), used as a control. Crowns from each group were divided into 2 subgroups representing 2 shapes of cingulum rest seat design: round design subgroup (n=12) with 0.5-mm radius of curvature and sharp design subgroup (n=12) with 0.25-mm radius of curvature for the rest seat preparation. The crowns were cemented with resin cement to a composite resin die on a steel nut. After 24 hours of storage in water at 37°C, the specimens were statically loaded to fracture with a custom metal retainer on top of the cingulum rest seat by using a universal testing machine at a crosshead speed of 1.5 mm/min. Two-way ANOVA and the Tukey honestly significant difference tests were used to control the familywise error rate (α=.05). Representative specimens were examined using an optical stereomicroscope at ×10 magnification and a scanning electron microscope to determine the failure patterns and fracture mechanism. RESULTS: The results of the ANOVA test indicated statistically significant differences by materials and rest seat designs (P<.001). The mean ±standard deviation maximal load capacity was 773.5 ±255.0 N for group MZ, 1124.9 ±283.9 N for group EM, 2784.1 ±400.5 N for group SM, 2526.9 ±547.1 N for group LP, and 3200.8 ±416.8 N for group ZC. The round design subgroups had an approximately 30% higher mean failure load than the sharp design subgroups for all surveyed crowns. CONCLUSIONS: The present in vitro study demonstrated that zirconia-based groups fractured at twice the load as the lithium disilicate group. Of the 3 zirconia-based groups, group ZirCAD had a statistically greater fracture resistance than the other groups. Designing the cingulum rest seat to have a broad round shape provides a statistically significant higher fracture resistance than a sharp-shape design (P<.05).

An in vitro comparison of photogrammetric and conventional complete-arch implant impression techniques.

STATEMENT OF PROBLEM: Conventional impression techniques for recording the location and orientation of implant-supported, complete-arch prostheses are time consuming and prone to error. The direct optical recording of the location and orientation of implants, without the need for intermediate transfer steps, could reduce or eliminate those disadvantages. PURPOSE: The objective of this study was to assess the feasibility of using a photogrammetric technique to record the location and orientation of multiple implants and to compare the results with those of a conventional complete-arch impression technique. MATERIAL AND METHODS: A stone cast of an edentulous mandibular arch containing 5 implant analogs was fabricated to create a master model. The 3-dimensional (3D) spatial orientations of implant analogs on the master model were measured with a coordinate measuring machine (CMM) (control). Five definitive casts were made from the master model with a splinted impression technique. The positions of the implant analogs on the 5 casts were measured with a NobelProcera scanner (conventional method). Prototype optical targets were attached to the master model implant analogs, and 5 sets of images were recorded with a digital camera and a standardized image capture protocol. Dimensional data were imported into commercially available photogrammetry software (photogrammetric method). The precision and accuracy of the 2 methods were compared with a 2-sample t test (α=.05) and a 95% confidence interval. RESULTS: The location precision (standard error of measurement) for CMM was 3.9 µm (95% CI 2.7 to 7.1), for photogrammetry, 5.6 µm (95% CI 3.4 to 16.1), and for the conventional method, 17.2 µm (95% CI 10.3 to 49.4). The average measurement error was 26.2 µm (95% CI 15.9 to 36.6) for the conventional method and 28.8 µm (95% CI 24.8 to 32.9) for the photogrammetric method. The overall measurement accuracy was not significantly different when comparing the conventional to the photogrammetric method (mean difference = -2.6 µm, 95% CI -12.8 to 7.6). CONCLUSIONS: The precision of the photogrammetric method was similar to CMM, but lower for the conventional method as compared to CMM and the photogrammetric method. However, the overall measurement accuracy of the photogrammetric and conventional methods was similar.

A segmented implant surgical guide connected with magnets.

The 2-piece surgical guide introduced in this article is designed for guiding implant placement in the edentulous arch. With the aid of embedded magnets, this guide can be easily and quickly assembled and disassembled during the surgical procedure without losing the predetermined guidance for the implant placement.

Freedom of rotation of selected overdenture attachments: an in vitro study.

STATEMENT OF PROBLEM: Attachments for mandibular overdentures may not allow for adequate freedom of rotation. PURPOSE: The purpose of this study was to subject 3 commonly used stud types of resilient attachments to rotation and to measure and compare the degree of rotation that occurs before the attachments bind. MATERIAL AND METHODS: Three stud attachments (ERA, Locator Extended Range, O-Ring System) with their corresponding metal housings were evaluated, first for a single implant system and secondly for a 2-attachment parallel implant system for the mandibular arch. Both the manufacturers' most and least retentive components were tested in each system. In the Locator system, the medium retention (3 lbs) for the standard is the most retentive of the extended range. A total of 60 test specimens, 5 per group, were fabricated. The attachments were connected to a 115-mm rod and attached to a testing device composed of a base, implant analogs, and a vertical plate holding graph paper. A free-sliding plastic block, which supported the metal rod parallel to the base, was removed to release the rod. The distance the rod fell was photographed and input into a computer with software designed to measure linear movement. Three-way ANOVA with heteroscedasticity consistent standard errors was used for data analyses (α=.05). RESULTS: Significant differences found among the most retentive components showed that the Locator had a greater freedom of rotation than the ERA (P=.001). The most retentive components in all systems showed that 1 attachment had a greater freedom of rotation than 2 (P=.005). When testing the least retentive components, all systems were significantly different (P<.003), with the freedom of rotation ranking being O-Ring System > Locator Extended Range > ERA. For all 3 systems, the least retentive components showed the most freedom (P≤.008). CONCLUSIONS: The least retentive components offer greater rotation than the most retentive components for all attachment systems tested and for both single and double attachments. Using the most retentive components, the Locator Extended Range attachment allows greater rotation than does the ERA for both single and double attachments. Using the least retentive components, the ORS attachment allows the most rotation and ERA the least for both single and double attachments. Using the most retentive components, one attachment allows greater rotation than does 2 attachments for the attachment systems evaluated.

A comparison of the rigidity of five mandibular major connectors for partial removable dental prostheses via load deflection.

STATEMENT OF PROBLEM: The rigidity of the major connector for partial removable dental prostheses (PRDP) has not been well defined. There is little scientific evidence to support specific standards relative to the dimensions of a casting and their effect on long-term function of the PRDP or the supporting tissues. PURPOSE: The purpose of this study was to compare in vitro the effects of load deflection of 5 clinically used mandibular major connector forms. MATERIAL AND METHODS: A partially edentulous dental cast of a patient with the mandibular anterior teeth remaining was used as the test model. Lingual plate (LP), continuous bar (CB), sublingual bar (SB), lingual bar (LB), and dental lingual bar (DLB) specimens (n=5) were waxed on identical refractory casts, then cast in cobalt base alloy and finished. Each specimen was placed on a custom test table and statically loaded in the contralateral second molar region with 500 g in the horizontal and vertical planes. Deflections were measured at the terminal abutment rest seat, anterior to the load. Comparisons between group means were evaluated using the Dunnett T3 method, assuming unequal variances, for all pairwise tests at a .05 significance level and for 95% confidence intervals for group mean differences. Differences within the groups for vertical versus horizontal deflections were compared using a paired t test and Bonferroni correction (alpha=.05 for all tests). RESULTS: Horizontal deflection between LP and LB, CB and LB, and SB and LB were significant, with the first in each comparison having the least amount of deflection. A significant difference was also seen in the vertical deflection, with the first in each comparison observed to have deflected the least for LP versus CB, LP versus LB, LP versus DLB, CB versus DLB, LB versus DLB, and SB versus DLB. The amount of vertical deflection was significantly greater than the horizontal deflection for all 5 mandibular major connectors (P< or =.033). CONCLUSIONS: In the horizontal plane, a significant difference in the amount of deflection was seen between the LP and LB, CB and LB, and LB and SB. In the vertical plane, the LP deflected significantly less than the CB, LB, and DLB. In addition, the DLB deflected significantly more in the vertical plane when compared to the CB, LB, and SB. All 5 mandibular major connectors had a significantly greater amount of vertical deflection as compared to horizontal deflection.

Managing the maxillary partially edentulous patient with extensive anterior tooth loss and advanced periodontal disease using a removable partial denture: a clinical report.

The treatment modality, a continuous occlusal rest removable partial denture, not only restored missing teeth but also stabilized the remaining dentition in a patient with advanced periodontal attachment loss. By engaging the guiding planes at the mesial surfaces of the abutments anteriorly and also the distal surfaces of the abutments posteriorly, the remaining teeth, with varying amounts of mobility, were splinted together by the framework. This conservative treatment option allows flexibility for easy repair during the life span of the prosthesis.

Laser welding of removable partial denture frameworks.

PURPOSE: To identify and measure distortions inherent in the casting process of a Class III mandibular cobalt-chromium (Co-Cr) framework to illustrate the problems faced by the laboratory technician and the clinician and to measure the changes that occur during the correction of the fit discrepancy using laser welding. MATERIALS AND METHODS: Five identical castings of a Co-Cr alloy partial denture casting were made and measured between 3 widely separated points using the x, y, and z adjustments of a Nikon Measurescope. The same measurements were made after each of the following clinical and laboratory procedures: sprue removal, sectioning of the casting into 3 parts through the posterior meshwork, fitting the segments to the master cast, picking up the segments using resin, and laser welding of the 3 segments. RESULTS: Measurements of all 5 castings showed a cross-arch decrease after sprue removal, an increase after fitting the segments to the master cast, and a slight decrease after resin pickup and laser welding. CONCLUSIONS: Within the limitations of this study, the findings suggest that precise tooth-frame relations can be established by resin pickup and laser welding of segments of Co-Cr removable partial denture frameworks.

Removable partial denture design using milled abutment surfaces and minimal soft tissue coverage for periodontally compromised teeth: a clinical report.

This clinical report describes the treatment of a partially edentulous patient with periodontally compromised teeth using a combination of single crowns and a removable partial denture (RPD). The RPD was designed to combine the benefits of milled surfaces and hygienic principles while allowing modification and addition of artificial teeth, should natural teeth be lost.

Lingual retention and the elimination of the visible clasp arm.

UNLABELLED: The modern removable partial denture (RPD), in response to the increased esthetic demands of our patients, is no longer acceptable if it requires visible buccal and facial clasp arms for retention. By eliminating the bracing arm and having its functions taken by paralleled guide planes and precise, positive rests, the retentive element, a lingual circumferential wire clasp arm, will provide the same amount of retentive force as if it were placed on the buccal surface and will not be visible. Designs for the major connector either plate the lingual surface of the abutment or, as in a lingual bar, leave that surface open. Both of these situations are adaptable to the lingual retentive clasp and are illustrated in this article. CLINICAL SIGNIFICANCE: Design features such as metal clasps needed for the retention of RPDs can be very unsightly, creating an esthetic problem. This article reviews the concepts for creating lingual clasp arms that enhance the esthetic quality of prostheses.

A survey of edentulous patient preference among different denture esthetic concepts.

STATEMENT OF PROBLEM: Despite the fact that solutions to functional problems are available, successfully restoring the appearance of an edentulous patient remains a challenge. PURPOSE: The aim of this survey was to determine if edentulous respondents could differentiate among three denture esthetic concepts and if there was an overall preference among the three. MATERIALS AND METHODS: Six edentulous test subjects were selected based on age and smile criteria. One wax tooth arrangement was completed according to each of the three esthetic concepts for a total of three wax tooth arrangements per test subject. The three esthetic concepts followed were natural, supernormal, and denture look. Standardized full-face digital photographs were made of each arrangement (three) during maximum smile for each subject (six). These 18 photographs were included in a questionnaire. Respondents were asked questions about their preference among the three randomly ordered concept photographs for each of the six subjects. A total of 147 completed questionnaires were analyzed. RESULTS: Ninety-six percent of respondents were able to differentiate among the three esthetic denture concepts. Natural tooth arrangements were preferred by 55% of the respondents, supernormal tooth arrangements were preferred by 19%, and the denture look arrangements were preferred by 26%. CONCLUSION: Within the limitations of this survey, the questionnaire respondents differentiated between the three esthetic denture concepts. The tooth arrangement most closely resembling the anatomical average was selected by 55% of the respondents. Preference for a particular concept changed when responses to each test subject set were considered individually. Demographic factors do not significantly affect patient preference. CLINICAL SIGNIFICANCE: Three esthetic concepts for complete denture construction have been differentiated. Questionnaire respondents preferred appearances that are far from the anatomical average 45% of the time.

A clinical investigation of the fit of removable partial dental prosthesis clasp assemblies.

STATEMENT OF PROBLEM: Removable partial denture frameworks are complex castings fabricated from high-shrinkage alloys. This may result in difficulty in achieving accurate fit. PURPOSE: The purpose of this study was to evaluate which component(s) of the clasp assembly contact the abutment tooth clinically. MATERIAL AND METHODS: New and existing frameworks with at least 1 conventional prepared occlusal rest were evaluated for fit. The spacing between the bottom of each of the rests of the 50 clasp assemblies and the corresponding rest seat were recorded with vinyl polysiloxane. The fit of each rest was determined with the use of a dial caliper by measuring, in micrometers, the thickness of the record between the rest and the bottom of the rest seat. The Kennedy classification was also recorded. A 2-sample t test was used to evaluate the difference in fit between tooth-tissue supported and tooth-supported designs (alpha=.05). RESULTS: The average space between the rest and prepared rest seat was 193 +/- 203 microm, with a range of 0 to 828 mum. Twenty tooth-tissue frameworks had an average space of 136 +/- 160 microm and 30 tooth-supported frameworks had an average space of 230 +/- 222 microm. The 2-sample t test showed no significant difference for fit between tooth-tissue supported and tooth-supported frameworks (P=.1081). Twenty-four percent of rests had contact in the prepared rest seat. CONCLUSION: The majority of rests evaluated did not contact the intended surfaces.

The milled surface as a precision attachment.

The creation of paralleled, milled surfaces in natural or restored abutment teeth, coupled with removable partial denture castings that have optimal contact with these preparations, results in a path of insertion and removal that is controlled in a manner similar to one using conventional precision attachments. Because the milled surfaces are primarily extracoronal, little if any, additional tooth reduction is required for adequate mouth preparation. Avoiding additional tooth reduction minimizes later abutment fracture. The great increase in stability and resistance to rotational movements, when combined with conventional posterior clasping, provides a reasonable alternative to the precision attachment in providing maximal esthetics for the partially edentulous patients.

Clinical outcome of the altered cast impression procedure compared with use of a one-piece cast.

STATEMENT OF THE PROBLEM: An altered cast impression procedure to improve the support of distal extension removable partial dentures is widely taught, but not often used in dental practice. PURPOSE: The purpose of this study was to determine the efficacy of an altered cast compared to a one-piece cast with regard to base support, abutment health, and patient comfort over time. MATERIAL AND METHODS: Seventy-two patients receiving a mandibular bilateral distal extension removable partial denture were assigned randomly for treatment using either a one-piece or an altered cast. All impressions and associated laboratory procedures were made by one investigator. A second investigator evaluated extension, support, and adaptation of the denture bases by observation of border length and lifting of the indirect retainer from its seat. The space between the soft tissues and the base when the framework was related to the teeth was measured cross-sectionally at half the length of the denture base. Mobility, gingival index, and sulcus depths at 6 locations around each abutment tooth were recorded at insertion and again 1 year later. Chi-square tests were used to evaluate differences between the treatment groups (alpha=.05). RESULTS: There was 0.15 mm less space between the ridge crest and base in the altered cast group (P<.01), and underextension of the base occurred only in the one-piece cast group (P=.01). At insertion, no tissue-ward movement was observed in 85% of the prostheses when anterior-posterior rotation was attempted. Fifteen subjects (21%) were lost to recall at 1 year, but they were distributed equally between the 2 groups. Of the remaining 57 prostheses, 42% exhibited decreased base support and 33% had increased gingival inflammation; the deepest probing depth decreased in 61%, mobility decreased or remained the same in 80% of the direct abutments, and 88% of the subjects were satisfied. None of these findings were related to the impression procedure. CONCLUSION: The altered cast impression procedure does not offer significant advantages over the one-piece cast, provided the standards used in this study are met. These include a completely extended impression, use of magnification to adjust and ensure complete seating of the framework, and coverage of the retromolar pad and buccal shelf by the base.

The conversion partial denture: a clinical report.

The treatment alternative described maximizes the benefit of remaining teeth while allowing simplified alteration of the prosthesis if abutments are lost during the life span of the removable partial denture (RPD). A conversion partial is an RPD whose tooth-frame assembly components are individually fabricated and then joined with an acrylic resin major connector. The conversion RPD optimizes retention and stabilization of a terminal dentition and can be easily converted to an immediate complete denture.

Posterior implants for distal extension removable prostheses: a retrospective study.

Common complaints associated with the Kennedy Class I (bilateral free end) and Class II (unilateral free end) removable partial denture situations are lack of stability, minimal retention, and unesthetic retentive clasping. Some of the same complaints have been reported for implant overdentures with only anterior implants. Starting in 1995, 10 of these patients were treated at the University of Washington with posterior osseointegrated implants to provide stability and/or retention of the removable prostheses, eliminating the need for clasps when possible. This article describes implant alternatives and prosthesis designs and presents a follow-up clinical evaluation of at least 1 year consisting of patient satisfaction, radiographic examination, and soft tissue health. Two groups were evaluated. Group 1 included patients whose implants were used as vertical stops for mandibular distal extension prostheses. Care was taken to ensure that the implants were not loaded laterally by creating a single-point contact at the center of a modified healing abutment. In these cases, sufficient retention was available from the anterior teeth and/or implant abutments. Group 2 included patients whose implants required retention because of lack of adequate tooth abutments. In those cases, a resilient type of attachment was used, which allowed for a small divergence from the path of insertion. Results indicated consistent increased satisfaction in all patients, minimal component wear, no radiographic evidence of excessive bone loss, and stable peri-implant soft tissues.

The milled implant bar: an alternative to spark erosion.

Patients who cannot tolerate total coverage of the hard palate or whose maxillary arches are poorly formed, because of congenital, developmental or surgical defects, may be unable to wear a conventional complete denture. These patients can be successfully treated with implant-supported prostheses that cover only a minimal amount of palatal tissue. With spark-eroded castings, very precise restorations can be constructed to fit such implant supports. However, these castings are so expensive that cost precludes their use for many patients. This article presents an alternative approach, developed with the refined techniques used for removable partial dentures, which can yield results similar to those for spark-eroded castings at a fraction of the cost. The clinical and laboratory procedures involved in this technique are described.

Custom precision attachment housings for removable partial dentures.

Limited space for extracoronal attachments is a serious complication in the design and fabrication of a precision attachment removable partial denture. A custom metal-ceramic housing joined to the partial framework with laser welding offers strength and improved esthetics while requiring minimal space. This article presents the technical aspects of custom housing construction.