G.A. Holland

Marginal fit of castable ceramic crowns

The objective of this study was to measure marginal fit of castable ceramic versus gold crowns. Full veneer gold and ceramic crowns were made on Ivorine dies. Crowns were cemented, embedded, sectioned faciolingually and mesiodistally, and photographed for measurement of absolute marginal discrepancies (cavosurface angle to casting margin) to evaluate fit. Results revealed no statistically significant differences in fit among four locations around the margins of either ceramic or gold crowns. There was no statistically significant difference in the combined absolute marginal discrepancy (fit) between ceramic and gold crowns. The variance of the combined absolute marginal discrepancy (fit) of the ceramic crowns was significantly different than that of the gold crowns (p = 0.01 level). The standard deviation (variability) of the gold crowns was more than twice that of the ceramic crowns. Randomized block ANOVA demonstrated statistically significant differences among individual gold crown specimens, but none among individual ceramic crown specimens.

Evaluation of condylar position from temporomandibular joint radiographs.

This investigation evaluated the accuracy of three different transcranial TMJ radiographic records of condylar position and joint space dimension. In addition, the study evaluated how reliably observers determined condylar position and joint space dimensions from the radiographs. The results of this investigation support the following conclusions. The classification of condylar position is not the same at different sagittal locations within a TMJ. Skull position and radiographic projection must be identical if joint space measurements from serial radiographs are to be compared. The actual joint space dimensions and the anatomic anterior/posterior position of the condyles in the glenoid fossae cannot be accurately recorded by the radiographic techniques used in this investigation. Condyle/fossa relationships cannot be classified reliably by subjective evaluation of TMJ radiographs.

Effects of preparation relief and flow channels on seating full coverage castings during cementation

Machined steel dies were used to study the effects of three die modifications on seating full coverage castings during cementation. The die modifications consisted of occlusal channels, occlusal surface relief, and axial channels. Fourteen specimens having one or more forms of die modification were compared with two control specimens having no die modifications. Statistical analysis of the data revealed that the addition of four axial channels to the simulated preparation on the steel die produced a significant reduction in the mean marginal discrepancy during cementation. Occlusal modifications alone failed to produce significant reductions in marginal discrepancies when compared with the control specimens. Occlusal modifications in conjunction with axial channels failed to produce further significant reductions in marginal discrepancies when compared with those reductions observed in specimens having only axial channels.

A photoelastic analysis of the stress distribution in bone supporting fixed partial dentures of rigid and nonrigid design.

Within the limitations of the experiment, the following conclusions can be made from this investigation: 1. Under conditions of vertical loading, the rigid fixed partial denture design does not permit independent response by either abutment. The nonrigid fixed partial denture design allows the abutments some independence in response to vertical loading 2. The stress distributions and concentrations produced in the supporting bone are favorably altered by the placement of a fixed partial denture of rigid or nonrigid design. 3. The distribution of stresses in the supporting bone varies with the number and location of the loading sites. 4. Under conditions of vertical loading, the Ney and Stern nonrigid fixed partial denture designs exhibit no significant differences in stress distribution or concentration.

Photoelastic analysis of stress induced from insertion of self-threading retentive pins

W hen teeth have lost significant amounts of their original structure, it is common to use pins as auxiliary support for the restorative material. Three basic systems are used for the retention of auxiliary pins: (1) cement (threaded or nonthreaded pins are luted into a prepared channel that is slightly larger in diameter than the pin); (2) friction (nonthreaded, tapered pins are forced into a channel that is slightly smaller than the pin); and (3) threads (threaded pins are twisted into a channel that is prepared slightly smaller in diameter than the pin).’ Of the three systems, self-threading pins are used most frequently because of their *excellent retentive properties.2 However, teeth may be damaged as a result of their use; cracking and crazing of dentin has been observed in association with insertion of self-threading retentive pins in extracted teeth.3 Two-dimensional photoelastic models have been used to observe stresses associated with the placement of various kinds of loads on the three types of retentive pin systems and to investigate stress concentrations associated with pin-retained amalgam restorations.4a 5 A photoelastic study that specifically evaluated the self-threading self-shearing retentive pin has shown the presence of high stress zones in the lateral and most apical portions of the pin channel; the apical zone exhibits the most stress.6 It is important to minimize the stresses induced during placement of pins because of the relatively small amount of tooth structure between the pulp and the external tooth surface. It is particularly important that the high stress zone be minimized to reduce the potential risk of fracture and resultant pulpal involvement or tooth failure, The goal of effective pin placement should be adequate retention with elimination of the high stress zone.

Photoelastic stress analysis of supporting alveolar bone as modified by nonrigid connectors

A two-dimensional photoelastic model was constructed to represent the mandible with a missing first premolar and first molar. The model contained a canine, second premolar, and second molar as abutment teeth supported by simulated periodontal ligament and photoelastic bone. Six FPDs were constructed, one of rigid design and five of nonrigid design with varying location and orientation of the nonrigid connector. Each of the six prostheses was subjected to six different loading conditions. Patterns of stress for each loading condition were recorded photographically and evaluated. This study indicated that the rigid FPD distributed stresses vertically and evenly. The nonrigid D of canine and nonrigid M of molar designs distributed stresses almost as well as the rigid FPD. They also resisted rotational movements and resultant horizontal stress better than other nonrigid designs. Compared with the other designs, prostheses with nonrigid connectors at the pier exhibited greater apical and horizontal stress particularly with one-point loading on the pier. Considering the limitations of this study, the following conclusions were drawn: 1. The pattern of stress is dependent on incorporation and location of a nonrigid connector. 2. The pattern of stress reflected the condition of loading. 3. The pattern of stress is independent of the orientation of the nonrigid connector at the distal aspect of the pier abutment. 4. The placement of a nonrigid connector at the mesial surface of the pier is least desirable.

An alternative orientation of nonrigid connectors in fixed partial dentures

Nonrigid connectors have been advocated for fixed partial dentures. However, space limitations may require overreduction of the preparation or overcontouring of the retainer to place the keyway within the retainer wall. An inverted orientation of the nonrigid connector can resolve these problems. With this design, the key is attached to the distal surface of the mesial retainer in a dual-abutment fixed partial denture, and the keyway is incorporated in the mesial surface of the pontic. This inverted orientation offers several advantages with few disadvantages.

Marginal discrepancy changes at various stages of construction of three-unit porcelain-fused-to-metal fixed partial dentures

Abstract Misfit at margins during fixed partial denture construction may occur at a variety of stages. The objective of this work was to monitor the vertical and horizontal changes at 3 locations on both abutments of a 3-unit fixed partial denture during various stages of fabrication. Distances were measured between fiduciary marks after casting, degassing, opaquing, body porcelain application, glazing, and soldering. There was no distinct pattern to the direction and magnitude of changes occurring during the presoldering steps. Measured changes ranged overall from −45 to +80 μm, but most absolute changes at any stage before soldering averaged less than 17 μm. The more significant changes occurred during the soldering step due to shrinkage of the solder. The largest changes during the soldering step occurred in the vertical direction.

Evaluation of the frozen-slab technique for cementing cast restorations

C ooling the mixing surface of a glass slab to temperatures above the dew point improves the properties of zinc phosphate cement. Moisture condensation, however, produces a cement with inferior physical proper-u-s when conventional powder-liquid ratios are used.’ Cooling the mixing surface to 10” C or lower and increasing the powder-liquid ratio to establish a proper cementing consistency results in a cement mix that retains strength and solubility properties at acceptable levels.‘-’ Under these same conditions, the working time is reportedly doubled while the setting time is significantly reduced. Many studies involving the frozen-slab technique have dealt with orthodontic considerations. Little information has been reported on the use of the technique for cementing fixed restorations. The objective of this study, therefore, was to evaluate the frozen-slab technique for use in cementing cast restorations. A series of commercially available zinc phosphate cements were evaluated for film thickness, temperature rise during setting, and pH levels.