Jack I. Nicholls

In vitro comparison of intact endodntically treated teeth with and without endo-post reinforcement

In an in vitro experiment, the failure loads of 59 intact endodontically treated teeth with and without Kerr Endo-Post reinforcement were compared. Fifty-eight teeth fractured below the cementoenamel junction. One tooth fractured through the pulp chamber with a chisel fracture involving both the crown and root. Teeth without posts fractured through the middle or coronal one third of the root. Teeth with posts fractured through the body of the post. No statistically significant reinforcement was demonstrated by cementing a Kerr Endo-Post No. 100 into a sound endodontically treated tooth.

Further investigation of spreader loads required to cause vertical root fracture during lateral condensation

The roots of 54 extracted human mandibular incisorswere instrumented, measured, and then filled with gutta-percha by lateral condensation on an Instron testing machine until vertical root fracture occurred. A spreader load as small as 1.5 kg (3.3 lb) produced a fracture, and 13% of the sample fractured at a load of 3.5 kg (7.7 lb) or less. Computer bivariate correlation analysis revealed significant positive linear correlations (p

An in vitro study of spreader loads required tocause vertical root fracture during lateral condensation

Forty-four extracted human maxillary central incisors, maxillary canines, and mandibular canines were filled with gutta-percha by lateral condensation on an instron testing machine until vertical root fracture occurred. A spreader load as small as 7.2 kg produced a fracture in a maxillary central incisor. Sixteen percent of the sample fractured at a load of less than 10 kg. No statistically significant correlation was found between fracture load and root dimensions. However, a statistically significant linear correlation did exist between the fracture load and the load rate at fracture (p

Evaluation of the marginal fit of a zirconia ceramic computer-aided machined (CAM) crown system

STATEMENT OF PROBLEM The marginal fit of crowns is a concern for clinicians, and there is no conclusive evidence of any one margin configuration yielding better results than others in terms of marginal fit. PURPOSE The purpose of this study was to evaluate the marginal fit with respect to gap and overhang of Y-TZP (yttria-stabilized tetragonal zirconia polycrystals) ceramic crowns and compare them with lithium disilicate pressable and complete metal crowns. The influence of margin configuration on the marginal fit was also evaluated. MATERIAL AND METHODS Impressions were made of premolar dies prepared with shoulders or chamfers with a 20-degree total occlusal convergence (n=30). Type IV stone dies were then distributed into test groups (n=10) for the fabrication of Cercon Y-TZP, IPS Empress II, and complete metal (noble type IV alloy) crowns. The crowns were then subjected to marginal gap and overhang evaluation at 6 designated margin locations using a computerized digital image analysis system. The data were calculated and statistically analyzed using 2-way ANOVA and Bonferroni post hoc tests (α=.05). RESULTS The overall mean (SD) marginal gap at the marginal opening for the crowns was 66.4 (42.2) μm for Cercon, 36.6 (32.1) μm for IPS Empress II, and 37.1 (22.3) μm for complete metal (control). Overall mean (SD) overhang was -15.2 (47.5) μm for Cercon, -22.1 (34.7) μm for IPS Empress II, and 30.9 (31.3) μm for complete metal (control). The ANOVA revealed significant effects by material and no significant effects by marginal configuration for marginal gap. There were significant differences in the marginal overhang values between the 2 margins, but no significant differences were found between the material groups for overhang. CONCLUSIONS The Cercon system showed significantly larger (P<.05) marginal gaps than both the IPS Empress II and complete metal (control) crowns, but no significant differences were found in marginal overhang among the 3 material groups. There was no significant difference in marginal gap between the 2 margin configurations, namely, chamfer and shoulder, for all test groups; however, there were significant differences (P<.05) in marginal overhang values between chamfer and shoulder margins.

Fracture load and mode of failure of ceramic veneers with different preparations.

STATEMENT OF PROBLEM Fracture is a clinical failure modality for ceramic veneers. Whether design of tooth preparation can affect the strength of ceramic veneers remains controversial. PURPOSE This in vitro study evaluated fracture load and mode of failure of ceramic veneers, with 4 tooth preparation designs, that were bonded on extracted human maxillary central incisors. Identical parameters were also measured on unrestored intact teeth for comparison. MATERIAL AND METHODS Fifty maxillary central incisors were randomly divided into 5 equal groups. Each group was assigned a different tooth preparation design: (1) no incisal reduction, (2) 2 mm incisal reduction without palatal chamfer (butt joint), (3) 1 mm incisal reduction and 1 mm height palatal chamfer, (4) 4 mm incisal reduction and 1 mm height palatal chamfer, and (5) unrestored (control). Forty teeth were prepared to accommodate ceramic veneers of equal thickness and incisocervical length. Stone dies were fabricated and veneers made from IPS Empress ceramic. Ceramic veneers were bonded and all teeth mounted in phenolic rings with epoxy resin. Fracture loads were recorded with a mechanical testing machine. RESULTS Mean fracture loads (SD) in kgf were as follows: group 1, 23.7 (6.11); group 2, 27.4 (9.63); group 3, 16.4 (3.44); group 4, 19.2 (6.18); and group 5, 31.0 (10.38). Modes of failure were also analyzed for both ceramic veneers and teeth. One-way ANOVA with multiple comparisons revealed 3 significant subsets: groups 1-2-5, groups 4-1, and groups 3-4 (P <.05). Groups 1 and 2 had no ceramic veneer fractures; group 3 had 3 ceramic veneer fractures, and group 4 had 6 ceramic veneer fractures. CONCLUSION Groups 1 and 2 recorded the greatest fracture loads that were comparable to an unrestored control.

Flexure fatigue of 10 commonly used denture base resins

The flexure fatigue properties of 10 denture base resins (four different types) were tested. Each specimen underwent two-way testing in a water bath maintained at 37 degrees +/- 1 degree C. A load of 3,650 gm was applied a 342 flexures per minute until the specimen broke. The resin types listed in order of increasing resistance to flexure fatigue are polymethylmethacrylate (PMM) pour resins, PMM thermosetting resins, vinyl resins, and a PMM grafted resin. Although small samples size and scatter of results limit broad application of this data, it is of some significance that the grafted resin Lucitone 199 tended to withstand repeated flexure when compared to the other resins.

A comparison of strains generated during placement of five endodontic posts

Twenty-five extracted human maxillary central incisors were randomly divided into five equal groups. Crowns were removed 1 mm incisal to the cementoenamel junction. Endodontic treatment was carried out and each tooth was affixed with two strain gauges. The strain gauge wires were connected to a Wheatstone Bridge circuit. Each group of five teeth was restored with either a Para-Post Plus (the control post), Flexi-Post, Vlock post, Kurer Fin Lock Anchor, or a Radix Anchor. Strains generated during post placement were recorded and compared for the five groups using a one-way analysis of variance. The maximum strains accompanying placement of the Kurer Fin Lock Anchor and the Radix Anchor were significantly higher than those induced by placement of the other posts. Also, when the threaded posts were allowed to contact the bottom of the prepared channel, high strains resulted.

Distortion of ceramometal fixed partial dentures during the firing cycle

1. Distortion does occur in the body of curved, long-span fixed partial denture frameworks during the porcelain firing cycle. 2. This distortion is a result of changes in the metal as well as the contraction of fired porcelain. 3. The greatest distortional changes occur during the degassing stage and the final glaze stage of the porcelain firing cycle. 4. Distortion incurred by the application and firing of the porcelain is reversible. When the porcelain is chemically removed from the framework, there is an elastic rebound. 5. Distortion is clinically important in that it may lead to detectable marginal openings. 6. The distortion pattern observed in the curved fixed partial denture is a closing of the posterior or lingual dimensions and labial movement in the anterior dimension. This indicates the effect of the contracting porcelain on the metal framework.

A study of the flexibility of wrought wire clasps

T he 18-gauge platinum-gold-palladium (PGP) wrought wire clasp arm is often used as a direct retainer for distal-extension removable partial dentures instead of a cast chromium-cobalt clasp arm, when the retentive undercut is on the mesial-buccal part of the abutment tooth. The wrought wire clasp arm can flex in any direction so that stresses caused by movement of the removable partial denture are dissipated. This helps to protect the abutment tooth from harmful forces. The 18-gauge PGP clasp wire has a fusing temperature sufficiently high that it can be safely cast against by gold and some chromium-cobalt alloys and soldered without changing its physical properties. An 18-gauge clasp wire is thick enough to resist breakage and allow tapering for additional flexibility.’ However, clinical observations of some wrought wire clasps on removable partial dentures constructed at the School of Dentistry, University of Washington, have indicated that these clasps may be less flexible than expected. Several reports in the dental literature have documented similar findings.

Tensile bond of resin cements to porcelain veneers

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