Dayananda Samarawickrama

The effect of veneering and heat treatment on the flexural strength of Empress® 2 ceramics

OBJECTIVES The aims of the study were to test and compare the biaxial flexural strength and reliability of Empress 2 ceramics after heat treatment and the addition of the veneering material and to characterise their microstructures. METHODS Forty disc specimens (2 x 14 mm) and forty disc specimens (1 x 14 mm) were produced by heat pressing in the EP 500 press furnace. Group 1 (2 x 14 mm Empress 2 core) was as heat pressed and group 2 (2 x 14 mm Empress 2 core) was subjected to the recommended firing cycles. Groups 3 and 4 (1 x 14 mm Empress 2 core) were veneered with the dentine material and heat-treated as per group 2. Groups 1, 2 and 3 were lapped to 800 grit silicon carbide paper on the compressive surface only and group 4 on both the compressive and tensile test surfaces. Twenty disc specimens per group were tested using the biaxial flexure test at a crosshead speed of 0.15 mm/min. Specimens were characterised using X-ray diffraction (XRD) and secondary electron imaging (SEM). RESULTS Mean biaxial flexural strengths (MPa+/-SD) were group 1: 265.5+/-25.7; group 2: 251.3+/-30.2; group 3: 258.6+/-21.4 and group 4: 308.6+/-37.7. There was no statistical difference between groups 1, 2 and 3 (p>0.05), but differences for group 4 (p<0.05). XRD and SEM revealed lithium disilicate and lithium orthophosphate in the Empress 2 core material and an amorphous glass and some evidence of a crystalline phase in the dentine material. CONCLUSIONS; Veneering or heat treatment of Empress 2 ceramics did not significantly affect the mean biaxial flexural strength (p>0.05) or reliability. Surface modification of the Empress 2 core material increased the mean biaxial flexural strength (p<0.05).

Assessment of shoulder dimensions and angles of procelain bonded to metal crown preparations

The metal ceramic crown is the most popular extracoronal restoration in the United Kingdom. These restorations may fail because of fracture or esthetics. A potential cause of failure is the quality and width of the facial shoulder preparation. In this study 24 extracted human teeth were prepared to receive metal ceramic crowns by one of three dentists. Preparations were replicated and scanned in the midfacial plane by a coordinate measuring machine with a noncontact probe. The x, y, and z surface coordinates were recorded. The results indicated a mean (+/-SD) shoulder width value of 0.752 mm (+/-0.174 mm) and a shoulder angle of 108.54 (+/-15.06) degrees. From these data it would appear that there are deficiencies in shoulder preparations, particularly in width. These inadequacies may have implications for longevity of the restoration and periodontal health in a clinical situation.

Taper of Full-Veneer Crown Preparations by Dental Students at the University of the West Indies

PURPOSE The ideal taper recommended for a full-veneer crown is 4° to 14°, but this is very difficult to achieve clinically, and studies on taper achieved by dental students have found mean taper measurements ranging from 11° to 27°. The objective of this study was to examine and compare the taper of teeth prepared for full-veneer crowns by dental students on typodonts in the laboratory and on patients, and also to compare the results with those of other dental schools. MATERIALS AND METHODS Preparations were scanned by specialized metrology equipment that gave the taper of the preparation in a buccolingual (BL) and mesiodistal (MD) plane. RESULTS No undercut was detected on any of the laboratory specimens; however, 12.5% of clinical specimens were undercut. The mean taper of the laboratory anterior specimens were 26.7° BL and 14.9° MD, and the laboratory posterior specimens were 18.2° BL and 14.2° MD. The mean taper of the clinical anteriors were 31.6° BL and 16.8° MD, and the clinical posteriors were 16.8° BL and 22.4° MD. CONCLUSIONS This study shows that although the taper achieved by dental students in the University of the West Indies when preparing teeth for full-veneer crowns was outside the ideal range of 4° to 14°, it is comparable to those achieved by dental students in other schools.