Lifong Zou

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.

Geographic distribution of porcelain veneer preparation depth with various clinical techniques.

STATEMENT OF PROBLEM Various clinical techniques have been advocated for uniform reduction of the tooth surface before a porcelain veneer restoration. Often these techniques do not produce a consistently uniform labial reduction. PURPOSE The purpose of this study was to identify the degree of inconsistency on a geographic scale in the depth of labial reduction for porcelain veneers, resulting from the use of 3 clinical techniques. The technique of co-ordinate metrology was used to map the variations in the depth of the preparation. MATERIAL AND METHODS A single operator using 3 techniques (dimples as depth guides, freehand, or depth grooves as depth guides) prepared 90 noncarious, unrestored extracted teeth to receive porcelain veneers (n=30). Impressions of the prepared and unprepared teeth were scanned with a co-ordinate measuring machine. In-house software was used to color-code the plotted images on the basis of the depth of preparation. Profile measurements were also made along the mid-labial sagital plane at the mid-labial, incisal, and cervical regions, as well as along the mesial proximal and distal proximal areas along the mid-labial horizontal plane. The ideal depth range for the labial reduction was chosen to be 0.4 to 0.6 mm. One-way analysis of variance and the Bonferroni test were performed to determine the significance (P<.05) in the difference between the means of reductions achieved with the 3 techniques. RESULTS There was no statistically significant difference in the mean percentage area prepared to the ideal depth range (0.4 mm-0.6 mm), between dimple (44.59%), freehand (36.35%), and depth groove (38.43%) techniques. The difference in the mean percentage area of reduction greater than 0.6 mm between dimple (12.98%), freehand (29.66%), and dimple and depth groove (37.32%) techniques were statistically significant (P=.0000), but not between freehand and depth groove techniques. With the profile measurements it was seen that there were statistically significant differences in the mean depth between dimple (0.45 mm) and depth groove (0.63 mm), and freehand (0.51 mm) and depth groove in the mid-buccal (P<.0004) and cervical (dimple = 0.48 mm, freehand = 0.52 mm, depth groove = 0.63 mm) (P<.0005) regions. There was statistically significant difference (P<.0000) in the mean depth between the dimple (0.39 mm) and freehand (0.30 mm), dimple and depth groove (0.50 mm), and freehand and depth groove techniques in the incisal area. In the mesial proximal region statistically significant difference (P<.0034) in the mean depth was found between the dimple (0.52 mm) and freehand (0.68 mm), and dimple and depth groove (0.64 mm) techniques only. In the distal proximal region, there was no statistically significant difference in the mean depth between dimple (0.55 mm), freehand (0.66 mm), and depth groove (0.64 mm) techniques. CONCLUSION The use of dimple technique showed a trend to greater consistency and fidelity in labial reduction to a depth of 0.4 to 0.6 mm. The 3 techniques for veneer preparations studied were associated with varying degrees of inconsistency in the distribution of depth of preparation within a tooth and between teeth in the same technique group.

The reliability of thermoform retainers: a laboratory-based comparative study.

AIM Despite a very significant increase in the use of thermoform retainers, within orthodontics, to date, there is no evidence regarding the quality of fit between the dental cast and retainer. The current study aims to compare the fit of different thermoform retainer materials, under laboratory standardized conditions. MATERIALS AND METHODS A prospective, laboratory-based study design was adopted to compare the fit of four different commercially available thermoform retainer materials. A master dental cast was fabricated in dental stone, to which three and five specific reference points were located on the central incisors and upper first molars, respectively. The master cast was then duplicated to produce a total of 40 such dental casts. The performance of the following four test materials was investigated: ACE, C+ (Raintree Essix Glenroe), True Tain (True Tain Inc.), and Iconic Clear (DB Orthodontics). For each material, 10 thermoform retainers were fabricated within the dental laboratory. Subsequent scanning and analysing of the casts and thermoform retainers was performed in a dedicated area, using the co-ordinate measuring machine, in order to calculate their respective fit at the incisor and first molar regions. RESULTS At all eight recorded landmark points, the mean values for the following thermoform materials: ACE, True Tain, and Iconic Clear, a similar trend was observed. At all eight points, there was a significant difference between the performance of the four different materials (P < 0.001), with the greatest difference observed in the mean values in relation to material C+. CONCLUSION Statistically significant differences were observed in the fit behaviour of all four thermoform materials being tested.

Royal London space analysis: plaster versus digital model assessment

Aim With the advent of digital study models, the importance of being able to evaluate space requirements becomes valuable to treatment planning and the justification for any required extraction pattern. This study was undertaken to compare the validity and reliability of the Royal London space analysis (RLSA) undertaken on plaster as compared with digital models. Materials and methods A pilot study (n = 5) was undertaken on plaster and digital models to evaluate the feasibility of digital space planning. This also helped to determine the sample size calculation and as a result, 30 sets of study models with specified inclusion criteria were selected. All five components of the RLSA, namely: crowding; depth of occlusal curve; arch expansion/contraction; incisor antero-posterior advancement and inclination (assessed from the pre-treatment lateral cephalogram) were accounted for in relation to both model types. The plaster models served as the gold standard. Intra-operator measurement error (reliability) was evaluated along with a direct comparison of the measured digital values (validity) with the plaster models. Results The measurement error or coefficient of repeatability was comparable for plaster and digital space analyses and ranged from 0.66 to 0.95mm. No difference was found between the space analysis performed in either the upper or lower dental arch. Hence, the null hypothesis was accepted. The digital model measurements were consistently larger, albeit by a relatively small amount, than the plaster models (0.35mm upper arch and 0.32mm lower arch). Conclusion No difference was detected in the RLSA when performed using either plaster or digital models. Thus, digital space analysis provides a valid and reproducible alternative method in the new era of digital records.

Dimensional Measurement for Dentistry

Dentistry requires development in hand skills throughout the undergraduate and postgraduate programs both in the pre-clinical and clinical settings (Allred, 1977). The level of expertise achieved depends on the level of training, the natural ability of the dentist and also experience. Tooth preparation for a crown is a common procedure in general dental practice and it is essential that dental students are able to perform this procedure competently before they graduate. The skill training of these techniques is in itself subjective, as there is use of “eyeballing” of the preparation rather than a definite measure. There is clearly a need to obtain more formal dimensional assessment of crown preparations. This will assist in the feedback to and the training of dentists and also in the practise of dentistry.

A three-dimensional soft tissue analysis of Class III malocclusion: a case-controlled cross-sectional study

Objective The present study used the optical surface laser scanning technique to compare the facial features of patients aged 8–18 years presenting with Class I and Class III incisor relationship in a case-control design. Materials and methods Subjects with a Class III incisor relationship, aged 8–18 years, were age and gender matched with Class I control and underwent a 3-dimensional (3-D) optical surface scan of the facial soft tissues. Results Landmark analysis revealed Class III subjects displayed greater mean dimensions compared to the control group most notably between the ages of 8–10 and 17–18 years in both males and females, in respect of antero-posterior (P = 0.01) and vertical (P = 0.006) facial dimensions. Surface-based analysis, revealed the greatest difference in the lower facial region, followed by the mid-face, whilst the upper face remained fairly consistent. Conclusion Significant detectable differences were found in the surface facial features of developing Class III subjects.