R.G. Chadwick

Comparison of a novel photogrammetric technique and modified USPHS criteria to monitor the wear of restorations

This paper describes monitoring the wear of restorations borne by partial dentures over a 12 months period using a novel photogrammetric technique and modified United States Public Health Service (USPHS) criteria. The performance of Class II restorations of Dispersalloy was compared with that of similar restorations of either KetacFil or Occlusin. The photogrammetric technique highlighted differences in performance not detected by the modified USPHS criteria. It is concluded that the photogrammetric technique should prove valuable in the in vivo assessment of the performance of restorative materials but that further refinement of the method is required particularly with regard to the orientation of replicas for sequential measurements.

A comparison of the shear bond strengths to a resin composite of two conventional and two resin-modified glass polyalkenoate (ionomer) cements

The inability of resin composite to bond directly to dentine has compromised the clinical success of this restorative material. This problem may be overcome by the so-called laminate technique in which glass polyalkenoate cement is placed upon dentine and then covered with a resin composite. The cement both adheres to dentine and affords micromechanical attachment to the composite. The introduction of the resin-modified glass polyalkenoate cements (RMCs) offers the potential for chemical union between cement and composite. This investigation examined the shear bond strengths of P-50 resin composite to four glass polyalkenoate lining materials, with and without the application of an intermediate bonding agent (Scotchbond 2). Two of the cements were RMCs (Vitrebond, XR-Ionomer) and the others were conventional base materials (Baseline, Ketac-Bond). The bond between P-50 and Vitrebond with or without Scotchbond 2 was significantly (P < 0.01) stronger and more consistent (P < 0.05) than that observed for all other materials. The treatment of the conventional materials and XR-Ionomer with Scotchbond 2 significantly (P < 0.01) improved the bond strengths to P-50. It is concluded that Vitrebond formed the most favourable cement-resin composite bond and that the other materials studied should be used in conjunction with an effective intermediate bonding agent, such as Scotchbond 2.

The effect of placement technique upon the compressive strength and porosity of a composite resin

This paper examines the effect of two placement techniques upon the compressive strength and level of porosity of specimens of a composite resin. Those specimens prepared by a condensation technique had a significantly lower mean compressive strength (P less than 0.05) and value of Weibull modulus (P less than 0.05) than specimens prepared using a smearing technique. No significant differences in the overall mean percentage area occupied by porosity and mean number of pores were demonstrated (P greater than 0.05) between the two placement techniques. Specimens prepared using the smearing technique, however, had significantly smaller mean maximum pore diameters (P less than 0.05) than those prepared using the condensation technique. It is therefore desirable when placing composite resins to reduce to a minimum handling procedures that are likely to influence the levels of porosity within the material.

Surface pH of resin-modified glass polyalkenoate (ionomer) cements

The recently developed group of materials known as light-activated, or resin-modified, glass polyalkenoate (ionomer) cements have been produced in response to clinical demands for a command set cavity base material. This study monitored the surface pH of three commercially available resin-modified glass ionomer cements over a 60-min period following either mixing alone or mixing followed by a 30-s exposure to a curing lamp. The results indicate that each material behaves in a unique manner. For all materials and conditions the pH reached after a 60-min period was significantly (P < 0.001) higher than the initial value. Light curing the materials significantly increased (P < 0.01) the surface pH of two of the materials (Baseline VLC and Vitrebond) as compared to the same materials in the uncured state. In the case of XR-Ionomer, however, no significant (P > 0.05) effect of light curing upon the surface pH was apparent. The precise clinical consequences of a low surface pH are unclear but may be an aetiological factor in postoperative pulpal sensitivity. It is therefore recommended that a sublining of a proprietary calcium hydroxide lining material should be placed routinely beneath these materials and every effort made to ensure effective light curing.

The permeability of dental gloves following exposure to certain dental materials.

OBJECTIVES This investigation sought to study the permeability of three commonly used clinical gloves when exposed to chemical agents. METHODS A total of 35 gloves of each type were selected at random and were assigned to one of seven equally sized treatment groups. The effective barrier properties were investigated following treatment for 15 min with one of: acid etchant, Trim, cavity varnish, Hibiscrub, Hydrex and Vitremer using a dye permeability test. Following the chemical insult a 0.02% solution of erythrosine dye was introduced into each glove and the outer glove surface was washed with 10 ml of distilled water at intervals of 30, 60, 90 and 120 min. The percentage absorption of the collected washings, at 530 nm, was determined using a spectrophotometer. The results were compared with those values obtained from untreated gloves. RESULTS Acid etchant and Hydrex had little effect on any of the gloves. Trim caused an increase in permeability of the vinyl gloves but had little or no effect upon those made of latex. Cavity varnish increased the permeability of both Biogel D and Tru-Touch but had no effect on Featherlite. The integrity of the latex gloves was diminished by Hibiscrub but little effect was observed in the case of the vinyl based glove. The permeability of all three gloves was increased by exposure to Vitremer liquid. CONCLUSIONS Manufacturers should provide details of potential glove chemical interactions to assist dentists in glove selection.

Strength-probability-time (SPT) diagram— an adjunct to the assessment of dental materials?

This investigation sought to construct and compare strength-probability-time (S-P-T) diagrams for four dental materials. Three of these were resin composites and one was dental plaster. In the case of dental plaster a total of 90 compressive specimens as fabricated whereas for each of the other materials a total of 75 specimens was prepared. The compressive strength of equal sized groups of each material was then determined at the crosshead (XHD) speeds of 1, 5 and 10 mm min-1 respectively. The data was subjected to Weibull analysis to relate the probability of failure to the applied stress. Where strong correlations were found between the (i) mean compressive strength and crosshead speed, (ii) individual compressive strengths and failure times, the data was used to determine the crack velocity exponent (n) and produce a S-P-T diagram. Although only one of the materials (P-50) evaluated fulfilled all the necessary criteria and yielded a value of n = 16.13 (7.22), it is suggested that this method may enable comparisons to be made amongst other materials satisfying the required conditions. As such diagrams are based upon a crack growth law they may be of value in assessing the likely clinical wear resistance of new formulations. Consideration, however, should always be given to what levels are deemed acceptable for the intended clinical application of the material. Thus, before this technique can be employed fully, to the evaluation of new restorative materials, further work is necessary to determine appropriate design criteria.

Close range photogrammetry--a clinical dental research tool

Photogrammetry is the art, science and technology of obtaining reliable information about physical objects through processes of recording and interpreting photographic images. This review outlines the principles of the technique and summarizes the various methodologies and applications in clinical dental research.

Assessment of a procedure for detecting minute levels of tooth erosion

Over a period of some years, the components of a system for discerning erosion on childrens teeth have been progressively developed, for use in an extensive project seeking correlations between erosion and various perceived risk factors. The aim was the detection of minute levels of erosion, based on mappings of the palatal surfaces of the maxillary central incisors in children. Significant challenges were encountered, the primary problem being the impracticality of placing control marks that would aid the realignment of successive measurements. The paper describes the erosion detection system and initial experiences based on the results of the first 100 subjects measured after 9 months. The procedures detected the occurrence of erosion of 50 μm magnitude on about one-quarter of the teeth over the 9 month period, at a precision estimated to be ±15 μm. The occurrence of some anomalous incidents prevented the procedure from being fully automatic, and it necessitated human examination of a graphical diagram derived from the surface matching program, but it was nevertheless superior to current practices of examining impressions or replicas entirely by eye.

Fibre reinforcement of two temporary composite bridge materials--effect upon flexural properties.

OBJECTIVES Temporary bridges must survive for their anticipated time in clinical service. Fibre reinforcement offers a potential means of increasing the chances of achieving this. This study sought to investigate this. METHODS This in vitro study determined the three point flexural strength and flexural fatigue characteristics of beam specimens (25 mm long × 3 mm wide × 2 mm deep) of a chairside temporary crown and bridge material (Protemp 4, 3M ESPE) and a laboratory resin composite (Ceramage, Shofu Inc.) in unreinforced and reinforced states. Reinforcement was provided by Everstick Crown and Bridge (Stick Tech Ltd.) and Ribbond THM (Ribbond Inc.). All testing was performed using an Instron Universal testing machine following one week of storage in distilled water at 37 °C. RESULTS Fibre reinforcement with Everstick C&B significantly (P < 0.001) increased the mean flexural strengths of both materials. The flexural fatigue limits of both Protemp 4 and Ceramage were significantly (P < 0.001) increased by fibre incorporation. CONCLUSIONS The observed fibre resin coupling (where Everstick C&B was used) and knitting together of fractured specimen fragments to restore specimen function (specimens where Ribbond THM was used) demonstrated the potential to enhance the durability of temporary bridges made of the materials studied here. CLINICAL SIGNIFICANCE The incorporation of fibre reinforcement in temporary bridge materials has the potential to enhance the longevity of temporary bridges and thus reduce the need for emergency treatment.

Linear interpolation error in measured surfaces in a dental erosion study.

An extensive palatal erosion study, which is currently being undertaken at the University of Dundee, aims to detect erosion through the precise mapping of replicas of childrens incisors. All perceived sources of error are being scrutinised closely. The error due to the interpolation of surface positions from discrete point measurements has been investigated so that its contribution to the erosion estimates can be gauged. The calculation of the error statistics made use of covariance functions to express the undulation of the tooth surfaces. It has been found that, for the measurement point spacing of 150 μm being used in the current project, the interpolation error for an incisor is typically about 25 μm, to be compared with the root-mean-square error due to tooth impression and replication procedures, assessed as being in the order of ±3 μm, and replica measurement, assessed as being in the order of ±5 μm. The interpolation error constitutes a surprisingly large proportion of the overall system error. The same error estimation technique can be applied usefully and without difficulty to other research studies concerning erosion or wear in natural or restorative dental surfaces.