Fractography of a bis-GMA resin.

The fracture behavior of a bis-GMA resin was studied by means of the double-torsion test. The fracture parameter measured was the stress-intensity factor. Fracture occurred in either a stick-slip (unstable) or continuous (stable) manner, depending upon the test conditions. When stick-slip propagation occurred, the fracture surfaces showed characteristic crack-arrest lines. The fracture surfaces were examined by use of a reflected-light optical microscope. The stress-intensity factor for crack initiation was found to be related to the size of the crack-arrest line which, in turn, could be related to the Dugdale model for plastic zone size. The evidence supported the concept that the behavior of the crack during propagation was controlled by the amount of plastic deformation occurring at the crack tip.

An investigation into the fracture behavior of a particulate-filled bis-GMA resin.

Dental composites are being increasingly used to restore occlusal surfaces of posterior teeth. If these restorations are not to fracture in service, an understanding of their fracture behavior is essential. The fracture parameter used is the stress-intensity factor at crack instability (KIC). This has been calculated by means of the double-torsion test in a mechanical testing machine. The fracture behavior of the composite was studied by varying: the amount of filler (7%, 15%, 26%, and 41% by volume); the surface treatment of the filler (coated or uncoated); the environment (air and water); and the cross-head rate (0.05, 0.5, 5, and 50 mm/min). Fracture was found to occur in either a continuous (stable) or stick-slip (unstable) manner. The stick-slip behavior was due to blunting of the crack tip, which was controlled by the yield behavior. When there was no significant blunting, continuous crack growth occurred. An unique relationship was found to exist between the yield stress and the stress-intensity factor for all conditions of testing.

The mechanics of asymmetric retraction loops used in fixed appliance therapy.

In order to move a tooth bodily without tipping, both a force P and an anti-tilt couple N of a suitable constant magnitude are required. Although many different spring components are available to orthodontists for this purpose, a comparison of the merits of different designs has not been possible because analytical solutions to the deformation behaviour have only been generally available for the atypical case in which the loop is centrally positioned. The asymmetric case, with two redundancies, for open and closed U-loops both with or without helices has been solved using the complementary (strain) energy method on the assumption that the effect of a longitudinal tension or compression on the bending of a cantilever beam by lateral forces (the tie-bar effect) is small. Unless the arms are of equal length it is then found that unequal couples M and N act at the posterior and anterior ends of the appliance, and in addition equal but oppositely directed forces Q are induced at either end. Check measurements were carried out on enlarged model components using a special jig utilizing a mixed dead-load and strain-gauge system. Agreement with theory was generally within the known errors of measurement. The main characteristics of these retraction components in the clinical situation can consequently be predicted. The characteristics examined are the lateral and vertical stiffnesses (S and dQ/dy, respectively), the N/P and Q/P ratios, the self-righting ability (SRA) and dN/d beta, the change in anti-tilt couple per degree change in gable angle (the angle of the mesial arm with respect of a horizontal in the unactivated appliance).

Dental biomechanics and the dental curriculum.

Attention is drawn to the numerous biomechanical concepts which provide the scientific basis for the many precepts taught in the various branches of clinical dentistry. It is suggested that dental biomechanics should logically be a subject area in its own right and that a practical way to achieve this would be for it to be gradually incorporated into the allied and complementary subject of dental materials science.

Accuracy of dental surveyors.

Maximum lateral forces as measured by strain gauges mounted on the analysing rod of a Nesor surveyor for four operators during surveying procedures ranged from 81 to 110 cN (mean 92.4 cN). Maximum lateral forces used when trimming blockout and inlay wax ranged from 159 to 184 cN (mean 169 cN) and 137 to 196 cN (mean 172 cN), respectively. Mean tilting errors produced in the vertical surveying arm by lateral loads of 98 and 196 cN (i.e. weights of 100 and 200 g) were determined to be 0.29 and 0.42 degrees for five Nesor surveyors and 0.21 and 0.36 degrees for five Krupps surveyors, respectively. All surveyors had been in constant use for at least 12 years. Play in the bearings of the horizontal arm due to loads of the same magnitude applied obliquely produced an additional mean error of approximately 0.5 degrees in the Krupps surveyors. Finally, discrepancies of up to approximately +/- 0.04 mm were found in 10 undercut gauges in each of the nominal sizes 0.25, 0.50 and 0.75 mm. It is concluded that surveyors and their accessories should be checked and serviced regularly, and possible sources of error pointed out to potential users.

Value of guide planes in partial denture retention.

The retention of five nominally identical partial denture casting fitted to a master model was measured on an Instron testing machine in the direction of the guide planes and also at angles inclined to them. It was found that: the maximum retention was considerably reduced when the clasps were removed; the guide planes offered some frictional resistance to displacement, the magnitude being critically dependent on the fit; for frameworks with or without clasps a significant increase (P less than 0.01) in retention occurred when the pull was at 22 degrees but not at 12 degrees to the guide planes, and that there was no statistically significant difference between the retention in the forward and backward directions for either a 12 degrees or a 22 degrees tilt of the dislodging force.

A laboratory investigation of the role of guide planes in the retention of cast cobalt-chromium alloy partial denture frameworks.

OBJECTIVES: This in vitro study examined the resistance to displacement offered by guide planes and the displacement mechanics of a bilateral bounded saddle cast cobalt chrome alloy removable partial denture framework under conditions simulating an average initial fit. METHODS: A specially designed model mouth jig fitted with strain gauges to monitor the forces across the guiding plane/guiding surface interfaces was used. Guiding planes of enamel, silver tin amalgam and dental composite with seven combinations of guide planes and three angles of withdrawal were examined. Guiding plane size was also examined for dental enamel. RESULTS: A statistically significant difference in retention was found for all factors examined. Retention increased with increasing angle of withdrawal. Size effects were found but considered unreliable. CONCLUSIONS: It was concluded that with a tight fit guiding plane retention may be readily predicted for any given material, combination of mesial, distal and lingual guiding planes and angulation conditions by the application of mechanical principles providing the basic parameters are known. Attention is drawn to the reasons for further work to fully understand the clinical situation.