R.L. Sakaguchi

The wear of a posterior composite in an artificial mouth: a clinical correlation

Abstract The wear of a posterior composite against a maxillary palatal cusp was studied in an artificial mouth. The coefficient of wear for the composite was 2.58×10 −5 . A retrospective clinical correlation with composite wear in the artificial mouth showed a correlation coefficient of 0.84 at 1 year of wear. The artificial mouth studies support a parabolic relationship between depth of composite wear and time. The ratio of 6 months depth of wear compared to 3 years was found to be 41% which supports the Leinfelder finding (5) of 49%. However, the correlation with the linear studies of Braem (2,3) was good as far as the mean depth of wear at 1 year was concerned. The disagreement between the linear and parabolic studies is small during the early wear process, but becomes serious during a longer term. It is important for future clinical wear studies to resolve the question of the nature of the wear rate curve in posterior composites, if accurate prediction of long term performance is to be achieved.

The wear of dental amalgam in an artificial mouth: a clinical correlation

Abstract The wear of dental amalgam by a smear mechanism and amalgam transfer onto the opposing cusp was confirmed by simulated studies in an artificial mouth. The coefficient of wear for dental amalgam was 4.89×10−5. Calculation of the coefficient of friction for the enamel/amalgam couple indicated that friction was overcome by shallow slippage and shear within the amalgam. On the basis of these facts, the good wear performance of both the amalgam and the opposing enamel was rationalized. A retrospective clinical correlation of amalgam wear in an artificial mouth showed a correlation coefficient of 0.938 as far as the mean wear values were concerned. Further the wear rates in both studies were linear except in a narrow region at time zero. The variances, however, in the 2 studies were very different. The desirability of in vitro studies to reproduce the same variance, as well as the same mean values, is a point for further discussion and experimentation.

The wear of dental porcelain in an artificial mouth

Abstract Simulated occlusal wear studies in an artificial mouth involving enamel occluding on porcelain demonstrated a high coefficient of wear for dental porcelain; in agreement with other workers, an abrasive wear process is postulated. Volume loss due to wear showed good linearity as a function of the number of masticatory cycles with slight flattening at higher masticatory levels. However, the depth of wear curve showed a pronounced deviation from linearity with flattening of the wear rate with time. A parabolic relation exists between volume and depth of wear and correspondingly between time and depth. Based on the coefficient of wear, the intrinsic wear of porcelain appears to be about one order of magnitude greater than that experienced by dental amalgam.

A simple model of crack propagation in dental restorations

Although natural teeth often exhibit microcracks, they rarely demonstrate bulk fracture. However, conventional full-crown restorations periodically exhibit failure due to fracture. Presented here is evaluation of a simple model of crack propagation that estimates crack growth during cyclic loading. A finite element model of a premolar tooth provides the tensile stresses adjacent to cusp loading. If the crack propagation rates for natural teeth, porcelain-fused-to-metal crowns and composite crowns are compared with the wear rates of their respective materials as determined in an artificial mouth, it is evident that the low wear rate of composites may predispose them to fracture. Natural teeth disperse occlusal stresses throughout the dentin so that the effect of high occlusal stress is minimized. Porcelain tends to wear the opposing dentition, which reduces areas of high occlusal stress. Composite, however, demonstrates crack propagation rates higher than those of either natural teeth or porcelain. This, in addition to its low wear rate, might predispose the material to fracture. This model should be used only as a qualitative indicator of fracture tendency. The high calculated crack propagation rates in composites may explain the observed clinical failures and microchipping at the area of occlusal contact, as noted in SEM analysis.

Deformation of crowns during cementation

Deformation of crowns during cementation was investigated by a simple loading system of defined crowns with silicone fluids as cements. Deformation of the crowns was measured by long stain gauges that encircled the cervical margins. Die spacing was simulated by etching the die. Venting was simulated by removing a small screw. Deformation of the crowns was decreased by decreasing the viscosity of fluid, increasing the thickness of the crowns, and venting. Etching the die decreased the seating times of the crowns, but did not alter the level of deformation. Terminal cementation with zinc phosphate cement confirmed the presence of crown deformation during cementation. The results have consequences for bonded and all-ceramic crowns, and explain several clinical phenomena. It is suggested that low viscosity cements, low seating forces, and die spacing be used to decrease the deformation of crowns during seating. The importance of passive fitting of the crown to the tooth is stressed.

Frictional effects between natural teeth in an artificial mouth

Abstract A method for measuring frictional forces on enamel of natural teeth and restorative materials was developed using the Artificial Oral Environment. Enamel/enamel systems were tested using different oral fluids in a minienvironmental chamber capable of introducing biologic fluids between the occluding surfaces. Matched, extracted opposing human premolars mounted in physiologic occlusion under an occlusal load of 3 pounds were bruxed at approximate masticatory velocities. A load cell measured the resulting horizontal forces and an X-Y record was attained. 31 independent measurements of friction in both buccal and lingual directions were performed with the teeth dry, and with human saliva, Xerolube, and distilled water intervening. It was found that typical values for enamel/enamel coefficients of friction, μ, were in the range of 0.1–0.42. μ was independent of different fluids within any one enamel/enamel couple (coefficient of variation was typically 10%). However, the coefficient of friction of the enamel pair was highly dependent on surface texture. Roughening virgin enamel led to a 3 fold increase in μ. Conversely surfactants present in mineral oil reduced the friction of roughened enamel by 3 fold. Where the wear process is by abrasion it is likely that the reduction of the tangential forces due to friction could lead to reduced loss of contour. It is likely that finishing procedures in dentistry are important in this process. Finally, the production of low friction restorative materials are clearly indicated as a future development.

Hard tissue lubrication by salivary fluids

Abstract The frictional coefficients for human parotid (HPS) and human submandibular/sublingual (HSMSL) salivas and water were measured for five subjects. Twenty different biomechanical conditions were used which approximated the range of force and movement of the human mandible. The experimental set-up comprised a bruxing motion under servohydraulic control; a microchamber with a small recirculating volume (3 ml minimum) under physiologic temperature control; and a maxillary element capable of measuring frictional forces at the enamel interface. The relative movement of enamel-on-enamel demonstrated the slip-stick phenomenon which was consistent with the negative velocity gradient of the coefficient of friction under all conditions. Studies on the variation of the frictional forces as a function of vertical load confirmed that the mechanism of lubrication was thin film, under the experimental conditions. HSMSL saliva had the greatest potential to vary the lubrication mechanism. At low speeds and higher force, it is suggested that boundary lubrication may offer the greatest potential in the development of enhanced lubrication of the hard tissues in the oral cavity.

Wear resistance of isosit and polymethyl methacrylate occlusal splint material

Abstract Simulated occlusal wear utilizing the artifical mouth included natural enamel against isosit and polymethyl methacrylate splint materials. The results reported in volume loss (mm 3 ) demonstrated the isosit material to be 55–67% more resistant to wear than polymethyl methacrylate. No wear occurred on the enamel surface opposing either splint material.