Angular asymmetries of the human face.

PURPOSE: The determination of an acceptable occlusal plane is essential for the development of esthetic prosthodontic restorations. However, since most faces are not symmetric, a method was developed for measuring facial angular asymmetry, i.e., the divergence from the vertical or horizontal of the line joining the midpoint of the intercanthal line and the philtrum of the lip, the interpupillary line, the intermeatal line, the lip commissure line, and the intercuspid line. MATERIALS AND METHODS: Standardized frontal images (mouth closed, smiling, and biting on a wooden spatula) of 100 subjects were taken using a digital camera. These images were downloaded into a computer, and the angles between the various facial lines and the horizontal were measured. The subjects were grouped by sex, age, and history of trauma and orthodontic treatment. RESULTS: No statistically significant differences were found between the mean values for each group. CONCLUSION: Asymmetry of the face can be measured using digital camera imaging and computer analysis. A range of facial asymmetries that can influence the choice of occlusal plane during prosthodontic treatment exists. Thus, the use of an occlusal plane parallel to the ala tragus and interpupillary lines, as often advocated by prosthodontists, may result in less than ideal esthetics in the final restoration.

Fracture toughness of water-aged resin composite restorative materials.

OBJECTIVES: The purpose of this study was to assess the effects of aging experimental dimethacrylate resin composites in water at 37 degrees C for periods up to 6 wk by measuring the variations in fracture toughness (K(c)), elastic modulus (E), fracture energy (G(c)), and water sorption. METHODS: Six experimental resins were formulated from dimethacrylate resins, and were filled to 86 wt% (ca. 70 vol%) with treated inorganic filler to form six experimental composites. The fracture toughness was determined using a double torsion technique, the elastic modulus was measured in flexure, and the fracture energy was calculated from the fracture toughness and elastic modulus. RESULTS: As a result of aging in water, K(c) and the G(c) increased, and the elastic modulus decreased, but all values approached a plateau near 6 wk. Water sorption also occurred during this period, mainly during the first 2 wk. SIGNIFICANCE: Variations in the mechanical properties are interpreted as being due to plasticization of the resin matrix by water, which appears to lower the yield stress and increase in the size of the plastic zone ahead of the crack, thereby causing the observed increase in G(c) and K(c). After approximately 6 wk, no further changes in properties occurred.