No abstract

A search has been conducted over a period of years for a test that could be used to predict the clinical behavior of dental porcelains under impact forces. At least eight different tests used in other fields have been tried, such as the Charpy and Izod (swinging pendulum-flexure), and Olsen (dropping weight-flexure and compression), but all failed to yield results that were definitive in terms of dental serviceability. In early 1954, Mr. P. W. Lee, in further attempts at correlation, discovered that special geometric porcelain forms used in conjunction with a dropping-weight test showed distinctions between porcelains that could be related to their clinical behavior.Concurrently with this development of a test technique, a vacuum-fired porcelain tooth had been developed, and clinical reports indicated it to be less brittle than either the translucent air-fired porcelain then on the market or the more opaque porcelains of former years that were also air-fired. Work was directed therefore toward refining and calibrating Mr. Lee's test to correlate with these findings. Work done by Vines, Semmelman, Lee, and Fonvielle1 on porcelain densification predicted a stronger porcelain under impact forces, and this was another reason for development of such a test. EXPERIMENTAL METHODSIn the technique proposed by Lee, impact forces are applied to porcelain specimens by means of a freely falling steel weight or "tup" of known hardness (Fig. 1), dropping within the confines of vertical guides marked in increments of 1 cm. to measure the height of fall (Fig. 2). The axis of the vertical guide chute is aligned by means of leveling foot screws on the base. The guide supports and specimen holder are constructed (Fig. 3) so as to allow the tup to deliver the impacting blow squarely upon the securely clamped specimen. Friction is minimized by chrome plating the inner area of the guide supports. The cylindrical tup has a hook on one end to facilitate lifting inside the chute (Fig. 4), and the impacting end is machined to a 16 micro-inch finish.Several geometric specimen shapes were considered, but the two finally selected for study corresponded approximately to an incisal contour and a cuspal contour (Fig. 5). These conical forms were chosen as an approximation of incisal and cuspal configurations and because they could be duplicated repeatedly with mechanical accuracy. Since the brittleness of the specimen could be affected by temperature, all specimens were preconditioned and tested at 750 ± 50 F. Preliminary investigations showed dental porcelain to be subject to minimum fatigue effects; so impacting was begun at heights well below the fracture point. One blow was