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Introduction

Fracture strength tests of anatomically shaped restorations are commonly used in dental literature to assess their load bearing capacity. While the aim of these tests is to mimic as close as possible the loading mechanism observed in the oral cavity, they remain far from that goal. Several key points were previously addressed in order to reproduce more realistic results as the shape of the loading indenter, presence of a stress distribution cushion, loading angle and speed, representing the periodontal ligament, and presence of water . Nevertheless, neither the failure load nor the facture patterns observed in fracture strength tests mimicked those observed in clinical failure .

Two important factors are responsible for the previously observed differences. First, it has to be noted that under clinical conditions, the restoration is subjected to average chewing forces which have been accurately estimated in different regions of the mouth . These forces change continuously in magnitude and direction following the dynamic movement of the mandible . Most importantly, these forces are distributed over the occlusal table of the restoration and point contact with the antagonist cups is prevented by the presence of the food substance. In case of clenching or intentional heavy biting, the loaded restoration is protected from excessive loads by the vertical contacts obtained from neighboring teeth during maximum inter-cuspation . Secondly, maximum peak forces are observed only for a fraction of the second in every chewing cycle and immediately followed by quick relief. Even in presence of an unexpected hard object, jaw closure preventing mechanism is immediately activated to prevent point contact, otherwise traumatic fracture is observed .

Under laboratory conditions, a continuously increasing load, controlled by the crosshead speed of the loading device, is delivered by the loading indenter over a selected location of the restoration until fracture is observed. The loaded restoration is actually squeezed between two solid fixtures, the indenter from one hand and the attachment unit on the other hand. With a continuously increasing load over a relatively longer loading time, the stress breaking function of the intermediate cushion, if placed between the indenter and the restoration, or the artificially made periodontal ligament will be lost as both would have been compressed beyond their elastic limit ending in point contact with the indenter. As axial loading continues, marked surface damage under loading indenter is observed ending in sudden explosive failure of the restoration at unrealistically very high loads . Knowing that clinically failed dental restorations are usually broken in two or three intact pieces without marked occlusal damage and at much lower loads , fracture strength values observed in the laboratory could only be used for screening purposes and not to reflect insight on the expected clinical performance .

Clinical failure, with exception of traumatic and impact fractures, is a function of several parameters that interact all together leading to a slow process of damage accumulation in the fractured restoration. Cyclic loading at an estimated average load and under the influence of a chemically and thermally fluctuating environment starts to weaken the restoration by stressing its internal structure especially at interface regions . While the influence of a single individual load cycle is definitely negligible, repeated cycles for hundreds of thousands and even millions have a marked deteriorating effect. Survival statistics can predict expected failure for a percentage of population, restorations in this case, if given loading parameters, number of cycles, and some material properties .

Luckily enough, the brittle nature of ceramics allow precise recording of all fracture events on the fractured surface and when the process is relatively slow; the fractured surface becomes a time map of the entire process. Fractographic analysis of fractured specimen can predict with high accuracy the size and location of critical crack, presence of water, relative number of loading cycles, entire pathway of the crack from moment of origin to later deflections, presence of regional stresses, and most importantly the load at failure if the fracture toughness of the material was previously determined .

In the present study, general guidelines were addressed in order to present a useful model for a laboratory fracture strength test of anatomically shaped restorations. First, long term cyclic loading at sub-critical loads was applied for relatively a high number of cycles. Following a review of literature an average chewing load of 25 kg was selected in this study representing almost 70% of the maximum biting force in the anterior region of the mouth . A suggested service time of seven years was selected as a plateau level indicating success of the restoration if no fracture was observed. It was estimated that a normal adult would perform an average of 500,000 loading cycles/year. Thus a 7-year service time would require 3.5 million cycles .

Secondly, stress concentration was prevented at any site of the restoration using a multi-level strain accommodating protocol which allowed for distribution of the loading strain (compression deformation associated with load application) over different components:

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  0.6 mm artificially created periodontal ligament using heavy consistency polyether impression material (Impregum, 3M ESPE, St. Paul, MN, USA) injected in the supporting socket before seating the root portion of the resin dies. This layer permitted macro-movement of the loaded specimens giving room for limited change in angle direction between the restoration and the loading indenter .

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  0.15 mm thick cement film thickness which prevented sharp contact points between the inner surface of the zirconia framework and the supporting tooth. These misfit contacts could lead to generation of internal Hoope stresses during loading resulting in barrel type fracture. Moreover, a continuous and even cement film thickness would provide better stress distribution under the cemented restorations especially at sharp angles as cusp tips or occluso-axial line angles.

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  2 cm heavy spring coil inserted between the loading indenter and the load cell. This spring acted as a piston and allowed the absorption of impact forces delivered by the loading indenter during every load cycle thus protecting the restoration from sudden rise in loading forces and subsequent surface contact damage. Additionally, the spring maintained continuous contact with the loaded restoration which prevented excessive abrasion, considering the high numbers of selected loading cycles.

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  0.5 mm high tensile strength (17.3 MPa) neoprene rubber sheet (Rubber sheet roll, Shippenburg, PA, USA) was inserted between the loading indenter and the surface of the restoration to prevent sharp contacts and to represent the consistency of the food substance.

A third key point to consider is the anatomy of the loading cycle. Dynamic tracings of the mandibular movements were previously analyzed and time–space–speed plots have been accurately presented . Unfortunately, representing these complicated movements require advanced chewing simulators which are not available for the majority of dental studies. Nevertheless, a simple tear drop shaped loading cycle that accounts for speed of load application and removal (0.9 cycle/s) could be incorporated into any loading device. A slow loading frequency (40–45 Hz) was chosen for this study to insure full compression of the heavy spring and full application of the selected load. The test started with a short thermo-cycling program (1000 cycles between 6 and 55 °C) followed by 10,000 load cycles at 5 kg to allow for proper seating of the loaded restoration before commencing with the determined load (accommodation phase).

Finally, simulation of chemically active environment is crucial for biomechanical degradation of material properties. Water acts as a lubricant, cooling medium, hydrolytic agent, and its rule in the process of slow crack growth cannot be over looked. Presence of water at sharp crack tip facilitates advancement of crack front compared to failure observed under inert environment .

The aim of the presented work was to assess accumulating damage and nature of fracture process during cyclic loading of bilayered zirconia restorations in an attempt to mimic clinical failure and to understand failure process.

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Materials and methods

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Materials and methods