The purpose of this study was to validate a new laboratory method to test all-ceramic systems with regard to the proportion of failures.
Sixteen standardized mandibular molar crowns consisting of two different materials (IPS Empress, IPS e.max Press) were adhesively luted on CAD/CAM milled PMMA abutments (first lower molar, circular chamfer). All crowns were loaded applying an eccentric force in a Willytec chewing simulator (steel stylus, Ø 2.4 mm, 2 mm lateral movement from fossa to cuspal tip) with stepwise increase of the load (3, 5, 9 kg, 100,000 cycles each, 0.8 Hz) and simultaneous thermocycling (5 °C/55 °C × 417 per phase). Another four crowns of each material were subjected to force measurements with a 3D force sensor during dynamic loading of each loading phase using two different lateral movements (from fossa to cusp and vice versa).
The cumulative forces for the three directions in space were much higher compared to the static load of the chewing simulator (maximal force at 3 kg 60 N, 5 kg 160 N, 9 kg 240 N). There was no statistically significant difference in the mean or maximal force between the two materials or two different lateral movements. During dynamic loading, no fractures occurred in the molar crowns made of IPS e.max Press, whereas 50% of the IPS Empress crowns showed failures (75% fractures and 25% chippings) (log-rank test p = 0.002). Most of the Empress crowns fractured during the third loading phase (9 kg).
The forces that the dead weights exerted during dynamic loading were 2–3 times higher than those during static loading. None of the lithium disilicate ceramic molar crowns fractured, whereas half of the leucite reinforced molar crowns failed during dynamic loading.
In prosthetic dentistry, there is a growing tendency toward replacing metal-based restorations with all-ceramic restorations. Single crowns consisting of various ceramic materials (lithium disilicate, leucite, aluminium oxide) have been successfully used for 10–20 years with good clinical survival rates and they have become a standard dental treatment option .
The ultimate test for prosthetic materials is the clinical trial which is, however, costly and requires a large deal of organizational work. Meaningful conclusions can only be drawn if an adequate number of crowns or fixed dental prostheses (FDPs) are incorporated and observed for at least 3–4 years. Therefore, it would be beneficial to take advantage of validated laboratory tests that allow an accurate prediction of the clinical performance of prosthetic dental materials.
Standard laboratory tests involve flexural strength or fracture toughness tests on standardized bars or the loading of crowns and FDPs until failure in a universal testing machine. These tests are useful to discriminate between material variants, compare the values with clinically proven materials and estimate the failure risk through Weibull statistics and similar approaches. As no artificial aging process or dynamic loading of the specimens is involved in these tests, the prediction of long-term success is limited or misleading. Examples of inadequate clinical performance with high fracture rates include posterior crowns made of Dicor as well as posterior bridgework made of In-Ceram .
Already in the late nineties, Kelly demanded that all-ceramic materials tested in the laboratory should produce failures that are comparable to those in clinical situations . He identified several important factors that are essential to (carry out) meaningful laboratory tests: (1) contact area of stylus with specimen, (2) clinically relevant crowns cemented on a defined substrate, (3) cyclic loading, and (4) wet conditions. Therefore, laboratory tests should include the testing of standardized crowns that are luted on an adequate substrate and subjected to cyclic loading in a wet environment.
An all-ceramic material with one of the longest clinical track records is the pressable leucite-reinforced ceramic IPS Empress, which has been on the market for more than 15 years. In 1998 the pressable lithium disilicate all-ceramic material IPS Empress 2, which demonstrated a higher mechanical strength than its predecessor and was suitable for three-unit fixed dental prostheses (FDPs) in the anterior region, was introduced in the market. Due to its opacity, this material needed to be veneered. In 2007 an all-ceramic material based on the same strengthening mechanism (lithium disilicate) but with a higher degree of translucency was launched on the market (IPS e.max Press). With this material fully anatomical restorations without veneering can be produced.
The purpose of this study was to validate a new laboratory method to test standardized fully anatomical mandibular molar crowns when submitting them to eccentric loading with a stepwise increase of the load in a chewing simulator. For the eccentric loading a 2 mm lateral movement has been chosen to accelerate the aging and stressing process. Two different all-ceramic materials were used: the leucite-reinforced material IPS Empress and the lithium-disilicate material IPS e.max Press. For the loading, a dual-axis chewing simulator was employed. During eccentric loading, the force was determined with a 3D force sensor. Force measurements on flat composite specimens that were submitted to cyclic vertical loading of a ceramic cusp with lateral movement in the same chewing simulator have shown that the force impulse that is created when the cusp hits the specimen is about three times higher compared to the dead weight .
The following hypotheses were formulated:
The measured forces are much higher compared to the static loads of the weights applied on the bars of the chewing simulator.
The proportion of failures is different between the two ceramic materials.