Simulation of cumulative damage associated with long term cyclic loading using a multi-level strain accommodating loading protocol

Abstract

Objective

To assess step by step the associated cumulative damage introduced in zirconia veneered restorations after long term cyclic loading using a new multi-level strain accommodating loading protocol.

Methods

40 zirconia veneered crowns received thermal and cyclic loading (3.5 million cycles at maximum load of 25 kg representing 70% of the critical load of the veneer ceramic). The used loading protocol allowed for reproduction of the combined damping action of the periodontal ligament, food substance, jaw deformation, and free movement of the mandibular joint. Speed of load application and release was obtained from the chewing cycle of adult patients. Principles of fractographic analysis were used to study the behavior and origin of critical crack and associated structural damage.

Results

The multi-level strain damping effect prevented generation of cone cracks and contact damage under the loading indenter commonly associated with fracture strength tests. 29 specimens (73%) survived 3.5 million cycles without fracture, 9 specimens (22%) demonstrated cohesive fracture of the veneer ceramic and limited axial fracture of the framework was observed in two specimens (5%). Of all fractured specimens, 2 restorations (5%) failed after 500,000 cycles while the rest survived at least 3 million cycles before fracture was observed. Fractographic analysis revealed initial wear and abrasion below the loading area, subsurface micro-cracking of the glass matrix followed by slow crack growth that traveled in a stepping pattern till deflection at zirconia veneer interface.

Significance

Cyclic loading using multi-level strain accommodating model can reproduce clinical failure. With exception to manufacturing errors, zirconia veneered restoration survived a simulated 7-year service time without fracture.

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:

  • 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 .

  • 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.

  • 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.

  • 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.

Materials and methods

Fabrication of bilayered zirconia restorations

A maxillary central incisor received a full crown preparation according to the following criteria: 1.5 mm incisal reduction, 1.2 mm axial reduction, and 0.9 mm round chamfer finish line. Resin dies (duplicates) were obtained using polyether impression material (Impregum Penta, 3M ESPE, Seefeld, Germany) and posterior composite resin restorative material (Z250, 3M ESPE). The resin dies were scanned and standardized zirconia frameworks were prepared using a CAD/CAM system (Cercon, Degudent, Hanau Wolfgang, Germany). The sintered copings were veneered using press-on ceramic material (Ceram Press, Dugodent) according to the anatomy of the unprepared central incisor. All restorations were cemented on the dies using a resin cement (Panavia F 2.0, Kuraray, Osaka, Japan) which was light polymerized using high intensity light emitting diode unit (Blue Phase C9, Ivoclare Vivadent, Shaan, Liechtenstein).

Cyclic loading and biomechanical degradation

The cemented restorations received cyclic loading (3.5 million cycles) at sub-critical load (25 kg) starting with the accommodating loading phase (5 kg applied for 10,000 cycles). The load was applied in sinusoidal pattern representing a slow chewing rate at 40 cycles/min . A stainless steel indenter (3 mm spherical tip) that provided 0.8 mm 2 contact area with the loaded restoration was changed every 50,000 cycles. Generation of cone cracks was prevented using the previously discussed multi-level strain accommodating protocol. The suggested loading protocol was performed using a computer controlled custom made pneumatic testing machine which was composed of three loading stations each equipped with a pneumatic driven piston and an electrical timer controlling speed of load application. Pressure gauges controlled maximum delivered force which was calibrated using a digital load cell for every loading station, Fig. 1 .

Fig. 1
Schematic diagram illustrating test setup used to load the specimens.

A thermally controlled water bath (37 °C) was incorporated to ensure continuous immersion of every loaded specimen in the following mediums which were changed every 100,000 cycles: water, table vinegar (10% acetic acid, pH 2.4), 0.1 mol buffered sodium hydroxide (pH 13), and 6% alcohol (pH 7.4) .

Assessment of accumulating structural damage

The fracture surface of each broken restoration was immediately ultrasonically cleaned, dried, gold sputter coated, and examined under scanning electron microscope (XL30, Phillips, Eindhoven, the Netherlands). Small fragments and small veneer chips, separated from the restorations demonstrated basically mirror surfaces and were thus not included in fractographic analysis. In case a restoration survived 3.5 million cycles without fracture, the test was stopped and the restoration was axially sectioned using a diamond coated saw and a precision cutting machine (MicraCut 120, Metkon, Germany). Cut sections were examined at different magnifications and angles to evaluate structural damage. Grinding damage produced during sectioning procedure was characterized by parallel lines in a single flat plane which were much different from the stepping pattern observed for slow crack growth landmarks.

Materials and methods

Fabrication of bilayered zirconia restorations

A maxillary central incisor received a full crown preparation according to the following criteria: 1.5 mm incisal reduction, 1.2 mm axial reduction, and 0.9 mm round chamfer finish line. Resin dies (duplicates) were obtained using polyether impression material (Impregum Penta, 3M ESPE, Seefeld, Germany) and posterior composite resin restorative material (Z250, 3M ESPE). The resin dies were scanned and standardized zirconia frameworks were prepared using a CAD/CAM system (Cercon, Degudent, Hanau Wolfgang, Germany). The sintered copings were veneered using press-on ceramic material (Ceram Press, Dugodent) according to the anatomy of the unprepared central incisor. All restorations were cemented on the dies using a resin cement (Panavia F 2.0, Kuraray, Osaka, Japan) which was light polymerized using high intensity light emitting diode unit (Blue Phase C9, Ivoclare Vivadent, Shaan, Liechtenstein).

Cyclic loading and biomechanical degradation

The cemented restorations received cyclic loading (3.5 million cycles) at sub-critical load (25 kg) starting with the accommodating loading phase (5 kg applied for 10,000 cycles). The load was applied in sinusoidal pattern representing a slow chewing rate at 40 cycles/min . A stainless steel indenter (3 mm spherical tip) that provided 0.8 mm 2 contact area with the loaded restoration was changed every 50,000 cycles. Generation of cone cracks was prevented using the previously discussed multi-level strain accommodating protocol. The suggested loading protocol was performed using a computer controlled custom made pneumatic testing machine which was composed of three loading stations each equipped with a pneumatic driven piston and an electrical timer controlling speed of load application. Pressure gauges controlled maximum delivered force which was calibrated using a digital load cell for every loading station, Fig. 1 .

Nov 28, 2017 | Posted by in Dental Materials | Comments Off on Simulation of cumulative damage associated with long term cyclic loading using a multi-level strain accommodating loading protocol
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