Risks of Ceramic and Cement Failure on Posterior Onlay Restoration

Objectives: To study the risks of ceramic and cement failure on posterior onlay restorations using an anatomically detailed 3D finite element (FE) analysis. Three types of ceramic materials were investigated. Methods: A mandibular molar was microCT scanned and segmented to obtain a 3D solid model. The model received a 2 mm occlusal reduction with cusp steepness of 45 degrees relative to the occlusal surface. The complete model consisted of 5 parts: enamel, dentin, supporting bone, ceramic onlays and luting agent. All parts were meshed using 4-noded tetrahedron elements. Mechanical properties of the materials were obtained from literatures. Nodes along the supporting bone surface were constrained in all degree of freedoms. A vertical 100 N ramp-and-hold load was applied on the central occlusal surface via a 6mm diameter rigid sphere. The model was solved in LS-DYNA explicit. Three simulations using different ceramic materials: Feldspathic porcelain (Feld), Leucite-reinforced glass ceramic (Emps) Lithium disilicate-reinforced glass ceramic (Emax) were analyzed. The ceramics were luted using a resin-based luting agent (RelyX ARC). Ten elements that showed the highest maximum principal stress (Smax) were located and the average value was calculated for each material. The risk of ceramic failure (RCF) and the risk of cement failure (RCEF) were calculated by dividing the maximum principal stress (Smax) by its tensile strength (TS). An inverse of RCF or RCEF was defined as relative safety factors (RSF) of the materials. Results: Simulation results revealed different maximum principal stress magnitude for different type of ceramics (Table). Stress in luting agent remained similar in both models. Lithium disilicate-reinforced glass ceramic had higher stress concentration but lower risk of failure compare to other ceramics. Conclusions: Selection of ceramic materials significantly influences the magnitude of stress and risks of ceramic fracture on posterior onlay restorations.

	Ceramic				Cement
	Smax(MPa)	TS(MPa)	RCF	RSF	Smax(MPa)	TS(MPa)	RCEF	RSF
Feld	46.11	100	0.46	2.17	9.76	135	0.07	13.83
Emps	56.25	160	0.35	2.84	10.65	135	0.08	12.68
Emax	66.14	400	0.17	6.05	10.06	135	0.07	13.42