The aim of the study was to compare the strength of thin-walled molar crowns made of various materials under simulation of mastication.
Five 3D FE models of the first lower molar with the use of contact elements were created: intact tooth; tooth with a zirconia crown; tooth with a porcelain crown; tooth with a gold alloy crown and tooth with a composite crown. The computer simulations of mastication were conducted. For the models, equivalent stresseswere calculated using the modified von Mises failure criterion (mvM). Contact stresses at the adhesive interface between the cement and tooth structure under the crowns were analyzed.
Equivalent stresses in the crowns, did not exceed the tensile strength of their material. The mvM stresses in resin cement under the zirconia crown were 1.3 MPa, and under the composite crown they increased over 6 times. The tensile and shear contact stressesunder the stiff crowns (ceramics and gold alloy), were several times lower than those under the composite one. The maximum mvM stresses in the tooth structure for the zirconia crown were only 2.8 MPa, whereas for the composite crown were 6.4 MPa. The higher elastic modulus the crown was, the lower the equivalent stresses occurred in the composite luting cement and in the tooth structures. Also contact stresses decreased with the increasing stiffness of the crowns.
Under physiological loads, the thin-walled crowns perfectly luted to molars, made of zirconia ceramic, gold alloys and composite resin are resistant to failure. Prosthetic crowns made of stiff materials are less prone to debonding than those made of composite resin. Prosthetic crowns made of a material with a higher elastic modulus than enamel will strengthen the dental structures of molar teeth.
For many years molar prosthetic crowns were fabricated of metal alloys. This is especially true of high noble alloy crowns which were once considered the gold standard in prosthodontic reconstruction of posterior teeth . Long-term observation confirms good performance of gold alloy crowns in clinical practice . In recent years metal crowns became less popular purely because of esthetics and rising gold prices. More and more frequently, metal-free ceramic crowns are now used as an alternative to gold alloy restorations.
All-ceramic crowns often are made of leucite-reinforced ceramic . This types of glass ceramic ensures high esthetics and biocompatibility, but it is brittle (fracture toughness K 1C value of 1.3 MPa*m 0.5 ) and has low flexural strength (109.1–153.6 MPa) . Glass ceramic materials can perform successfully in the anterior region but the long-term outcomes of posterior crowns are not so encouraging . Zirconia-based ceramic is characterized by higher flexural strength (up to 1200 MPa) and fracture toughness (K 1C = 9–10 MPa*m 0.5 ) . This material is indicated for posterior crowns but due to its high opacity requires veneering with glass ceramics. High strength zirconia core can be manufactured through CAD/CAM technology and subsequently veneered conventionally. According to in vivo observation, the clinical survival of zirconia-based restorations are comparable to metal–ceramic restorations . Recently, a new idea of milling solid monolithic (full-contour) zirconia crowns has occurred .
Prosthetic crowns can also be fabricated with composite resin. Although some authors recommend composite crowns as a permanent restoration , the application of these restorations should be limited to interim purpose . This is due to its occlusal wear especially in molar region , gradual loss of marginal adaptation , discoloration and increased plaque accumulation .
Strength and durability of tooth restoration depends on many factors, such as crown material, its thickness, remaining tooth structure, crown bonding and quality of laboratory fabrication . Standard preparation of a tooth for a monolithic crown requires equal hard tissue reduction of all axial walls and 1.5–2 mm of occlusal clearance. Chamfer preparation of 0.8 mm is indicated for both all-ceramic and composite crowns . Unfortunately, such preparation leads to 67.5% of tooth structure removal .
The authors tried to find the answers for the following questions. Can the amount of removed tooth structures be reduced? Which materials can be used for thin-walled posterior crowns to preserve a crown’s strength and reduce marginal leakage during occlusal loading? Which crown’s material provides the posterior tooth structure with the highest durability?
The objective of this investigation was to compare the strength and adhesion of thin-walled molar crowns made of various materials under masticatory simulation. Our null hypothesis was that molars with crowns made of stiffer materials (zirconia ceramic and gold alloy) are more resistant to failure and debonding than restorations made of low elastic moduli materials (composite resin). Crowns made of materials with higher elastic modulus also better strengthen tooth structures under loads.
Materials and methods
Double-layer impressions of upper and lower arch of a patient with normal occlusion using polyvinylsiloxane material (Express, 3M/ESPE, St. Paul, MN, USA) were taken. Occlusal registration in central and lateral positions of the mandible with wax (Aluwax Dental Products Co, Allendale, MI, USA) were recorded. Two lower and one upper working casts with separate dies (Girostone, Amann Girrbach GmbH, Pforzheim, Germany) were prepared. Using a laser scanner (Dental 3D Scanner D700, 3ShapeA/S, Copenhagen, Denmark) the occlusal surfaces of three die stone teeth were scanned: first lower molar and two opposing teeth (first upper molar and second premolar). The obtained scans were then processed with software (3Shape Dental Designer CAD; 3Shape A/S). Additional examinations of the first lower molar were made with computerized tomography (GXCB-500/i-CAT; Gendex Dental Systems, Des Plaines, IL, USA). Digital files with coordinates of the occlusal surface points of the examined teeth and the points on the enamel–dentin–pulp junction of the lower molar (obtained from the CBCT), in horizontal layers (every 1.0 mm), were introduced into the finite element analysis FEA software (ANSYS v. 10; ANSYS Inc., Canonsburg, PA, USA). These points were connected with splines, and the cross-sections of the molar were created. Connecting these cross-sections allowed for the creation of a solid model of the first lower molar ( Fig. 1 a) . The cervico-occlusal length of the crown was 7.5 mm, the bucco-lingual diameter was 10.5 mm, and the length of the roots was 14 mm . A 0.2 mm periodontal ligament was modeled around the roots. The lower molar tooth was anatomically inclined 15 degrees lingually and 8 degrees anteriorly .