The purpose of this review was to assess the influencing factors which affect laboratory tests that evaluate the effectiveness of luting agents on the retention of crowns in prepared dentin and – based on the results of the review – to propose a reasonable experimental setup.
Materials and methods
The database MEDLINE was systematically searched for laboratory methods that evaluated the effectiveness of luting agents by pulling off crowns from prepared extracted teeth.
Eighteen studies were included into the systematic review. The studies varied largely with regard to tooth type (molars, premolars), number of specimens (9–25), stump height (3–6 mm), convergence angle (4.8–33°), standardization and measurement of preparation surface, seating force (25–200 N), artificial ageing, crosshead speed for tensile force and statistical analysis. The coefficient of variation of the test results varied from 3% to 100%. The most important influencing factors for the crown dislodgement were stump height and convergence angle as well as the luting agent. Panavia and RelyX Unicem generally produced the highest values followed by glass ionomer and zinc phosphate cements. When pooling and normalizing the data, the mean difference between glass ionomer and resin-based materials as well as between glass ionomer and zinc phosphate cements was statistically significant (Wilcoxon, p < 0.05). Seating force, roughness, type of cutting bur and use of a desensitizing agent had all a negligible effect on the test results. Artificial ageing like thermocycling had no influence with glass ionomer cements whereas for resin-based cements thermocycling and prolonged water storage generated similar a failures stress than thermocycling alone. The comparison with clinical results did not reveal conclusive evidence that the results of the laboratory methods completely reflect the results of prospective clinical trials in conjunction with single crowns and fixed dental prostheses. A reasonable experimental setup includes: at least 20 specimens per group, stump height 3 mm, convergence angle 20°, thermocycling of specimens (5000×), avoidance of shearing forces during dislodgement and failure probability statistics (Weibull).
The results of pull-off tests with crowns that are cemented with luting agents varied largely and reflect only partially the results from clinical trials. The most influencing factors (standardized tooth preparation, avoidance of shearing stress during dislodgement of crown) had to be controlled to get meaningful results.
Dental materials for the luting of indirect restorations mainly have to fulfil three requirements: (1) to fill the space between the indirect material and the prepared tooth; (2) to keep the restoration in place (retention) and prevent dislodging; and (3) to provide adequate aesthetical conditions for the indirect restoration . It is important that the luting material fills the space between the restoration and the tooth structure to prevent tooth hypersensitivity, biofilm infiltration, marginal discoloration, and eventually secondary caries. The retention of the restoration depends on many variables. Macromechanical retention is mainly determined by the geometrical configuration of the tooth preparation. Different geometrical variables contribute to the macromechanical retention of a tooth prepared for a full-coverage crown: (1) convergence angle of tooth stump; (2) surface area; (3) apical-coronal height of stump; (4) retentive guiding grooves or notches .
The smaller the convergence angle, the larger the height and the larger the surface area of the prepared tooth, the higher the macromechanical retention will be. Retentive guiding grooves can considerably increase the degree of retention.
Luting agents for indirect restorations can primarily be classified into the following two categories : (1) so-called conventional water-based luting agents, such as zinc phosphate or glass ionomer cements, (2) polymerizing cements based on composite resin technology in conjunction with adhesive systems or with self-adhesive properties.
Recently, self-adhesive resin composite luting agents have gained popularity due to their ease of application and adhesive properties, which, however, are lower than those of materials that use an additional adhesive system . Another factor that interferes with the adhesive performance is the curing method: self-cured luting agents exhibit lower bond strength values than light-cured agents. In materials such as metal alloys and oxide ceramics, which do not permit light to be transmitted through the material, the luting agent can – except for the margins – only be cured with the self-curing mode.
Factors that provoke dislodgement and retention loss are shear forces that interact with the restoration such as (1) deep occlusal grooves and fissures, (2) hyper-balancing contacts during articulation, (3) high occlusal forces and/or parafunctional habits (gnashing, bruxism, etc.).
The luting agent should help to prevent the restorations from being dislodged. There is an inverse relationship between the adhesive properties of the luting agent and macromechanical retention . The lower the macromechanical retention, the higher the adhesive properties have to be to compensate for the low degree of retention. Other material factors of the luting agent can act as co-variables in the dislodgement of a restoration as outlined below:
flexural strength: the higher the flexural strength the lower the risk ;
shrinkage: the lower the shrinkage the lower the risk ;
water uptake: water uptake decreases the mechanical properties of some luting agents ;
expansion: minimal expansion due to water uptake can increase the retention as it enhances the interlocking between restoration and prepared tooth .
Another variable is the restorative material itself. Restorations made of materials with a low modulus of elasticity such as (fiber-reinforced) composites show an increased degree of debonding when used with conventional luting agents or with luting agents that demonstrate low adhesive properties .
The adhesive properties refer to both the bond to the prepared tooth as well as to the bond to the indirect material that covers the tooth. In the case of a full-coverage crown the substrate is mostly dentin and/or artificial materials that are used to build up the core. These materials can be resins, ceramic, gold alloys or amalgam whereas the crown material can be a resin, a ceramic or a metal alloy.