J Funct Biomater. 2023 Jul 5;14(7):354. doi: 10.3390/jfb14070354.
The restoration of class II cavities is predominantly carried out with composite materials. Due to the high failure rate in restoring this type of cavity, composite materials with much-improved properties and new application techniques have been promoted.
The study aimed to analyze the mechanical behavior of several topical composite materials (nanocomposites, nanohybrids and ormocer) using different application techniques. In a lower second molar, a class II occlusal cavity was prepared. As filling materials, we used the following combinations: Admira Fusion and Admira Fusion Flow, Grandio and Grandio Flow, Filtek Supreme XT and Filtek Supreme Flow. These were applied using a snow plow, injection molded and Bichacho techniques. Three-dimensional scanning of the molar with the prepared cavity was performed, and then scanning of each layer of added composite material was performed, obtaining three-dimensional models.
The virtual molar models were analyzed with software specific to the finite element analysis method, where their physical-mechanical properties were entered and assigned to the components of the virtual molar. Simulations at high forces specific to bruxism were then carried out and analyzed, and compared. The values of displacements and strain, for all six analyzed situations, are relatively small (range from 5.25 × 10-6-3.21 × 10-5 for displacement, 6.22 × 10-3-4.34 × 10-3 for strain), which validates all three methods and the materials used. As far as the stress values are concerned, they are similar for all methods (250-300 MPa), except for the snow plow and injection-molded techniques using Grandio and Grandio Flow composites, where the maximum von Mises stress value was more than double (approximately 700 MPa). When using the combination of Grandio and Grandio Flow materials, the 1 mm thickness of the fluid composite layer was found to have a major influence on occlusal forces damping as opposed to 0.5 mm. Therefore, the Bichacho technique is indicated at the expense of the snow plow and injection-molded techniques.
The composite materials used by us in this study are state-of-the-art, with clear indications for restoring cavities resulting from the treatment of carious lesions. However, their association and application technique in the case of Class II cavities is of clinical importance for resistance to masticatory forces.
Practice Question
Results
#1. The most common cause of fracture at the isthmus of a Class II restoration is
AI Response to a Question:
The most common cause of fracture at the isthmus of a Class II restoration is b) a sharp axio-pulpal line angle.
Key Reasoning:
- Sharp axio-pulpal line angles create stress concentration points in the restoration. Occlusal forces are poorly distributed, leading to cracks or fractures at the structurally vulnerable isthmus.
- Preparation design is critical: textbooks emphasize rounding internal line angles to reduce stress and improve restoration longevity.
- Other factors, such as narrow isthmus width (c) or moisture contamination (d), are less directly linked to fractures at this specific site.
Answer: b) a sharp axio-pulpal line angle