November 8-12, 2015
The much improved strength of dental restorative materials has brought more flexibility to the design of dental prosthesis, leading to the concept of minimally invasive dentistry. Fiber-reinforced composite (FRC) systems, for example, which provide excellent tooth-colored appearance, allow the amount of tooth removal to be minimized. Clinically, the ideal indication of FRC restoration is single tooth replacement. Depending on the position of the missing tooth, the periodontal status of the neighboring teeth, and the patient’s occlusal force or any existing parafunctional habits, the following two fixed partial denture (FPD) designs can be prescribed: a 3-unit dental bridge supported by two abutments, one at each end of the edentulous area, or a cantilevered dental bridge supported by only one abutment. However, according to a 2009 systemic review, the mean survival rate of FRC restorations was only 73.4% at 4.5 years. The two major failure modes were reported as debonding at the tooth-retainer interface and structural fracture; the latter could occur at the loading point, the pontic and the connectors linking the retainer and the pontic. Several studies have shown that the main contributing factor of clinical failure is suboptimal fiber position and orientation.
In the present project, we apply a bio-inspired stress-induced material transformation (SMT) technique to the two main FRC designs, i.e. the 3-unit bridge and 2-unit cantilever. Using this technique, the mechanical property of a structure under optimization can be modified according to the local stresses in an evolutionary manner. Structural optimization is performed using ABAQUS via a user-defined material subroutine. For the 3-unit FRC bridge, regions with high stresses are iteratively reinforced with stronger fiber materials and, to reduce the risk of delamination, reinforcing fibers are closely aligned with the plane of the maximum principal stress in all locations. For the more challenging cantilevered design with only a single retainer, a two-step approach is adopted. The first step involves optimizing the shape of the cavity preparation/retainer on the abutment to lower the interfacial tensile stress at the abutment-pontic connection to reduce the risk of debonding. Then, with the optimized retainer, the user subroutine is applied to the restoration to seek an optimal fiber layout.
Results from the optimization suggest that the fiber has to be placed at the top of the connector in the cantilevered design and a U-shaped fiber substructure has to be placed at the bottom of the pontic in the 3-unit bridge. Compared to the conventional designs, the peak tensile stress is reduced by ~30% and ~45% for the 3-unit and cantilevered FRC FPD, respectively. Furthermore, the accompanying cavity design for the cantilevered bridge reduces the peak tensile stress at the tooth-denture interface by ~70%. In the in vitro validation tests, both optimized designs demonstrate higher fracture resistance. For the 3-unit bridge, acoustic emission measurement shows that the optimized design has, on average, fewer micro-cracking events than the conventional design during loading (38 vs. 2969). For the 2-unit cantilevered design, the optimized design has a ~108% higher mean failure load than the conventional step-box design (203.35 ± 28.02 N vs. 97.32 ± 21.10 N).
Yung Chen and Alex Fok, "Structural optimization of fiber-reinforced composite dental bridges" in "Composites at Lake Louise (CALL 2015)", Dr. Jim Smay, Oklahoma State University, USA Eds, ECI Symposium Series, (2016). http://dc.engconfintl.org/composites_all/42