[Design of non-metallic crown for primary molars and analyzation of stress distribution: a finite element study]

Objectives: To study the design of nonmetallic crowns for deciduous molars by means of computer aided design and to analyze the key parameters of the nonmetallic crowns of deciduous molars using finite element method. Methods: The three-dimensional model of a mandibular second primary molar was constructed by using a micro-CT system. The thickness of the crown was limited to 0.5 mm and four different crown shapes (chamfer+anatomic, chamfer+non-anatomic, knife edge+anatomic and knife edge+non-anatomic) were designed. Then, the crown shape was limited as chamfer+non-anatomic and five different thicknesses of the crown (0.50, 0.75, 1.00, 1.25, 1.50 mm) were designed, and three different materials, including polyetherketoneketone (PEKK), polymethylmethacrylate (PMMA) resin and resin-infiltrated ceramic, were applied to make the crown. Stress distribution and fatigue of each component of the model under vertical and oblique loadings were analyzed by using finite element method. Non-axial retention analysis was performed on chamfer+non-anatomic crowns, made of PMMA resin, with thicknesses of 0.50, 0.75, 1.00, 1.25 and 1.50 mm. Results: Among the four crown shape designs, the chamfer+non-anatomic type crown showed the lowest von Mises stress and the highest safety factor. By comparing three different materials, the resin-infiltrated ceramic group showed obvious stress concentration on the buccal edge of the crown and the PEKK group showed stress concentration in the adhesive layer. Results of non-axial retention analysis showed that the torques required by the crowns with five thicknesses at the same rotation angle were as follows: 4 856.1, 4 038.1, 3 497.3, 3 256.3 and 3 074.3 N⋅m, respectively. The comparison of areas of the adhesives fracture among groups were as follows: 0.5 mm group < 0.75 mm group < 1.00 mm group < 1.25 mm group < 1.50 mm group. Conclusions: In the design of nonmetallic crowns for primary molars, the edge of the crown should be designed as chamfer, the shape of the inner crown should be non-anatomical and the minimum preparation amount of the occlusal surface should be 1.00 mm. Among the three materials, PMMA resin, of which elastic modulus is similar to the dentin and the dental adhesive, might be the most suitable material for the crowns of primary molars.