Dowel design optimization in an endodontically treated single-rooted tooth: a finite element stress analysis

Improper dowel design can cause the failure of a restoration in an endodontically treated single-rooted tooth. A three-dimensional finite element stress analysis was used to evaluate stress patterns on an endodontically treated single-rooted tooth restored with a tapered dowel and core and a complete crown under an angular load applied to the tip of the crown. Results showed that, on the loading side, peak dentin and cement tensile stresses were located at the level of the alveolar crest and at the crown margin, respectively. Increasing the diameter and/or stiffness of the dowel decreased these peak stresses; however, it increased the dentin and cement stresses at the dowel apex level, and at the bone level, respectively. The role of the dowel length was to change the location of the stress concentrations in the dentin occurring at the dowel apex. A large and short dowel or a large and stiff (200 GPa) dowel might cause the root fracture to occur across the dowel apex region. The ferrule had no effect on both the dentin and cement stresses below the alveolar crest level. Increasing the ferrule height diminished the stress propagating in the cement layer from the crown margin toward the tooth-core junction. If root fracture occurred, the model predicted a horizontal fracture when the cement layer was perfectly intact. A vertical root fracture was predicted when the cement was broken at the crown margin on the loaded side. The ferrule effectively prevented the vertical fracture. Reducing the bone height resulted in increasing the dentin and cement stresses at and below the bone level. Extending the crown margin toward the bone level increased the cement stress at the crown margin. With an application of the composite theory, it could be demonstrated that while dowels might increase the stiffness of the system, it might not strengthen the tooth under a transverse loading. In conclusion, dowel diameter, dowel length (below the alveolar crest), dowel material, and ferrule are important factors in predicting the failure of the dowel-core restoration. All factors have to be considered together in the dowel design.