Endodontic access in all-ceramic dental restorative crown materials
Gorman, Catherine M.
University College Cork
A diagnosis of irreversible pulpitis can occur after a crown has been permanently cemented. This necessitates the need for endodontic treatment, often with the crown in situ. Increasing trends indicate that all-ceramic crowns are provided as the preferred restorative option to metal-ceramic crowns. This is because modern all-ceramic restorations can now provide excellent aesthetic solutions combined with high mechanical strength properties, compared with earlier, weaker ceramic materials. It is a considerable operative challenge for the dental practitioner to prepare an endodontic access cavity in vivo, due to the high mechanical properties of modern dental ceramic materials. The inherent nature of ceramic materials is that they are brittle, sensitive to damage and moisture, with failure occurring in an unpredictable manner. The difficulties in performing endodontic treatment in all-ceramic crowns and subsequently repairing the access cavity is relatively unexplored in the dental literature, more research is needed to inform clinical practice in this area. A systematic review of the literature aimed to identify influential treatment factors of endodontically accessed and repaired all-ceramic crowns and report the evidence of damage around the endodontic access cavity as a result of preparing the cavity in an all-ceramic crown. Eight studies were selected to address the aims. The inadequate volume of literature was highlighted with, the earliest relevant publication identified in 1962 and since the last electronic search (2016) only 26 additional references were identified in the subject area. Potentially noteworthy strength controlling factors were identified to be related to the crown material, its baseline strength, the grit size of the diamond bur used to create the access cavity, the ratio of access cavity to crown dimension, the cement used to lute the crown and the presence of radial cracks after access cavity preparation. The effect of two variables, namely, cavity dimension and modulus of elasticity of the resin composite repair material on the equibiaxial flexural strength of lithium disilicate glass-ceramic (IPS e.max® Press, Ivoclar Vivadent) material was investigated. Disc specimens with representative access cavities were used as a model system to examine these variables. Within the study limitations, the results indicate that cavity size and not the repair material, influence the equibiaxial flexural strength. The shear bond strength of the resin composite material used to repair the access cavity in a lithium disilicate glass-ceramic was determined to be comparable to those values as found in the literature (see Appendices). Model mandibular first molar crowns were fabricated from lithium disilicate glass-ceramic to examine the impact of cavity size on failure load. The failure load for the intact crowns and crowns with a rhomboidal (based on the presence of three-canals) or rectangular (based on the presence of four-canals) endodontic access cavity, with and without a resin composite repair were measured and analysed statistically. Within the limitations, the results show that a rectangular access cavity significantly reduces the failure load which was then restored to the original values upon repair with resin composite. The preparation of a rhomboidal access cavity did not reduce the failure load compared with the intact crown. The novel use of Finite Element Analysis (FEA) was successfully demonstrated in this subject area. Solid geometric models of lithium disilicate glass-ceramics (LDGC) crowns with three endodontic access options repaired with a resin composite (Tetric EvoCeram®, Ivoclar Vivadent) were modelled. The models were subsequently subjected to clinically relevant loads and a stress analysis was performed using FEA. This work showed that high curvature access cavity designs produced the highest stress scenario and therefore should be avoided. In an attempt to compliment the in vitro study computer models of LDGC discs were modelled using Finite Element (FE). The models were successfully validated, similar variables were modelled and concentric ring loading conditions were applied as per the in vitro study. It was determined that the size and not the stiffness of the repair material was more critical to the strength of LDGC discs. In conclusion, this study has addressed some of the aspects of problems encountered when endodontic access cavities are prepared in all-ceramic dental crowns, however it is an area where substantial literature is lacking and therefore further research is warranted.
Gorman, C. M. 2018. Endodontic access in all-ceramic dental restorative crown materials. PhD Thesis, University College Cork.