THE FERRULE EFFECT
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Figure 9. Completed fiber-reinforced composite post-and-core. The placement of a 1.0 mm circumferential ferrule on sound tooth structure ensures the mechanical retention and resistance. |
Figure 10. An optimal adhesive integration between the components of the post-retained system which provides a structural integrity for intraradicular rehabilitation. |
The successful rehabilitation of any endodontically treated tooth using the post-retained system requires the consideration of one specific structural design characteristic: the ferrule effect. The stability of the crown is influenced by the preparation design for endodontically treated teeth. Preserving tooth structure during preparation is paramount in preventing stress concentrations at the cementoenamel junction of the endodontically restored tooth and provides resistance to tooth fracture. The completed crown preparation should have a ferrule design that encapsulates the endodontically re-stored tooth complex. This collar effect provides an antirotational feature for the stability of the crown. Clinical studies have demonstrated and confirmed the importance of this coronal tooth “collar” on the mechanical resistance and retention form of the endodontically restored tooth complex.14 The general guideline is a 1.0 to 2.0 mm preparation on sound tooth structure. Procedures that provide a shoulder on tooth structure, and an axial preparation on the core buildup, will have an insufficient ferrule design. In cases where there is insufficient sound tooth structure for a ferrule design, it is necessary to obtain this dimension through periodontal crown lengthening and/or forced-tooth-eruption procedures.
CURRENT TECHNIQUES: DISCUSSION
Currently, an increased demand for clinically convenient post-and-core systems to replace lost tooth structure has provided the clinician with a plethora of simplified “one-visit” post-and-core restorative options. However, in view of the previous considerations, it is understandable that clinicians have uncertainties about selection of restorative materials and techniques to achieve optimal results for post-and-core build-up procedures.
According to studies by the CR Foundation (formerly known as Clinical Research Associates), the fiber-reinforced systems are superior to metal prefabricated posts. In the last few years, there has been a major shift away from metal custom cast post-and-cores toward resin-based composite cores.15 Prefabricated composite post systems are replacing metal post systems because an adhesive procedure with the fiber-reinforced composite post system adds strength to the tooth-restorative interface after bonding. Therefore, the fiber-reinforced post has an advantage after assembly. The fiber-reinforced composite post system has a similar modulus of elasticity to the dentin after bonding, whereas the metal post assembly has an appreciably higher modulus of elasticity. Figures 2 to 10 demonstrate the utilization of a fiber-reinforced composite post-and-core system to restore a fractured endodontically treated maxillary right central.
CONCLUSION
Although the quest for the ideal material to restore lost tooth structure continues to be a focus of modern dental research, the aforementioned review indicates there are many post-and-core materials and techniques that are available to the clinician for a variety of clinical procedures and thus each clinical situation should be evaluated on an individual basis.1