Dentistry – Method or material for testing – treating – restoring – or... – Crown
Reexamination Certificate
1999-04-21
2002-04-16
Wilson, John J. (Department: 3732)
Dentistry
Method or material for testing, treating, restoring, or...
Crown
C433S224000, C433S225000
Reexamination Certificate
active
06371763
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention generally relates to a dental post and core system for endodontically-treated teeth. More specifically, this invention relates to a passive dental post and core system having a flexible inelastic post, wherein the post is made from a material having a plurality of distributed fibers, such as, for example, medical grade optical fibers, other medical grade fibers or other fiberglass materials, which are held together in a matrix in a resin, such as a polyester resin or a vinyl ester resin.
In the preferred embodiment, the flexible post has a modulus of elasticity less than or equal to that of tooth dentin, to prevent widespread damage to a tooth in a traumatic event, when a conventional post would flex less than the tooth dentin, causing tooth fracturing where the flexible dentin violently contacts the inflexible conventional post.
Also in the preferred embodiment, the endodontic post of the present invention is cylindrical, rather than wedge shaped as in many non-metallic posts, because of its less stressful impact and its decreased wedging effect, which can cause immediate and/or residual root fractures.
While the fibers may be axially aligned, preferably at least one of the fibers extends non-axially aligned with respect to a straight axis extending from the apical end to the opposite coronal end of a root of a tooth.
In one embodiment, the present invention includes an endodontic dental reinforcement post for endodontic and reconstructive pin therapy comprising a prefabricated bundle of loosely compacted fibers in a cured resin, with the post extending from an apical end to a coronal end of a tooth canal.
For example, the fibers may be a bundle of fibers, a longitudinally twisted bundle, a twisted braid, a woven lattice, a helically wrapped bundle of fibers, or a composite of randomly dispersed fibers in a binder.
In this preferred embodiment, at least one of the fibers extends non-axially aligned with respect to the straight axis of a root of a tooth.
For example, in a bundle of fibers, while some of the fibers may extend parallel to the straight axis of the root, at least one or more of the fibers extend in an axial direction which is not parallel to the straight axis of a root of a tooth. That is, at least one or more of the fibers extends in a transverse or angled direction away from the straight axis of the root of a tooth.
With respect to a longitudinally twisted bundle, a twisted braid, a helically wrapped bundle of fibers, the twisting or helical wrap of the fibers causes many, but not necessarily all, of the fibers to extend non-axially. Concerning a woven lattice of fibers, while one set of fibers could extend axially parallel to the straight axis of the root, the other intersecting set of fibers extends in a direction which is non-axially aligned with respect to the straight axis of the root. Even if most of the weft of a weave of a plurality of fibers extends parallel to the straight axis of the root, at least one or more fibers constituting the warp of the weave of fibers extends non-axially with respect to the straight axis of the root of the tooth.
Moreover, concerning a composite of randomly dispersed fibers, there is always the possibility of one or more of the fibers being axially aligned to the straight axis of the root of a tooth. However, in order to be randomly dispersed, at least one or more of the fibers extends non-axially with respect to the straight axis of the root of a tooth.
Preferably, the post is radio-opaque and bears a color simulating that of a natural tooth.
Rigid dental post and core systems are widely utilized to restore endodontically-treated teeth. Post and core restorations are routinely used to create an adequate foundation for the final restorative step, which may be a crown, inlay, or a fixed partial denture abutment. Generally, a post is provided for retention and lateral stability of the restoration. The core provides support for the crown. Two general types of post and core systems are known in the art: “active” or screw-in type systems and “passive” type systems. Active post and core systems mechanically engage the walls of the root canal and tooth dentin. Passive post and core systems are bonded in endodontically treated teeth utilizing cements and the like.
Two major problems are encountered when restoring an endodontically-treated tooth. Firstly, the tooth is more susceptible to fracture, and secondly, there is generally less coronal structure with which to work. The greater susceptibility of a tooth to fracture after endodontia may result from the tooth being more brittle. However, studies of the changing mechanical properties of pulpless teeth do not generally support this theory equating dryness with reduced mechanical strength. It appears that the greater susceptibility for fracture in an endodontically-treated tooth results from mechanical weakening of the tooth during root canal therapy and refinement of the root canal. Improvements in restoration techniques that reduce mechanical weakening are therefore desirous.
An endodontically-treated tooth is generally severely compromised either due to trauma or neglect. Thus, traumatic fractures, removal of old restorations and carious tissue, and preparation of root canal access may not leave enough tooth to maintain the “dome effect” of the tooth or to retain a crown.
The stress concentrations in a tooth resulting from the rigid post and core systems of the prior art also play a vital role in tooth fracture. Stress concentrations can be impacted through system design and/or restoration techniques. Various studies and investigations into the susceptibility of endodontically-treated teeth to fracture and the contribution of rigid dental post and core systems to such fracture have been conducted. “
A Comparison of Intracanal Stresses in a Post Restored Tooth Utilizing the Finite Element Method
”, Cailleteau, Johnny G., Rieger, Monty R. and Akin, J. Ed,
Journal of Endodontics
, Vol. 18, No. 11, November 1992, pp. 540-544, reports that placement of a rigid post within a tooth alters the pattern of stress along the root canal as compared with an intact tooth. Instead of strengthening the tooth the post stiffens the coronal posted section and shifts the flexure point apically. The effect of this stiffening causes the non-posted apical portion of the tooth to deform at the post apex, resulting in a stress increase in that portion of the canal wall. Also, the cyclic loading and unloading of an incisor during mastication requires consideration of fatigue failure. Since the maximum bending stresses occur in connection with the apex of the post, any inclusions or defects within the wall of the dentin near the apical end of the post would create stress concentrations that increase the risk of a fatigue crack formation. Defects and microfractures introduced during endodontic treatment and post access preparation could become areas contributing to stress concentrations. Studies have also shown that more intact tooth structure provides better resistance to fracture than a metallic post. There is also evidence that stresses in the tooth tend to increase as the post diameter increases.
A flexible post eliminates these problems and a cylindrical flexible post performs even better. A post and core system utilizing a flexible post shifts the stress concentrations coronally, eliminates the introduction of defects during post access preparation and post placement, and leaves more of the tooth intact.
The main function of a post is to provide retention to the core. Relieved of its expectation to facilitate resistance to tooth fracture, the post can be designed to optimize its retentive properties. Several factors govern the retentiveness of endodontic posts. The shape of the post and its length are among the essential factors.
For example, unlike the preferably flexible cylindrical post of the present invention, tapered dowels have been found to be significantly less retentive than parallel-sided posts. While inflexible metallic posts are generally
Masyr Samuel
Sicurelli, Jr. Robert J.
Walker Alfred M.
Wilson John J.
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