The publications and other materials used herein to illuminate the background of the invention, and in particular, cases to provide additional details respecting the practice, are incorporated by reference.
Existing systems for anchorage of dental constructions such as crowns and large fillings to remaining roots are based on the use of dental posts mostly made of high rigidity materials like metal alloys, ceramics or to reduce the rigidity they can be made of completely polymerized fiber reinforced composite materials as disclosed e.g. in U.S. Pat. No. 5,964,592. Common characteristics of these posts are that when they are delivered and thus when they are inserted, they are in their final straight shape and that they are rigid. Occasionally they exhibit tapered or structured surfaces for increased mechanical retention for the composite luting cements. The earliest fiber reinforced composite (FRC) posts were made of carbon/graphite fibers with epoxy polymer matrix resulting in flexural modulus of 40 to 60 GPa. The most recent FRC posts are made of glass or silica fibers with an epoxy or dimethacrylate polymer matrix having flexural modulus between 28 to 40 GPa. Clinically the advantage of FRC posts has been that these do not cause fracturing of roots even if they have been used in short post lengths compared to the length of the clinical crown. On the other hand, the fracture incidence with metallic and ceramic posts has been high. The biggest disadvantages of completely polymerized FRC posts are the low bonding strength between the FRC post and the composite luting cement or composite core material, and the inability to bend along the curved root canal. All commercially available FRC posts require preparing the root canal to the standardized form of the FRC post. The preparation reduces the quantity of dentine and thus, reduces strength of the remaining root. These shortcomings cause frequent debonding of the FRC posts from the core and cement and hamper seriously the preparation of curved root canals. As a result of the present preparation techniques perforation of the tooth root and the periodontal ligament often occurs resulting in an elevated risk of infection of periodontal tissues. On the other hand, if there is need for long post lengths, which is the case when using metallic or ceramic posts to reduce the functional stress at the end point of the post, the root canal sealing has been shown to deteriorate considerably. This can cause an infection, e.g. a periapical periodontitis. Therefore a post system with an improved bonding characteristic that can be placed in curved individually shaped root canals even with short post lengths would be a welcome improvement.
It is known that polyethylene fiber products are being marketed for making posts in situ. The polyethylene fibers are inserted into the root with a special handheld instrument by pushing the fiber ribbon from the middle. The root canal is filled with a dual curing composite to wet the fibers and to bond the post to the tooth root surface. The potential advantages of this system are that there is no interface between the post polymer matrix surface and the cement since it is of the same material and that there is no need to prepare straight cavities for the posts. Disadvantages of a polyethylene fiber post system are that wetting of the fibers by the polymer matrix and bonding of the fibers to the polymer matrix are inadequate (Vallittu, Ultra-high-modulus polyethylene ribbon as reinforcement for denture polymethyl methacrylate. Dent Mater 1997;13:381-382), the control of wetting the fibers inside the root is impossible and use of woven fibers results in less than optimal orientation of the fibers in the root.
The state of the art fiber reinforcement material in dentistry is preimpregnated unidirectional glass fiber material as disclosed e.g. by Sicurelli & Masyr (WO 98/52486). There are four such materials available commercially: Jeneric Pentron's Fibrekor®, Ivoclar-Vivadent's Vectris®, Stick Tech's Stick® and everStick™. These differ from the other fiber materials in two respects: the bonding between the fiber surface and the polymer matrix is significantly higher than with polyethylene fibers and the wetting of the fibers with the polymer matrix is complete. This results in flexural strengths of up to 1280 MPa as compared to 350 MPa of the best polyethylene product and in elastic modulus of up to 28 GPa as opposed to 3-5 GPa of polyethylene composites.
The disadvantage of unidirectional glass fibers is their poor controllability in the clinical handling process. Considering a root canal of dimensions approximately 2 mm opening diameter and 1 mm end diameter, 5-10 mm length and being of considerable curvature imposes various problems for the insertion of a unidirectional, non woven, non twisted glass fiber bundle of approximately 1000 to 6000 individual fibers, impregnated with a low viscosity monomer liquid. The fibers fray, bend and tangle with each other when one tries to push them inside the canal. Once spread, it is next to impossible to collect the fibers back into order and try again.
Very similar devices to posts are root canal anchors called root canal screws that are actually just very short posts. These are manufactured in the form of a metallic screw of maximum length of approximately 10 mm and minimum length of 3 mm. In fact the division between an anchor and a post is not clear. In both cases however the root canal is prepared with a separate straight drill to make a close fit cavity for the screw. On the other hand, the screws can also be placed into other dentinal cavities and canals, such as those prepared in a vital tooth. These vital tooth screws are called parapulpal posts and they are most often used to improve retention of fillings to remaining teeth.
The indication for an anchor is the need for added fixation of a partial crown or filling of large dimensions. Fillings are generally attached through mechanical or chemical retention or both. The strength of chemical retention depends on bonding surface area and roughness and the chemical nature of the bond. Mechanical retention depends solely on the shape and surface roughness of the cavity. One could say that the less of the tooth is left for mechanical or chemical bonding the more important it is to create increased retention of posts and anchors. In this sense a crown with very little tooth support left is an extreme case of a filling and is a clear indication for a root canal post. The other extreme is a one-wall filling that in practice usually does not need any additional retention from a post nor an anchor. With larger cavities involving 2 or 3 walls, larger than that being already considered crowns, the achievable retention is dramatically reduced and there is an increasing need to create more retention artificially.