Dentistry – Method or material for testing – treating – restoring – or... – By filling – bonding or cementing
Reexamination Certificate
2000-08-02
2003-08-05
Lucchesi, Nicholas D. (Department: 3732)
Dentistry
Method or material for testing, treating, restoring, or...
By filling, bonding or cementing
C433S029000, C433S215000
Reexamination Certificate
active
06602074
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention relates to light curing apparatus and processes for preparing dental restorations.
2. Description of Related Technology
Shrinkage of light- and self-cured dental composites used for direct, intraoral restorations has long been a concern of dental investigators because of the potential for micro-leakage to occur at a tooth-restoration interface. When composite restorations cure, they shrink as a result of the polymerization of the resin monomers in the composite. The volumetric shrinkage of composites may be in the range of approximately 3-4%. Micro-leakage resulting from such shrinkage can contribute to recurrent tooth decay, staining, and sensitivity.
Investigators have demonstrated that self-cured dental composites display less shrinkage stress than light-cured composites, because self-cured composites exhibit the ability to flow to allow the relaxation of stress during a relatively slow curing period, usually several minutes. (See Davidson et al, JDR, 63:1396-1399(1984); Feilzer et al., Dent. Mater., 9:2-5 (1993)). When a composite is light-cured it quickly polymerizes to a fairly high cross-link density, for example, within just a few seconds. Such quick polymerization does not provide sufficient time for the composite to relax and relieve the stress.
Light-induced polymerization shrinkage has long been a concern in the dental industry because of its potential to cause “debonding” at the tooth-restoration interface, especially when adhesive strengths are not optimal. With an adhesive bond strength of sufficient magnitude, the polymerization shrinkage can be redirected to reduce the stress at the restoration-tooth interface. This redirected polymerization shrinkage, however, can create internal stresses in the restorative material (composite resin) or the remaining tooth structure detrimental to the long term success of the restored dentition.
The use of advanced dental adhesives may cause the fracture of the more brittle tooth enamel at the margin of the restoration in response to the polymerization shrinkage. As a result of the enamel fracture, micro-leakage can then progress to the detriment of the restored dentition. Enamel is an anisotropic brittle substance consisting mainly of rods or prisms, having a high elastic modulus and low tensile strength resulting in a very rigid structure. (See The Art and Science of Operative Dentistry, 3rd. Ed., C. M. Sturdevant, T. M. Robertson, H. O. Heymann, and J. R. Sturdevant editors, 1995, Mosby-Year Book, Inc., St. Louis Mo., pp. 12-18). Forces or stresses applied perpendicular to the direction of the enamel rods can much more easily fracture enamel as compared to parallel directed stress. The upper portion of Class I or Class II (MOD) restorations, above the dentin-enamel junction (DEJ), typically will have enamel rods parallel to the bonding line to be formed between adhesive and enamel. Because of the relatively weak nature of enamel, if stressed perpendicular to the rod direction, Class I or larger Class II restorations are susceptible to fractures within the enamel structure relatively close to the bond line forming cracks parallel to the bond line.
Of the several different classes of restorations, the Class I or Class II (e.g., MOD) type with their high amount of bonded surface area has in particular been the focus of much research due to its inherent high stress potential with good adhesives. Several studies on methods to counteract stresses in dental restorations have been conducted, including varied composite insertion methods into the dental cavity (e.g., bulk and incremental), composite shrink minimization, stress relaxation by flow to allow built-up stress to decrease, and strain measurements by numerous groups. See C. M. Kemp-Scholte and C. L. Davidson, J. Dent Res., vol. 67, p. 841, (1988); J. R. Bausch, et al., J. Prosthetic Dent., vol. 48, vol. 59 (1982); A. J. Feilzer, et al., J. Dent. Res., vol. 66, p. 1636; C. Davidson, J. Prosthetic Dent., vol. 55, p. 446 (1986); A. Feilzer, et al., J. Dent. Res., vol. 68, p. 48 (1989); C. M. Kemp-Scholte and C. L. Davidson, J. Prost. Dent, vol. 64, p. 658 (1990); C. M. Kemp-Scholte and C. L. Davidson, J. Dent. Res, vol. 69, p. 1240 (1990), C. L. Davidson, et al., J. Dent. Res., vol. 63, p. 1396 (1984); M. R. Bouschlicher, et al., Amer. J. Dent. vol. 10, pp. 88-96 (1997). It has been found that slow, self-cured composites reduce the rate of shrinkage stress formation which may cause enamel cracking.
In contrast, the visible-light curing process currently used with dental composites is a very fast, free-radical-initiated process where the bulk of the polymerization reaction typically is completed within just a few seconds. This quick process has always been considered a great advantage to the clinician but can lead to very high stress rates within the tooth structure. Such high rates of stress formation are known to cause premature failures in brittle materials (See Organic Coatings: Science and Technology, Vol. II, Z. W. Wicks, el al., John Wiley & Sons, New York, pp 105-31 (1994) and references cited therein; see also, An Introduction to the Mechanical Properties of Solid Polymers, I. M. Ward and D. W. Hadley, John Wiely & Sons, New York, Chapter 12 (1993) and references cited therein).
A study conducted by the inventors to illustrate stress formation problems with light-cured composites as compared to self-cured composites included the steps of placing an increment of a light-cure composite (
LITEFIL™, Bisco, Inc., Schaumburg, Ill.) into a Class I cavity of approximately 3×3×3 mm up to the dento-enamel junction. After a light cure of ten seconds at 600 milliwatts per square centimeter (hereinafter mW/cm
2
), a second increment of the same light-cure composite was added to the cavo-surface margin and the composite was cured using a high intensity light (e.g., ten seconds at 600 mW/cm
2
) on each of the buccal, lingual, and occlusal surfaces. An enamel crack was observed on one side of the restoration (see FIG.
4
). The inventors performed an identical study using a dual-curing (self-light) dental composite (DUO-LINK, Bisco, Inc., Schaumburg, Ill.) for the second increment and allowing the self-cure process to proceed for five minutes before light curing. In this latter study, no enamel cracks were observed (See FIG.
5
). No cracks were observed in dentin for either the light-cured or self-cured samples studied. (The reason why enamel cracked and dentin did not crack is most likely due to dentin's higher flexibility (lower modulus). Thus, it was determined that the slower, self-curing composite could provide excellent structural integrity compared to light-cured composites. However, self-cured composites are not clinically ideal for use as occlusal surface restorative materials. Thus, for occlusal surface restorative applications, light-cured composites remain the most desirable option. However, latest trends with light curing are towards curing dental composites even faster, using higher and higher intensity curing systems, which can only create higher stressed restorations.
Commercial curing systems usually use fixed intensities, typically at 400 to 600 mW/cm
2
, which are not adjustable by the user. Additionally, commercial curing systems employ curing times that are usually pre-programmed at a minimum of ten seconds to a maximum of sixty seconds in ten second increments. Such curing systems require dental clinicians to use high curing intensities for durations which will result in high stress formation. Shorter than ten second duration curing times are not provided for by commercially-available curing systems.
Known devices used to cure visible-light-initiated dental composites are often called light-curing (LC) units or guns, since the hand-held portion of most units looks like a gun with a trigger for activating light. A typical visible light curing unit includes a base unit that typically sits on a counter in a dental operatory and houses the electronics that operate the light. The ba
Suh Byoung I.
Vinson Wayne
Bisco Inc.
Howrey Simon Arnold & White , LLP
Lucchesi Nicholas D.
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