Dental Pulp Capping and Cavity Lining Composition and Preparative Method

This invention relates to an improved dental pulp capping and cavity lining composition and its method of preparation. The beneficial effect of calcium hydroxide on tooth pulp and secondary dentine formation has been well known in dentistry for many decades. The original calcium hydroxide suspensions were later mostly replaced by hard setting calcium hydroxide formulations.

Three prior art disclosures for which United States Patents were subsequently granted have special relevance to the present invention. U.S. Pat. No. 2,516,438 advocates that a mixture of excess calcium hydroxide and eugenol be used as pulp capping material. This was prior to the advent of polymerizable, acrylic functional, dental restorative materials, the setting of which is unfortunately inhibited by compositions containing eugenol, all of which contain some residual free eugenol even after many years. It is reported in the Science of Dental Materials (R. W. Phillips) 1973, Chapter 7, page 93, that as much as 5 percent free eugenol, from an original eugenol content of 16.4 percent, may be present in a zinc oxide and eugenol mixture, even ten years after mixing, and a similar finding would be expected in the case of calcium hydroxide and eugenol.

Since the presence of excess available free calcium hydroxide has been found beneficial in the stimulation of reparative secondary dentin formation over exposed dental pulps, U.S. Pat. No. 3,047,400 advocated the continued use of calcium hydroxide, but now reacted with non-eugenol phenolic derivative, to achieve compatibility with the newly introduced, unfilled acrylic restorative materials.

After 1965, filled acrylic functional resin based composite restoratives were introduced and rapidly gained wide acceptance. Concurrent with this development the use of the acid-etched enamel bonding technique also gained very wide acceptance, creating a need for pulp capping and cavity lining materials to be acid resistant. U.S. Pat. No. 4,080,212, places major emphasis on the prime objective stated as "the prevention of discoloration", even foregoing all the inherent advantages of a two paste system in order to achieve this objective.

Accordingly, it is an object of this invention to provide a dental pulp capping composition which when placed over moderately inflamed dental pulp will reduce inflammation and induce dentinal bridge formation thus increasing the probability of pulp healing.

It is also an object of this invention to provide a dental cement composition capable of fast curing in an oral environment and having high crushing strength to resist amalgam condensation pressure.

It is another object of this invention to provide a pulp capping and cavity lining material which is resistant to the type and concentration of acids used for etching tooth enamel and which also exhibits complete compatibility with composite restorative materials.

It is a further object of the present invention to provide a novel capping and cavity liner material in the form of a two paste system.

The above objects and other advantages are achieved based upon the use of condensates of suitable alkyl salicylates with suitable aldehydes, reduced in viscosity by blending with a further quantity of a similar alkyl salicylate, and reacted with calcium hydroxide or calcium oxide, in stoichiometric excess over the resin condensate blend. The resulting product comprises a rigid semipermeable, layer of a calcium phenolate complex, containing excess available calcium hydroxide dispersed therein.

The resin condensates found most useful in accordance with this invention are prepared by the condensation of an alkyl salicylate with formaldehyde or its polymer. These resin condensates are prepared by the methods used for many years for the preparation of conventional phenol formaldehyde resins and described in numerous chemical literature references.

When combined with calcium hydroxide the reaction time of the resin condensate increases with an increase in size of the alkyl groups. At the same time the compressive strength decreases with an increase in the size of the alkyl groups. Satisfactory results were obtained with a range of alkyl groups having from one to six carbon atoms, that is up to and including hexyl salicylate.

Acetaldehyde, as well as formaldehyde, was found suitable for synthesis of the resin condensate. The chosen resin condensate is then blended with a further quantity of the preferred alkyl salicylates to obtain a blend of suitable viscosity for the preparation of a paste. This alkyl salicylate acts as a reactive diluent and thus ensures that adequate physical properties are obtained, following the reaction with the calcium hydroxide, after mixing.

This resin condensate blend is made into a radiopaque paste by mixing with one or more inert fillers, at least one of which should be radiopaque, and all of which should be essentially insoluble in water.

Since radiopacity is approximately proportional to the square root of the atomic weights of the constituent elements, insoluble barium compounds, particularly finely divided barium sulfate, are preferred radiopaque fillers which may be beneficially augmented with other inert, insoluble fillers, such as titanium oxide, sub-micron silica, iron oxide pigments, etc.

The second paste, containing calcium oxide or hydroxide extended with zinc oxide, utilizes an inert liquid vehicle as the matrix liquid. In order to ensure desirable physical properties, it is essential that the calcium hydroxide-salicylate condensate reaction of the present invention be formulated using a stoichiometric excess of calcium hydroxide. Furthermore, this unreacted excess of calcium hydroxide is dispersed throughout the set material and is a source of partially leachable free calcium hydroxide which is available to assist in the stimulation of secondary dentine formation. Additionally, the set and hardened layer of material, which is a complex calcium polysalicylate, will resist penetration or disintegration by acid etchants used in dentistry, and the presence of excess dispersed calcium hydroxide will neutralize any such acid inadvertently coming into contact with the pulp capping material, thus protecting the pulp from accidental acid percolation.

The ranges of composition which have been found suitable for synthesis of the resin condensate and the catalyst paste formulated from it are as follows:

An aldehyde to salicylate ratio of about 0.5 to 1.0 moles of aldehyde per mole of salicylate, with the preferred ratio being about 0.55 to 0.75 moles of aldehyde per mole of salicylate.

A resin condensate to alkyl salicylate ratio of between about 40 and 80 parts by weight of resin condensate to between about 60 and 20 parts by weight of alkyl salicylates, with the preferred ratio being between about 65 and 80 parts by weight of resin condensate to between about 35 and 20 parts by weight of alkyl salicylate.

A catalyst paste matrix liquid to filler ratio of between about 30 and 70 parts by weight of matrix liquid to between about 70 and 30 parts by weight of filler, with the preferred ratio being between about 40 and 60 parts by weight of matrix liquid to between about 50 and 40 parts by weight of filler.

The ranges of composition which have been found suitable for formulation of the base paste can be varied within fairly wide limits, but in order to preserve adequate physical properties consistent with good paste flow, the powder to liquid ratio of the paste should be optimized. In addition, in order to ensure availability of an adequate excess of free calcium hydroxide, a substantial stoichiometric excess of the latter over the resin condensate should always be utilized.

The consistency of the mixed material is a free-flowing, creamy, paste which can be conveniently placed, without pressure, over exposed pulp tissue or a prepared dentine surface in a tooth cavity, and when in contact with the tooth structure warms quickly to body temperature, decreasing in viscosity and spreading easily. The mixed material adheres to the moist dentine surface and functions well as a protective barrier.

A preferred example of a two-paste formulation suitable for use in the present invention is listed below:

When required for use, suitable quantites of catalyst and base paste are mixed in equal proportions. The setting time of the material is controlled by the addition of a small amount of water, providing work time flexibility, so that formulations can be prepared which are usable at high ambient temperatures and humidities, yet will set rapidly when applied to the base of a cavity preparation. The following examples serve to illustrate the methods of preparation of the resin condensate, catalyst paste and base paste:

A calcium hydroxide containing base paste of the following formulation was prepared:

This formulation is disclosed in Example V of U.S. Pat. No. 3,047,408 and is later used to compare the composition of the present invention with a commercially available material manufactured according to the U.S. Pat. No. 3,047,408.

Methyl salicylate-formaldehyde resin condensate was prepared according to the following procedure: 22.8 parts of methyl salicylate, 3 parts of paraformaldehyde, 3 parts of glacial acetic acid and 0.6 parts of zinc chloride were placed in a boiling flask fitted with reflux condenser, thermometer, stirrer and heating mantle. The mixture was heated slowly with stirring until gentle reflux conditions were reached and maintained for approximately 48 hours until the last traces of paraformaldehyde sublimation disappeared.

After the condensation is completed the resin is purified by multiple washings with deionized water to remove the water soluble materials from the condensate. The resin was then stripped of all volatile material under vacuum, with the temperature being finally raised to 250.degree. C.

The resin condensate remained as a light brown viscous liquid, which when cooled below 60.degree. C. formed a solid amorphous resinous mass.

Six catalyst pastes were prepared using various phenolic derivatives, including mono and disalicylate esters, and a 65% solution of the resin condensate from Example II in methyl salicylate, to the following general formula:

The ingredients were mixed together and homogenized on a three roll mill. Equal parts of each catalyst paste were mixed in turn with an equal volume of the base paste from Example I and the setting times measured with the following results:

The setting time of all of the above formulations based on monoalkyl salicylates is too fast to have an practical use in denistry and therefore only the last two formulations were evaluated for compressive strength. A trace of water was used to accelerate materials prepared according to the present invention. The following results were obtained:

The same method as described in Example II was used for preparation of the following resin condensates:

Methyl salicylate/acetaldehyde resin condensate

Isobutyl salicylate/formaldehyde resin condensate

Isobutyl salicylate/acetaldehyde resin condensate

Hexyl salicylate/acetaldehyde resin condensate

Hexyl salicylate/formaldehyde resin condensate

Ethyl salicylate/acetaldehyde resin condensate

Ethyl salicylate/formaldehyde resin condensate

It was observed that the necessary reaction time seemed to increase with increasing size of the alkyl group of the salicylate ester, and also when acetaldehyde was substituted for paraformaldehyde. All of these resin condensates were tested, following dilution with various different monoalkyl salicylates, and their physical properties were similar, but with a trend towards lower compression strength with increasing size of the alkyl group of the salicylates ester.

The acid resistance of the material manufactured according to the U.S. Pat. No. 3,047,408 was compared to the material of Example IV. Thin layers of both materials were spread on the glass slab and allowed to set for 1 minute at 37.degree. C. and 100% relative humidity. One drop of 50% phosphoric acid was placed on each material and after 1 minute removed with an excess of water. The commercially available material according to U.S. Pat. No. 3,047,408 showed a deep indentation where the acid was placed with porous structure at the bottom. The material according to the present invention only lost its luster without any detectable indentation even under magnification.

The above examples are not intended to limit the scope of the invention or the applications to which this invention may be directed. It is to be understood that although the invention has been described with specific reference to particular embodiments thereof, it is not to be so limited, since changes and alterations therein may be made which are within the full intended scope of this invention as defined by the appended claims.