Catalyst – solid sorbent – or support therefor: product or process – Catalyst or precursor therefor – Organic compound containing
Reexamination Certificate
1996-07-29
2002-02-19
Henderson, Christopher (Department: 1713)
Catalyst, solid sorbent, or support therefor: product or process
Catalyst or precursor therefor
Organic compound containing
C502S159000, C502S167000, C525S379000, C525S387000
Reexamination Certificate
active
06348429
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates generally to the fields of polymer chemistry and medicine. More particularly, the present invention is directed to self-curing acrylic-or vinyl-based cements particularly suitable for orthopedic, dental and other applications, as well as methods for the preparation and use thereof.
Self-curing cements are based on the discovery many decades ago that polymerization of monomers initiated by diacyl-peroxides and a tertiary amine proceeds at much lower temperatures than polymerizations initiated by thermal decomposition of the peroxides only. The amine reacts with the peroxide in the monomer medium at ambient temperature, resulting in the formation of free radical intermediates which initiate the polymerization.
This mode of polymerization was first generally employed in products for use in dentistry. For practical, functional and safety reasons, the monomer used was methyl methacrylate. Polymerization was frequently effected in the presence of polymethyl methacrylate powder; the polymer powder (which is soluble in the monomer) was used to thicken the monomer mixture with its own polymer product to form a dough which was easily moldable at ambient temperatures into various complex shapes otherwise difficult to attain by standard polymer processing methods. Another reason for addition of the polymer powder to the monomer was to lower the shrinkage that occurs to a significant extent during any vinyl polymerization. Finally, addition of the polymer to the monomer lowered the considerable heat of polymerization, minimizing the porosity of cast articles due to overheating of the polymerizing system above the boiling point of the monomer.
For use in dentistry, acrylic cements had been developed for casting of dentures, tooth caps, bridges, etc. The cements consisted essentially of methyl methacrylate (MMA), benzoyl peroxide (BPO), dimethyl-p-toluidine (DMpT) and polymethyl methacrylate (PMMA). The ingredients were mixed in appropriate amounts to form a dough which was then processed into a final rigid shape. Notwithstanding their widespread usage, these cement compositions had several significant shortcomings. In particular, the amines present in the compositions resulted in coloration of the resultant products, which is especially unwelcome in dentistry where attaining of natural color of teeth is an important consideration.
These acrylic-based cement compositions were subsequently employed in orthopedic surgery, primarily for fixation of artificial joints in hip replacements to fill the void between the cavity in the bone and the artificial hip inserted into the cavity. The use of acrylic cements in orthopedic surgery poses the most demanding requirements on these materials, particularly with respect to biocompatibility of both the precursor compositions and final products.
These cements have significant shortcomings with respect to their suitability for use in both dentistry and orthopedic surgery. For many dental applications, however, some of these shortcomings may be of lesser importance because all processing prior to final application is done external to the body. One shortcoming of particular concern in orthopedic uses is the toxicity of the cements, resulting from toxic initiation compounds used, toxic reaction products formed therefrom and residual monomer remaining after incomplete polymerization. Another shortcoming is the shrinkage of the cement during the setting, which can result in loosening of the fixation of the artificial hip. A third deficiency is the porosity of the acrylic cement products, resulting in part from the mixing technique and in part from formation of bubbles due to overheating in microregions of the polymer bulk; this porosity decreases the mechanical properties of the product cements. Finally, polymerization of the setting cement at temperatures well above body temperature damages bone tissue. Any or all of these factors may contribute to shortened lifetime of a joint replacement.
Even after many years of experience and research since the introduction of orthopedic cements, there have been no innovations of significance which would satisfactorily remedy all of the deficiencies encountered with the heretofore-known compositions. Accordingly, there remains a need in the art for improved acrylic-based cement compositions, particularly for biomedical or other applications.
It is an object of the present invention to provide improved acrylic-based compositions and methods for the preparation and use thereof which do not suffer from the drawbacks attendant to the prior art compositions and methods.
It is a particular object of the present invention to provide acrylic-based cement compositions and methods for the preparation and use thereof in which there is the possibility of greater control of polymerization in all stages up to completion.
It is a further object of the present invention to provide acrylic-based compositions for use in particular in biomedical applications which do not require major modifications in current techniques, such as mixing and insertion methods.
SUMMARY OF THE INVENTION
In accordance with one aspect of the present invention, initiation systems are provided comprising at least one peroxide or amine which has a higher molecular weight than the heretofore-employed peroxide BPO and amine DMpT, respectively. In particular, derivatives of peroxides and amines bearing large substituents relative to BPO and DMpT and polymeric peroxides and amines are suitably employed in the reaction mixture. Through the use of these initiator systems in which one or more of the components has reduced mobility relative to the materials employed in the conventional mixtures, it is possible to control the exothermicity and the peak temperature of the polymerization, while nonetheless achieving timely and adequate polymerization of the material. Concurrently, the choice of components for use in the initiation system advantageously results in a reduction in the generation of waste products and in the potential toxicity of the composition. In general, initiators with substituents attached to the benzoyl peroxide or the amine skeleton and equivalent to at least about 4 additional carbon atoms are preferred.
In accordance with another aspect of the invention, porosity and shrinkage of the cement products are addressed through selection of appropriate comonomer mixtures. In accordance with one embodiment, hydrophilic comonomers are employed under suitable copolymerization parameters which lead to formation of a random copolymer. This type of copolymer allows water to diffuse slowly through the matrix, swelling the material slightly to compensate for shrinkage, but without non-uniform softening of the matrix which might significantly compromise the mechanical strength. In accordance with another aspect of this embodiment of the invention, monomers with lower vapor tension than methyl methacrylate are used as comonomers (optionally, in combination with the hydrophilic comonomers) in order to reduce the formation of pores. These monomers are chosen such that the boiling point of the reaction mixture is increased but the alkyl ester chain length does not significantly alter the properties of the final polymer. Use of these monomers further makes it possible to achieve a more substantial degassing of the reaction mixture, thereby also reducing pore formation during the mixing process.
DETAILED DESCRIPTION OF THE INVENTION
Pursuant to one aspect of the present invention, alternative initiation systems are provided in order to eliminate various disadvantages of the conventional BPO/DMpT system. In these improved initiation systems, higher molecular weight amines and/or peroxides are employed. These compounds do not diffuse as easily as BPO and DMpT out of the cement (for example, into the surrounding tissue when used in biomedical applications), and thus these compounds and their reaction products are less likely to be extracted out of the cured cement. Moreover, the decrease of mobility results in a relative
Gough David Arthur
Lim Drahoslav
Rourke Andrea M.
Fulbright & Jaworski L.L.P.
Henderson Christopher
The Regents of the University of California
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