Nanocrystalline apatites and composites, prostheses incorporating them, and method for their production

Abstract

Methods for synthesis of nanocyrstalline apatites are presented, as well as a series of specific reaction parameters that can be adjusted to tailor, in specific ways, properties in the recovered product. Particulate apatite compositions having aveage crystal size of less than 150 nm are provided. Products also can have a surface area of at least 40 m2 / g and can be of high density.Hydroxyapatite material is investigated in particular detail. Compositions of the invention can be used as prosthetic implants and coatings for prosthetic implants.

Description

RELATED APPLICATION[0001]Notice: More than one reissue application has been filed for the reissue of U.S. Pat. No. 6,013,591. The reissue aplications are patent application Ser. Nos. 10 / 044,801 (the present application), and 10 / 863,863, which is a continuation of patent application Ser. No. 10 / 044,801. <?insert-end id="INS-S-00001" ?>[0002]This non-provisional application claims the benefit under Title 35, U.S.C. §119(e) of co-pending U.S. provisional application Ser. No. 60 / 035,535, filed Jan. 16, 1997, entitled “Nanocyrstalline Apatites and Compositions, Prostheses Incorporating Them, and Method for Their Production” by Jackie Y. Ying et al., incorporating herein by reference.FIELD OF THE INVENTION[0003]The present invention relates generally to bioceramics and more particularly to a class of apatite materials and composites incorporating these materials that are useful as prosteses, or coatings for prosthesis, and methods for production of these materials.BACKGROUND OF THE ...

AI Technical Summary

Benefits of technology

[0022]By carefully controlling processing parameters affecting the molecular and structural development of hydroxyapatite such as precursor type, precursor concentration, addition rate of precursors, aging time, reaction and aging temperature, and pH during synthesis, as well as by controlling parameters affecting the agglomeration of ceramic particles such as washing and drying of the as-synthesized gel, a loosely agglomerated nanocrystalline hydroxyapatite powder is obtained. By minimizing particle size, packing and densification is enhanced resulting in the fabrication of densified nanocrystalline hydroxyapatite by using a simple pressureless sintering process at relatively low sintering temperatures. By reducing crysllite size, ceramics become more ductile as the volume fraction of grain boundaries increases allowing grain boundary sliding. Nanostructured hydroxyapatite also allows superplastic net-shaped forming for inexpensive production. Furthermore, by achieving smaller crystalline sizes, defect size is reduced. With minimized flaw sizes, nanocrystalline hydroxyapatite is densified with minimalor no sintering additives at substantially lower temperatures and demonstrates improved strength compared to the conventional polycrystalline hydroxyapatite. Thus, nanocrystalline hydroxyapatite possesses greater reliability and better mechanical properties compared to conventional hydroxyapatite with a coarser microstructure. Additionally, hydroxyapatite can be structurally reinforced by nanocomposite processing such as incorporating nanocrystalline zircona into hydroxyapatite. Additionally, carbonate icons be substituted for phosphate ions in hydroxyapatite to yield carbonate apatite, both Type A and Type B.

[0023]Using wet chemical processing as the basis, synthetic appro

Problems solved by technology

The technique typically requires a traumatic in vivo polymerization reaction within the cup of a hip joint, and the use of a metal hall joint within the cup which can result in stress shielding (described below), causing bone dissolution.

Uneven wear rates between the metal ball joint and the polymer sockets can cause the polymer to disintegrate within the body causing even more rapid dissolution.

As a result, the interface between the metal ball joint and bone often loosens over time causing the patient great discomfort.

The result is that hip joint replacement using current state-of-the-art technology may have to be performed more than once in a patient.

Although many ceramic compositions have been tested as implants to repair various types of the body, few have achieved human clinical application.

Problems associated with ceramic implants typically involve the lack of a stable interface with connective tissue, or a lack of matching of the mechanical behavior of the implant with the tissue to be replaced, or both (L. L. Hench, “Bioceramics: from Concept to Clinic”, J. Am. Ceram. Soc., 74, 1487-1510 (1991)).

In the case of bioinert bioceramic materials, only a mechanical interlock is obtained, and if the mechanical fixation between the surrounding tissue and implant is not strong enough, then loosening of the bioceramic can occur causing necrosis of the surrounding tissue along with total implant failure.

However, if movement occurs, the fibrous capsule surrounding the implant can grow to become several hundred microns thick and the implant can loosen, leading to clinical failure.

Problems long associated with resorbable bioceramics are the maintenance of strength, stability of the interlace, and matching of the resportion rat eto the regeneration rate of the host tissue.

This imposes a sever limitation on these compositions.

One problem assoicated with hard tissue prosthesis, for example, artificial bones or bone portions, is “stress shielding”.

The higher modulus of elasticity of the implant results in its carrying nearly all the load.

That is, stress shielding weakens bone in the region where a load applied to the bone is lowest or in compression.

Though hydroxyapatite is the most common bioceramic, applications forits use have been limited by its processability and architecture design conceptualization.

Conventional processing lacks compositional purity and homogeneity.

Because hydroxyapatite is difficult to sinter, dense hydroxyapatite structures for dental implants and low wear orthopedic applications typically have been obtained by high-temperature and / or high-pressure sintering with glassy sintering aids which frequently induce decomposition to undesirable phases with poor mechanical stability and poor chemical resistance to physiological conditions.

Thus, conventionally-formed hydroxyapatite necessitates expensive processing and compromises structural integrity due to the presence of secondary phases.

Existing mehtods require high forming and machining costs to obtain products with complex shape.

Furthermore, typical conventional hydroxyapatite decomposes above 1250° C. This results in a material with poor mechanical stability and poor chemical resistance.

However, Jarcho, et al. report low volume fraction of pores, and report considerable grain growth during sintering even at firing temperatures of 1000° C. Jarcho, et al. achieved 99% density in some cases, but using a techniquethat can be impractical for forming desired shapes. M. Akao, et al., in “Mechanical Properties of Sintered Hydroxyapatite for Prosthetic Applications”, J. Mater. Sci., 16, 809-812 (1981), report the compressive flexural torsional and dynamic torsional strengths of polycrystalline hydroxyapatite sintered at 1300° C. for three hours and, compare the mechanical properties of the product with those of cortical bone, dentine, and enam

Thus, the creation of new forms of hydroxyaptitic having improved mechanical properties would have significant use, but the results of prior art attempts have been disappointing.

However, synthesis of nanocrystalline or nanocomposite materials is difficult.

Significant effort has been put into such synthesis and it is likely that in many or most attempts particle sizes on the nanometer scale are not recovered due to aggloemration.

While hydroxyapatite is used widely, and a hydroxyapatite formulation having mechanical and morphological properties advantageous for prostheses would be very useful, attempts to date have failed to produce reliable structural hydroxyapatite implants.

Images