Use of hydrogels to fix bone replacements

Details Use of hydrogels to fix bone replacements Use of hydrogels to fix bone replacements

1995-04-27

1997-07-08

Prebilic, Paul B.

Prosthesis (i.e., artificial body members), parts thereof, or ai

Implantable prosthesis

Bone

606 76, 606 63, 623901, A61F 228

Patent

active

056455925

DESCRIPTION:

BRIEF SUMMARY
BACKGROUND OF THE INVENTION

1. Field of the Invention
The present invention relates to the coating of orthopedic replacements such as intramedullary nails, screws, hip replacements, etc., with hydrogels and other biocompatible/biodegradable materials which expand in the presence of liquids. Expansion of the outer coating causes the replacement to be fixed into position once inserted into the intramedullary bone cavity. The same principle applies in the case of similarly coated screws and nails, which are thus able to maintain their mechanical hold where they are inserted in intracortical holes, and improve the function of normal spiral screws. Coating can be done by direct polymerization of the monomer by immersion in the case of coatings made with linear polymers. The degree of pressure exercised after expansion is controlled by the degree of cross-linking of the polymeric network, by the thickness of the coating, and by the presence of organic or inorganic inclusions in the coating.
2. Description of Related Art
Current treatments for bone fractures involve the use of stiff plates which lend mechanical support to the join until the tissues have had time to heal. The plates are fixed to the bone by spiral screws screwed into the bone itself (Amis et al.: Fatigue fracture of a femoral sliding compression screw-plate device after bone union. Biomaterials, Vol. 8, 1987). As the plate is stiffer than the bone, it bears most of the strain inflicted on them, whereby the bone is actually protected from any strain. This protection results in anomalous regrowth of the natural tissue and prevents the early mending of the bone parts because of the formation of callus during the process of reconnection (Szivek et al.: A study of bone remodelling using metal-polymer laminates. J. of Biomedical Materials Research, Vol. 15, 853-865, 1981). Over time, the extreme stiffness of the plate can cause atrophy and osteoporosis.
Another problem arises from the type of fastener used to fix the plate to the bone. These are usually metal screws driven directly into the bone, and often cause local trauma by tearing the tissues holding the screw. This often causes inflammation and further problems connected with the healing of the tear.
The most widely used cements are based on acrylates, e.g., PMMA, in the monomeric or mixed monomeric-polymeric phase which are then polymerized in vivo (M. F. Refojo: Materials for use in the eye, in Polymers in Medicine and Surgery, R. L. Kronenthal et al., eds. Plenum Press, New York, Vol. 8, page 313, 1975). Direct polymerization gives rise to a series of disadvantages essentially linked to the difficulty of controlling the reaction from the outside (W. Petty: Methyl methacrylate concentrations in tissues adjacent to bone cement. J. of Biomedical Materials Res., Vol. 14, 427-434, 1980). The exothermic character of the polymerization reaction causes the formation of hot spots reaching unbearably high temperatures for the surrounding tissues, which consequently become degraded (Kliment et al.: Use of spongy hydron in plastic surgery. J. Biomed. Mater. Res., Vol. 2, 237, 1968). In order to avoid this degradation, the reaction is made as mild as possible, but this in turn decreases the degree of conversion with a consequent increase in the percentage of unreacted products which cannot be eliminated (Willert et al.: Measurements of the quantity of monomer leaching out of acrylic bone cement into the surrounding tissues during the process of polymerization. Biomedical Applications Of Polymers, H. P. Gregor, ed., Plenum Press, 1975). The absorption of unreacted acrylic monomers is a highly toxic phenomenon, and can lead to very serious consequences (Silvestre et al.: Failure of acrylic bone cements under triaxial stresses. J. of Materials Science, Vol. 25, 1050-1057, 1990).
Furthermore, in the last few years, medical research has disclosed that the pathology of articular arthrosis is also occurring in younger patients (S. Spainer: Histology and Pathology of Total Joint Replacement, in Total Joint Replace

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