Synthetic resins or natural rubbers -- part of the class 520 ser – Synthetic resins – At least one aryl ring which is part of a fused or bridged...
Reexamination Certificate
1998-09-16
2001-05-15
Medley, Margaret (Department: 1714)
Synthetic resins or natural rubbers -- part of the class 520 ser
Synthetic resins
At least one aryl ring which is part of a fused or bridged...
C524S415000, C525S415000, C525S450000
Reexamination Certificate
active
06232384
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to bone fixation material easily biodegrading and bioabsorbing into the living body organism and methods for preparing the materials. More particularly, the present invention relates to bone fixation materials for bone medical treatment use in plastic surgery, oral surgery, thoracic surgery and in addition animal bone treatment and methods for preparing these materials.
2. Description of the Related Art
Artificial biomaterials having a high bending strength are employed in order to redress bone fractures or support chests for fractured bone treatment in plastic surgery, oral surgery, and thoracic surgery. It is necessary that the bone fixation materials such as plates and screws are applied in the period that the bone is redressing and are removed as soon as possible after healing in order to prevent the bone from weakening.
Nowadays, although the bone fixation materials in clinical use are almost all composed of metals or ceramics, there are problems in which materials cause the bone to degrade, because they have too high of a level modulus, or happen to biodegrade or the metal ions are possibly dissolved into the bone. If materials having similar modulus to the bone, and which biodegrade and bioabsorb into a living organism, are employed in order to redress the bone fracture, various ill effects on the redressing of the bone will disappear because no alien substances take place in the living body over a long time.
According to the above conception and expectations, lactic acid polymers which have the characteristic of biodegrading and bioabsorbing into the living body organism are gradually being employed right now as bone fixation materials in clinical application use. It has been well-known for many years that the lactic acid polymers could be employed as bone fixation materials. For example, Japanese applied patent 59-97654 discloses a method to synthesize a lactic acid polymer, but fails to disclose a lactic acid polymer composite having a high level modulus and high level bending strength.
Japanese patent 3-63901 discloses molding conditions in which a lactic acid polymer composite contains hydroxyapatite and has the characteristic of biodegrading and bioabsorbing into the living body organism improved by uniaxial extension, mechanical properties such as modulus and bending strength useful for bone fixation materials. PCT patent WO/05312 discloses other molding conditions similar to said patent, in which a lactic acid polymer rod was uniaxially extended to a relative high draw ratio (7 to 12 times) at a relatively high temperature and attained a high level bending strength, and a high level modulus by fibrillation. Japan applied patent 1-198553 discloses bone cementing materials, of which lactic acid polymer was composed of a viscosity-average molecular weight of more than 200,000 after molding and uniaxial extension. Furthermore, Japanese applied patent 9-135892 also discloses molding conditions, in which molecules were oriented parallel not to the main, but to the plural axes during molding in order to reduce anisotropy of the bending strengths in a composite.
However, the patents mentioned above only concern initial bending strength and the hydrolyzing durability of bone fixation materials and fail to disclose its bending strength in a living body organism and its durability. There is not enough description concerning biodegradation and bioabsorption into the living body organism after redressing, and especially no description of the method to promote biodegradation and the bioabsorption after the bending strength has left. It takes only about 6 to 8 months to perfectly redress a bone fracture. However, the bending strength of the bone connecting materials characteristically biodegrade and bioabsorb, gradually decreasing after embedding into the body in accordance with the decrease of the molecular weight, and decrease to 0 within 4 months. The molecular weight of the materials reduces from about 200,000 at first to several thousands, to several tens of thousands, while a lot of granular crystallite still remains for 3 to 5 years (J. E. Bergsma et al. Biomaterials, 16(4),267-274,1995). As a result, when the bone connecting materials mentioned above are employed for a material to redress bone fractures, it takes, for example, from 3 to 5 years for the fibrous tissues to perfectly retrieve bone tissue, because the velocity of biodegradation and bioabsorption is too slow. In addition, recreated bones can not invade into and embed with parts of the bone connecting materials even after healing and, therefore, vomicae in the bone remains almost forever. It causes a reduction in mechanical tenacity which bears a load at first. Added to the above problem, many serious late complications can occur after healing (J. E. Bergsma et al. Biomaterials, 16(1), 25-32,1995).
SUMMARY OF THE INVENTION
The present invention proposes a bone fixation material having an initial bending strength at break of more than 250 MPa, maintaining this strength for more than 3 months in a living body organism after which it characteristically biodegrades and bioabsorbs into the organism and disappears within a range of 6 months to 3 years. It is preferred that the material includes a polymer which characteristically biodegrades and bioabsorbs into the living body organism, a hydroxyapatite and an alkaline inorganic compound. It is highly preferred that the chain molecules of the polymer are extended and oriented parallel to the major axis or plural axes, and, it is preferred that the polymer be a lactic acid polymer composed of a L type and D, L type mixture or copolymer of 70 to 100% in a mole percentage of the L type, or a copolymer of a lactide and a glycolide. It is also highly preferred that the composite ratio of the polymer and the hydroxyapatite is in a weight ratio range of 99:1 to 60:40, and the composite ratio of the hydroxyapatite and the alkaline inorganic compound is in a weight percentage range of 99.9:0.01 to 80:20.
Furthermore, the present invention proposes a method of making a bone fixation material which includes: (A) providing a melt which has the characteristic to biodegrade and bioabsorb into the living body organism in itself, and which comprises a polymer with the characteristic of biodegrading and bioabsorbing into the living body organism, a hydroxyapatite and an alkaline inorganic compound, (B) molecularly orienting the melt by applying it, at an extendible and moldable temperature, to a process of extension or molding selected from the group consisting of a uniaxial extension, an extruding extension, a milling and compression molding, and (C) extending chain molecules of the polymer and orienting them parallel to the major axis or plural axes. It is preferred that the extension is a ram extrusion at high pressure or a hydrostatic extrusion at a temperature range of 80 to 160° C.
DETAILED DESCRIPTION OF THE INVENTION
The bone fixation material of the present invention includes a polymer with the characteristic of biodegrading and bioabsorbing into the living body organism, a hydroxyapatite and an alkaline inorganic compound. The polymer is a lactic acid polymer, preferably a L type lactic acid polymer having more than 95% optical purity. However, it is highly preferable that it is in an optical purity range of 80 to 95%, to prevent the polymer from having too high crystallinity. Of course, the crystallinity relates to the initial modulus and bending strength at break and a bone fixation material having high tenacity and modulus is easily composed from a high crystallinity polymer. However, as objective tenacity and modulus are possibly derived from the orientation of the molecular chain in a crystalline state during molding, the crystallinity is preferably lowered in order to better bioabsorb it into and eliminate it from the organism as soon as possible after biodegradation. It is highly preferred in the present invention that the polymer, as a raw material, is a lactic acid polymer compos
BMG Inc.
Jordan and Hamburg LLP
Medley Margaret
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