Prosthesis (i.e. – artificial body members) – parts thereof – or ai – Implantable prosthesis – Bone
Reexamination Certificate
1999-08-24
2002-04-02
Milano, Michael J. (Department: 3738)
Prosthesis (i.e., artificial body members), parts thereof, or ai
Implantable prosthesis
Bone
C623S023560
Reexamination Certificate
active
06364909
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to a method of producing restructured bone, and more particularly to a method causing bone to bond to an implant containing a calcium phosphate component, and a method to control the restructuring of bone through the use of an implant containing calcium phosphate.
BACKGROUND OF THE INVENTION
The need for bone implants, joint replacements, and regrowth of missing or damaged bones is great because of bone damage due to accidents, cancer surgery and genetic defects. The ideal permanent bone implant would be compatible with living tissue and would be able to withstand the stresses typically placed upon bones during normal movement. Such an implant has proved difficult to develop, however.
Most materials when used in vivo cause tissue reactions such as inflammation, the presence of macrophages, a fluid filled capsule, and a fibrous tissue covering. Only a few materials are sufficiently inert in the body to be used for prostheses. Even the bio-inert materials are walled-off by a fibrous capsule. For allergic individuals one or more of the undesirable responses above may occur. The fibrous capsule allows some movement to occur as it develops. This can cause movement that is accommodated by a thicker capsule, which allows even more movement, which creates an even thicker capsule and leads, progressively, to failure.
Another problem with bone implants is their uniform structure that does not match the inherent non-uniformity of bone. This, and the difference in elastic properties between the implant and the bone, can lead to problems of stress distribution in the bone. High local stresses cause bone resorption and low local stresses lead to osteoporosity and weak bone.
Joint replacements are in high demand, especially for hips and knees. Approximately 250,000 total hip replacements are performed in the United States each year. Approximately 25,000 revisions are also performed each year because the failure rate is about 10%, although many surgeons expect a 12 to 15 year prosthesis life. This high failure rate has many causes. The most common is pain and loosening under stress. This is usually aseptic loosening and has several underlying causes. These include bone deterioration at the bone/implant interface and inflammation at the joint capsule as the result of particulate debris from the polymethylmethacrylate (“PMMA”) cement or from wear of polyethylene or metal components. Some prostheses use polycrystalline aluminum oxide as one or more of the articulating components to reduce wear debris. The use of polycrystalline aluminum oxide is very expensive because of the complex shapes in use and the brittle nature of the material. Single crystal aluminum oxide in the form of sapphire or ruby has even less wear. Another cause of failure is the use of PMMA cement to anchor the prosthesis in such a way that the trabeculae of porous bone become filled with cement, which blocks the blood supply needed for bone repair and limits the life of the implant.
The number of bone replacement surgeries is expected to increase because patients are living longer, and the number of older people in the population will increase dramatically in the next two decades. Other joints such as elbows and shoulders also require replacements, which are being conducted at an increasing rate.
Joint replacements that are compatible with native bone tissue would have increased lifetimes and improved functioning. Problems with current hip replacements include the need to remove the ball and stem of the femur to accommodate typical prostheses, and the use of tissue-incompatible materials such as polyethylene, metal, and polymethylmethacrylate cements. Typical methods of replacing joints may also cause problems because of excessive reaming of the acetabulum, which should be avoided because it causes problems in the event of future replacements.
Bone regrowth is often desired when the native bone tissue is missing or is damaged, especially in circumstances in which an implant would not be feasible. In addition to human circumstances this may include repair of hollow bones such as avian bones, or repair of damage to the long bones of some mammalian species such as dogs and cats, which does not normally occur if the length of missing bone is more than 1½ times the external diameter of the bone. Typically bone regrowth is encouraged by removing bone from either the patient or another individual and grafting it in the damaged site, but such bone grafting is complicated, involves multiples surgeries, and if allogenic bone is used there may be problems of infection or rejection of the graft. Bone substitutes have been used, but typically lack tissue compatibility and may produce undesirable foreign body response, especially if they release particulates due to friction or chemical reactions.
Typical materials used for implants, joint replacements, artificial bone grafts, and fixation devices such as bone screws and posts include metals such as titanium, 316-L stainless steel, Al6V4 titanium, and cobalt-chrome alloys, organic materials such as very high molecular weight (VHMW) polyethylene and polymethylmethacrylate, and ceramics such as alumina and zirconia. Although these materials are bioinert, and have minimum solubility in tissue fluids, they all invoke a foreign body response to some degree, and none of them are bonded directly by osseous tissue. Over time, movement of the implant causes the fibrous capsule to thicken, which causes tissue degeneration, leading to more movement and progressive failure of the implant. Other problems with these materials include excessive wear and particularization of metal and organic materials, and the brittleness of typical ceramics.
Attempts to avoid or lessen capsule formation, which is the major cause of implant and replacement failure, include the use of hydroxyapatite coatings, or metal beads or mesh to encourage tissue ingrowth into the implant or other orthopedic device. Joint replacements are often fixed into place with polymethacrylate cement that is in contact with the osseous tissue. Cement is inserted under pressure before the prosthesis component is inserted to ensure that the cement fills the space between the prosthesis and the tissue. The insertion of cement into trabecular bone penetrates and displaces the soft tissue in the trabeculae, effectively shutting off the blood and nutrient avenues for repair of the trabecular walls and leading to progressive tissue degeneration.
Materials that are not inert but are not walled off by a foreign-body capsule have especially desirable tissue response. The only crystalline materials of this nature are calcium phosphates such as hydroxyapatite [Ca
10
(PO
4
)
6
(OH)
2
], fluorapatite, oxyapatite, tricalcium phosphate [Ca
3
(PO
4
)
2
], and calcium pyrophosphate [Ca
2
P
2
O
7
]. The natural mineral in bone is impure hydroxyapatite, which contains water, but ceramics typically have less water due to the high temperature firing processes used to make them. The tissue response for ceramics with calcium-to-phosphorous ratios between 1.0 and 2.0 is known to be suitable for hard tissue.
Calcium phosphate materials are often called osteoconductive, meaning that they stimulate bone growth, as opposed to osteoinductive, which refers to the production of osseous tissue in soft tissue sites. The lack of a fibrous capsule and the ability of bone to bond directly to the calcium phosphates makes them very interesting for prosthesis applications. Tissue response is critical, and if calcium phosphates can be used to achieve a bond between the implant and the native hard tissue, they make long implant life a possibility.
The calcium phosphates are brittle materials. Brittle materials fail as the result of stress concentration resulting from flaws present in the material. Reducing the flaws in manufacturing improves the mechanical strength. Thus it is possible to produce strong calcium phosphates such as tricalcium phosphate and hydroxyapatite. Unfortunately, their bioactivity a
Dickenstein Shapiro Morin & Oshinsky LLP
Iowa State University & Research Foundation, Inc.
Milano Michael J.
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