Prosthesis (i.e. – artificial body members) – parts thereof – or ai – Implantable prosthesis – Bone
Reexamination Certificate
2001-05-18
2004-02-17
Snow, Bruce (Department: 3738)
Prosthesis (i.e., artificial body members), parts thereof, or ai
Implantable prosthesis
Bone
C623S023560, C623S023610, C623S023630
Reexamination Certificate
active
06692532
ABSTRACT:
SUMMARY OF THE INVENTION
The present invention relates to a composite material which is especially useful to repair bone in joints and other load bearing positions.
Operations to replace defective hip joints are now well-known and are performed routinely in the United Kingdom and western world and total hip arthroplasty (THA) is currently one of the most successful operations performed. The operation involves the removal of the patient's defective hip joint and its complete replacement with a prosthetic joint. The patient's quality of life is generally greatly improved throughout the time period that the prosthetic joint remains functional, and from the Swedish hip register the success rate at ten years post operatively lies at around 90%. Total knee arthroplasty is fast approaching similar levels of success. However, a major disadvantage of the prosthetic joint is its finite lifetime and the ultimate need for replacement of the prosthetic. The limitation on the lifetime of the prosthetic arises when the articulating surfaces become worn and debris from the worn prosthetic attracts a macrophage response. Chemicals released by the macrophages unfortunately tend to cause degradation of the bone around the prosthetic implant site, causing loosening of the prosthetic joint which can be difficult to combat. The success rate of subsequent revision hip surgery is significantly lower than for primary THA.
Various approaches have been made to overcome the bone defect around a hip joint. Examples include the use of a large custom-made prosthesis, but this is an expensive approach and has yielded poor results.
Autograft would provide the best bone for re-incorporation in impaction grafting, but donor site morbidity usually prevents harvesting autograft from an individual at the same time as performing their revision hip surgery. Femoral heads removed at the time of primary hip surgery are a ready, sterile supply of allograft, which is the next most preferable graft type. The immunogenic incompatibility between donor and recipient is not usually a problem and is further attenuated by the act of freezing. The increased demand and the move for centres to perform their own revision operations has prompted many smaller centres to set up facilities to perform their own bone banking of allograft femoral heads at the time of primary hip arthroplasty.
Cadaver allografts have also been used to pack the bone defect and ideally large amounts of bone could be harvested in a clean manner from cadaveric donors and then sterilised. However allografts carry a high risk of disease transmission (eg HIV and CJD) and those from cadavers often fail to incorporate into the host skeleton, proving to be of limited value. Nonetheless, utilisation of allograft bone is increasing as the number of revisions of failed joint arthroplasty rises and techniques for bone replacement gain wider acceptance. Future demands for allograft bone are expected to rise as the number of primary joint arthroplasties performed per annum increases and this will be further exacerbated as operations are offered to younger patients and the population as a whole, live longer. Current estimates place the total number of hip replacements performed world wide as over 800,000 per annum.
There is therefore a large and increasing demand for stores of bone graft.
BRIEF SUMMARY OF THE INVENTION
Bone graft alone, either morcellised or whole, has had some success in replacing lost bone stock. However, limited supply and increasing concerns regarding transmission of pathogens has prompted interest in synthetic materials. There has been an increasing interest in bone substitute materials, although their current use and future role have yet to be defined, together with cost-benefit analysis.
Inert materials with high mechanical strength have been tested clinically. Apatite-wollastonite (A-W) glass ceramic has been used in combination with milled allograft and fibrin glue with some success in revision THA's. Direct bonding between bone and A-W glass ceramic granules was seen histologically. There is however no replacement of the inert material with time to replace lost bonestock, should a subsequent revision be necessary. Interest in bio-active materials has evolved to address this problem. Osteogenic Protein-1 (BMP-7) (Stryker Biotech) is a growth factor in the TGF-&bgr; superfamily and has been shown to stimulate bone producing cells in vitro and in vivo. It may also enhance bone incorporation around implants, whereas Hydroxyapatite (HA) may be an alternative to bone allograft. Pro-Osteon has been investigated as a bone void filler in several studies.
It is an object of the present invention to provide a material suitable for packing bone defects, for example in any fractured or broken bone, including facial bone, the jaw and teeth. The material is especially useful for packing bone defects in load bearing positions such as in primary joint arthroplasties (for example around prosthetic hip or knee joints) whilst simltaneously enabling bone regeneration to occur within the treated defect.
In one aspect, the present invention provides an admixture of biocompatible water-soluble glass (BWSG) particles and morsellised bone particles, wherein the particle size range and particle size distribution is pre-selected to be capable of forming an aggregate. The particle distribution may be selected according to the Fuller curve for maximal packing of particles.
Critically we have found that addition of BWSG, either as a bulking agent in a 50/50 mix by volume or by adding the particle size(s) needed to achieve the Fuller requirements, increases the shear strength of the bone. Desirably at least 10% by volume, more usually 25% or 40% by volume, of BWSG is present in the mixture.
As an example, the admixture may comprise particles of diameter 0.1 mm to 10 mm, preferably 0.2 mm to 8 mm, especially preferably 0.5 mm to 6 mm.
Where the diameters of the majority of the particles fall within the preferred range of 0.5 mm to 6 mm, the following typical particle distribution (which conforms to the Fuller curve) would produce a well-graded mixture:
particles 6.0 mm to 5.0 mm=7.0%
particles 5.0 mm to 4.0 mm=9.0%
particles 4.0 mm to 3.0 mm=11.5%
particles 3.0 mm to 2.5 mm=7.5%
particles 2.5 mm to 2.0 mm=9.0%
particles 2.0 mm to 1.5 mm=11.5%
particles 1.5 mm to 1.0 mm=16.5%
particles 1.0 mm to 0.5 mm=28.0%
The admixture of the present invention exhibits excellent mechanical stability and are able to “cement” the prosthesis into healthy bone tissue in a manner similar to a bridge pile sunk into a gravel aggregate.
Improved results have been obtained where the bone particles are washed before use. We believe that washing removes the “wet slurry” produced due to increased fat and marrow release in finely ground bone particles. The presence of “wet slurry” decreases the shear strength of the composite.
The critical properties of the aggregate formed by the admixture are particle size distribution, angle of internal friction, dilatancy and degree of fluid saturation. The fluid may be any sterile fluid (such as water or a protein solution) but advantageously may contain soluble growth factors able to promote bone repair and/or certain bone stem cells or tissue engineered bone forming cells.
To produce the strongest aggregate (or aggregate most resistant to shear stress), the material of the present invention should have the following characteristics;
1. “Ideal” particle size distribution;
2. Low state of Hydration;
3. Sequential layered impaction of well mixed material;
4. Impacted with a large amount of Joules/Volume; and
5. Rigidly contained (use of Meshes).
“Ideal” particle size distribution refers to a mixture of different particle sizes that produces the strongest aggregate. As explained above, this has been determined by Fuller who mathematically determined the graphical curve (Fuller Curve) of particle distribution that represents the sequence of spheres to fit the “gaps” which if carried to infinitely sm
Gilchrist Thomas
Healy David Michael
Fite Holdings Limited
Miller Cheryl
Nixon & Peabody LLP
Snow Bruce
LandOfFree
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