Prosthesis (i.e. – artificial body members) – parts thereof – or ai – Implantable prosthesis – Bone
Reexamination Certificate
2001-03-22
2003-12-09
Robert, Eduardo C. (Department: 3732)
Prosthesis (i.e., artificial body members), parts thereof, or ai
Implantable prosthesis
Bone
C623S017160
Reexamination Certificate
active
06660038
ABSTRACT:
FIELD OF THE INVENTION
The invention relates to an implant for orthopedic applications. More particularly, the invention is related to skeletal reconstruction cages formed from bone for filling vacancies in bone tissue.
BACKGROUND OF THE INVENTION
Bone grafts have become an important and accepted means for treating bone fractures and defects. In the United States alone, approximately half a million bone grafting procedures are performed annually, directed to a diverse array of medical interventions for complications such as fractures involving bone loss, injuries or other conditions necessitating immobilization by fusion (such as for the spine or joints), and other bone defects that may be present due to trauma, infection, or disease. Bone grafting involves the surgical transplantation of pieces of bone within the body, and generally is effectuated through the use of graft material acquired from a human source. This is primarily due to the limited applicability of xenografts, transplants from another species.
Orthopedic autografts or autogenous grafts involve source bone acquired from the same individual that will receive the transplantation. Thus, this type of transplant moves bony material from one location in a body to another location in the same body, and has the advantage of producing minimal immunological complications. It is not always possible or even desirable to use an autograft. The acquisition of bone material from the body of a patient typically requires a separate operation from the implantation procedure. Furthermore, the removal of material, oftentimes involving the use of healthy material from the pelvic area or ribs, has the tendency to result in additional patient discomfort during rehabilitation, particularly at the location of the material removal. Grafts formed from synthetic material have also been developed, but the difficulty in mimicking the properties of bone limits the efficacy of these implants.
As a result of the challenges posed by autografts and synthetic grafts, many orthopedic procedures alternatively involve the use of allografts, which are bone grafts from other human sources (normally cadavers). The bone grafts, for example, are placed in a host bone and serve as the substructure for supporting new bone tissue growth from the host bone. The grafts are sculpted to assume a shape that is appropriate for insertion at the fracture or defect area, and often require fixation to that area as by screws or pins. Due to the availability of allograft source material, and the widespread acceptance of this material in the medical community, the use of allograft tissues is certain to expand in the field of musculoskeletal surgery.
Various spinal conditions are managed, in part, by the introduction of bone grafts. For example, degeneration in the intervertebral discs of the cervical spine and the joints between the vertebrae can result in abnormal pressure on the spinal cord that must be relieved with surgical intervention. It is known to ease undesirable pressure by surgically removing the degenerated tissue, such as the vertebrae, and replacing the surgically-created void with a bone graft. Other reasons for surgical removal of spinal tissue include disease such as cancer or other trauma. The procedure of removing vertebral bodies and the discs between each vertebra is known as a corpectomy, i.e., a removal of the body. A bone autograft suitable for this purpose is often taken from a patient's pelvis or leg bones. Typically, the graft is in the form of a strut or block of bone, which is shaped to fit into adjoining vertebral bodies to fill the empty space and maintain proper spacing between remaining vertebrae. The strut also preserves proper anatomic orientation, while promoting bony fusion with surroundings for subsequent stability.
Fusion procedures may be performed in the cervical, thoracic or lumbar spine, and following placement of the bone graft, a unicortical locking plate is typically installed over the graft by screwing it into the adjoining vertebral bodies. The plate may enhance stability until bony fusion occurs, as well as prevent dislodgment of the graft.
The frequency of corpectomies has created a demand for improved implant designs as well as novel approaches to forming the implants, such as with allografts. In order to provide such implants, an understanding of the sources of allograft bone and the characteristics of bone is useful.
Different bones of the body such as the femur (thigh), tibia and fibula (leg), humerus (upper arm), radius and ulna (lower arm) have geometries that vary considerably. In addition, the lengths of these bones vary; for example, in an adult the lengths may vary from 47 centimeters (femur) to 26 centimeters (radius). Furthermore, the shape of the cross section of each type of bone varies considerably, as does the shape of any given bone over its length. While a femur has a generally rounded outer shape, a tibia has a generally triangular outer shape. Also, the wall thickness varies in different areas of the cross-section of each bone. Thus, the use of any given bone to produce an implant component may be a function of the bone's dimensions and geometry. Machining of bones, however, may permit the production of implant components with standardized dimensions.
As a collagen-rich and mineralized tissue, bone is composed of about forty percent organic material (mainly collagen), with the remainder being inorganic material (mainly a near-hydroxyapatite composition resembling 3Ca
3
(PO
4
)
2
.Ca(OH)
2
). Structurally, the collagen assumes a fibril formation, with hydroxyapatite crystals disposed along the length of the fibril, and the individual fibrils are disposed parallel to each other forming fibers. Depending on the type of bone, the fibrils are either interwoven, or arranged in lamellae that are disposed perpendicular to each other.
There is little doubt that bone tissues have a complex design, and there are substantial variations in the properties of bone tissues with respect to the type of bone (i.e., leg, arm, vertebra) as well as the overall structure of each type. For example, when tested in the longitudinal direction, leg and arm bones have a modulus of elasticity of about 17 to 19 GPa, while vertebra tissue has a modulus of elasticity of less than 1 GPa. The tensile strength of leg and arm bones varies between about 120 MPa and about 150 MPa, while vertebra have a tensile strength of less than 4 MPa. Notably, the compressive strength of bone varies, with the femur and humerus each having a maximum compressive strength of about 167 MPa and 132 MPa respectively. Again, the vertebra have a far lower compressive strength of no more than about 10 MPa.
With respect to the overall structure of a given bone, the mechanical properties vary throughout the bone. For example, a long bone (leg bone) such as the femur has both compact bone and spongy bone. Cortical bone, the compact and dense bone that surrounds the marrow cavity, is generally solid and thus carries the majority of the load in major bones. Cancellous bone, the spongy inner bone, is generally porous and ductile, and when compared to cortical bone is only about one-third to one-quarter as dense, one-tenth to one-twentieth as stiff, but five times as ductile. While cancellous bone has a tensile strength of about 10-20 MPa and a density of about 0.7, cortical bone has a tensile strength of about 100-200 MPa and a density of about 2. Additionally, the strain to failure of cancellous bone is about 5-7%, while cortical bone can only withstand 1-3% strain before failure. It should also be noted that these mechanical characteristics may degrade as a result of numerous factors such as any chemical treatment applied to the bone material, and the manner of storage after removal but prior to implantation (i.e. drying of the bone).
Notably, implants of cancellous bone incorporate more readily with the surrounding host bone, due to the superior osteoconductive nature of cancellous bone as compared to cortical bone. Furthermore, cancellous bone from diff
Angelucci Christopher M.
Boyer, II Michael L.
Higgins Thomas B.
Paul David C.
Pennie & Edmonds LLP
Robert Eduardo C.
Synthes (USA)
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