Prosthesis (i.e. – artificial body members) – parts thereof – or ai – Implantable prosthesis – Bone
Reexamination Certificate
1998-11-12
2001-12-25
Isabella, David J. (Department: 3738)
Prosthesis (i.e., artificial body members), parts thereof, or ai
Implantable prosthesis
Bone
Reexamination Certificate
active
06332896
ABSTRACT:
BACKGROUND OF THE INVENTION
1. The Field of the Invention
The present invention relates generally to the field of artificial joints. The invention relates more particularly, but not exclusively, to a femoral cementless hip stem prosthesis that provides enhanced bone ingrowth stimulation and subsidable engagement within the femur.
2. The Background Art
It is known in the art to replace the hip joint with an artificial hip stem replacement. Numerous artificial implants are available that can be installed to replace the natural hip joint with an artificial ball and socket combination. A passage called the medullary canal is reamed or bored in the upper end of the femur. A stem or femoral component of an artificial implant is inserted into the reamed portion of the medullary canal in a secure, seated position. A neck member extends outward and away from the stem and terminates in a spherical knob for insertion into the acetabulum of the hip in rotational contact therewith about the three major orthogonal axes.
A hip prosthesis generally includes a collar or support plate disposed upon a proximal portion of the stem. The under surface of the support plate in most hip prostheses is flat and is disposed at an angle to the major direction of load, as shown for example in U.S. Pat. Nos. 5,314,489 (issued on May 24, 1994 to Hoffman et al.) and 4,888,023 (issued on Dec. 19, 1989 to Averill et al.). The planer geometry and angular orientation of prior art collars offer only limited capacity for force transfer. Prevailing hip stem philosophy dictates that the proximal portion of the stem provide the bulk of the force transfer, with the collar providing only minimal bone contact simply to prevent the stem from sinking too deeply into the femur. The thinking is that if the collar becomes fully seated it could prevent the proximal stem portion from fully engaging, resulting in less stress being transferred to the prosthesis/bone interface. Thus, prior art collars are designed to be flat and are configured so as to avoid fully-seated contact with the cortical bone.
The prior art femoral components are characterized by a number of disadvantages. The major load is transferred radially outward from the proximal stem portion in tension, generating “hoop stress” as it is referred to in the art. However, the femur is designed to accept loads from the hip joint in compression and not tension, and the tensile hoop stresses cause thigh pain in the patient. Further, the primary radial contact is less stable and allows for micromotion of the stem, making it difficult for the stem to achieve a position of stability. The micromotion decreases vital bone growth at the contact interface, further inhibiting stabilization. The planer geometry of the collar fails to permit settling of the collar in tandem with settling of the proximal stem portion which further inhibits stabilization. The flat collar actually tends to block settling. The prior art also fails to adequately inhibit osteolysis caused by wear debris introduced into the femoral canal at the site of the collar.
Relatively recent attempts to improve the state of the art include U.S. Pat. No. 4,944,762 (issued on Jul. 31, 1990 to Link et al., referred to herein as the “'762 patent”), which represents an attempt to improve the transfer of forces between the under surface of collar and the resection surface of the femur. However, such attempts are actually designed to prevent settling, as tacitly admitted in the '762 patent in col. 2 at lines 1-5. The '762 patent explains therein that the resection surface should be meticulously shaped to enable secure interlocking with the under surface. However, a secure interlock between the under surface of the collar and the resection surface prevents the prosthesis from settling to a position of stability. The '762 patent thus solves one problem but introduces others.
OBJECTS AND SUMMARY OF THE INVENTION
It is therefore an object of the present invention to provide a proximal prosthetic component for use in the replacement of joints.
It is another object of the invention, in accordance with one aspect thereof, to provide such a prosthetic component that replaces hoop stress with increased compressive force.
It is a further object of the invention to provide such a prosthetic component that is simple in design and manufacture.
It is an additional object of the invention, in accordance with one aspect thereof, to provide such a prosthetic component that enables increased surface area of contact with bone and corresponding reduction in the occurrence of gaps at the bone/prosthesis interface.
It is still another object of the invention, in accordance with one aspect thereof, to provide such a prosthetic component that reduces micromotion and thereby increases bone growth stimulation.
It is yet another object of the invention, in accordance with one aspect thereof, to provide such a prosthetic component which inhibits introduction of wear debris into the femoral canal.
It is a still further object of the invention, in accordance with one aspect thereof, to provide such a prosthetic component that provides enhanced bone ingrowth stimulation.
It is an additional object of the invention, in accordance with one aspect thereof, to provide such a prosthetic component that enables subsidable engagement within the femur.
The above objects and others not specifically recited are realized in a specific illustrative embodiment of a cementless femoral hip stem component. The hip stem component includes an elongate stem, a proximal body, and a collar disposed on the proximal body and extending outward therefrom in a sideways direction to form a ledge. The proximal body defines at least a majority of a frustoconical shape. The ledge formed by the collar includes a tapered undersurface, beneath which extends the proximal body. The tapered undersurface of the ledge and the proximal body extend in different directions and thereby cooperatively define a double-flared contact surface configured to enable subsidable engagement of said double-flared contact surface with the femur at two different rates of subsidence within the femur. An abrupt, male corner is formed by the stem and the proximal body and is sufficiently abrupt to provide enhanced bone ingrowth stimulation.
Additional objects and advantages of the invention will be set forth in the description which follows, and in part will be apparent from the description, or may be learned by the practice of the invention. The objects and advantages of the invention may be realized and obtained by means of the instruments and combinations particularly pointed out in the appended claims.
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Hubbard Barry
Kee Jerry
McTighe Timothy
Mraz Paul
Clayton Howarth & Cannon, P.C.
Isabella David J.
Ortho Development Corporation
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