Prosthesis (i.e. – artificial body members) – parts thereof – or ai – Implantable prosthesis – Bone
Reissue Patent
1995-09-18
2001-07-10
Isabella, David (Department: 3308)
Prosthesis (i.e., artificial body members), parts thereof, or ai
Implantable prosthesis
Bone
Reissue Patent
active
RE037277
ABSTRACT:
BACKGROUND OF THE INVENTION
This invention relates to implants for total knee arthroplasty, and more particularly, to an improved stemmed tibial prosthetic implant.
Total knee arthroplasty involves providing new articulating surfaces for the tibia, femur, and patella. The most common technique for providing new articulating surfaces for the tibia involves resecting an upper portion of the tibia and then attaching a prosthetic implant to the tibia over the resected surface. The tibial implant includes a base adapted to abut the resected surface of the tibia and an articulating portion which includes the new articulating surfaces. The base of the implant has a shape that is adapted to generally approximate the shape of the resected surface of the tibia so that the implant generally aligns with
and completely covers
the resected surface when the base is properly attached to the tibia.
The implant may also include a plurality of pegs extending from the lower or inferior surface of the base. The pegs are adapted to extend into the bone of the tibia when the implant is secured to the resected surface and provide enhanced torsional stability about the longitudinal axis of the tibia. In addition or alternatively to pegs, a stem may extend from the inferior surface of the base so as to extend a substantial distance into the tibia when the implant is attached over the resected surface. Tibial stems provide further torsional stability and axial strength by providing more surface area for contact with the bone. The stem also provides a “keel” effect to prevent the tibial implant from toggling or wobbling.
The tibia itself comprises an outer layer of hard cortical bone and a central canal of relatively soft cancellous bone. Much of the strength of the tibia is provided by the cortical bone. In a normally shaped tibia, the central canal is offset from the center of the tibial articulating surfaces or the center of the tibial plateau. The stems of prior tibial implants
used for normally shaped tibias
were, however, positioned centrally to the implant base. Although the central location of the stem allowed a particular implant to be used for either the right or left knee, this stem position resulted in serious drawbacks. The primary drawback was that the centrally located stem was substantially offset from the center of the tibial canal itself when the base of the implant was aligned with the resected tibial surface. In fact, stems located centrally to the base occasionally contacted the posterior
and/or lateral
cortical bone of the tibia. This interference with the cortical bone sometimes prevented the base of the implant from seating flush against the resected surface of the tibia, thereby inhibiting adequate initial stability and fixation which is essential for successful knee arthroplasty.
SUMMARY OF THE INVENTION
It is a broad object of the invention to provide a stemmed tibial implant that overcomes the above-described problems and others associated with prior tibial implants
used for patients having normally shaped tibias
.
In order to accomplish this object, a tibial implant
according to the invention
for use in a normally shaped tibia
includes a stem that is offset from the center of the implant base.
The offset is within a particular range for use in a normally shaped tibia.
Similarly to prior tibial implants, the present implant includes a base, an articulating portion, and attachment means for securing the implant to a patient's tibia during the implantation procedure. However, the tibial implant according to the invention also includes a stem extending from a point on the inferior surface of the base that is offset from a center point of the base. This offset enables the stem to extend into the central tibial canal when the implant is attached to the
normally shaped
tibia with the inferior surface of the base abutting and aligning with the resected surface. The offset stem does not extend into or otherwise interfere with the cortical bone of the tibia. Also, optimal keel effect is achieved when the stem is central to the condylar weight bearing surface, providing equally dense bone on either side of the stem.
The optimum stem offset varies from patient to patient. Normally, however, the stem is offset anteriorly or forward of the center point of the base and also medially or to the inside with respect to the center point of the base.
Although the optimum stem offset varies from patient to patient, normally shaped tibias will accept in the central tibial canal a stem that is offset anteriorly or forward of the center point of the base and also medially or to the inside with respect to the center point of the base.
The anterior offset is preferably in the range of approximately 59% to 68% of the total anterior-posterior medial condyle depth dimension as measured from the outermost posterior edge of the base. The medial offset is preferably in the range of approximately 52% to 55% of the total base medial-lateral width dimension, as measured from the outermost lateral edge of the base.
The stem may be cylindrical in shape, square, or any other shape desired for the particular application. The preferred stem, however, has a cruciate transverse cross-sectional shape and comprises a central root portion with four webs extending at different angular orientations from the root portion. Each web extends generally toward a different corner of the base and includes an enlarged portion proximal to the base and a relatively narrow portion at the end of the stem distal to the base.
These and other objects, advantages, and features of the invention will be apparent from the following description of the preferred embodiments, considered along with the accompanying drawings.
REFERENCES:
patent: Re. 29757 (1978-09-01), Helfet
patent: 4205400 (1980-06-01), Shen et al.
patent: 4215439 (1980-08-01), Gold et al.
patent: 5137536 (1992-08-01), Koshino
patent: 0189253 (1986-07-01), None
Drawing of knee implant (Prouty); date unknown (1 page).
Drawing of Intermedics Natural Knee System Device; dated 1987 (1 page).
Drawing of Johnson & Johnson Orthopaedics′ P.F.C.; dated 1990 (2 pages).
Baldwin James L.
Whitlock Steven I.
Culbertson Russell D.
Hayes Medical, Inc.
Isabella David
Shaffer & Culbertson
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