Prosthesis (i.e. – artificial body members) – parts thereof – or ai – Implantable prosthesis – Bone
Reexamination Certificate
1998-09-04
2001-12-04
Milano, Michael J. (Department: 3738)
Prosthesis (i.e., artificial body members), parts thereof, or ai
Implantable prosthesis
Bone
C606S095000
Reexamination Certificate
active
06325830
ABSTRACT:
STATEMENT REGARDING FEDERALLY-SPONSORED RESEARCH
Not Applicable.
FIELD OF THE INVENTION
The present invention relates to a device used in hip arthroplasty, and more particularly to a structure for creating a cement restriction or blockage within a bone.
BACKGROUND OF THE INVENTION
Arthroplasty procedures, such as a total hip replacement, can require the removal of the femoral head and neck, followed by implantation of an artificial hip stem into a reamed portion of the femoral medullary canal. Some hip arthroplasty procedures call for the use of bone cement to secure the hip stem within the medullary canal. For procedures that call for cement, it is generally undesirable to allow the cements to infiltrate the medullary canal to an uncontrolled depth and volume. Therefore, a hip arthroplasty procedure can include the step of placing an obstruction within the medullary canal in an attempt to restrict or block the flow of cement.
Not infrequently, the obstruction is merely a partially hardened or cured ball of cement placed into the canal and held in place by friction fit with the wall of the canal. This makeshift obstruction is easily dislodged by the distal end of the hip stem if the cement ball is not inserted deep enough into the canal. Additionally, the ball of cement is readily displaced when pressurized cement is added to the medullary canal to bind the stem in place. If the cement ball is fractured and/or if it falls beyond a narrow central region of the femur known as the isthmus, the pressurized cement does not properly infiltrate the bone and air pockets or pores are created in the cement. The imperfection laden hardened cement thus provides a poor interlock with the bone and stem and it is susceptible to cracking. Poor mechanical interlock and cement failure causes the stem to loosen. This undesirable occurrence often requires that the joint be replaced in a procedure known as a revision.
Revision surgery and/or procedures requiring a “long” hip stem are especially problematic with an application that calls for pressurized cement. Specifically, the distal end of a revision stem ultimately extends further into the medullary canal than an original “normal” stem because additional bone is cut-away during removal of the original stem in preparation to prepare for implantation of the revision stem, or poor quality bone stock forces a larger stem to be used to secure the stem more distally in the canal to reach better quality bone to achieve implant stability. Whereas the distal end of the original stem may extend to a point before or above the isthmus, and thus above the ball of cement, the distal end of the revision stem may extend beyond the isthmus.
Structures other than cement balls are also known for creating a blockage within a medullary canal. For example,
FIG. 1
illustrates a known device
10
including a tapered body
12
having a first end
14
, a second end
16
, and fins
18
that extend radially from the body. Each fin
18
is resilient and can be flexed toward the first end
14
or the second end
16
of the body
12
as shown in the illustration by dashed lines. Although it is possible to maintain one or more fins
18
in a flexed condition by applying pressure to the fin(s) or placing them in a confined space to elastically deform them, once the pressure is relieved or the device is removed from confinement, the fin(s) will always return to their original position unless they have been plasticily deformed. Thus, the fins
18
and the device
10
can be described as only having a single stable state.
In use, a single stable state device
10
can be well suited to the tasks of creating a blockage within a reamed medullary canal
20
above an isthmus region
22
as shown in FIG.
2
. It will be noted that the fins
18
are deformed different amounts depending on where they are within the tapered medullary canal
20
. The body
12
and the fins
18
can have a thickness such that even when the fins are fully compressed against the body, the device
10
is broader than the isthmus
22
to prevent the device from being readily pushed beyond the isthmus. Thus, in a typical pressurized cement application, the pressurization of the cement does not dislodge the device.
By contrast with an above-the-isthmus application, the device
10
is totally unsited for beyond-the-isthmus applications as shown in FIG.
3
. Specifically, once some of the fins
18
of the device
10
move beyond the isthmus, there is less and less mechanical interlock with the bone and even the application of low pressure causes the plug to be dislodged. Were the device
10
to be deliberately passed beyond the isthmus and then pulled back up into the narrow passage as shown in
FIG. 4
, the flexed fins
18
would urge the device down and away from the isthmus.
SUMMARY OF THE INVENTION
The present invention overcomes the disadvantages of known techniques and devices by providing a cement restrictor that is particularly well suited for revision arthroplasty. An appropriately dimensioned cement restrictor can create a fixed obstruction at any selected point within a long bone, particularly at points beyond the isthmus.
The cement restrictor includes a single or multiple finned body having a first stable state and a second stable state. In the first stable state, the cement restrictor is narrower than in the second stable state. While the cement restrictor is readily transitionable from the first stable state to the second stable state, the transition can be irreversible.
An illustrative embodiment of the cement restrictor includes a body having a first end and a second end. Bistable fins extend radially from the body and are irreversibly movable from a first stable state to a second stable state. The fins are concave with respect to the first end of the body in the first stable state and convex with respect to the first end of the body in the second stable state. The diameter of each fin is larger in the second stable state than in the first stable state.
Other embodiments of inventive cement restrictors are shown that include shape memory material that changes shape or dimension(s) in response to temperature and/or stress.
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Brochure entitledCemented Hip Systems Surgical Technique, Johnson & Johnson Orthopaedics, pp. 1-9, dated 5/96.
Article entitledShape Memory Alloys, by C.M. Wayman, pp. 49-56, reprinted from Materials Research Society MRS Bulletin, vol. 18 No. 4, Apr. 1993.
Mastrorio Brooke W.
Ostiguy Pierre S.
DePuy Orthopaedics, Inc.
Milano Michael J.
Nutter & McClennen & Fish LLP
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