Prosthesis (i.e. – artificial body members) – parts thereof – or ai – Implantable prosthesis – Bone
Reexamination Certificate
1996-10-24
2001-04-17
Isabella, David J. (Department: 3738)
Prosthesis (i.e., artificial body members), parts thereof, or ai
Implantable prosthesis
Bone
Reexamination Certificate
active
06217617
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Technical Field
The present invention relates to an implant and to a method of securing an implant to a bone. More particularly, the present invention relates to an implant for securing to bone with bone cement, and to a method of securing an implant to a bone with bone cement.
2. Description of the Prior Art
Implants are often secured to bone with a material such as polymethylmethacrylate (PMMA), hereinafter referred to as “bone cement” or simply “cement”. The cement bonds between a surface of the implant and a surface of the bone to secure the implant to the bone.
Often the bone cement is applied as a slab, for example between adjoining flat surfaces of a patellar implant and a patella. There may be defects in the cement such as an air bubble, an impurity or unreacted PMMA powder. It is known for slabs of bone cement to crack. initiates at the site of a defect. Or, a slab of bone cement could fail and crack from other causes such as stress or trauma. If bone cement is present in a continuous mass such as a slab, a crack propagates through the slab, resulting in complete failure of the cement system.
Another concern with bone cement is the maximum force (such as torque) which can be transferred, through the cement, between the implant and the bone. In the case of a slab, the maximum torque is related to the adhesive interfacial shear stress between the cement and the implant.
SUMMARY OF THE INVENTION
It is desirable to be able to minimize propagation through the bone cement of any cracks which may unavoidably occur. To this end, in accordance with the present invention, compartments are formed in the face of the implant adjacent the bone, and in the face of the bone. The two sets of compartments are aligned to form cement cells extending between the implant and the bone when the implant is secured to the bone. The cement cells receive cement therein to form plugs of cement extending between the implant and the bone to secure the implant to the bone.
In one embodiment the present invention is an implant for securing to a bone to form a plurality of cement cells. The implant has a bone engagement portion for engagement with the bone. The implant has surfaces defining a first plurality of cement compartments in the bone engagement portion for alignment with a second plurality of cement compartments in the bone to form a plurality of cement cells extending between the implant and the bone when the implant is secured to the bone. The cement cells receive cement therein to form plugs of cement extending between the implant and the bone to secure the implant to the bone.
The implant may include one or more fill ports for supplying cement to one or more of the cement compartments. The implant may include means for allowing excess cement to flow from the cement compartments away from the bone engagement portion, such as exit channels extending from the cement compartments in a direction away from the bone engagement portion. The implant may include ridge portions around the cement compartments in the implant to block flow of cement out of the cement compartments and between the implant and the bone. There may be respective alignment markers on the bone and on the implant for aligning the cement compartments in the implant with the cement compartments in the bone.
The bone engagement portion of the implant is preferably in abutting engagement with the bone so that there is not a continuous layer of cement between the implant and the bone across the full extent of the interface between the implant and the bone. This discontinuity in the layer of bone cement inhibits crack propagation.
In another embodiment, the present invention is a combination including an implant having a bone engagement portion for engagement with a bone and a first plurality of cement compartments in the bone engagement portion; a bone having a second plurality of cement compartments; the cement compartments in the implant being aligned with the cement compartments in the bone to form cement cells extending between the implant and the bone; and plugs of cement in the cement cells extending between the implant and the bone and securing the implant to the bone.
In another embodiment, the present invention is a method of securing an implant to bone comprising the following steps: providing an implant having a bone engagement portion for engagement with the bone and having surfaces defining a first plurality of cement compartments in the bone engagement portion; making a second plurality of cement compartments in the bone; placing the bone engagement portion of the implant adjacent the bone; aligning the cement compartments in the implant with the cement compartments in the bone to form cement cells extending between the implant and the bone; and placing cement in the cement cells to form plugs of cement extending between the implant and the bone to secure the implant to the bone. Preferably, the method includes blocking cement from entering between the implant and the bone sufficiently to ensure that there is not a continuous layer of cement between the implant and the bone over the full extent of the interface between the implant and the bone.
The cement compartments in the bone may be formed by drilling, broaching, or pressing into the bone. The cement compartments are preferably cylindrical because that shape can easily be made with a standard drill. However, other shapes are possible.
With a compartmentalized system, the strength of the implant/bone interface is governed by not only the adhesive shear stress but also the pure shear stress of the plugs of bone cement. The interface strength is enhanced by the pure shear strength of the plugs of bone cement. For an aligned compartment system to fail in shear the cement plugs must fail in shear which requires substantially more stress than that needed to cause failure of a slab.
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Isabella David J.
Tarolli, Sundheim, Covell Tummino & Szabo L.L.P.
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