Prosthesis (i.e. – artificial body members) – parts thereof – or ai – Implantable prosthesis – Bone
Reexamination Certificate
2002-08-20
2004-06-29
Philogene, Pedro (Department: 3732)
Prosthesis (i.e., artificial body members), parts thereof, or ai
Implantable prosthesis
Bone
C623S023250, C623S023260, C623S023120
Reexamination Certificate
active
06755866
ABSTRACT:
TECHNICAL FIELD OF THE INVENTION
The present invention relates generally to the field of orthopaedics, and more particularly, to an implant for use in arthroplasty.
BACKGROUND OF THE INVENTION
The invention relates to implantable articles and methods for manufacturing such articles. More particularly, the invention relates to bone prostheses and processes for manufacturing the same.
There are known to exist many designs for and methods for manufacturing implanted articles, such as bone prostheses. Such bone prostheses include components of artificial joints, such as elbows, hips, knees, and shoulders. An important consideration in the design and manufacture of virtually any implantable bone prosthesis is that the prosthesis has adequate fixation when implanted within the body.
Early designs of implantable articles relied on the use of cements, such as polymethylmethacrylate to anchor the implant. The use of such cements can have some advantages, such as providing a fixation that does not develop free play or does not lead to the erosion of the joining bone faces post-operatively. However, the current trend is to use the cements to a lesser extent because of their tendency to lose adhesive properties over time and the possibility that the cement contributes to wear debris within a joint. Implantable articles, which are not implanted with the use of cements, are implanted by providing a cavity for the implant that matches the geometry of the implant and then press fitting the implantable article into the cavity.
Whether utilizing cement or the press-fitting technique for the implantable articles, a problem has been observed which relates to the proper distribution of stresses within the prosthesis and throughout the surrounding bone. This problem can best be described with references to Wolff's Law. This Law is generally based on the human anatomy of principles of atrophy. According to the principle of atrophy, when a human tissue is under utilized, it will atrophy or deteriorate. Conversely, as human tissue is utilized, it will grow and strengthen. According to Wolff's Law, this phenomenon can describe the conditions on human bone, particularly when that bone is used in conjunction with an implantable prosthesis.
According to Wolff's Law, atrophy is defined as the thinning of the cortex with retention of normal cortical texture. According to Wolff's Law, hypertrophy will occur at the area of highest stress surrounding an implant. The thickening of the cortex is a very desirable event in the post-operative patient. For many implants within a long bone, the location of hypertrophy is often at that distal end of the implant. This is caused by the artificially raised stresses at the point of sudden transition from the flexible distal femur to the artificially stiffened proximal femur. This is true both for press fit and cemented stems. This phenomenon of hypertrophy does result in excellent adhesion in the diaphysis but results in a less than desirable condition between the implant and the long bone in the metaphesis.
If too little stress is applied to the bone, resorption can occur leading to atrophy of the affected area. Too much stress may also lead to resorption in atrophy, or may result in an undesirable atrophy of the affected area. Accordingly, there exists a need for an improved joint prosthesis, that addresses the needs and problems of prior joint designs as it relates to the distribution of stress.
The phenomenon of atrophy resulting from insufficient loading at certain portions of the implant and bone interface has been referred to as stress shielding.
Stress shielding has been addressed by putting a porous coating only on the proximal portion of the joint prosthesis and using a highly refined surface on the distal surface of the prosthesis. The porous coating is utilized to encourage the growth of hard tissue around the implant. The bone attachment usually occurs and growth is promoted when the surface of the implantable bone prosthesis is regular or textured. The interactions of newly formed hard tissue in or around the texture surface of the implantable bone prosthesis has been found to provide a good fixation for the prosthesis within the bone.
The designs where the porous coating is placed only on the proximal portion of the bone of the joint prosthesis attempt to duplicate the natural transmission of the load in the long bone. Such design may be associated with leg pain.
Attempts to reduce stress shielding include various attempts to make the stem more flexible. These efforts have included the clothespin design or central opening in the prosthesis. Another attempt is the use of altering the cross section of the stem of the prosthesis along its length. Another method of making the stem more flexible is the use of alternate materials.
These efforts, which are aimed at making the stem more flexible, are expensive and in each case allow the distal portion of the prosthesis to rub against the inside of the bone canal as the bone flexes. This may be painful to the patient and may induce a natural bone growth near the distal end of the prosthesis as the prosthesis rubs against the inside of the bone canal.
These attempts at making the stem more flexible by making the stem smaller or of a material with less strength may increase the probability that the stem will fracture in the long bone. Further, the use of the more flexible material may require that their size be larger and they may require that a larger amount of bone must be reamed from the bone canal.
The natural anatomy of the long bone is arcuate and curved in the central portion of the long bone. For use in revision surgery and when a fracture has occurred in the long bone, prostheses have been developed which are fitted into the more distal curved portion of the long bone. Such stems have been designed with curved distal portions. It can be difficult to prepare the long bone canal to accept a curved stem and accurate insertion can therefore be challenging. Devices with these features are expensive to produce and a large number of sizes and shapes are required to fit the individual anatomy including the need to obtain the proper ante-version in the patient.
Efforts have been made to reduce the instance of point stress associated with the distal portion of the prosthesis stems. As mentioned earlier, the point stress may lead to leg pain and also such high point stress is located at the distal portion of the prosthesis stem. The distal point stress may lead to fracture resulting from trauma or sub optimal bone quality. Bullet-shaped tips have met with some success but still cause some stress concentration at the distal portion of the implant.
While attempts have been made to improve the physiological loading of the bone to reduce stress shielding, the prior art efforts have met with limited success.
Accordingly, a new prosthesis is needed which improves the physiological loading of the bone and thereby reduces stress shielding type pain, as well as the probability of a stem fracture of the long bone.
SUMMARY OF THE INVENTION
The present invention is an element in the form of a linear bearing that is placed in the canal of the long bone. It is designed to mate with the stem of a prosthetic joint or an intermedullary rod. Typically, stems or rods used with this device would be smaller in diameter than existing orthopedic implants and would not contact the interior cortex of a long bone. Instead, the present invention would allow the stem or rod to guide or slide up and down in a central bushing.
One or more of the linear bearings may be used in conjunction with a single stem or intermedullary rod. The device allows a long bone, for example, a femur, to bend under load without shielding the bone from the normal stress patterns nearly as much as conventional designs of orthopedic joint implants do. The present invention places compressive forces almost exclusively in the proximal portion of the prosthetic joint. The linear bearing may include an articulating feature within the canal of the long bone
Bonderer David A
Depuy Products, Inc.
Philogene Pedro
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