Prosthesis (i.e. – artificial body members) – parts thereof – or ai – Implantable prosthesis – Bone
Reexamination Certificate
1999-06-21
2001-08-14
Mancene, Gene (Department: 3732)
Prosthesis (i.e., artificial body members), parts thereof, or ai
Implantable prosthesis
Bone
C623S023110, C623S023150
Reexamination Certificate
active
06273915
ABSTRACT:
BACKGROUND OF THE INVENTION
This invention relates generally to femoral head-neck prostheses and methods for their implantation.
Total hip replacement became a clinical reality for the first time in November, 1963, because of “cement fixation” of the components. The femoral head and neck were removed, the upper marrow canal of the femur was cleaned out (i.e., marrow contents removed), acrylic cement was poured into the marrow canal of the femur, and the metal femoral component was inserted into the liquid cement. In 10 to 15 minutes, the acrylic (methylmethacrylate) cement hardened and provided fixation for the femoral stem. The acrylic cement is similar to the acrylic dentists use to make dentures. Today, cement fixation of femoral components is still the most common means of fixation of the implant to the bone.
Cement fixation is the ultimate in form-filling contact with the bone. The liquid cement touches the entire inner surface of the upper femur. This type of fixation is generally successful for the short term (ten years), however in the long run, deterioration of the bone occurs and the cement and femoral component may loosen. Bone loss is caused by the cement and implant splinting the upper femur, preventing the upper femur from being subjected to natural bending. This is particularly a problem with younger patients (i.e., less than 50 years old).
Non-cemented or “press-fit” femoral components basically try to do the same thing as cemented implants; achieve solid fixation between the implant and the bone by maximally filling the medullary canal with the metal implant. In other words, the thinking is that the more closely and completely the metal implant fills the medullary canal, the better the fixation will be, and the more successful the result will be. However, experience shows this is not always the case.
The non-cemented femoral stems are larger and thicker than their cemented counterparts because the more flexible layer of acrylic cement is replaced with metal. Because a non-cemented stem is made of the same material and has a greater diameter than the cemented stem, it is stiffer. The greater the stiffness, the worse the splinting of the upper femur from the normal bending deflection that occurs in walking (strain). Although acceptable clinical results are achieved with non-cemented intramedullary femoral stems, non-cemented stems enerally have a more rapid rate of bone loss in the upper femur due to strain deprivation or what is commonly but incorrectly referred to as “stress shielding.”
In summary, the fixation of all conventional intramedullary total hip femoral components depends on maximally filling the upper femur and the medullary canal with either cement and metal or metal alone. Not coincidentally, bone loss occurs with all of these implants.
SUMMARY OF THE INVENTION
Among the several objects and features of the present invention may be noted the provision of a femoral head-neck prosthesis which protects the femur from bone loss; the provision of such a prosthesis which provides stable seat between the prosthesis and the neck of the femur; the provision of such a prosthesis which accommodates compression at its interface with the upper femur; the provision of such a prosthesis which inhibits splinting of the upper femur; the provision of such a prosthesis which inhibits total axial fixation of the prosthesis below the seat; the provision of such a prosthesis which receives loads from the hip almost completely in compression; and the provision of such a prosthesis which has a longer useful life.
Further among the several objects and features of the present invention may be noted the provision of a method for implanting a femoral head-neck prosthesis which considers the historical loading of the femur; the provision of such a method which results in loads applied to the prosthesis being transmitted in a substantially natural way to the femur; the provision of such a method which permits a stable interface between a collar of the prosthesis and the femur neck for transmission of loads to the neck and upper femur; the provision of such a method which inhibits total axial fixation of the prosthesis; the provision of such a method which substantially reduces bending moments on the prosthesis as implanted; and the provision of such a method which causes the prosthesis to be loaded almost completely in compression.
Generally, a femoral prosthesis for implantation in a femur comprises a neck adapted to receive a prosthetic head thereon, a collar on which the neck is mounted and a stem extending from the collar on the opposite side of the collar from the neck. The prosthesis has a longitudinal axis corresponding to the longitudinal axis of the stem. The stem is constructed and arranged to fix the prosthesis from movement about its longitudinal axis and about axes perpendicular to the longitudinal axis, and to inhibit axial fixation of the prosthesis upon implantation in the femur, thereby to achieve substantially natural loading of the upper femur.
Another aspect of the present invention is a method for implanting a non-cemented femoral head-neck prosthesis in a femur, the femur having a shaft and a neck at the upper end of the shaft at the medial side of the femur. Generally, the method includes the steps of determining the axis of the medial trabecular stream of the femur, and cutting the neck of the femur to form a seat on the femur neck. A first bore is drilled through the shaft of the femur to extend from the neck of the femur down toward the lateral side of the femur along a line substantially parallel to the axis of the medial trabecular stream. A second bore is drilled through the shaft of the femur to extend from the neck of the femur down toward the lateral side of the femur along a line substantially parallel to the axis of the medial trabecular stream but spaced from the line of the first bore. A stem of the prosthesis is inserted in one of the first and second bores extending through the shaft to the lateral side of the femur, with a portion of the stem being received in the other of the first and second bores.
A fundamental aspect of this device is the stem being implanted in line with the normal loading trajectory of each individual hip in accordance with my prior U.S. Pat. No. 4,998,937. With the stem implanted in this orientation, the main forces on the implant will be end-on (i.e., in compression). In other words, with each step, the 500 pounds of force that a 150 pound man generates at the hip with normal walking is directed along the axis of the implant. The goal is to have the femoral neck receive 100% of the load (joint reaction force) through the collar. Force on the ball of the implant forces the collar against the resected femoral neck. The goal is to have the collar transmit all the load to the femoral neck so that the bone will receive 100% of the normal strain (bending).
Because so much load is transmitted through the collar, it is also important that the collar-bone interface be stable. In my previous patented (stem/barrel/plate) design, the barrel/sideplate component stabilized the stem and collar. The barrel prevented the stem from toggling (forces which move the ball front to back or side to side) or rotating. The collar/stem component was free to be dynamically compressed against the femoral neck because the stem in the barrel offered little or no resistance to axial movement relative to the femur.
In my new invention, the upper femur takes the place of the barrel/sideplate in the prevention of toggling and rotation of the implant. The implant is constructed to mate with the machined upper femur so that toggle and rotation are controlled, but compression is permitted. The thick proximal stem makes contact with the inside of the femoral neck to prevent toggle and rotation. After cutting the femoral neck and removing all of the marrow contents, if you sight along the axis of loading, you will see a cavity in the femur which is generally oval in cross section. This oval cross-section can be substantially filled with two overlapping
Mancene Gene
Priddy Michael B.
Senniger Powers Leavitt & Roedel
LandOfFree
Femoral head-neck prosthesis and method of implantation does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Femoral head-neck prosthesis and method of implantation, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Femoral head-neck prosthesis and method of implantation will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-2466505