Questions about joint implants
What are joint replacements made of?
Artificial hip or knee joints consist of alloys of titanium and cobalt chrome. The metal parts that join to the bone are designed to be relatively soft and capable of directly ingrowing or adapting to being cemented to the bone.
Typically alloys of cobalt chrome and polyethylene coupling comprise the bearing (the part that moves inside the artificial joint). Newer bearings consist of ceramic and metal couplings. In such bearings, two ceramic surfaces glide against each other, or two metal surfaces glide against each other. The bearings are engineered to reduce wear as much as possible.
We have had very successful results using trabecular metal, a soft metal that is woven into a spongy mass looks much like a pad used to scour pots and pans. The stiffness of trabecular metal compares to bone, and the porous material allows bone grows to readily ingrow.
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Why do artificial joints wear over time?
All bearings wear with use. During everyday walking, most of us will load our hip and knee joints through at least two million cycles of use per year. People who are athletic or walk more than usual will load their joints even more. Cyclic loading is hard on an artificial joint, even though the joints are engineered to produce extremely low rates of wear.
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Where do the wear particles from the artificial joint go?
All modern orthopaedic bearings produce microscopic wear particles typically localized to the soft tissue envelope around the artificial joint. This layer of tissue, called the synovial layer or the capsule, forms around the prosthetic joint after surgery. Cells in this layer act like a "biological sink" by absorbing and storing the wear particles. Some particles shed into the circulation system and are transported to other organs, such as the spleen, heart, lymph nodes, etc. Based on previous data, we do not know of any adverse effect of the dissemination of wear particles from a normally-functioning joint replacement.
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What kind of implant will be used for my total joint replacement?
For total knee replacements, we typically use the Zimmer brand of knee, called the posterior stabilized total condylar knee replacement. We use all uncemented components that bind directly to bone. We prefer to use a soft, highly porous metal such as tantalum that readily attaches to bone and that is similar to bone in its stiffness. By eliminating the use of bone cement entirely, we avoid the possibility of premature loosening of cemented implants and simplify the surgery. The posterior stabilized total condylar knee replacement has excellent long term data and the design is evolutionary, incorporating all the improvements in total knee design over the past three decades.
For total hip replacements, we typically use the Zimmer Trilogy brand of pelvic component, also called the acetabular component, which represents the socket part of the artificial hip, and a tapered titanium alloy femoral stem. This type of design has withstood the test of time and has superb results, again with simplification of the procedure.
The bearing surface preferred in our practice is highly cross-linked polyethylene. It resists wear and outlasts the remaining lifetime of most patients. It is an evolutionary product that utilizes all we have learned about polyethylene since the first artificial joints were implanted in this country. This bearing material contains a number of geometric options not available with any other bearing material. These options simplify the surgery, and increase the margin of safety and performance of the artificial joint.
If you have a strong preference about a certain type of joint prosthesis, or a certain type of material, please discuss it with us.
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How do the implants work?
Hips:
You have a natural socket called the acetabulum in the pelvis bone. The hip ball normally sits in this socket. By reaming away the damaged bone and cartilage, fresh bone is exposed, and a slightly oversized hemispherical socket made of a sturdy, inert titanium alloy is impacted into your bony socket. Usually this impaction does not require
fixation with screws. Into this titanium shell, a bearing surface of polyethylene, ceramic, or metal is then attached mechanically. Next, the femur (thigh bone) is prepared by removing the diseased or deformed femoral head (the ball of your hip joint). The cavity inside the femur bone is enlarged, and a slightly oversized femoral implant is securely impacted into the femur. Onto this part, an artificial ball is attached. This ball is made of a cobalt-chrome alloy, or ceramic, and it fits perfectly into the bearing earlier fitted inside the artificial socket. Once the ball is placed in the socket, a complex series of decisions must be made with regard to leg lengths, implant stability, muscle tension, and range of movement of the hip joint. After making adjustments, if all looks good, the wound is closed.
Knees: The knee moves in different planes by gliding, rolling, sliding and rotating with every step. The bone at the end of the femur makes up part of the knee joint. This is prepared to match a properly sized femoral component that is impacted on the end of the femur. Likewise, the top part of the shin bone, the tibia, is prepared to fit a properly-sized tibial base plate, usually made of titanium. Then, a polyethylene spacer is attached to the tibia. The components are designed so that the cobalt-chrome femoral piece moves against the polyethylene insert. The thickness of the insert is dictated by the ligaments of the knee, which are preserved during the operation. Knee replacement is probably better referred to as knee resurfacing, because the ends of the diseased joint are resurfaced with metal and plastic, while the supporting ligaments are preserved.
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Can I be allergic to the total knee or hip replacement materials? Such occurrences are exceedingly rare. Most commonly, what is often called an allergy is a missed, subtle infection. True metal allergies are usually well known to the patient ahead of time. In these rare and unusual cases, it is possible to use non-metallic components. For example, one can use an all-polyethylene component for the knee cap, and the tibial tray. For the femoral component, a part made of alumina or zirconia ceramic is used. Such components are usually allowed by the Food and Drug Administration (FDA) on a case-by-case basis only.
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What are ceramics, and what is the advantage of using them in total hips and knees?
Ceramics are synthetic materials, used in industrial applications. They are made from very fine particles of alumina and zirconia. These two types of ceramics are commonly used in orthopaedic surgery applications. Fine particles of either material are pressed together under extreme pressure and heat. The resulting material is very hard, and can be polished to a very smooth surface that has lower friction than any metal that we use in orthopaedic bearings. As a result, when ceramic is used with polyethylene in total knees, or when ceramic is used with ceramic itself as in total hip replacements, the wear rates are very favorable and the long term outlook for the implant is improved.
We have completed a trial of ceramic hips, and these bearings are now available for use by any surgeon in the United States who performs total hip replacements. Ceramic parts have fewer options in terms of size variations and other parameters that the surgeon may find useful. Other bearing surfaces, such as highly cross-linked polyethylene, offer the same superior wear characteristics as ceramics. At the present time, our preferred bearing surface is the highly cross-linked polyethylene on cobalt-chrome, because a number of style and sizing options are available to fit the needs of patients.
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Should I have a ceramic knee for my prosthesis?
The FDA has not released this implant for general use yet. One clinical study is in progress, sponsored by a company called Kinamed. We do not currently enroll patients in this study. It is possible that in the future, ceramic knees will become popular in the United States. Right now, they are used mostly in Europe and Japan. The advantage of placing a ceramic implant instead of a cobalt-chrome implant in total knees is the reduced wear produced by the ceramic.
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