Surgery

Advances in Total Joint Replacement



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I'm a nurse practitioner. I've worked in orthopedics for almost seven years, and most of that time has been spent working with hip and knee replacement patients. In that time there have been major advances in the technology of total joints (especially in hip replacements) and I'm going to let you know about them. To begin with, I'll talk a little about the basics of total joint replacement, what the surgery involves, and the actual implants that are used. Then I'll talk about some of the improvements that have been made in those implants and the surgery.

First, we'll discuss the basics. Total joint replacement is indicated when the joint is damaged to the point that activities of daily living (ADLs) are impaired by pain or stiffness. The classic phrase used is "bone-on-bone arthritis" but you don't always have to be that bad to need a new joint. The most commonly replaced joints are probably hips, knees, and shoulders. There are replacements for ankles, elbows and finger and toe joints as well. When a joint is replaced, the surgeon removes bone and cartilage on either side of the joint, and fixes the replacement joint in place. This does not mean that the surgeon merely takes a certain number of millimeters of bone, but rather shapes the bone on either side of the joint to accept the implant. For example, in the case of knee replacements, the end of the femur (thigh bone) is cut square and then the front and back corners are mitered off as well, but only 5-10 millimeters are removed. The implant is then (in the case of knees) cemented into place.

Now we'll talk a little about the design of the implants themselves. Total hip replacements were invented in the United Kingdom in the 60's and '70's, and still look very similar today to that original design. A common design looks for all the world like a metal railroad spike with a metal ball welded on at an angle, coupled with a metal cup about the size of half a racquetball (the range is from golf ball to tennis ball) with a plastic liner. The spike fits down inside the top of the femur (after some cutting and drilling), while the metal shell fits up into the acetabulum (hip socket). Some implants need to be cemented into place, and others have a rough or porous coating that allows bone to grow in and fix it into place. Some femoral components are all one piece, and others are put together from several different pieces, allowing the surgeon to customize the fit while the patient is on the table.

Total knees came about later in the 70's. A total knee is made of two main pieces, and a few smaller pieces.. The first is a cap that replaces the end of the femur. Its is made of metal, usually titanium or an alloy of cobalt and chromium. It looks like what the end of the femur would look like if it wasn't damaged by arthritis, and is polished mirror smooth. The other half fits into the top of the tibia or shinbone. It is a kidney-shaped metal plate that usually has a short pin that extends down the shaft of the bone an inch or so. It usually has some wings extending from the sides of the pin to stop the plate from rotating. A plastic piece is snapped into the plate that is the bearing surface. The shiny metal femoral part rubs on the plastic every time the knee bends. Finally there is a plastic button that is cemented onto the inside of the kneecap to help the muscle that straightens the leg to work better. (This part is done almost always in the United States, and much less often in Europe, due to several factors.)

There are other materials used in some other artificial joints, such as silastic rubber or pyrocarbon for finger joints, but the advances I'm going to talk about in the bearing surfaces for total hips and knees, and their designs. First, notice that both total hips and total knees have pieces of metal rubbing against pieces of plastic. Common sense says that the plastic is going to wear away eventually, and it does. The plastic in both cases is UHMWPE, or Ultra-High Molecular Weight PolyEthylene. To give you an idea of what it looks like, Low Density PolyEthylene (LDPE) is used to make gallon milk jugs in the United States, and LDPE or HDPE (High Density polyethylene) is used to make things like shampoo bottles. UHMWPE is much stronger than either. Implant companies have come up with several different methods of strengthening the plastic even more, such as radiation treatments, or special molding with heating and cooling cycles that make the plastic molecules bond together more tightly (called cross-linking). Generally most knees and hips use highly cross-linked polyethylene as standard. Because of these advances, a standard total knee or hip replacement can be expected to last up to 15 years.

But there's more. In the past 5 years, the FDA has approved several new "hard bearing" surfaces for total hips. One uses various forms of ceramics, mostly aluminum oxide (alumina) or zirconium oxide (zirconia) or blends of the two. Remember how a conventional hip has metal rubbing on plastic? Well in the newer ceramic hips, the plastic cup liner has been replaced with a ceramic liner, and the metal ball is replaced with a ceramic ball as well. The ceramic is almost as hard as diamond (yes, people are working on diamond surfaces, but we're not there yet) and both pieces are made of the same stuff. Neither side will wear away first. They are also much smoother than their metal or plastic counterparts, and so friction is much lower as well. European studies for ceramic on ceramic hips suggest that they will last much longer than conventional metal on polyethylene designs, possibly as much as 20 or even 30 years.

Another hard bearing hip is one that uses a metal ball and a metal socket, with no plastic. They are also very long lasting, and have several benefits over both ceramic on ceramic and conventional metal on poly hips. First, there is a simple fact that the smaller the head on a total hip, the more likely it is to dislocate, or come out of joint. Due to limitations in the current technology, ceramic heads cannot be made larger than about 38 millimeters, or about an inch and a half. The natural head of the femur is much larger, often on the order of 50 or even 60 millimeters. Metal heads can be made that large with existing technology. Also, metal is ductile, which means it can be molded or polished. It turns out that metal on metal hips tend to almost self-repair by polishing away imperfections or rough spots as they rub together. Ceramic does not do that, because it is hard and brittle. That brittleness can also be an issue, and there are cases where the ceramic liner has broken, requiring more surgery to replace it. It must be pointed out that metal on metal hips also have disadvantages one being that increased levels of metal atoms have been measured in the blood of metal on metal hip recipients after vigorous activity. The long-term effects of this are not well understood, but many doctors will not put a metal on metal hip into a woman of childbearing years for that reason.

The other big advance in total hips is the resurfacing arthroplasty. Instead of sticking a spike down the inside of the thighbone, just the head of the femur is ground down (with a device like a giant pencil sharpener) and a metal cap is cemented on. The concept is almost the same as capping a chipped tooth. Currently, this system is only approved in the United States for treating the femur side of the hip joint, which then is put back into the natural hip socket. As such, it is not helpful in bad cases of arthritis. There is also a risk of the femur breaking just below the new implant. In Europe, this system is being used with a metal socket being placed in the pelvis as well, creating a metal on metal total hip, with quite good results. Also, in the event of a failure (such as the femoral neck fracture I just mentioned) the system can be converted to a regular metal on metal total hip quite easily.

When it comes to total knee replacements, it seems that more work has gone into the design of the implants rather than the materials. Total knees have benefited greatly from the improvements made in the polyethylene, but there is not much happening with things like ceramic components. This may be because the knee is the largest joint in the body, and subjected to an impact of 5 times your bodyweight every time your heel strikes the ground when you walk. In my case that would be over half a ton! If we take an average of a million steps a year, then each knee would be subjected to a cumulative load of 250,000 tons a year (500,000 steps per leg per year multiplied by ton a step)!

Instead, most time and money has gone into making the implants behave as much like a natural knee as possible. This means things like making the contact area between the top and bottom components as large as possible to spread the load. Another new design allows the knee to rotate a few degrees as well as hinge back and forth. Several companies have variations on this theme. One design has a platform that rotates slightly sitting on the top of tibia, and other use precision machining of femoral component to allow the rotation to occur at certain points in the stride.

To sum things up: total joint replacement has gone through significant changes in just the last 10 years. It is considered one of the most successful elective surgeries there is. Total hips and knees can be expected to last up to 15 years with conventional designs and even longer with some of the newer designs of total hips. If you have joint pain that is limiting your ability to live your life, it might be time to talk to a healthcare professional. You can locate an orthopedic surgeon in your area by searching the Internet for the following terms: "American academy of orthopedic surgeons," or "American association of hip and knee surgeons." There are also lots of information and pictures available on the websites for the implant companies (search for names like "Biomet," "Zimmer," "Howmedica," "Sulzer," and "Stryker.")



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