1. We don't take out your whole knee, just the ends of the bones.
When you look at a knee replacement from the side, you will see a thin layer of metal covering the end of the femur (thigh bone) and a thin metal tray covering the top of the tibia (shinbone). A plastic insert is locked into the tibial tray and has two grooves to accommodate the two bumps of the femur. There may or may not be a plastic button on the underside of the patella (kneecap). So really, it's more like a knee resurfacing, rather than a full replacement. Occasionally, we need to replace the whole knee, but those cases involve massive bone loss, such as with bone tumors, infections or revision surgeries.
2. There are partial knee replacements available.
The knee is composed of three compartments--medial, lateral and patellofemoral (between the kneecap and thigh bone). Arthritis can involve some or all of these areas. If only the patellofemoral joint is involved, there are isolated patellofemoral replacements. These consist of a plastic button for the undersurface of the patella and a metal implant for the groove. If only the medial or lateral compartment is involved, unicompartmental replacements are available. These consist of a metal covering for half of the femur and a plastic tray for half of the tibia. All of the partial knee replacements require good ligamentous balance of the knee. If there is too much malalignment or instability, these implants will fail. In general, partial knee replacements are meant to last about 10 years, and may be viewed as temporizing measures until a total knee is needed.
3. There are "female" knees available.
Anatomic studies using CT scans have shown that the average female knee is different from the male knee. The female knee is narrower from side to side. The patellar groove has a higher quadriceps angle, or Q angle. This is due to the fact that females have wider hips and their quadriceps approach the knee from a greater angle. Lastly, the female knee has a less-pronounced anterior prominence. There are now "gender-specific" implants available on the market. But remember, this represents the average female knee. Some female knees are better fit with "male" implants and vice versa. Remember, we've been putting "male" knees into females for decades, with good results. Also, as of yet, no long-term studies have shown a difference between "regular" and "gender-specific" knee replacements.
4. These are not "bionic" knees.
Knee replacements are meant to allow you to walk, sleep and do your daily activities more comfortably. They can withstand low-impact activities, such as doubles tennis. They are not meant to withstand high-level sports, such as running.. In the end, they are still made of metal (cobalt-chrome or titanium alloys) and plastic (polyethylene). In the lab, they have been proven to withstand millions of cycles during testing. So if you go out and run marathons, you will use up your cycles sooner. Then, you might need a revision of your knee replacement, which is always more difficult to perform than the first one. Under low demand, knee replacements can last for 15 to 20 years and more.
5. In terms of motion, what you started with is usually what you end up with.
The biggest predictor of how much you can bend after surgery is how much you could bend before surgery. People with arthritis develop contractures. Because it hurts to move an arthritic knee, people lose the desire and ability to bend and fully straighten their knees. The surgeon tries to obtain as much motion as possible during the surgery. But with long-standing arthritis, the muscles and ligaments have tightened up over time. Once the joint surfaces are replaced, the tightness can remain, and most people will end up with plus or minus 10 degrees of their preoperative motion. You need 65 degrees of flexion to walk, 70 degrees to climb stairs and 90 degrees to go down stairs.
