Knee replacements are a more difficult operation to get right than hip replacements, because the knee is a more complicated structure. Your hip is a ball and socket joint that rotates around one center of rotation. As long as you get that one point right, you have a good hip replacement. A knee bends like a hinge, but it also has subtle motions of sliding, slipping and rotating as it bends. Getting the subtle motions right in the knee replacement is a complicated undertaking.

I’m a fan of robotic knee replacement. We get a CAT scan of your knee and build your knee replacement virtually before the surgery. By taking the hour or two to build the replacement virtually, we have developed a plan for exactly how your knee replacement will be to optimize the movement. During surgery, we use a robotic arm that I operate.

As you may know, a knee replacement is a resurfacing of the bone. We remove a very small amount of surface, maybe a quarter of an inch, on the end of your bones so it is a new surface. Metal components are put on both sides of the joint and between these two metal components is a Teflon plastic spacer.

The robot is used to remove those thin slices of bone from the end of the joints. In removing the bone, there are a multitude of very minute adjustments one has to make. In a successful knee replacement, you’re dealing with millimeters or micromillimeters and the robot allows us to have the precision of half a millimeter and half a degree, which is the difference between a B+ and an A+ knee.

With all knee replacements, we are trying for the difficult goal of a knee that feels like a “normal knee.” How well the knee replacement is done influences how the knee will function long term. How close will it feel to a normal knee? How long will it last? If we get the replacement right, it will feel very close to a normal knee and our expectations would be it would last a lifetime.

Replaced knees wearing out

We used to tell patients to be careful, protect their replaced knees and not overdo it. The idea was that the more wear a knee had the sooner it would wear out, like a tire, but that thinking has started to reverse itself.

Knees have historically worn out for one of two reasons: the plastic liner wearing out or the prosthesis not staying in place. Technology has improved and if the knee is put in correctly now, the liner should last at least 30 years. The issue of the liner wearing out has been resolved, and the prosthetics attaching to the bone is now a greater issue. The traditional technique is to use epoxy cement, which is still used everywhere and I’ll still use it to some degree. There’s also new technology called press-fit where the surface of the prosthesis is 3-D printed to have an architecture that mimics the architecture of the bone itself, and your bone integrates into the prosthesis. After the bone grows into the prosthesis, in theory it will never wear out and never decouple.

So the main modes of failure of knee replacements over a long time have largely been resolved. While not every knee replacement is going to last for 30, 40 or 50 years, looking at longevity of knee replacements there is a failure rate in the first five years and then it levels out. If knee replacements last the first few years they tend to last a long, long time.

Activities with a knee replacement

We used to think that the more you did on the knee replacement, the greater likelihood it would wear out. However, studies are showing that the people who are more active have knee replacements that last longer. While I used to tell patients to take care of their knee replacement, now I tell patients to be reasonable but go for it. If you’re a tennis player, gently wade into the water to play tennis. If you’re a runner, gently wade into the waters of running. For instance, start with a walk-run program and slowly over time increase the distance and the strenuousness with which you do your activity. I don’t think anybody would recommend you play contact sports, but repetitive activities of a significant level of strenuousness like running are allowed.

There are not studies that say we did a hundred knee replacements with level one activity and a hundred knee replacements with level two activity and so on and compare them over the course of 20 years. That data is not out there, so I’m getting out on a limb a little bit when I talk about this. We’re slowly but surely trying to get people back to very high levels of activity if not “absolutely normal” activities. It all comes down to getting the knee replacement right.

Recovery timeline

Recovery from a knee replacement is never a picnic. There are multiple techniques of moving the tissue around during a knee replacement. To the extent that we can, we want to do a minimally invasive procedure. However, at the end of the day, you do need to get into the knee joint and remove the surfaces of the knee. The process will never be atraumatic, but year-by-year we get closer to a recovery process that people find manageable.

For the first few weeks you’re going to be slow and we don’t want you to overdo it. Managing the swelling of your knee is important. At about week three we try to start pushing you. Some people can push it pretty hard and go fast while others move slower but by six weeks, everyone is making good progress. You’ll see recovery month by month, little by little, up to three months, six months and even some up to a year.

Frequently asked questions

Are there upper age restrictions or lower age restrictions on knee replacements?

While we will perform knee replacements for patients under 50, it is rare. We’re a little more aggressive using a partial knee replacement instead of a total knee replacement for patients in their 40s. It’s very common to do knee replacements for patients in their fifties. On the other side, I think my record is a knee replacement on a 93-year-old, and my partner Dr. Shaia gave my mother a knee replacement when she was 87 and she’s doing well. As long as the patient is healthy and we think that they can tolerate the procedure, there really is no age limit.

How do you determine if someone should have a partial replacement or a full replacement?

It’s not an easy question to answer, and there is a lot of nuances to be discussed with your surgeon. In general, if the wear and tear is isolated to one area of your knee and does not encompass the entire knee, you may be a candidate for a partial knee replacement.

With a partial knee replacement, your recovery is surprisingly quick, and a partial knee replacement feels more like a normal knee. The downside of a partial knee replacement is that it won’t necessarily last your lifetime because the rest of your knee that was not replaced may become arthritic and painful.

What kind of anesthesia is used?

Almost everyone receives a nerve block, which numbs the nerves that go to your leg and your knee. Nerve blocks last 12-24 hours and we expect a fairly good response to nerve blocks to control your knee pain.

As well as the nerve block, we inject your knee with numbing medication at the end of the surgery to provide additional pain control.

Finally, you receive either spinal anesthesia or general anesthesia. Both methods have merits. With spinal anesthesia, you wake up clearly; however, it takes some time to wear off and you generally need a catheter for at least a few hours. Which type of anesthesia is right for you is a discussion to have with your surgeon.

After knee replacement, how soon can you go up and down stairs?

The physical therapist will work with you to practice going up and down stairs before you leave the hospital or surgery center after your surgery. You won’t be flying up and down the stairs, but you will be able to do them.

After knee replacement, how soon can you drive?

You’re not going to hurt your knee to drive, but you do need to make sure you can drive safely. If your left knee was operated on, you can probably drive after two or three weeks. If it’s your right knee, it could be three to four weeks. You’ll work with your doctor to determine when it’s safe for you to drive.

Do you cut along the top of the knee or the side of the knee?

The standard approach is the incision down the middle of the knee. During surgery, you need to be able to manage any type of unforeseen problem, and your approach to the knee can play a role in your options. If a surgeon goes into the knee through the midline approach down the center of the knee, it gives the surgeon the ability to handle any problem on the inside, middle or outside of the knee. If you make an incision on the outside of the knee and have to deal with an issue on the inside of the knee, you could have a problem.

Once you go through the skin, the traditional approach is to split the tendon and go into the knee that way. There is also an approach called the subvastus approach where you move the tissue to the side instead of cutting it. I’m a fan of the subvastus approach and use it regularly, although I don’t use it for certain patients who have a higher likelihood of additional complications.

The research says type of approach used makes very little difference in recovery.

We all want to recover from knee surgery as quickly as possible. However, it’s much more important that you have a strong, stable knee that lasts you a lifetime. My recommendation is to find a surgeon with a good reputation who you have a lot of faith in and a good rapport with. At OrthoVirginia, we have dozens of great joint replacement surgeons and I’d let any one of them do a knee replacement on me. Trust in what your surgeon says, and chances are very high that you’re going to have a good outcome.

How do you know if a knee needs to be replaced?

There are two major criteria for a knee replacement. One criteria is if you’re bone-on-bone on the x-ray. The other criteria is if you say that the knee is so significantly compromising your quality of life that you cannot sustain the level of function needed to live a happy, healthy life. The second criteria varies patient-to-patient and is different for everyone. If you tell me that you can’t play tennis anymore and you need your knee replaced, I might suggest other activities like pickleball or swimming. But if you say that you can’t go to Target or you can’t go on trips with your family, that’s what we’re here for.

During surgery, what steps are taken to prevent infection?

The infection rate of the facility is important. All facilities that do surgery have to report infections into a national database. The facilities answer to governing bodies that look into their infection rates. The sterility, cleanliness and hygiene of a facility is overseen from a national regulatory aspect.

We use antibiotics during surgery, to reduce the chances of infection.

There’s a period of two or three days after surgery where the knee doesn’t want to be pushed too hard. You want your incision to heal and the drainage to stop before you start pushing hard. Following the guidelines for when to rest, when to bend your knee, and when to walk will help make sure your knee heals well.

It used to be common for patients to need a unit of blood during a knee replacement, but with modern medications and techniques it’s been years since one of my patients needed additional blood after knee replacement. We’ve gotten better at controlling the problems around knee replacement that may lead to an infection, so the odds of an infection is very low – under one percent in my practice.