The implant itself is metal and plastic, sized to match your anatomy

A knee replacement implant has three main parts: a metal component that replaces the end of your thighbone, a plastic spacer that sits between the bones, and a metal or plastic component that replaces the top of your shinbone. The surgeon cements or press-fits these pieces into place during surgery. The implant is designed to mimic the shape and movement of a healthy knee joint, though the range of motion is usually slightly less than your original knee had.

The metal used is typically a cobalt-chromium alloy or titanium, chosen because they resist wear and don't trigger immune reactions in your body. The plastic spacer is usually made of ultra-high-molecular-weight polyethylene, a durable plastic that can handle decades of walking, climbing stairs, and bending. Some surgeons also place a patellar component—a plastic button—on the back of your kneecap, though not all knee replacements include this piece.

The implant is roughly the size of your actual knee joint, but the exact dimensions vary. Your surgeon will choose from several sizes during surgery based on measurements of your bone and the shape of your knee. A knee replacement typically weighs between 300 and 400 grams (about 11 to 14 ounces).

Key Takeaways

  • A knee replacement implant consists of metal components for the thighbone and shinbone, plus a plastic spacer between them, all cemented or press-fit into your bone.
  • The surgical scar runs down the front of your knee and is typically 8 to 10 inches long, though it fades significantly over 12 to 18 months.
  • Your replaced knee will bend and straighten, but usually not quite as far as your original knee did, and you may feel or hear a clicking sensation during movement.
  • The implant is designed to last 15 to 20 years with normal use, though some last longer and some may need revision surgery sooner.
  • You will set off metal detectors and may need to carry an implant card when traveling, since the metal components are permanent.

The surgical scar and what it looks like as it heals

The surgeon makes a single incision down the front of your knee, typically 8 to 10 inches long, running from just above your kneecap to just below it. This incision is necessary to access the joint and remove the damaged bone and cartilage. The scar will be red and raised for the first few weeks, then gradually flatten and fade over 12 to 18 months. After a year, most scars are pale and thin, though they rarely disappear completely.

The appearance of the scar depends partly on how your skin heals and partly on how well you care for the incision during recovery. Keeping the wound clean and dry, avoiding sun exposure to the scar, and following your surgeon's instructions about when to start gentle massage can all influence how the scar looks long-term. Some people's scars remain more visible than others, regardless of care—this is largely determined by genetics and skin type.

You will have stitches or staples removed about two weeks after surgery. Until then, the incision site will be covered with a dressing that your surgical team will change according to their protocol. Once the stitches are out, you can usually shower normally, though you may need to keep the scar dry for a few more days.

How your knee moves and feels after replacement

A replaced knee bends and straightens like a normal knee, but the motion is usually slightly more limited than before. Most people regain the ability to bend their knee to about 110 to 120 degrees, which is enough for walking, climbing stairs, and sitting in a chair. Some people achieve greater bending with physical therapy, and some never regain quite as much motion as they had before the arthritis developed.

You may feel or hear clicking, popping, or creaking in your replaced knee during movement. This is normal and does not mean something is wrong. The sounds come from the way the plastic and metal surfaces interact, and they typically do not cause pain. Some people notice the sensation more than others, and it often becomes less noticeable over time as you adjust to the implant.

The replaced knee will feel different from your natural knee in subtle ways. You may notice that it does not feel quite as "alive" or responsive as your other knee, or that it takes slightly longer to respond to your brain's signals. This sensation usually improves over several months as your nervous system adapts to the new joint. Most people stop noticing the difference within a year.

Strength and stability in the replaced knee

when ready after surgery, your replaced knee will be weak and unstable because the muscles around it have been affected by surgery and immobility. Physical therapy is essential to rebuild strength in the quadriceps (the muscle on the front of your thigh) and the hamstrings (the muscles on the back of your thigh). These muscles are what hold your knee stable during walking and other activities.

Most people notice significant improvement in strength and stability within 6 to 12 weeks of starting physical therapy. By three to six months, many people can walk without a cane or crutches and can climb stairs with a normal gait. Full strength recovery typically takes six to twelve months, though some people continue to gain strength for up to two years after surgery.

The replaced knee itself is mechanically stable because the implant is anchored to your bone. However, the soft tissues around the knee—the ligaments, tendons, and muscles—are what give you the sensation of stability and control. If you had ligament damage before surgery, your surgeon may have addressed it during the replacement procedure, but some instability may remain if the ligaments were severely compromised.

What you can and cannot do with a replaced knee

Most people with a knee replacement can walk, climb stairs, swim, cycle, golf, and do low-impact exercise like yoga or water aerobics. These activities do not damage the implant and are generally encouraged as part of long-term joint health. Many surgeons recommend staying active to maintain strength and flexibility.

High-impact activities like running, jumping, contact sports, and heavy labor are typically discouraged because they can accelerate wear on the plastic spacer and potentially loosen the implant. Some people do run or play sports after knee replacement, but this increases the risk that the implant will need revision surgery sooner. Your surgeon will give you specific guidance based on your age, bone quality, and the type of implant used.

You should avoid kneeling on the replaced knee if possible, especially in the first year after surgery, because pressure on the front of the knee can irritate the scar tissue and the patellar component. Many people find they can kneel comfortably again after 12 to 18 months, though some never feel comfortable doing so.

How long a knee replacement lasts

A knee replacement implant is designed to last 15 to 20 years with normal use. Some implants last longer—studies show that 80 to 90 percent of knee replacements are still functioning well after 20 years. However, some implants fail sooner due to infection, loosening, wear, or fracture of the bone around the implant.

The lifespan of your implant depends on several factors: your age at the time of surgery, your weight, your activity level, the quality of your bone, and the specific design of the implant. Younger, more active people tend to wear out implants faster than older, less active people. If you had your knee replaced in your 50s, you may need revision surgery (a second surgery to replace the worn-out implant) in your 70s or 80s. If you had it done in your 70s, the implant may last for the rest of your life.

Revision surgery is more complex than the original replacement because the surgeon must remove the old implant and the bone that has grown around it, then place a new implant. Recovery from revision surgery is typically longer and more difficult than recovery from the first replacement.

Metal detectors and traveling with a knee replacement

Your knee replacement will set off metal detectors at airports, courthouses, and other security checkpoints because the metal components are permanent and cannot be removed. You should inform the security officer that you have a knee replacement before you go through the detector. Most security personnel are familiar with this and will either wave you through or do a manual pat-down instead.

You can request an implant card from your surgeon's office after surgery. This card identifies you as having a knee replacement and can be shown to security personnel. Some surgeons provide this automatically; others only give it if you ask. The card is not required, but it can speed up the security process if you travel frequently.

Metal detectors used by handheld security wands will also react to your implant. Again, informing the officer beforehand prevents confusion. The metal in your knee replacement is not affected by metal detectors and will not be damaged by them.

Frequently Asked Questions

Will I be able to see the implant under my skin?

No. The implant is buried deep inside your knee joint, surrounded by bone, muscle, and soft tissue. You will not see it or feel it as a lump under your skin. You will only see the surgical scar on the front of your knee.

Can I get an MRI after knee replacement?

Most modern knee replacements are MRI-safe, but you must tell the MRI technician that you have an implant before the scan. Some older implants or certain components may not be safe in an MRI machine. Your surgeon can tell you whether your specific implant is MRI-compatible, and this information is usually on your implant card.

What does it feel like to walk on a replaced knee?

In the first few weeks after surgery, walking feels stiff and painful. Over several months, as swelling decreases and strength returns, walking becomes easier and more natural. Most people report that by six months, walking feels nearly normal, though some always notice a slight difference compared to their other knee.

Will my replaced knee set off alarms at stores?

No. The security gates at store entrances use a different type of sensor than airport metal detectors and will not react to your knee replacement. You can shop and move through retail spaces without any issues.

Can the implant break or crack?

The metal components are very strong and rarely break under normal use. The plastic spacer can crack or wear through, but this usually happens gradually over many years rather than suddenly. If the plastic wears through, you would typically need revision surgery to replace the worn component.