Understanding Knee Replacement

The Operation

What actually gets replaced?

Despite the name, a total knee replacement does not remove your entire knee.

A better way to think about the operation is as a resurfacing of the damaged joint.

The worn cartilage and a small amount of underlying bone are removed from the end of the thighbone, or femur, and the top of the shinbone, or tibia. These surfaces are then covered with artificial components.

The basic parts of a total knee replacement are:

• The femoral component: a metal component that covers the end of the thighbone
• The tibial component: a baseplate that sits on top of the shinbone
• The polyethylene liner: a specialized plastic bearing that sits between the metal components and acts as the new joint surface
• The patellar component: an optional plastic surface that can be placed on the underside of the kneecap

These components work together as you straighten and bend your knee.

The Implants

What is the new knee made of?

Modern knee replacements generally use a combination of metal and highly engineered plastic.

The femoral component

The component on the end of the thighbone is usually made from a durable metal alloy. It is shaped to create smooth surfaces that move against the plastic liner as the knee bends and straightens.

The tibial component

The top of the shinbone is typically covered by a metal baseplate. The polyethylene liner attaches to this component.

The polyethylene liner

The liner is made from specialized medical-grade plastic called polyethylene. You can think of it as one of the new bearing surfaces of the knee.

The metal femoral component moves against this plastic as you bend and straighten your leg.

What happens to the kneecap?

Your natural kneecap, or patella, remains in place.

Depending on the patient, the implant system, and the condition of the joint, the damaged cartilage on the underside of the kneecap may be resurfaced with a plastic component.

Patellar resurfacing is not performed identically in every knee replacement, and there are successful approaches both with and without routine resurfacing.

Does the brand of knee replacement matter?

There are several well-established knee replacement systems. Patients sometimes arrive asking for a particular implant, robot, or brand they have seen advertised.

I think the more useful questions are whether the implant has a good clinical track record, whether it provides the options needed for your anatomy, and whether the surgeon understands how to use that system effectively.

The implant is important, but the name printed on the box is only one part of the operation.

Fixation

How do the implants stay in place?

Knee replacement components can be attached to bone using cemented or cementless fixation.

Cemented fixation

Bone cement creates a strong mechanical connection between the implant and the prepared bone. Cemented knee replacement has a long history of successful use.

Cementless fixation

Cementless implants are designed to achieve secure initial contact with the bone. Their porous surfaces are designed to allow your own bone to grow onto or into the implant over time.

Which is better?

I do not think of cemented and cementless fixation as a competition where one method has to be best for every patient.

Bone quality, age, anatomy, implant design, activity level, and the reconstruction we are trying to achieve can all influence the choice.

Balance & Stability

What happens to the ligaments?

A successful knee replacement is about more than replacing worn cartilage. The knee also has to be stable and balanced as it moves.

The ligaments on the inner and outer sides of the knee help control stability. During surgery, we evaluate the tension of these structures as the knee moves from straight to bent.

What happens to the ACL?

In a standard total knee replacement, the anterior cruciate ligament, or ACL, is generally removed.

Modern knee replacement designs account for this and use the shape of the implants, the polyethylene liner, and the remaining soft tissues to provide stability.

What about the PCL?

The posterior cruciate ligament, or PCL, may be preserved or substituted for depending on the implant design and the needs of the knee.

You may hear terms such as cruciate-retaining, posterior-stabilized, medial-congruent, or more constrained implants. These describe different ways an implant can work with — or substitute for — the knee's natural stabilizing structures.

Why does balancing matter?

An arthritic knee may be bow-legged, knock-kneed, stiff, or have stretched or contracted soft tissues.

During knee replacement, the goal is to restore a stable relationship between the bones and soft tissues throughout the knee's range of motion.

That is why knee replacement is not simply a matter of making bone cuts and putting in components. Alignment, rotation, joint-line position, ligament tension, stability, and motion all have to work together.

What about robotics and navigation?

Computer navigation, robotic systems, and other alignment technologies can provide additional information during knee replacement. They are tools that may help a surgeon measure alignment, component position, or soft-tissue balance.

The technology itself does not replace surgical judgment. Regardless of which tools are used, the goal remains the same: a well-positioned, stable, balanced knee.

What about partial knee replacement?

A partial knee replacement is different from a total knee replacement. Instead of resurfacing the entire joint, only the arthritic compartment is replaced.

Partial replacement can be an excellent option for carefully selected patients whose arthritis is truly isolated to one part of the knee. It is not the right operation when arthritis, deformity, or ligament problems involve more of the joint.

Goals & Expectations

What can a knee replacement realistically improve?

The primary goal of knee replacement is to relieve pain coming from an arthritic joint.

As that pain improves, patients commonly find it easier to walk, stand, use stairs, get up from a chair, exercise, travel, work, and participate in everyday life.

The operation can also improve significant deformity and restore motion that has been lost to arthritis.

Will the knee feel completely normal?

Not necessarily.

A knee replacement is an artificial joint. Even when it is functioning very well, some patients remain aware that the knee has been replaced.

There may be areas of numbness around the incision, occasional clicking from the components, or some residual stiffness. Recovery also takes time, and the knee can continue changing for many months after surgery.

How much motion will I have?

Motion after knee replacement depends on several factors, including how much motion you had before surgery, the severity of your arthritis and deformity, the reconstruction, and your recovery.

One of the most important early goals is regaining the ability to fully straighten the knee while progressively improving bending.

What might knee replacement not fix?

A knee replacement treats pain coming from the arthritic knee joint. It does not automatically correct every source of pain in the leg.

Pain coming from the hip, lower back, nerves, muscles, tendons, or other conditions may remain even after the arthritic knee has been replaced.

This is one reason getting the diagnosis right before surgery matters so much.

How long does a knee replacement last?

Modern knee replacements are designed for long-term use, and many continue functioning well for decades.

No artificial joint can be guaranteed to last forever. Longevity depends on factors such as age, activity, body weight, implant fixation, wear, bone quality, and whether complications develop over time.

Long-term follow-up remains worthwhile even when the knee feels good because wear or loosening can sometimes appear on X-rays before it causes major symptoms.

Understanding Risk

What are the important risks of knee replacement?

Knee replacement is a highly successful operation, but it is still major surgery. No surgeon can make the risk of a complication zero.

Important risks include:

• Infection around the incision or implanted joint
• Blood clots in the leg or, less commonly, clots that travel to the lungs
• Stiffness or difficulty regaining motion
• Persistent pain despite technically successful surgery
• Instability or abnormal movement of the replaced knee
• Fracture around the components
• Nerve or blood-vessel injury
• Wound-healing problems
• Implant loosening or wear over time
• Need for another operation if a major complication or future implant problem develops
• Medical complications involving the heart, lungs, kidneys, or other organ systems

Your individual risk is not determined by a generic list. Medical conditions, smoking, weight, nutrition, diabetes, medications, previous surgery, skin condition, bone quality, and other factors can all change the risk profile.

Part of preparing for knee replacement is identifying the risks we can improve before surgery and making sure the expected benefit of the operation makes sense for you.

Common Questions

Frequently asked questions

Is the entire knee removed?

No. A relatively small amount of bone and the damaged joint surfaces are removed. Most of your femur, tibia, kneecap, muscles, and tendons remain.

Is knee replacement more like resurfacing?

Yes. Although we call it a "total knee replacement," resurfacing is often an easier way to picture what actually happens during the operation.

Does everyone need the kneecap replaced?

Your kneecap itself is not removed. In some knee replacements, its undersurface is resurfaced with a plastic component. Whether this is done depends on the surgeon, implant system, and individual knee.

Do all knee replacements use cement?

No. Cemented fixation has a long history of successful use, while modern cementless implants are designed to allow bone growth onto or into their surfaces. The appropriate method depends on the patient and reconstruction.

Do I need a robot to have a good knee replacement?

No. Robotics and navigation are tools that can provide additional information during surgery. Successful knee replacement still depends on appropriate patient selection, accurate reconstruction, stability, soft-tissue balance, and sound surgical judgment.

Will I be able to kneel after knee replacement?

Many patients are physically able to kneel after recovery, although the front of the knee may feel uncomfortable or different. Numbness around part of the incision is also common and can make kneeling feel unusual.

Will I need another knee replacement someday?

Possibly, but not necessarily. Many modern knee replacements continue to function well for decades. Younger and more active patients have more years in which wear, loosening, infection, fracture, or another problem could eventually require revision surgery.

Can I be active after knee replacement?

Yes. Returning patients to an active life is one of the major goals of the operation. Walking, cycling, swimming, golf, hiking, and many other activities are commonly possible after recovery. Your specific activity goals should be discussed with your surgeon.