Your questions answered: Understanding knee and hip replacements
If you’re considering a knee or hip replacement, you probably have a few questions. This article answers the most common questions my patients ask – from what the new joint is made of, to how long it can last, and even whether it may set of an airport scanner.
What is my new knee or hip joint made of?
Both knee and hip replacement joints (implants) are made from a combination of strong materials – metals, ceramics and plastics – designed to work well inside the body.
Knee replacement joints are made of metal alloys (like cobalt-chrome or titanium) that attach to the femur (thighbone) and tibia (shinbone). A polyethylene (plastic) sits between the metal parts and acts like cartilage so the joint moves smoothly.
In hip replacements, a titanium stem fits inside the femur (thighbone). A cobalt-chrome or ceramic ball sits on the stem. A polyethylene (or ceramic) liner fits in the hip socket, creating a smooth surface for the ball to move against.
How does the replacement joint stay in place?
There are three main methods to secure a knee or hip implant:
- Cemented – the implant is fixed to the bone with a special bone cement.
- Uncemented (cementless) – the implant has a rough surface that allows your own bone to grow onto it.
- Hybrid – a combination of the cemented and uncemented methods.
What is a cemented implant?
A cemented knee or hip uses polymethyl methacrylate (PMMA) bone cement to “glue” the artificial joint to the bone. This approach is often ideal for older patients or those with poorer bone quality.
Advantages: Immediate stability, so the joint can usually bear weight soon after surgery, with good pain relief and excellent long-term results.
Disadvantages: In some cases, the cement may irritate surrounding tissue or loosen over many years, especially in younger, more active patients.
Cemented fixation is still the most common method in knee replacements with excellent long-term outcomes. Because cement can occasionally lead to small bone changes over time (osteolysis), surgeons sometimes recommend cementless techniques for younger patients.
What is an uncemented (cementless) implant?
Cementless implants rely on bone growth rather than cement. The implant’s surface is specially designed to encourage your bone to grow into it over several months, forming a strong biological bond. Sometimes small screws might be added for extra stability during healing.
Uncemented implants are usually recommended for younger, more active people who have healthy bones and good bone density.
Advantages: Uncemented implants encourage natural bone growth, avoid cement-related wear, offer long-term stability, and preserve bone for potential future surgery.
Disadvantages: Uncemented implants may be less stable right after surgery as it takes time for the bone to grow into the implant. The surgical procedure is also more technical. Implants are more expensive than cemented versions, although shorter operation times and fewer cement-related costs can help balance this out.
You might also notice a little movement or settling (called migration) in the first few months – this is normal and usually stabilises with healing.
What is a hybrid implant?
A hybrid implant combines both methods – for example, one component is cemented for immediate stability while another is cementless to allow long-term bone growth.
Advantages: This approach combines immediate stability with long-term biological fixation and can deliver excellent long-term outcomes.
Disadvantages: The procedure is more complex, and success depends on which components are cemented or cementless.
Hybrid fixation is particularly helpful when some bone areas are weaker (benefiting from cement) and others are strong enough for bone ingrowth.
What does my new joint weigh?
A hip or knee implant typically weighs around 450 grams (just under half a kilogram).
It’s only slightly heavier than the natural bone that gets removed, and most people don’t notice the difference after healing.
What if I have a metal allergy?
Metal allergies are rare but can happen if your body reacts to metals like nickel, cobalt, or chromium in an implant (called metal hypersensitivity).
Symptoms may include:
- Persistent pain or swelling
- Skin rash near the incision
- Stiffness or burning sensations
Your surgeon will first rule out other causes such as infection or loosening. Always tell your surgeon if you have a known metal allergy. Skin patch testing can be done before surgery, or we can choose a hypoallergenic (e.g. gold-coated) implant to avoid the issue altogether. Mild symptoms are often managed with topical steroids or anti-inflammatories; more serious cases may require revision to a hypoallergenic implant.
How long will my replacement joint last?
Most modern implants are designed to last 15 – 20 years or more. Studies show that around 8 in 10 knee replacements and 6 in 10 hip replacements are still working well after 25 years. With proper care and no early complications, we aim for your implant to last 20 years or longer. Longevity depends on your activity level, weight, bone quality, and overall health, but today’s materials and techniques mean implants are lasting longer than ever.
Can I run after a joint replacement?
Running or other high-impact sports aren’t usually recommended. The repeated high-impact forces can wear down the plastic parts of the implant more quickly. There’s also a higher risk of falls, fractures or ligament injuries which could damage the joint.
Remember, joint replacement is designed to relieve pain and restore normal function, not to enable extreme sports. Most people can safely return to low-impact activities such as walking, cycling, golf, or swimming. Moderate-impact sports like doubles tennis or skiing may be possible for some people, depending on your experience and fitness.
It’s best to talk to your surgeon or physiotherapist about what level of activity is right for you.
For more detail, see our article on what activities are allowed after knee and hip replacements.
Will my new joint click?
It’s quite normal for a knee to make clicking sounds in the first few months after surgery. This happens as swelling decreases and tissues adjust to the new joint. A hip replacement is less likely to click because of its ball-and-socket design. Clicking noises are not harmful if they are not painful. If you develop new pain, swelling, changes in the joint’s shape or movement, or a noise that starts suddenly after healing, contact your surgeon.
Will I set off airport metal detectors?
Yes, your implant may trigger a metal detector. Let security staff know in advance and allow extra time for a potential pat-down. Your surgeon can provide a medical letter confirming that you have a metal implant, which can make travel smoother.
Why should I keep a record of my implant details?
It’s a good idea to keep a note of type and manufacturer of your joint. This information is useful for future check-ups or if revision surgery is ever needed.
Moving forward with confidence
Joint replacement surgery can be life-changing – restoring comfort, mobility, and confidence. Understanding your new joint helps you make informed choices and take an active role in your recovery. If you have any concerns or would like to discuss your specific situation, please contact me or my team – we’re here to help.