The risks, with approximate frequency
| Risk | Approximate frequency |
|---|---|
| Deep infection requiring further surgery | ~1.4 to 1.6% in the first 2 years |
| Blood clot, DVT or pulmonary embolism | ~1 in 100 combined, symptomatic |
| Stiffness requiring manipulation under anaesthesia | ~2 to 5% |
| Nerve injury | Uncommon |
| Fracture during or after surgery | <1 in 100 |
| Persistent dissatisfaction despite a sound operation | ~10 to 15% |
| Revision within 10 years | ~5% |
| Revision within 15 years | ~9% |
| Revision within 20 years | ~14 to 15% |
| Death within 30 days | Very rare |
What each of these actually is
Deep infection. One of the most serious common complications. It usually requires further surgery, sometimes including removal and later replacement of the implant, and a prolonged course of antibiotics. Superficial wound infections, which are more common and less serious, are separate and usually resolve with antibiotics alone.
Blood clot. A clot can form in the leg (deep vein thrombosis) or travel to the lung (pulmonary embolism). This is why blood-thinning medication and early mobilisation are standard after surgery, both measures exist specifically to reduce this risk.
Stiffness and manipulation under anaesthesia. Some knees become stiff after surgery, meaning the bend does not improve as expected with physiotherapy alone. When this happens, a manipulation under anaesthesia (MUA), a short procedure to gently bend the knee while you are asleep, may be offered to restore movement, most often in the first few weeks to months after surgery. Risk is higher in patients with limited preoperative bend and in those who develop scar tissue around the joint.
Fracture. The bone can crack during the operation, particularly when fitting an implant into bone that is thinner or more brittle than average, or afterwards from a fall. Risk is higher with osteoporosis and increases with age.
Nerve injury. A nerve near the knee can be stretched or bruised during surgery, most often producing numbness or weakness that improves over weeks to months. Permanent nerve injury is uncommon.
Bleeding requiring transfusion. Uncommon with current practice, including the routine use of medication that reduces surgical bleeding.
Wound healing problems. More likely with higher BMI, diabetes, or smoking.
Heterotopic ossification. Bone occasionally forms in the soft tissue around the joint. Usually without symptoms; occasionally it limits movement.
Anaesthetic and medical complications. Cardiac, respiratory or other medical complications relate more to your general health than to the knee operation itself, and your anaesthetist will discuss these with you separately based on your individual health.
Postoperative delirium. A temporary state of confusion, more common in older patients, particularly after a general anaesthetic or in unfamiliar surroundings. It is distressing for families to witness and is rarely discussed beforehand. It usually resolves within days.
Death. Very rare. A complete account of risk should include it rather than omit it.
What increases your individual risk
Your own risk is not the average. Some factors can be changed before surgery; others cannot.
Can be modified
- Body weight
- Smoking status
- Diabetes control
- Anaemia
- Dental health, an active dental infection is a genuine infection risk and is usually addressed before surgery
- General fitness
Cannot be modified
- Age
- Prior knee surgery
- Inflammatory arthritis
- Immunosuppression
- Bone quality
This is not an abstract list. Getting some of these into better shape before surgery, where there is time to do so, genuinely changes your risk profile, and is covered in the preparation phase articles that follow.
Timing, if you are already considering an injection
If a knee replacement is likely within the next few months, this is worth raising before considering a corticosteroid injection into the joint. Professor Rodda will not perform a joint replacement within 3 months of a steroid injection into the joint.
Questions to ask
- Which of these risks apply particularly to me?
- Is there anything I can do beforehand to reduce my risk?
- What is your approach to reducing infection and clot risk?
- What would recovery from a complication actually involve, if one occurred?
- How is dissatisfaction assessed and followed up, if it occurs?
Common questions
How common is infection after knee replacement?
Deep infection requiring further surgery occurs in roughly 1.5 in 100 knee replacements within the first two years. Superficial wound infections are more common and generally resolve with antibiotics alone. Risk is higher with diabetes, smoking, higher body weight and other factors that can, in part, be addressed before surgery.
What is stiffness after knee replacement, and what is a manipulation under anaesthesia?
A small proportion of knees become stiff after surgery, meaning the bend does not improve as expected with physiotherapy alone. When it occurs, a manipulation under anaesthesia (MUA), a short procedure to gently bend the knee while you are asleep, may be offered to restore movement, usually in the first few months after surgery. Stiffness requiring MUA affects roughly 2 to 5 in 100 knee replacements.
What is the most under-discussed risk of knee replacement?
Persistent dissatisfaction despite a technically sound operation and no identifiable complication affects a meaningfully higher proportion of people than the surgical complications typically discussed, commonly cited at around 1 in 10. It is rarely raised before surgery and is worth understanding as part of informed consent.
Can I reduce my own risk before surgery?
Yes, in part. Body weight, smoking status, diabetes control, anaemia and dental health can all be improved before surgery and each affects your individual risk profile. Age, prior surgery and some medical conditions cannot be changed, but knowing your individual risk factors allows a more accurate conversation about what applies to you specifically.
Is postoperative confusion normal after knee replacement?
Temporary confusion, known as postoperative delirium, is relatively common after surgery in older patients, particularly following a general anaesthetic or in unfamiliar surroundings. It is distressing to witness but usually resolves within days. It is rarely discussed beforehand despite being a recognised and common occurrence.
Common questions.
How common is infection after a knee replacement?
Roughly 1 in 100 primary knee replacements develop a deep infection. It is more common in the knee than the hip because the joint is superficial. Risk is reduced by theatre practice and by addressing blood sugar, weight, smoking, dental infection and skin conditions beforehand.
What is a manipulation under anaesthetic?
If flexion stalls in the early weeks because scar tissue is forming, the knee is bent under anaesthetic to break down that scar. It is usually done within the first three months while the scar is still remodelling, and around 2 to 4 percent of knee replacements have one.
Why is part of my knee numb after surgery?
Small skin nerves on the outer side of the knee are unavoidably divided by the incision. A patch of numbness there is very common, does not affect how the knee works, and often improves over a year or two.
What can I actually do to reduce my risk?
Stop smoking, get blood sugar under control if you are diabetic, lose weight if your BMI is high, treat any dental or skin infection, correct iron deficiency, and build quadriceps strength before surgery. These measurably change your risk and there is usually time to act.
What if my knee still hurts afterwards?
A proportion of patients have a technically well-performed replacement that still does not feel right. Published satisfaction is around 80 to 90 percent. If pain persists it is investigated properly, including excluding infection, rather than assumed to be the way it will stay.
This page provides general information about a medical condition and a surgical procedure. It is not individual medical advice and does not replace consultation with a qualified practitioner. Outcomes vary between individuals and all surgery carries risk. A referral is needed for a consultation with Professor Daevyd Rodda; without one, you can start with the practice nurse practitioner, who can provide it.