Most knee joint operations are problem-free but about 1 person in every 20 may have complications. Most of these complications are minor and can be successfully treated.

The risk of complications developing will depend on a number of factors including your age and general health. In general, a younger patient with no other medical problems will be at a lower risk of complications.

It’s important to remember that any drugs used throughout your stay in hospital, for example anaesthetic or painkillers, may also have side-effects. Your surgeon or anaesthetist will be able to discuss these with you.

Blood clots

Blood clots which form in the deep veins in the leg (deep vein thrombosis, or DVT) can cause pain and/or swelling. This is because of changes in the way blood flows and its ability to clot after surgery. There are various ways to reduce the risk of this happening, including special stockings, pumps to exercise the feet and drugs that are given by injection such as heparin.

Blood-thinning drugs can increase the risk of bleeding, bruising or infection so your surgeon will need to balance these risks. Rivaroxaban, dabigatran and apixaban are tablets to help prevent DVT which have recently become available as an alternative to injections. The tablets are more convenient than injections, which makes them easier to take at home. However, they still carry a risk of bleeding.

Pulmonary embolism

In a very small number of cases a blood clot can travel to the lungs, leading to breathlessness and chest pains. In extreme cases a pulmonary embolism can be fatal. However, it’s usually possible to treat pulmonary embolism with blood-thinning drugs and oxygen therapy.

Wound infection

As with all operations, there’s a small risk that the wound will become infected. This happens in about 1 in 50 cases. Usually the infection can be treated with antibiotics. About 1 in 100 patients develops a deep infection, which may mean removing the new joint until the infection clears up. In extreme cases, where the infection can’t be cured, the knee replacement has to be removed permanently and the bones fused together so the leg no longer bends at the knee. Very rarely, the leg may have to be amputated above the knee and replaced with an artificial leg – but this is extremely unusual.

Nerve and other tissue damage

There’s a small risk that the ligaments, arteries or nerves will be damaged during surgery.

  • Fewer than 1 in 100 patients have nerve damage and this usually improves gradually in time.
  • About 1 in 100 have some ligament damage – this is either repaired during the operation or protected by a brace while it heals.
  • About 1 in 1,000 suffer damage to arteries that usually needs further surgery to repair.
  • In about 1 in 5,000 cases blood flow in the muscles around the new joint is reduced (compartment syndrome). This usually also needs surgery to correct the problem.

Bone fracture

The bone around the replacement joint can sometimes break after a minor fall – usually after some months or years and in people with weak bones (osteoporosis). This is extremely rare but when it happens further surgery is usually needed to fix the fracture and/or replace the joint components.


When a mobile plastic bearing is used there is a small risk of dislocation of the knee, and this would need further surgery.

Bleeding and wound haematoma

A wound haematoma is when blood collects in a wound. It’s normal to have a small amount of blood leak from the wound after any surgery. Usually this stops within a couple of days. But occasionally blood may collect under the skin, causing a swelling. This can either discharge itself, causing a larger but temporary leakage from the wound usually a week or so after surgery, or it may require a smaller second operation to remove the blood collection. Drugs like aspirin and antibiotics can increase the risk of haematoma after surgery.


For most people, pain gradually eases during the first few months after surgery. However, some people have ongoing pain or develop new types of pain. Research shows that 10–20% of people still have moderate or severe pain in the long term. This isn’t always caused by a technical fault or recognisable complication, and therefore it can’t be fixed by a repeat operation. This complication is known as complex regional pain syndrome. Some hospitals have pain clinics that can help with this.


Some people experience continuing or increasing stiffness after surgery. Usually this resolves with exercise, and as the swelling improves. Pain may contribute to this complication by stopping the patient from doing physiotherapy exercises and allowing scarring to glue together the soft tissues around the joint. Occasionally knee stiffness may be treated by a manipulation of the joint under anaesthetic, followed by intensive physiotherapy.