Blood clots after knee surgery

A blood clot is the complication people are quietly worried about in the first weeks after knee surgery, and one of the few where knowing the signs changes what happens next. After an ACL reconstruction the risk is low but not nothing, and the early symptoms overlap with what is entirely ordinary after an operation on the knee: swelling, soreness, a leg that feels heavy. What follows is what the research has found and which signs the health services tell people to act on.

This is general information, not an assessment of your leg. Your surgical team knows what was done to the knee, whether you were given anything to reduce clotting risk and what they want you to do about it. If you think you might have a clot, contact them, your GP or an urgent care service now rather than finishing the article.

How common are clots after ACL surgery?

A deep vein thrombosis, or DVT, is a clot in one of the deep veins of the leg, most often in the calf or the thigh. A pulmonary embolism, or PE, is what happens when part of a clot breaks away and lodges in the lungs. Studies group the two together as venous thromboembolism, VTE.

The honest answer to “how common” is that it depends almost entirely on how hard anyone looked.

Forlenza and colleagues went through a United States insurance database covering 11,977 people who had an ACL reconstruction between 2007 and 2017, counting the clots that caused symptoms and were treated. They found VTE in 1.01 per cent within 30 days of surgery and 1.22 per cent within 90 days — roughly one in a hundred.

Look for clots instead of waiting for them, and the number changes. Joo and colleagues scanned all 260 patients in their series with ultrasound a week after an isolated ACL reconstruction, whether or not anyone had symptoms, and found a DVT in 8.1 per cent. Erickson and colleagues, reviewing the studies that screened patients rather than only counting those who came back unwell, reported a DVT rate of 8.4 per cent among patients given no anticoagulant medication — and that 73 per cent of those clots caused no symptoms. Symptomatic pulmonary embolism in the same review was 0.2 per cent.

A 2025 systematic review in Orthopaedics and Traumatology: Surgery and Research, pooling six studies and 47,886 reconstructions, put the range of reported VTE rates at 0.4 to 11.0 per cent.

So: clots visible on a scan are not rare after this operation. Clots that cause symptoms and need treating are uncommon, around one in a hundred. Clots that reach the lungs are rarer still. None of which says anything about your own leg, which is why the signs below matter more than the percentages.

When the risk is highest, and what raises it

The timing inside Forlenza’s figures is worth reading twice: 1.01 per cent at 30 days, 1.22 per cent at 90. Most of the events of the first three months had already happened in the first month. The weeks when you move least carry most of the risk — and they are also the weeks when the leg is swollen and sore for ordinary reasons, which is what makes a clot hard to pick out.

Which individual characteristics raise the risk is less settled than you would expect. The 2025 review set out to examine age, sex, body mass index, high blood pressure, smoking, day-case versus inpatient surgery and tourniquet time. Of those it was able to pool sex, finding no significant difference between men and women; the rest it could not settle on the evidence available.

Outside the ACL literature the general risk factors for a DVT are better established, and the NHS lists them: a previous clot, a spell of being much less mobile than usual, being overweight, smoking, and oestrogen-containing contraception or HRT. Your surgeon will have weighed whichever apply to you before deciding whether you needed anything beyond getting moving.

The signs, and how they differ from an ordinary post-operative leg

This is the part worth remembering.

The NHS describes a DVT in the leg as throbbing or cramping pain in one leg, usually in the calf or thigh; swelling in that leg; warm skin around the painful area; and skin that looks red, darkened or blue around it.

The difficulty after an ACL reconstruction is obvious: the operated leg is already swollen, sore and warm around the knee. Two things help to tell them apart. One is location — a clot’s pain is typically in the calf rather than at the knee itself. The other is pattern: post-operative swelling and soreness ease gradually and respond to elevation, while a clot turns up as something new and one-sided that does not fit the shape of your recovery so far. The wider question of what is ordinary soreness and what is not is in normal soreness or a setback.

The standard list of things that mean contacting your surgical team, your GP or an urgent care service the same day is short:

• a calf that is hot, red, swollen or painful
• a fever, or redness or discharge at the incision
• new weakness or numbness in the foot
• a knee that locks

A pulmonary embolism is an emergency rather than a same-day call. The NHS describes breathlessness, chest pain that is worse when you breathe in or cough, a cough, coughing up blood, and a fast or irregular heartbeat. Its instruction is unambiguous: do not drive yourself to hospital. Call an ambulance — 999 in the United Kingdom, 911 in the United States — or have someone else drive you.

None of this is a reason to check your calf every hour. It is a reason to know in advance what you would do, so that if something changes you act the same day rather than waiting to see.

What is done to prevent clots

Practice varies more than patients expect, and most of the variation is legitimate.

In England, NICE guideline NG89 takes a threshold approach to arthroscopic knee surgery: prophylaxis is generally not needed where total anaesthesia time is under 90 minutes, and low molecular weight heparin should be considered — starting 6 to 12 hours after surgery and continuing for 14 days — where anaesthesia time runs longer than that, or where the person’s risk of a clot outweighs their risk of bleeding. Other countries and surgeons draw the line elsewhere, and some patients go home with aspirin or an injection as a matter of routine. If you were sent home with nothing, that is a decision rather than an oversight, and one to ask about rather than worry about.

What the protocols have in common is movement. The consensus rehabilitation guideline from van Melick and colleagues has the knee moving and the leg loading from the first days after surgery, for reasons to do with the knee itself — extension, swelling, quadriceps activation — and the NHS’s own recovery advice is built around getting up and about rather than resting the leg flat. Whatever is or is not prescribed alongside it, the early exercises are the part of this you control. The shape of those first days is in what is normal in the first week.

Where the app fits

No app can tell you whether you have a clot; that is a phone call, and a short one. What our free ACL tracker for iOS and Android can do is hold the record that makes the call easier: its pain diary takes a 0-to-10 score, the place on the body map where it hurts and a note, so if calf pain does appear you can say when it started and how it has changed, instead of reconstructing it from memory in a waiting room.

Most people get through the first weeks after an ACL reconstruction without meeting any of this. The reason to know the signs is not vigilance but speed. Clots are treated with anticoagulant medicine, which the NHS describes as stopping a clot getting bigger and preventing new ones forming; none of that can start until somebody knows. If something in your calf or your breathing is new and does not fit the recovery you have had so far, it deserves someone’s attention today — and your care team would far rather look at a leg that turns out to be fine.