Clot prevention after joint replacement is planned before the operation, and it continues after you go home.

Why the risk rises after hip and knee surgery

A deep vein thrombosis, or DVT, is a clot that forms in a deep vein, most often in the calf or thigh ¹. Surgery raises the risk in three ways: slowed blood flow, a temporary increase in the blood's tendency to clot, and injury to the vein wall ¹. Preparing the bone to receive a hip implant can release substances that promote clotting ¹. The danger of a DVT is that part of it can break free and travel to the lungs. That is a pulmonary embolism, and it can be fatal ¹.

A Norwegian series followed 5,607 major hip and knee operations, all with heparin prophylaxis in hospital ². Within six months, 2.7% of patients developed a symptomatic clot, and 70% of those were diagnosed after leaving hospital ².

Prevention usually combines movement, compression, and medication

The American College of Chest Physicians guideline recommends that every patient having major orthopaedic surgery receive some form of clot prevention rather than none ³. It lists several options: low-molecular-weight heparin, fondaparinux, the oral anticoagulants dabigatran, apixaban, and rivaroxaban, low-dose unfractionated heparin, adjusted-dose warfarin, aspirin, or an intermittent pneumatic compression device ³. It recommends a minimum of 10 to 14 days ³. It suggests low-molecular-weight heparin in preference to the alternatives, and adding a compression device during the hospital stay ³. Patients at higher bleeding risk may be managed with compression alone ³.

Early walking is the third element. AAOS patient guidance lists mobility, compression stockings or pneumatic devices, and anticoagulant medication as the three main strategies ¹. Getting up and moving soon after surgery increases circulation in the legs ¹.

Which medication, at what dose, and for how long is the surgical team's decision for each patient. It rests on bleeding risk, kidney function, other medications, and history. This page does not recommend any medication, and nothing here should change what you take.

Aspirin versus anticoagulants: the trials do not fully agree

Two large randomized trials reached different conclusions. In a Canadian trial of 3,424 hip and knee replacements, every patient took rivaroxaban for five days ⁴. They were then randomized to continue it or switch to low-dose aspirin ⁴. Symptomatic clots occurred in 0.64% of the aspirin group and 0.70% of the rivaroxaban group, and aspirin was found non-inferior ⁴.

An Australian registry-nested trial of 9,711 patients compared aspirin alone with enoxaparin ⁵. Symptomatic clots within 90 days occurred in 3.45% of the aspirin group and 1.82% of the enoxaparin group, and aspirin did not meet the non-inferiority standard ⁵. The two trials differ in design: one gave everyone an anticoagulant first, the other did not. Where your surgical team lands is a judgment about you and about this evidence, and it is theirs to make.

How long the risk window lasts

The risk of a clot is highest in the first two weeks after surgery, roughly days two to ten ¹. It persists at a lower level for about three months ¹. In the Norwegian series, deep vein thrombosis presented at a median of 21 days after hip replacement and 20 days after knee replacement ². The cumulative risk lasted up to three months after hip surgery and about one month after knee replacement ².

Pulmonary embolism can come earlier. In a series of 11,148 replacements under non-warfarin prophylaxis, the median day of diagnosis for symptomatic PE was day three, and the 90-day rate was 0.71% ⁶. This spread in timing is why the CHEST guideline suggests extending prophylaxis for up to 35 days ³. Some clots produce no symptoms at all ¹.

Symptoms in the leg need a same-day call to the surgical team

AAOS guidance for hip and knee replacement says to notify the surgeon immediately if any of these appear ⁷⁸:

• Increasing pain in the calf, or pain in the leg that is unrelated to the incision ⁷⁸.
• Tenderness or redness above or below the knee ⁷.
• New or increasing swelling of the thigh, calf, ankle, or foot ⁷⁸.

Swelling that does not improve with elevation, distended veins, and warm or discolored skin are also described as signs of DVT ¹. So is a firm cord along the vein ¹.

Symptoms in the chest are an emergency

A clot that reaches the lungs can cause sudden shortness of breath, sudden chest pain, chest pain with coughing, or coughing up blood ¹⁷. AAOS guidance describes these as signs needing immediate medical attention ⁸. Call emergency services rather than waiting for a clinic to open.

Questions worth asking your surgeon before the operation:

• What clot prevention does your program use, and for how many days?
• Does anything in my history change that plan, such as a prior clot or a bleeding problem?
• How soon will I walk after surgery, and who supervises it?
• Which symptoms do you want a same-day call about, and which should go straight to emergency services?