Swelling after a knee replacement peaks in the first days, eases over weeks, and can linger for months.

How long swelling lasts

The American Academy of Orthopaedic Surgeons describes moderate to severe swelling in the first few days or weeks after a knee replacement, and mild to moderate swelling for about three to six months ¹.

A study that measured knee swelling with bioimpedance in 85 patients gives the shape of that curve. On the first day after surgery, swelling was about 35 percent above its pre-operative level; it then fell steadily, but at day 90 the knee was still about 11 percent above where it had started ². So a knee that is still a little swollen at three months is inside the measured range, not outside it.

Why swelling matters beyond comfort

In the same study, swelling was associated over time with quadriceps weakness and with slower walking speed ². A second study measured 24 patients a week before surgery and again at discharge, about two and a half days after the operation. The increase in knee circumference correlated with the loss of knee-extension strength, and after adjusting for age and sex, swelling explained about 27 percent of that loss ³. Changes in knee-extension strength and knee flexion together explained 57 percent of the fall in fast walking speed ³.

The practical reading is that the weak, heavy thigh of the first weeks is partly a swollen knee inhibiting its own muscle, not only a muscle that has been cut and stretched. That is one reason the guidance treats swelling as something to manage rather than simply wait out ¹.

What helps: elevation, ice, compression, and movement

The guidance recommends elevating the leg slightly and applying ice, and says compression stockings may also help ¹. Its exercise guide adds ankle pumps for two to three minutes, two to three times an hour in the early days, continued until the swelling in the ankle and lower leg has gone ⁴.

The evidence for ice is real but modest. The 2025 Cochrane review of cryotherapy after knee replacement pooled 22 trials with 1,839 participants. Cooling may slightly reduce pain at 48 hours, 3.2 against 4.8 on a ten-point scale, may reduce blood loss, and may improve knee motion at discharge by about 8 degrees, all at low certainty ⁵. Seven studies measured swelling: it was about 7 millimetres less at the kneecap between days two and six with cooling, but there was no difference at six weeks or three months ⁵. The review's authors concluded that these benefits may be too small to justify the treatment, and noted that the trials were hard to blind and mostly at high risk of bias ⁵.

Walking is the other half of the plan, and the same measurement study that tied swelling to slower walking is why it matters: the goal is short, frequent walks with the prescribed aid, not long ones. Our page on walking and stairs after knee replacement sets out what the first six weeks of walking look like.

Swelling that is not routine

The guidance draws one line clearly: tell your doctor about new or severe swelling, because it may be the warning sign of a blood clot ¹. The signs it lists for a clot are pain in the leg or calf unrelated to the incision, tenderness or redness above or below the knee, and increasing swelling of the calf, ankle or foot ¹. It lists separately the signs that a clot has travelled to the lungs, which need emergency care: shortness of breath, sudden chest pain, or chest pain with coughing ¹.

Swelling of the wound itself, with increasing redness, tenderness, drainage, persistent fever above 100 degrees, or shaking chills, is on the infection list rather than the swelling list ¹. Our page on when to call after joint replacement sets the lists side by side.

• Swelling that is settling week by week, most in the knee, less in the calf: expected ¹².
• Swelling that is new, severe, or in the calf, ankle or foot: call the surgical team the same day ¹.
• Swelling with shortness of breath or chest pain: emergency care ¹.

What to ask your surgeon

• How much swelling do you expect in my first two weeks, and what would make you want to see the knee?
• Do you want me to use ice, and for how long at a time?
• Should I wear compression stockings, and for how many weeks?
• How do I tell swelling that is settling from swelling that is a clot?

The measurements here describe groups of knees. Your surgeon decides from yours.