If both of your knees are worn out, the question comes up early: can I have them both done at once and get it over with? It is a fair question, and not a simple one. The honest answer is a trade-off, and this month’s papers lay out both sides of it with unusually large numbers.
What the studies looked at
Surgeons call it bilateral knee replacement, and there are two ways to do it. Simultaneous means both knees under one anaesthetic, in one operation. Staged means one knee now and the other later, usually weeks to months apart. The main paper, published in 2026, pooled every comparative study it could find from 2000 to August 2025: 53 studies and 572,881 patients, about 244,000 of whom had both knees done together and 329,000 one at a time. It compared deaths in the weeks after surgery, blood clots, transfusion, infection, revision, and length of stay.
Because no single analysis settles a question like this, it is read here alongside four others: a second 2025 meta-analysis that also costed the two approaches, a 2023 one that reports the actual death rates rather than just the ratio between them, a 2025 analysis of how long to wait between staged operations, and a large single-surgeon series followed for fifteen years that reached a more reassuring conclusion. A 2026 American database study showing how practice has changed completes the picture.
What they found
- The short-term risk of dying is higher with both knees at once, from a low base. The main analysis found the odds roughly 2.35 times higher (95% CI 1.69 to 3.27). The 2025 meta-analysis found 2.43 times for deaths in the first year. The 2023 analysis gives the figures that keep this in proportion: 0.66% of simultaneous patients died, against 0.43% of staged patients. A doubling of a small number is still a small number, and both framings are true at once.
- Clots and transfusion are more common with both at once. Deep vein thrombosis was about 1.45 times as likely, and the need for a blood transfusion about four times as likely, which is what you would expect from twice the surgery in one sitting.
- But staging has its own costs. Patients who had one knee at a time had more superficial and deep infections, more fractures around the implant, and more reoperations. They also had two anaesthetics, two recoveries, and two exposures to every risk that comes with an operation. Simultaneous surgery saved about 67 minutes of total operating time and, in the American data, several thousand dollars.
- Function was the same either way. Neither approach produced a better knee. Pain, stiffness and function scores did not differ once recovered.
- If you stage, the gap matters. Across 15 studies, having the second knee between six weeks and six months after the first carried lower mortality than doing both together, and three to six months ranked best. Going back within six weeks, or waiting more than six months, both came with more lung and infection complications.
- With careful selection the difference can disappear. One surgeon’s series of 18,327 patients, followed for fifteen years, found deaths of 0.2%, 0.3% and 0.4% for simultaneous, staggered and staged surgery, not significantly different. That is a single high-volume unit choosing its simultaneous patients carefully, and it is probably the best case rather than the typical one.
- The profession has voted with its feet. In the United States, simultaneous surgery fell from 19% of bilateral cases in 2016 to 7% in 2021, and the patients still having it were younger and had fewer other illnesses. It is being reserved, not abandoned.
What it means for you
In this practice the usual advice is one knee at a time. Doing both together is considered only for patients who are otherwise very well, and this evidence is the reason: the extra risk is small in absolute terms, but it is real, it is consistent across every large analysis, and it falls on the heart, the lungs and the blood in the days after surgery, which is exactly where an older patient or one with other illnesses has the least reserve.
That is not the same as saying simultaneous surgery is wrong. For a fit patient with two bad knees, one operation, one anaesthetic and one recovery is a reasonable thing to want, and the data on infection and reoperation show that staging is not free either. What the evidence supports is selection, not a rule. If both your knees need replacing, the useful questions to bring to the consultation are about you: your age, your heart and lungs, diabetes, weight, and how you would manage a recovery with no good leg to stand on. If the answer is to stage, the second knee is best planned for somewhere between six weeks and six months after the first, with three to six months the sweet spot in the data.
A caveat worth knowing
Every one of these studies is observational, and the bias runs in both directions. Surgeons choose simultaneous surgery for fitter patients, which makes it look safer than it is for the average patient; at the same time, a patient who dies after a first knee never becomes a “staged” patient at all, which makes staging look safer than it is. The authors of the main analysis say plainly that the true risk of staging may be underestimated for that reason. The data are mostly American and Asian, with Australian registry patients not separately reported, and no study measured the practical burden of recovering on two new knees at once. A randomised trial would be the way to settle it, and there is not one. For now, three meta-analyses of more than half a million patients point the same way, and the sensible reading is the one most surgeons have arrived at: both knees at once for the very well, one at a time for everyone else.