Both knees can be replaced in one operation, and the evidence says that choice carries a cost.

There are two ways to replace both knees

A simultaneous bilateral knee replacement replaces both knees during one anaesthetic and one hospital stay. A staged bilateral replacement does one knee, lets it recover, and then does the other. In a 2013 consensus statement, 81 percent of the expert panel agreed that if a patient is not a candidate for the single operation, the second knee should be scheduled no sooner than three months after the first ¹.

The same panel agreed, by 81 percent, that replacing both knees in one operation is a more invasive and complex procedure with a higher risk of adverse events than a single knee replacement, in an unselected group of patients ¹. No large randomised trial compares the two approaches directly, so the comparison rests on observational studies and the meta-analyses that pool them ¹.

What the evidence says about mortality and complications

A 2021 systematic review pooled 29 studies covering 257,284 patients, 104,207 of whom had both knees replaced at once. Simultaneous surgery was associated with higher 90-day mortality, with an odds ratio of 2.24, and with more pulmonary embolism, venous thrombosis, and neurological complications ². Staged surgery, in the same analysis, was associated with more superficial and deep infection, and revision rates within a year were equivocal ².

A 2023 meta-analysis of 37 studies reported the absolute figures: overall mortality was 0.66 percent after simultaneous surgery and 0.43 percent after staged surgery, an odds ratio of 1.55 ³. Both numbers are small. The difference between them is what the choice is about.

The studies do not agree on everything. An earlier meta-analysis of 14 studies found higher mortality and more neurological complications with simultaneous surgery, but no significant difference in infection, pulmonary embolism, deep-vein thrombosis, or cardiac complications ⁴. Its authors cautioned that the poor quality of the underlying studies limited the analysis ⁴.

Safety has changed over time. A US population analysis of an estimated 258,524 bilateral knee replacements between 1999 and 2008 found in-hospital mortality falling from 0.42 to 0.16 percent ⁵. Pneumonia, pulmonary embolism, and some cardiac complications did not fall, and rose once adjusted for shorter hospital stays ⁵.

Who is typically considered a candidate

Because the added risk is mainly medical, selection is mainly medical. The consensus panel agreed that surgeons and hospitals should use more restrictive selection criteria for the single operation, and that patients with a revised cardiac risk index above 3 should be excluded ¹. In the fuller statement, 93 percent opposed same-day surgery for patients with active ischaemic heart disease or a reduced ejection fraction, and 93 percent favoured avoiding it in patients at ASA class III or higher ¹.

The panel also listed conditions many members treat as reasons to stage: moderate to severe pulmonary hypertension, oxygen-dependent lung disease, a body mass index above 40, renal insufficiency, chronic liver disease, poorly controlled diabetes, and a history of stroke ¹. On age, 63 percent supported excluding patients over 75, though a majority felt biological age should count rather than calendar age ¹.

One institution adopted criteria of this kind in 2006. Patients selected for the single operation became younger with fewer comorbidities, and major complications per 1,000 hospital days fell from 31.5 in 1998 to 7.9 in 2014, with fewer cardiac complications the largest contributor ⁶.

The consensus statement ends on a rule the whole panel agreed to: when the orthopaedic case for one operation conflicts with the medical case against it, the medical concern for the patient's safety prevails ¹.

How recovery differs

The single operation means one anaesthetic, one hospital admission, and one rehabilitation. In the pooled data, the mean hospital stay was 2.1 days shorter ². The trade is that both legs are recovering at once, with no sound leg to stand on in the first weeks.

The first knee is usually well along before the second is done, since the consensus interval is at least three months ¹. The cost is a second operation, a second anaesthetic, and a second stretch of rehabilitation, and in the pooled data a higher rate of infection ².

• Simultaneous: one recovery, shorter total hospital time, higher short-term mortality and clot risk in the pooled studies ²³.
• Staged: two recoveries, the second no sooner than three months, with more infection reported across the pooled studies ¹².

What to ask your surgeon

• Given my heart, lungs, kidneys, and diabetes, would you consider me for both knees at once, and why?
• What selection criteria do you and this hospital use for the single operation?
• If we stage, how long would you wait between the two, and what would decide that?
• How would the first weeks at home differ if both knees are recovering at once?
• Which of the risks in the studies do you think applies most to me, and which least?

Your surgeon decides from your exam, imaging, and history, and from a medical assessment, which this decision depends on more than most.