BMI Cutoffs for Knee and Hip Replacement: Complications, Weight Loss and GLP-1s

If you have been told your BMI is too high for a knee or hip replacement, you are running into a rule that is common but not universal. Most surgeons in one survey used a cutoff, and the average landed just above a BMI of 40 ^[3]. The cutoff exists because complication rates climb with BMI, and they climb most steeply above 40 ^[1][2]. What is much less settled is whether losing weight before surgery actually brings those risks down, and whether a GLP-1 medication taken beforehand changes the picture.

The honest summary

• There is no single national BMI requirement. Cutoffs are mostly set by individual surgeons and hospitals. In a 2023 survey of California surgeons, 75% used one, and the mean cutoff was 40.5 for hip and 41 for knee replacement ^[3].
• The risk behind the rule is real. In 22,808 joint replacements, a BMI above 40 independently predicted more complications, including roughly double the odds of superficial infection (OR 2.11) and acute kidney injury (OR 1.79) ^[1].
• Risk rises in steps, not only at 40. In 268,663 patients, complications increased with each BMI band from 30 to 35, 35 to 40 and above 40 ^[2].
• Losing weight before surgery has mixed evidence. A review of three small randomized trials reported fewer complications with weight loss ^[4], but a Mayo Clinic cohort of 3,665 knee patients found that reaching common weight loss goals was not associated with better outcomes ^[5].
• GLP-1 data is observational and early. In people with a BMI of 40 or more, GLP-1 use around knee replacement was linked to fewer 90-day infections and readmissions ^[7], and a meta-analysis found no significant link with aspiration or pneumonia ^[6]. No randomized trial has tested a GLP-1 before joint replacement.

Is there a BMI requirement for knee or hip replacement?

Not a formal, universal one. The cutoff you hear about is usually a policy set by your surgeon, their practice or the hospital. A 2025 study surveyed members of California’s statewide orthopedic association in March 2023 ^[3]. Three quarters of respondents used a BMI cutoff for hip and knee replacement, and 91% said they were wholly or partly responsible for setting it themselves. The mean cutoff was 40.5 for hip replacement and 41 for knee replacement. The same paper notes that many surgeons decline patients with a BMI of 35 or above.

The survey grouped surgeons’ reasons into four categories: risk of complications, surgical logistics, concerns about facilities or resources, and surgeon perception ^[3]. Only the first of those is about your body. The others are about equipment, staffing and judgment, which is why a different surgeon or a higher-volume center may give you a different answer.

Why 40? What the complication data shows

The number 40 is not arbitrary. A study of 22,808 primary hip and knee replacements found that complications rose with BMI, and that a BMI above 40 was an independent predictor of several of them after adjusting for other factors ^[1]. The unadjusted comparison looked like this:

Two things stand out. First, the relative increases are large, but most of the absolute rates remain low. Second, the risk does not switch on at 40. A larger national analysis of 268,663 joint replacements split people with obesity into three bands (30 to 34.9, 35 to 39.9, and above 40) and found a stepwise increase in complications as BMI rose ^[2]. The group above 40 had longer operations and hospital stays and higher rates of readmission, reoperation, blood clots, kidney problems, superficial and deep infection, and wound breakdown. The same pattern held, to a lesser degree, between the 35 to 39.9 and 30 to 34.9 bands.

That is the core logic of a cutoff: 40 is where the curve is steep enough that many surgeons decide the tradeoff is no longer favorable for an elective operation.

Does losing weight before surgery lower the risk?

This is where the evidence gets uncomfortable. It seems obvious that if a high BMI raises risk, lowering it should reduce risk. The data does not clearly say so.

On the encouraging side, a 2024 systematic review found three randomized trials, with 198 patients in total, that compared a weight loss program before hip or knee replacement with usual care ^[4]. Two used diet changes and one used bariatric surgery. All three produced significant weight and BMI reductions, and the intervention groups had fewer complications after surgery, though length of stay did not differ. That is a small evidence base.

On the other side, a Mayo Clinic study followed 3,665 patients with a BMI of 30 or more who had knee replacement, with a mean follow-up of 6 years ^[5]. Few lost much weight beforehand: only 9% lost 20 pounds or more. Reaching common weight loss goals was not associated with better outcomes. In the adjusted analysis, losing 10 to under 20 pounds was actually associated with a higher risk of joint infection than holding steady (HR 2.6), while gaining more than 5 pounds was associated with more complications (HR 1.5). The study was observational and cannot explain why modest loss tracked with more infection, but the authors concluded that weight loss before knee replacement may not be enough to improve outcomes for most patients with obesity.

Ozempic or other GLP-1s before knee or hip replacement

GLP-1 medications such as semaglutide and tirzepatide are now widely used for weight loss, so surgeons are watching how they affect joint replacement. The data so far comes from insurance claims and hospital records, not randomized trials.

The most direct study looked at people with a BMI of 40 or more having knee replacement ^[7]. Those who took a GLP-1 for at least three months before and after surgery were compared with matched patients who did not. The GLP-1 group had lower 90-day rates of joint infection (1.0% vs 1.8%), any medical complication (10.6% vs 12.7%) and readmission (5.3% vs 8.9%). Their complication rates were similar to those of patients with a BMI of 35 to 39.9. There was no difference in two-year surgical complications, and the authors called for randomized trials.

A 2026 meta-analysis pooled 13 studies covering 346,899 hip and knee patients ^[6]. GLP-1 use was associated with a lower risk of 90-day joint infection after knee replacement (OR 0.75), a lower risk of 90-day revision after hip replacement (OR 0.76) and fewer 90-day readmissions for both. It also found a higher risk of heart attack in the knee replacement setting (OR 1.39), a signal that needs more study rather than dismissal.

If you want the osteoarthritis side of the story, meaning whether a GLP-1 improves knee pain before anyone is talking about surgery, that is covered separately in the STEP 9 knee osteoarthritis trial.

Anesthesia, aspiration and holding doses

The main safety worry with GLP-1s and any surgery is that they slow stomach emptying, which raises concern about aspiration under anesthesia. The same meta-analysis flagged that concern and then tested it: across joint replacement studies, GLP-1 use had no significant association with aspiration or pneumonia (OR 1.09, 95% CI 0.71 to 1.67) ^[6]. That is reassuring, but it comes from observational records and does not replace your anesthesia team’s instructions.

The joint replacement studies above do not settle when to hold a dose before surgery. Follow the plan your surgeon and anesthesiologist give you. For the anesthesia guidance itself, see stopping GLP-1s before surgery.

Practical guidance

• Ask where the cutoff comes from. If a surgeon cites BMI, ask whether it is about your specific risk, equipment or policy. A second opinion at a higher-volume center is reasonable.
• Ask what number would change the answer. Get the target in writing so you are not guessing.
• Raise GLP-1 treatment with your doctor early. The study in patients with a BMI of 40 or more involved at least three months of use before surgery, not a last-minute start.
• Protect nutrition and muscle. Fast weight loss that leaves you undernourished is not the goal. Ask about protein and strength work before surgery.
• Tell your anesthesia team you take a GLP-1, the dose and when you took your last one, and follow their instructions on holding it.
• Do not hide weight changes. Gaining more than 5 pounds before knee replacement was linked to more complications in the Mayo Clinic cohort.