BMI and Joint Replacement

The short answer

BMI matters, and a BMI number by itself is not a reason you cannot have a joint replaced. Both of those things are true, and most patients only ever get told one of them.

Weight matters for one specific reason: infection risk. Not appearance, not judgement. A prosthetic joint infection is the complication that changes everything about an outcome, and higher BMI raises that risk measurably. That is the whole basis of the concern.

How BMI is calculated

BMI is a ratio of your weight to your height. In metric it is your weight in kilograms divided by your height in metres squared. In pounds and inches:

It is worth working out your own number rather than guessing, because patients are frequently surprised in both directions.

What the risk actually is

Patients are rarely given real figures, which leaves them either dismissing the issue or assuming the worst. Here is what the data supports.

Relative risk is alarming on its own, so it is worth seeing the absolute numbers alongside it. Long-term data following hip replacements out to 15 years puts the risk of infection at around 1% for a patient of normal weight, rising to around 4% in the highest BMI category.

Why BMI is an imperfect number

BMI does not distinguish muscle from fat. A man or a woman carrying significant muscle mass can have a BMI that looks alarming on paper without carrying the risk that number is supposed to represent.

This is why I do not apply the number mechanically. A BMI is a starting point for assessing somebody, not a description of them. Two patients at the same BMI can be in genuinely different surgical positions, and treating them identically because a formula returned the same value is not good medicine.

Where my thresholds sit

Being specific is more useful to you than being vague:

I do not have a strict cutoff of my own. What I have is a target, a facility policy that is not mine to override, and a range in which the answer depends on the individual person in front of me.

When operating anyway is the right call

There is a version of this decision that gets missed when BMI is treated as a gate: not operating is also a risk.

Some patients are so disabled by their arthritis that the cost of leaving it — immobility, deconditioning, weight gain that follows from being unable to move, loss of independence, the effect on everything else in their health — is greater than the additional infection risk of going ahead.

I have operated on patients in exactly that situation, and I would do it again. It requires an honest conversation where the risk is stated plainly and the decision is made together. What it does not require is pretending the risk is not there.

What the 2025 guidelines say

This is worth knowing about if you have been turned away, because the professional guidance moved recently.

In June 2025, the American College of Rheumatology and the American Association of Hip and Knee Surgeons published a guideline in Annals of Internal Medicine that conditionally recommends against delaying surgery solely for the purpose of reaching a target BMI.

The same guideline still supports delaying surgery for:

The recommendation is conditional, which in guideline language means the underlying evidence is not strong and the decision should depend on the individual patient. That is precisely the point — it moves the standard toward shared decision-making and away from a fixed number applied to everybody.

Modifiable risk factors

The frame I find most useful with patients is modifiable risk factors — the things we can actually change between now and surgery to improve the odds of a good result.

This is a collaborative exercise, not a set of hurdles. The practice has resources we can connect you with covering nutrition, activity, lifestyle and medication, and I would rather put you in touch with them than send you away to solve it alone. Some of what is worth trying before surgery is covered in what to try before a joint replacement.

GLP-1 medications

GLP-1 medications have been genuinely useful for getting patients to a safer starting point. Whether a patient is on one for diabetes, for prediabetes, or as part of a dedicated weight loss plan, they have made meaningful preoperative weight loss achievable in a way it often was not before.

There are two timing points that matter around surgery:

Tell us what you are taking and the exact timing gets confirmed with the anesthesia team as part of your preoperative planning. This is a common situation now and it is straightforward to manage — but only if we know about it.

If you were told no

If you have been told you are too heavy for a joint replacement and left without a plan, that is worth a second conversation — particularly given where the guidelines now sit.

The answer might still be that we work on some things first. It might be that your BMI is inflated by muscle and the number was never the real issue. It might be that your disability justifies going ahead. What it should not be is a number quoted at you with no discussion of what happens next.

A second opinion is a reasonable thing to seek here. Bring your X-rays and your recent labs, and we will give you a straight answer about where you actually stand.