Is There a Weight Limit for a Knee or Hip Replacement?

People ask whether there is a weight limit for a knee or hip replacement. There is, and it is a BMI rather than a number of kilograms. Together with blood sugar, it is one of two numbers that decide whether I will operate on a hip or a knee, before anything about the X-ray.

I will not do a hip or knee replacement if your HbA1c is above 8%, or if your BMI is above 45. Both are reasons for the operation to wait, not reasons for it never to happen. This page explains why, how long each takes to change, and what you can do in the meantime.

Why these two

A hip or knee replacement is a clean implant placed in a joint that has to stay free of infection for the rest of your life. Almost everything that raises the chance of a problem does it through the same few routes: poorer wound healing, a weaker defence against bacteria, a harder anaesthetic and a slower recovery.

High blood sugar impairs healing and the body’s response to infection. A deep infection of a joint replacement is uncommon, and it is also one of the most serious things that can happen to one, because it often means more operations and a long course of antibiotics.

A very high BMI raises the same risks, and adds to them: wounds are under more tension, the anaesthetic and the positioning are harder, clots are more likely, and the new joint carries more load every day.

Neither limit is a judgement about you. They exist because on one side of them the operation is safer and on the other it is not, and the aim is to do it when it is.

HbA1c: what it is and how it changes

HbA1c is a blood test that reflects your average blood sugar over roughly the last two to three months, weighted towards the most recent weeks. It is the number your GP uses to judge how well diabetes is controlled.

That is why it changes slowly. A good week will not move it, and neither will a crash diet in the fortnight before surgery. It takes about three months of better control to show.

If your HbA1c is above 8%:

• See your GP now, not close to the date. Treatment can often be adjusted, and some of the options work quickly
• Repeat the test after about three months, and bring the result to the rooms
• Keep a record of your medications, including any weight-loss or diabetes injections, because they affect fasting instructions. Ozempic and knee replacement surgery covers that one in detail

Some situations push blood sugar up for reasons outside your control, such as steroid tablets or a cancer treatment. If that applies, say so, because your GP or an endocrinologist may be able to manage it differently. It is still the number that matters, so the conversation is about how to get it down.

If you have not had an HbA1c recently, or you have never had one, ask your GP for one before you see me. It is a simple blood test.

BMI: what it is and what it means here

BMI is your weight in kilograms divided by your height in metres, squared. A BMI of 45 works out as roughly the following, and the limit, the maximum weight for your height, is anything above it:

BMI is a blunt measure, and it does not distinguish muscle from fat. At the numbers where it becomes a limit, it is a fair marker of risk.

Getting from there to here is hard, and harder when the knee or hip is what stops you moving. I know that. It is the single most common trap in this conversation: told to lose weight before surgery, unable to exercise because the knee hurts. It is not a failure of will.

What helps:

• Start with food. Most of the weight lost comes from what is eaten, not from exercise. A dietitian is better than another diet
• Use movement that spares the joint. Water walking, a stationary bike and short, regular walks. Weight and knee load and the weight section of understanding hip arthritis set out why even a modest change makes a difference to the joint
• Ask your GP about supervised programs and medication. Weight-loss medications are an option, and they come with trade-offs, covered in the Ozempic page, which is written about the knee and applies in the same way to the hip. Where weight is very high, a referral to a specialist weight-management service may be appropriate
• Protect your muscle. The aim is to arrive at surgery lighter and stronger, not lighter and weaker

What the research shows

You should know how firm the evidence is, rather than be told the limits are beyond argument. The studies are mostly observational. They agree on the direction and do not agree on a single cut-off.

Diabetes and obesity both raise the risk of infection. Two meta-analyses of risk factors for periprosthetic joint infection after hip and knee replacement, pooled together, found that diabetes raised the odds of infection by roughly 1.6 to 1.8 times, and obesity by roughly 1.5 to 1.6 times (Kong 2016; Resende 2018).

For HbA1c, the direction is clear and the exact line is not.

• In 1,645 patients with diabetes having a hip or knee replacement (1,004 knees and 641 hips), the one-year infection rate rose from 0.8% to 5.4% above an HbA1c of 7.7% (Tarabichi 2017)
• A meta-analysis found raised HbA1c was linked to more surgical site infection, but the pooled result was not statistically significant and did not support the conventional 7% cut-off. The authors described the optimal threshold as contentious (Shohat 2018)
• A 2025 study of 17,481 patients found an infection threshold of 7.8% for hip replacement and 9.7% for knee replacement, and concluded that HbA1c has limited predictive power on its own while remaining useful for assessing risk (Palmer 2025)
• A prospective study of 1,488 patients found no significant difference in wound complications at 6.5% (Citak 2020)
• A review of 69 studies recommended an HbA1c of 7.5% or lower before elective joint replacement, and delaying surgery until risk factors are optimised (MacMahon 2021)

The thresholds in that literature run from about 7% to 9.7%. A limit of 8% sits inside that range.

For BMI, risk rises with severity. The studies of the hip and of the knee point the same way, though almost none addresses a BMI of exactly 45. Most group patients at 40 and at 50.

Knee replacement.

• In 150,934 knee replacements from a US national quality database, patients with a BMI above 40 had more readmissions, reoperations, periprosthetic infections and wound breakdown than patients of normal weight (George 2017)
• In 464 knee replacements in patients with a BMI of 40 or more, those who started at 45 or above and lost enough weight to get below 45 had no measurable difference in infection risk from lower-BMI controls, while those who did not had about eight times the odds. The numbers were small and the confidence intervals wide, and the authors also argued that a limit of 40 may exclude some patients without lowering their risk (Spezia 2024)
• The same 2021 review recommended a BMI below 40 as the target before elective joint replacement (MacMahon 2021). A limit of 45 is higher than that, and the evidence for where the line should sit between 40 and 45 is thin
• In a 2025 audit of 2,674 cementless knee replacements, patients with a BMI of 40 to 44.9 had revision rates similar to non-obese patients, 0.7% against 0.9%. The numbers were too small to say anything about a BMI above 45 (Madden-McKee 2025)

Hip replacement.

• In 161,785 hip and knee replacements from a US national quality database, the effect of obesity on complications was greater for hip replacement than for knee replacement. Among morbidly obese patients, deep infection occurred in 0.84% of hips against 0.23% of knees (DeMik 2018)
• A meta-analysis of 10 studies and 46,080 hip replacements found that patients with a BMI of 40 or more had roughly four times the odds of periprosthetic infection of those below 40. The authors noted that hard BMI cut-offs can restrict access to care, and that a certain level of obesity may warrant them for patient safety (Rubin 2025)
• In 21,035 hip replacements, patients with a BMI of 50 or more had about seven times the odds of periprosthetic infection, and four times the odds of a pulmonary embolism, compared with normal-weight patients (Telang 2025)
• In 21,550 hip replacements followed for up to 15 years, patients with a BMI of 40 or more and no other infection risk factor had three to nine times the relative risk of infection, but a low absolute risk of 2% to 4%. Among patients with two or more other risk factors such as diabetes, the absolute risk reached 18% in the most severely obese group (Carender 2024). That finding is the reason the two limits are applied together, not separately

Restricting access on these grounds is common in Australia. A 2023 poll of Australian arthroplasty surgeons, with 77 responses, found that 91% restricted access to joint replacement for patients with modifiable risk factors. That included 72% for excessive BMI and 85% for poor diabetic control (Wall 2023).

It is not a statement that surgery does not help. In Australian registry data covering about 33,000 hip and knee replacements, patients with a higher BMI reported satisfaction comparable with other patients (Mulford 2023). The limits are about timing the operation for when it is safest.

Weight loss before surgery changes the risk. A 2024 meta-analysis reported lower rates of periprosthetic infection after knee replacement in patients who had previously had bariatric surgery (De Mauro 2024).

Meanwhile, the joint

The joint does not stop hurting while you work on this. The non-surgical options for the knee and the treatment options for hip arthritis are worth using properly, in order: exercise and physiotherapy, simple pain relief and a measured approach to injections.

One note for people with diabetes: a cortisone injection into the hip or knee raises blood sugar for several days. Tell whoever is giving it and expect higher readings for those days. Cortisone injections covers it.

What happens next

Once your HbA1c is 8% or lower, or your BMI is 45 or lower, bring the new result back and the operation can be planned. You are not starting again.

Other things also need sorting before surgery, whatever the numbers. The pre-op page and, for the knee, preparing for a knee replacement cover smoking, skin, teeth and the rest, and when I advise against knee surgery covers the situations in which the answer is not yet, or not at all.

If you would like to talk it through, phone the rooms on (08) 6332 6365 or use the contact page.

Studies cited

All are indexed in PubMed, and each link goes to the publisher’s page.

• Kong L et al. Risk factors for periprosthetic joint infection following primary total hip or knee arthroplasty: a meta-analysis. Int Wound J 2016. doi:10.1111/iwj.12640
• Resende VAC et al. Higher age, female gender, osteoarthritis and blood transfusion protect against periprosthetic joint infection in total hip or knee arthroplasties: a systematic review and meta-analysis. Knee Surg Sports Traumatol Arthrosc 2018. doi:10.1007/s00167-018-5231-9
• Tarabichi M et al. Determining the threshold for HbA1c as a predictor for adverse outcomes after total joint arthroplasty. J Arthroplasty 2017. doi:10.1016/j.arth.2017.04.065
• Shohat N et al. Inadequate glycemic control is associated with increased surgical site infection in total joint arthroplasty: a systematic review and meta-analysis. J Arthroplasty 2018. doi:10.1016/j.arth.2018.02.020
• Palmer RC et al. The limited utility of hemoglobin A1c as a predictor for periprosthetic joint infection following total joint arthroplasty. J Arthroplasty 2025. doi:10.1016/j.arth.2025.01.004
• Citak M et al. Elevated HbA1c is not a risk factor for wound complications following total joint arthroplasty: a prospective study. Hip Int 2020. doi:10.1177/1120700020926986
• MacMahon A et al. Preoperative patient optimization in total joint arthroplasty: the paradigm shift from preoperative clearance. HSS J 2021. doi:10.1177/15563316211030923
• DeMik DE et al. Complications and obesity in arthroplasty: a hip is not a knee. J Arthroplasty 2018. doi:10.1016/j.arth.2018.02.073
• Rubin J et al. A systematic review and meta-analysis of periprosthetic joint infection rates in morbidly obese patients undergoing total hip arthroplasty. J Am Acad Orthop Surg Glob Res Rev 2025. doi:10.5435/JAAOSGlobal-D-24-00306
• Telang S et al. Total hip arthroplasty among superobese patients: proceed with caution. Arthroplast Today 2025. doi:10.1016/j.artd.2025.101770
• Carender CN et al. Obesity and primary total hip arthroplasty: the absolute versus relative risk of periprosthetic joint infection at 15 years. J Arthroplasty 2024. doi:10.1016/j.arth.2024.03.033
• George J et al. Association between body mass index and thirty-day complications after total knee arthroplasty. J Arthroplasty 2017. doi:10.1016/j.arth.2017.09.038
• Spezia MC et al. Body mass index improvement reduces total knee arthroplasty complications among patients who have extreme, but not severe, obesity. J Arthroplasty 2024. doi:10.1016/j.arth.2024.08.054
• Madden-McKee CT et al. Body mass index of 45 is a safe cut-off for cementless total knee arthroplasty. J Arthroplasty 2025. doi:10.1016/j.arth.2025.12.038
• Wall CJ et al. Perception of perioperative risk for arthroplasty patients: a poll of Australian orthopedic surgeons. J Arthroplasty 2023. doi:10.1016/j.arth.2023.02.056
• Mulford JS et al. The association between body mass index and patient-reported outcome measures before and after primary total hip or knee arthroplasty: a registry study. ANZ J Surg 2023. doi:10.1111/ans.18449
• De Mauro D et al. Role of bariatric surgery in reducing periprosthetic joint infections in total knee arthroplasty: a systematic review and meta-analysis. BMC Musculoskelet Disord 2024. doi:10.1186/s12891-024-07288-2

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Next: Pre-op: preparing for your surgery — or preparing for a knee replacement.