Body Mass Index & Joint Replacement

Part of The Joint Replacement Guide — Phase 2: Considering Surgery · Article 12 of 44 · Hip & Knee

Body mass index (BMI) often comes up when hip or knee replacement is discussed. It may be considered when assessing surgical risk, but it can also become a source of frustration for people who feel that a single number is being given more importance than their pain, disability and overall health.

This article explains what BMI can—and cannot—tell us about joint-replacement surgery.

Key Points

  • BMI is one of several factors considered when planning hip or knee replacement. It does not provide a complete assessment of an individual’s health or surgical risk.
  • Some complications become more likely as BMI increases, particularly wound problems and infection. The size of the increase varies between studies and individuals.[1–3]
  • People across BMI categories generally experience substantial improvements in pain, function and quality of life after joint replacement.[1]
  • NICE states that people should not be excluded from referral for joint replacement because they are overweight or living with obesity.[1]
  • If weight loss would be beneficial, support should be realistic, respectful and individualised. Surgery should not automatically be withheld until an arbitrary BMI target is reached.

What Is BMI?

BMI compares weight with height:

BMI = weight in kilograms ÷ height in metres²

It is a convenient screening measurement, but it cannot distinguish between muscle and body fat or show how fat is distributed. It also does not directly measure fitness, nutritional health, frailty or how well conditions such as diabetes are controlled.

For these reasons, BMI should be considered alongside other information, including:

  • the effect of arthritis on your daily life;
  • heart and lung health;
  • diabetes control;
  • smoking;
  • anaemia and nutritional health;
  • physical fitness, mobility and frailty;
  • previous surgery; and
  • the anticipated complexity of the operation.

A BMI calculator can provide general information, but its result is not a decision about whether surgery is suitable.

Referral and Local Policies

NICE recommends that people with osteoarthritis should not be excluded from referral for joint replacement because of overweight or obesity measured using BMI.[1] The decision to refer should be based on a clinical assessment, including how substantially symptoms affect quality of life and whether appropriate non-surgical treatment has been ineffective or unsuitable.

Historically, local NHS commissioning policies have varied. A study of English policies in place in 2021 found BMI thresholds ranging from 30 to 45 kg/m². Some policies offered additional support, while others delayed or restricted access.[4] More recent research indicates that variation in local BMI policies has persisted.[5]

A local policy is not the same as a national NICE recommendation. Patients who are told that referral or surgery cannot proceed because of BMI alone can reasonably ask:

  • whether the decision relates to referral, hospital policy or an individual clinical risk assessment;
  • what specific risks apply to them;
  • what support is available;
  • whether exceptions or an individual review are possible; and
  • when and how the decision will be reconsidered.

What Does BMI Mean for Surgical Risk?

Research consistently finds associations between higher BMI and some complications after hip or knee replacement. These include wound problems, infection and, in some studies, readmission or further surgery.[2,3]

These are statistical associations across groups of patients. They cannot predict exactly what will happen to one person. Risk does not suddenly appear at a particular BMI, and people with the same BMI may have very different health profiles.

Relative-risk figures can also sound more alarming than they are. For example, describing a complication as “twice as likely” does not show whether the underlying risk has increased from 1 in 1,000 to 2 in 1,000 or from 1 in 20 to 1 in 10. Where possible, your team should explain your estimated absolute risk in understandable terms.

Although some risks may be higher, NICE’s review found that people who were overweight or living with obesity still experienced improvements in quality of life and patient-reported outcomes after joint replacement.[1]

BMI Calculator

This tool is for general information only and is not a substitute for assessment by your surgical or anaesthetic team. Local NHS policies on BMI and access to surgery vary by area — discuss your individual circumstances with your surgeon or GP.

About this calculator: BMI is a general screening measurement and does not provide an individual assessment of surgical risk or suitability. It cannot distinguish muscle from body fat and should be considered alongside your overall health, mobility and personal circumstances. NICE advises that overweight or obesity should not, by itself, prevent referral for consideration of joint replacement. Local pathways vary; ask your GP or surgical team what applies to you.

Anaesthesia

Body size can affect anaesthetic assessment and planning. Relevant considerations may include positioning, intravenous access, medicine dosing, breathing during and after surgery, and the possibility of obstructive sleep apnoea.

Having a higher BMI does not automatically mean that one type of anaesthetic is required. Spinal, regional and general anaesthesia each have potential advantages and disadvantages. NICE recommends offering people having primary hip or knee replacement an appropriate choice of anaesthetic techniques, taking account of their individual circumstances.[6]

If you use a CPAP machine for sleep apnoea, tell the pre-assessment team and follow the hospital’s instructions about bringing it with you. Let the team know if you snore heavily, have witnessed pauses in breathing during sleep or experience marked daytime sleepiness.

Recovery and Wound Care

Wound healing and infection are important areas of attention when BMI is higher. The exact level of risk depends on more than BMI alone and may also be affected by diabetes control, smoking, nutrition, skin condition, the complexity of surgery and other health problems.

The clinical team may plan:

  • appropriate antibiotics around the operation;
  • blood-clot prevention based on individual risk;
  • careful positioning and pressure-area care;
  • suitable mobility equipment;
  • closer wound observation; and
  • additional support after discharge where required.

These measures reduce risk but cannot remove it completely.

Rehabilitation

Early movement after hip or knee replacement supports recovery and helps reduce problems associated with immobility. Rehabilitation should be adapted to your starting level of strength, mobility, balance and confidence.

Equipment and exercises should be suitable for your needs. If standard chairs, walking aids or other equipment are uncomfortable or unsafe, raise this before surgery so alternatives can be arranged.

Patients with a higher BMI can still achieve substantial improvements in pain and function. Recovery varies widely, and it is not helpful to assume that BMI alone determines how quickly or how well someone will rehabilitate.

Hip-Specific Considerations

Higher BMI has been associated with an increased risk of dislocation after total hip replacement, although estimates vary considerably. A large systematic review found a more modest association than some older individual studies: BMI of 30 kg/m² or above was associated with a relative risk of approximately 1.38 compared with BMI below 30 kg/m².[7]

Dislocation risk also depends on several other factors, including previous hip or spinal surgery, muscle and neurological conditions, surgical approach, implant design and component positioning.[7] Your surgeon may take these factors into account when selecting the operative approach and implant.

Knee-Specific Considerations

For knee replacement, wound complications and infection are among the main concerns associated with higher BMI. Surgical exposure and positioning of the components may also be more technically demanding.

This does not mean that a good outcome is unlikely. Large studies considered by NICE found meaningful improvements in symptoms and quality of life across BMI categories.[1] Individual benefits and risks should be discussed together rather than considering complication risk in isolation.

Does Losing Weight Before Surgery Help?

Weight loss may improve general health, mobility, diabetes control and osteoarthritis symptoms in some people. A small systematic review of three randomised trials found that structured preoperative weight-loss interventions achieved weight loss and reported fewer postoperative complications.[8] However, the trials included only 198 people, used different interventions and do not establish that every patient must lose weight before surgery.

Other reviews have found that evidence about whether short-term preoperative weight loss improves joint-replacement outcomes remains limited or inconsistent.[9] Rapid weight loss can also reduce muscle mass or worsen nutrition, which may be unhelpful before a major operation.

The balanced approach is therefore:

  • offer evidence-based weight-management support where it is wanted and clinically appropriate;
  • agree realistic goals without blame or stigma;
  • protect muscle strength and nutritional health;
  • assess improvements in health rather than focusing only on a number; and
  • avoid treating failure to reach an arbitrary BMI target as an automatic reason to withhold referral indefinitely.

Do not undertake a crash diet or start weight-loss medication specifically for surgery without discussing it with an appropriate healthcare professional.

The Bottom Line

BMI provides useful but limited information. A higher BMI may increase certain surgical risks, but it does not show whether someone will benefit from joint replacement and should not be considered in isolation.

The appropriate conversation is not simply, “Is my BMI too high?” It is:

“What are my individual benefits and risks, which risks can reasonably be reduced, and what support will help me prepare safely?”

This article provides general information and does not replace an individual assessment by your surgical or anaesthetic team. Risks, hospital resources and local pathways vary.

References

Seward MW, Briggs LG, Bain PA, Chen AF. Preoperative nonsurgical weight loss interventions before total hip and knee arthroplasty: a systematic review. J Arthroplasty. 2021;36(11):3796–3806.e8. doi:10.1016/j.arth.2021.06.048. Available from: https://pubmed.ncbi.nlm.nih.gov/34247869/for that individual, and to give an honest picture of the risks involved.

National Institute for Health and Care Excellence. Osteoarthritis in over 16s: diagnosis and management. NICE guideline NG226. London: NICE; 2022. Recommendations 1.6.1–1.6.4 and rationale. Available from: https://www.nice.org.uk/guidance/ng226/chapter/Recommendations

Onggo JR, Onggo JD, de Steiger R, Hau R. Greater risks of complications, infections and revisions in the obese versus non-obese total hip arthroplasty population of 2,190,824 patients: a meta-analysis and systematic review. Osteoarthritis Cartilage. 2020;28(1):31–44. doi:10.1016/j.joca.2019.10.005. Available from: https://pubmed.ncbi.nlm.nih.gov/31705995/

National Institute for Health and Care Excellence. Evidence review for outcomes of joint replacement surgery dependent on body mass index. Evidence review P supporting NICE guideline NG226. London: NICE; 2022. Available from: https://www.nice.org.uk/guidance/ng226/evidence/p-outcomes-of-joint-replacement-surgery-dependent-on-body-mass-index-pdf-405016302429

McLaughlin J, Elsey J, Kipping R, et al. Access to hip and knee arthroplasty in England: commissioners’ policies for body mass index and smoking status and implications for integrated care systems. BMC Health Serv Res. 2023;23:77. doi:10.1186/s12913-022-08999-9. Available from: https://pmc.ncbi.nlm.nih.gov/articles/PMC9875525/

Ofosu K, Whale K, McLaughlin J. Persistent use of body mass index policies as a barrier to surgery: prevalence and analysis of policies across England in 2025. J Health Serv Res Policy. 2025. doi:10.1177/13558196251405207. Available from: https://journals.sagepub.com/doi/10.1177/13558196251405207

National Institute for Health and Care Excellence. Joint replacement (primary): hip, knee and shoulder. NICE guideline NG157. London: NICE; 2020. Section 1.3. Available from: https://www.nice.org.uk/guidance/ng157/chapter/Recommendations

Kunutsor SK, Barrett MC, Beswick AD, et al. Risk factors for dislocation after primary total hip replacement: a systematic review and meta-analysis of 125 studies involving approximately five million hip replacements. Lancet Rheumatol. 2019;1(2)–e121. doi:10.1016/S2665-9913(19)30045-1. Available from: https://pubmed.ncbi.nlm.nih.gov/38229338/

Lau LCM, Chan PK, Lui TWD, et al. Preoperative weight loss interventions before total hip and knee arthroplasty: a systematic review of randomized controlled trials. Arthroplasty. 2024;6:30. doi:10.1186/s42836-024-00252-4. Available from: https://link.springer.com/article/10.1186/s42836-024-00252-4

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