What is the recommended Body Mass Index (BMI) cut-off for knee total replacement?

From the Guidelines

The recommended Body Mass Index (BMI) cut-off for knee total replacement is not an absolute threshold, but rather a conditional recommendation based on individual patient factors, with a BMI of 40 kg/m² or higher being considered high-risk, as stated in the 2023 American College of Rheumatology and American Association of Hip and Knee Surgeons clinical practice guideline 1.

Key Considerations

• The decision to proceed with knee replacement surgery should be made through a shared decision-making process between the patient and their physician, taking into account the unique risks and benefits for the individual patient.
• Patients with a BMI of 40-49,35-39, or ≥50 should be conditionally recommended to proceed to TJA without delaying to achieve weight reduction to a BMI of <40, <35, or <50, respectively, as the evidence supporting a preoperative weight reduction and a rigid BMI or weight threshold is indirect and very low quality 1.
• The use of absolute BMI or rigid thresholds is discouraged, as not all patients have the necessary medical, financial, or social support and resources to effectively lose weight at all or within a suitable timeframe.
• Patients who have medical or surgical risk factors, such as obesity, diabetes mellitus, and nicotine use, should be counseled as to their increased risks, and preoperative attempts to modify these risk factors through efforts such as weight loss, glycemic control, or smoking cessation should be encouraged.

Recommendations

• In patients who have moderate-to-severe symptomatic OA or advanced symptomatic ON with secondary arthritis who are indicated for TJA and for whom nonoperative therapy has been ineffective, proceed directly to surgery without delay for additional nonoperative treatment of the joint problem.
• For patients who have obesity and moderate-to-severe symptomatic OA or advanced symptomatic ON with secondary arthritis who are indicated for TJA, do not delay surgery to meet a rigid weight or body mass index threshold, but rather educate patients on the increased risk of medical and surgical complications due to their obesity and counsel them on how to lose weight 1.
• In patients with diabetes mellitus and moderate-to-severe symptomatic OA or advanced symptomatic ON with secondary arthritis who are indicated for TJA, delay surgery to allow for improved glycemic control, as poor glycemic control is associated with increased risk of poor outcomes after TJA 1.
• In patients with nicotine dependence and moderate-to-severe symptomatic OA or advanced symptomatic ON with secondary arthritis who are indicated for TJA, delay TJA to achieve nicotine cessation or decreased use of nicotine products, as nicotine use is associated with increased medical and surgical risks in TJA 1.

From the Research

BMI Cut-off for Knee Total Replacement

The recommended Body Mass Index (BMI) cut-off for knee total replacement is a topic of ongoing debate. Several studies have investigated the relationship between BMI and outcomes following total knee arthroplasty (TKA).

• A study published in The Journal of Arthroplasty in 2022 2 found that a BMI cut-off of 40 kg/m2 would prevent 17.3% of all complications, while allowing 94.8% of complication-free surgeries to proceed.
• Another study published in the International Journal of Orthopaedic and Trauma Nursing in 2021 3 suggested that the use of arbitrary BMI thresholds for TKA surgery may not be justified, as the evidence on the relationship between obesity and TKA outcomes is mixed.
• A study published in The Journal of Knee Surgery in 2022 4 found that strict enforcement of a BMI cut-off for TKA is variable among surgeons, and that many morbidly obese patients may not be able to lose enough weight to become eligible for surgery.
• A study published in The Journal of Arthroplasty in 2020 5 found that patients with a BMI ≥40 kg/m2 who underwent TKA had a higher rate of surgical site infections, but overall complications were not higher in this group.
• A study published in BMC Musculoskeletal Disorders in 2022 6 found that patients with obesity reported similar improvements in pain, function, and quality of life following TKA as those with normal weight.

Key Findings

• The use of BMI cut-offs for TKA surgery is not universally agreed upon, and the evidence on the relationship between obesity and TKA outcomes is mixed.
• A BMI cut-off of 40 kg/m2 may prevent some complications, but may also limit access to surgery for many patients.
• Patients with obesity may still benefit from TKA, with similar improvements in pain, function, and quality of life as those with normal weight.
• The decision to undergo TKA should be made on a case-by-case basis, taking into account individual patient factors and risks.