How surgeons decide who can and cannot have surgery

Being told you might not qualify for knee replacement is rarely a final answer. Surgeons work from a structured framework that divides barriers to surgery into two distinct categories: absolute contraindications, where an operation cannot safely proceed regardless of optimisation, and relative contraindications — modifiable barriers that delay surgery rather than permanently closing the door.

Someone told they are 'not a candidate' because of their weight, smoking habit, or poorly controlled blood sugar is almost certainly facing a relative contraindication — one with a clear pathway through it. A patient with an active infection, by contrast, faces an absolute stop until that infection is fully treated. The two categories call for very different responses, and conflating them leads to unnecessary despair or, at the other extreme, avoidable risk.

In 2024, the Journal of Arthroplasty published a formal consensus paper categorising these conditions, giving surgeons a consistent clinical framework to apply. Even so, no single factor — not BMI, age, nor any individual diagnosis — automatically disqualifies a patient in every UK setting; institutional thresholds and commissioning policies vary considerably.

Conditions that permanently rule out knee replacement

Five conditions constitute hard stops — situations where no amount of pre-operative optimisation changes the surgical calculus.

• Active infection anywhere in the body. Bacteria from an untreated infection — whether in the knee joint, skin, urinary tract, dental, or respiratory sites — can seed a metal and plastic implant during surgery. Once established, periprosthetic joint infection carries roughly a 20% mortality rate, a risk no elective procedure can justify. Every candidate is screened for active infection before a date is set, including Staphylococcus skin colonisation.
• Severe uncontrolled systemic illness. Advanced cardiac failure or severe respiratory disease can make the physiological demands of anaesthesia and major surgery life-threatening. Surgery is deferred — sometimes permanently — until the risk-benefit calculation shifts.
• Complete extensor mechanism dysfunction. The quadriceps muscle and patellar tendon together extend (straighten) the knee. If this system is non-functional, a prosthesis has no mechanical foundation to work from; inserting one achieves nothing.
• Active deep vein thrombosis or pulmonary embolism. Operating on a patient with an active clot significantly amplifies thromboembolic risk. Surgery is postponed until the clot is treated and stable.
• Pain not originating from the knee joint. If symptoms arise from the spine, hip, vascular disease, or nerve damage rather than structural knee pathology, replacing the joint will not relieve them. Identifying the true pain source before listing a patient is therefore as important as any scan.

Modifiable factors that delay, not permanently deny

Four factors come up repeatedly in pre-operative assessments — not as closed doors, but as tasks to work through before a date can be set.

Smoking and nicotine use. Nicotine constricts blood vessels and slows wound healing, raising the risk of poor wound closure and infection at the surgical site. In a survey of 131 Australian GPs, 90% actively addressed smoking before referring patients for joint replacement, and 57% supported postponing the listing until the patient had stopped. UK surgical practice broadly reflects the same position.

Poorly controlled diabetes. Elevated HbA1c is a documented complication risk in surgical patients. Ninety-four per cent of the same surveyed GPs raised diabetic control before referral, and 84% supported deferral until blood sugar was within an acceptable range. For most patients this represents a wait of weeks to months, not an indefinite postponement.

Inability to participate in rehabilitation. Recovery from knee replacement depends heavily on post-operative physiotherapy compliance. Where cognitive impairment, severe physical limitation, or lack of carer support makes that unlikely, the pre-operative conversation becomes one of realistic expectations — not automatic exclusion, but an honest assessment of what the surgery can and cannot achieve for that individual.

IV corticosteroid therapy. Patients receiving intravenous steroids — for vasculitis, for example — face amplified bleeding and infection risks. The timing of surgery and any bridging arrangements are decisions made jointly between the patient's physicians and the surgical team.

Nutritional status. Low serum albumin appears in surgical guidance as a risk factor for poor healing. Where it is identified pre-operatively, the typical response is a referral to a dietitian, a period of nutritional support, and re-assessment before listing — a delay measured in weeks, not a permanent bar.

What BMI evidence and NHS policy actually mean for patients

Body weight generates more pre-operative conversation than almost any other modifiable factor — and more confusion, because the clinical evidence and NHS policy have been pulling in different directions.

The evidence is unambiguous: higher BMI is associated with measurable increases in surgical risk. An NHS Scotland study of 50,751 hip and knee replacement patients found that BMI ≥40 raised the odds of a prolonged hospital stay 1.43-fold; at BMI ≥50, that figure reached 1.89-fold, with 30-day readmission rates climbing to 17.5% in the most severely affected group. These are real risks that surgeons are right to discuss openly.

Policy is more complicated. NICE updated its arthritis guidance in October 2022 to state explicitly that BMI should not be used alone to exclude patients from referral to surgery. That is a significant national position. Yet 37% of UK integrated care boards still operate restrictive BMI thresholds — LLR ICB, for instance, sets a hard ceiling of BMI ≤45 and requires referral to weight management at BMI >35. Hertfordshire and West Essex frames BMI above 30 as a relative contraindication but encourages reduction rather than outright exclusion. Local policy and national guidance do not always agree.

The tension carries an equity dimension. The OAKS study, whose protocol was published in BMJ Open in 2026, is actively investigating this: people with BMI ≥35 carry approximately 19 times the population risk of needing total knee replacement — making blanket exclusion a question of access as much as clinical risk.

For patients, the practical position is this: BMI is a modifiable risk factor to address before surgery, not a fixed verdict. A referral to weight management is clinically reasonable; indefinite exclusion on BMI alone may not align with current NICE guidance. If a patient has been told they do not qualify solely because of BMI, it is worth asking their surgeon — or seeking a second assessment — whether that decision reflects national guidance or local commissioning policy.

Why failed conservative care must come first

Ruling out contraindications is only half of the candidacy picture. Before surgery is offered, there also needs to be a positive finding: that non-surgical options have been genuinely tried and found inadequate.

What counts as conservative care is broader than many patients realise — physiotherapy, oral analgesics and NSAIDs, structured weight management, corticosteroid or hyaluronic acid injections, and walking aids, used in combination rather than ticked off one at a time. The Hertfordshire and West Essex NHS ICB, for example, specifies a package including exercise, weight reduction, anti-inflammatories, injections, and walking aids as a prerequisite for referral.

'Failure' of that package is defined by impact, not duration. There is no fixed minimum of months or number of interventions required; the standard is functional — pain, mobility, and quality of life remain unacceptable despite optimal non-surgical management.

This requirement is a genuine clinical safeguard: some patients improve substantially once physiotherapy and weight management are properly structured, and surgery they had anticipated needing becomes unnecessary. For patients who have spent years managing knee osteoarthritis, the threshold is often already met. Arriving at a first consultation with a record of what has been tried — and for how long — is practical preparation that helps a surgeon form a clear picture quickly.

Partial knee replacement and its different patient profile

Unicompartmental knee replacement (UKR) targets only one of the three knee compartments — medial, lateral, or patellofemoral. It is not a scaled-down version of total knee replacement; it is a different operation designed for a different pattern of disease. A patient who does not qualify for TKR is not automatically a UKR candidate, and the reverse holds equally.

The clearest dividing line is disease extent. Total knee replacement is indicated when all three compartments are significantly affected. UKR becomes the option when structural damage is confined to a single compartment with the remainder of the joint meaningfully preserved. Patients with widespread tri-compartmental OA do not satisfy this requirement, regardless of their other characteristics.

Traditional UKR candidacy was tightly drawn. Guidelines once excluded patients with ACL deficiency, patellofemoral OA, high BMI, or age under 60. That picture is shifting. A 15-year follow-up study found that anterior knee pain and evidence of patellofemoral wear did not significantly affect outcomes in mobile-bearing UKR, directly challenging their status as contraindications. Age thresholds once capped at 60 are similarly contested by emerging evidence of acceptable outcomes in younger cohorts; BMI limits and ACL integrity are under comparable scrutiny, though no settled consensus has replaced the older rules.

For patients who are younger, more active, or not yet at the threshold that justifies a full joint replacement, UKR — or a joint-preserving procedure such as osteotomy — may be a more appropriate intermediate step rather than an earlier TKR. Compartment pattern, activity goals, and the surgeon's assessment of what is worth preserving all bear on this judgement. Imaging, clinical examination, and a specialist consultation are what translate those variables into a clear recommendation — not a checklist.

1. [1] Australian GPs' Perception of Modifiable Risk Factors for Joint Replacement Infection (2024). (2024). https://doi.org/10.31128/AJGP-06-23-6880 https://doi.org/10.31128/AJGP-06-23-6880
2. [2] Obesity and Arthritis of the Knee Study (OAKS): Protocol (BMJ Open 2026). (2026). https://doi.org/10.1136/bmjopen-2025-112478 https://doi.org/10.1136/bmjopen-2025-112478
3. [3] Impact of Obesity on Outcomes after Total Hip and Knee Replacement: NHS Scotland Study (2025). (2025). https://doi.org/10.1016/j.ijotn.2025.101216 https://doi.org/10.1016/j.ijotn.2025.101216
4. [4] High Mortality After TKA Periprosthetic Joint Infection Related to Preoperative Morbidity (2022). (2022). https://doi.org/10.1016/j.arth.2022.03.046 https://doi.org/10.1016/j.arth.2022.03.046