The short answer to the candidacy question

Partial knee replacement — or unicompartmental knee replacement (UKR) — may be an option if your knee arthritis is strictly confined to one section of the joint and certain structural conditions are met. If degeneration has spread across more than one compartment, a partial replacement is not appropriate; a total replacement is more likely to be the right choice.

The single most important piece of evidence is a weight-bearing X-ray. Imaging taken whilst you are standing reveals exactly which compartments are under load and whether the damage is genuinely isolated — something a scan taken lying down can understate. From that image, combined with a physical examination of your ligaments and the movement available in your knee, a surgeon determines whether UKR is feasible for you.

Candidacy cannot be established from symptoms alone. Inner-knee pain is suggestive but not sufficient — a consultant assessment is the necessary next step. It is also worth knowing that UKR requires specialist training: fewer than one in ten orthopaedic surgeons perform it routinely, which makes the operating surgeon's experience a meaningful part of the decision.

Why compartment anatomy is the starting point

The knee is divided into three compartments: the medial (inner side), the lateral (outer side), and the patellofemoral (where the kneecap meets the lower end of the thigh bone). UKR resurfaces only the arthritic compartment, leaving the others untouched — but that precision depends entirely on establishing which compartments are damaged and which remain healthy.

Medial compartment disease — specifically anteromedial osteoarthritis (AMOA) — is the most common indication, and the evidence base for medial UKR is substantially larger than for lateral or patellofemoral variants. Both lateral and patellofemoral UKR are performed in appropriately selected patients, but the long-term outcome data for these are less extensive than for the medial procedure.

Confirming compartment status requires a weight-bearing X-ray — taken whilst standing, so the joint reflects the load it carries in normal activity. Imaging taken lying down can underestimate joint-space narrowing in a loaded compartment and may miss relevant disease in a neighbouring one. MRI adds important detail on cartilage thickness, bone quality, and the integrity of the anterior cruciate and collateral ligaments — information that weight-bearing X-ray alone cannot provide. The two modalities are complementary rather than interchangeable, and together they give the surgeon a complete picture of which compartments are involved before any decision about UKR is made.

The clinical criteria surgeons use

During a surgical assessment for UKR, your surgeon works through several structural and clinical criteria — not a rigid exclusion list, but a way of confirming that the joint mechanics and soft-tissue architecture are genuinely compatible with a single-compartment implant.

Ligament integrity

An intact anterior cruciate ligament (ACL) is the classical requirement. Because UKR resurfaces only one compartment, the ACL provides the stability the implant depends on; a ruptured or functionally deficient ACL alters joint mechanics in ways a unicompartmental device is not designed to manage. That position has, however, evolved: for carefully selected patients — those with medial OA, a tibial slope below 10°, and no patellofemoral involvement — combined UKR with simultaneous ACL reconstruction has produced KOOS, Oxford Knee Score, and WOMAC outcomes comparable to TKR at ten years. The approach is not yet considered standard practice, but it means ACL deficiency is no longer an automatic disqualification for every patient. The collateral ligaments must also be functionally intact; they work with the ACL to maintain alignment, and their preservation is central to the more natural loading pattern that UKR patients often describe.

Deformity and range of motion

A correctable varus or valgus deformity is acceptable, but a fixed flexion deformity of 15 degrees or more prevents the implant from seating correctly and is a contraindication. Range of motion is assessed directly: at least 90 degrees of flexion, with less than 5 degrees of fixed flexion contracture, is typically required for the implant to function well — making this criterion concrete rather than a matter of clinical impression alone.

Mechanical rather than inflammatory arthritis

UKR is designed for mechanical osteoarthritis or localised avascular necrosis — conditions where structural damage is genuinely confined. Inflammatory arthritis (rheumatoid, psoriatic, and related conditions) is a firm contraindication because these diseases affect synovial tissue throughout the entire joint; damage that appears localised on imaging may progress in neighbouring compartments after a partial replacement.

Prior conservative management

Before surgery is considered, patients should have completed a meaningful trial of conservative care — typically at least six months of physiotherapy, weight management where relevant, and injection therapies such as steroid or hyaluronic acid. This threshold matters for two reasons: a significant proportion of patients improve sufficiently without surgery, and completing it confirms the pain has a structural basis that an implant is positioned to address.

Age and weight: where the evidence has moved

Two concerns bring patients to a consultation having already assumed they will not qualify: "I'm too young for a partial replacement" and "my weight will rule me out." Current evidence gives neither concern the weight it once carried.

On BMI, a study with a minimum seven-year follow-up found that obesity had no adverse effect on UKR clinical outcomes — directly contradicting earlier frameworks that treated elevated weight as a near-automatic exclusion. Longer-term data reinforce this picture: a 2023 study reporting 15-year survivorship of medial mobile-bearing UKA found no statistically significant difference in implant survival by sex, BMI, or age, with overall 15-year survival reaching 84.1% (95% CI 80.3–88%). Together, these findings suggest that weight and age, while they remain part of the surgical risk conversation, are not the determining factors they were once treated as.

The position on age has shifted in much the same way. Traditional guidance — often citing 60 as a lower bound — was driven partly by concerns about activity-related wear and partly by early registry data. Modern evidence does not consistently support a firm age cut-off. For younger or more active patients, the surgeon may weigh UKR against high tibial osteotomy (HTO), which preserves native anatomy and tends to carry a lower revision rate; UKR, by contrast, delivers less postoperative pain, fewer complications, and superior WOMAC scores in a meta-analysis of 38 studies covering nearly 9,000 knees. Range of motion favours HTO in that comparison — a nuance that matters for patients with high functional demands. The right choice between these two paths depends on structural anatomy, activity level, and individual risk profile, and is properly reached at assessment rather than assumed in advance.

BMI and age remain part of the decision — but as context-dependent factors within a broader clinical picture, not as thresholds that automatically close the door.

How UKR differs from total knee replacement in practice

For patients weighing up UKR against total knee replacement, the practical differences begin before the theatre.

Because UKR resurfaces only one compartment, the procedure involves a smaller incision, reduced intraoperative blood loss, and — for appropriate patients — is often performed as day surgery. In one NHS cohort, 48.5% of UKR patients were discharged on the day of surgery, with average cost savings of £577 to £1,429 per case compared with an inpatient pathway.

On formal outcome measures — Oxford Knee Score, Knee Society Score, and WOMAC — UKR achieves equivalent results to TKR at six months in well-selected patients under the same enhanced recovery protocol. What those scores alone do not easily capture emerges from a gait analysis study of patients who had undergone UKR on one side and TKR on the other: in every loading phase of walking, participants consistently placed more weight through the UKR limb — 22% more at weight acceptance and midstance — with loading patterns closer to those of a healthy knee than the TKR side produced. That measurable preference reflects the preserved ligaments and cartilage in the intact compartments continuing to guide movement as they naturally would.

A large US database study covering 104 million patients reported that conversion to TKR within the first year was the most common complication after UKR, occurring in approximately 39.9% of cases — a figure the authors attributed to suboptimal patient selection rather than inherent implant failure. It is a reminder that the procedure's outcomes depend heavily on identifying the right candidate in the first place, and why a thorough specialist assessment is not a formality.

TKR remains the correct procedure for bi- or tri-compartmental disease. UKR is not a lesser version of total knee replacement — it is a different operation for a different clinical situation, one that delivers equivalent measured function whilst preserving more of the knee's natural mechanics where the anatomy genuinely allows it.

What an assessment involves and when to seek one

The assessment itself is relatively straightforward. A first appointment typically involves a clinical examination — checking range of motion, ligament stability, and compartment-specific tenderness — alongside a weight-bearing X-ray. Where the soft-tissue picture is uncertain, an MRI is used to confirm that cartilage damage and ACL integrity are genuinely confined to a single compartment. In cases where objective functional data would add to the clinical picture, biomechanical gait assessment using MAI Motion® can provide measurable loading information to inform the decision.

Patients will usually be asked to complete the Oxford Knee Score (OKS) at their first appointment. The 12-item questionnaire covers pain and function across everyday activities, with 48 indicating no symptoms. It establishes the baseline against which any post-operative progress is tracked, and remains the main tool surgeons use to monitor how the knee responds over time.

If conservative management — physiotherapy, weight optimisation, or injection therapy — has not yet been completed, the assessment may confirm that those options should come first. A surgeon offering UKR is not bypassing that pathway; they are confirming whether a patient has reached the point where surgery is genuinely the next appropriate step.

In England, NHS waiting times for orthopaedic assessment were running at around 20 weeks in 2024, with over 800,000 patients on the trauma and orthopaedics list as of March that year. For patients who would prefer not to wait, Lincolnshire Knee is part of the MSK Doctors group and accepts patients without a GP referral, with consultations and diagnostics available at Sleaford NG34 and Grantham NG31. An assessment can be arranged at lincolnshireknee.co.uk.

1. [1] Obesity has no adverse effect on the outcome of unicompartmental knee replacement at a minimum follow-up of seven years. (2013). https://doi.org/10.1302/0301-620X.95B8.31370 https://doi.org/10.1302/0301-620X.95B8.31370
2. [2] Achieving discharge within 24 h of robotic unicompartmental knee arthroplasty may be possible with appropriate patient selection and a multi-disciplinary team approach. (2020). https://doi.org/10.1016/j.jor.2020.01.051 https://doi.org/10.1016/j.jor.2020.01.051
3. [3] The unicompartmental knee is the preferred side in individuals with both a unicompartmental and total knee arthroplasty. (2019). https://doi.org/10.1007/s00167-019-05814-7 https://doi.org/10.1007/s00167-019-05814-7
4. [4] Unicompartmental knee arthroplasty versus high tibial osteotomy for medial knee osteoarthritis: A systematic review and meta-analysis. (2023). https://doi.org/10.1177/10225536231162829 https://doi.org/10.1177/10225536231162829
5. [5] 15-Year Follow Up of Mobile Bearing Medial Unicompartmental Knee Arthroplasty. (2023). https://doi.org/10.1016/j.arth.2023.01.024 https://doi.org/10.1016/j.arth.2023.01.024