An Abnormal Outer Meniscus on MRI and Partial Knee Replacement:
What a New Review Found

What the study asked

In the most common pattern of knee osteoarthritis, the cartilage wears out first on the inner (medial) side of the knee, while the outer (lateral) side is still in reasonable condition. For selected patients with this pattern — sometimes called anteromedial osteoarthritis — a medial unicompartmental knee arthroplasty (UKA), or partial knee replacement, resurfaces only the worn inner compartment and leaves the rest of the knee and its ligaments in place. (Our article on TKA, UKA and PFA explains the differences between these operations.)

Because a partial replacement relies on the untreated outer compartment continuing to work well, surgeons pay attention to its condition. The lateral meniscus — the C-shaped cushion of cartilage on the outer side — often looks abnormal on MRI in older knees, even when it causes no symptoms. That can range from a change in signal inside the meniscus (a sign of wear) to a definite tear or slight bulging outwards (extrusion). Opinions have differed on whether such findings should count against a partial replacement.

Zhou and colleagues set out to gather the published studies that compared patients with and without a lateral meniscus abnormality before medial UKA, and to combine their results statistically.

Who was studied and how

This was a systematic review and meta-analysis: the authors searched eight English- and Chinese-language databases up to March 2026 and pooled data from the studies that met their criteria. They found eight studies, all of them retrospective cohort studies (looking back at patients already treated), from South Korea, China, Japan, France and the USA. Together these included 1,709 patients — 368 with a lateral meniscus abnormality and 1,341 without.

Those headline numbers need some context. One American study contributed 1,000 patients, of whom only 33 had an abnormal MRI, and it did not provide data for the main pooled comparisons. The pooled analyses themselves therefore drew on smaller numbers: 454 patients for function scores, 350 for range of motion, and 259 for outer-knee pain. Average follow-up across studies ranged from just over a year to ten years. Six of the eight studies used a mobile-bearing partial knee implant, and all of the studies that fed into the function and range-of-motion analyses did.

The outcomes compared were knee function scores after surgery, how far the knee could bend and straighten, pain on the outer side of the knee, revision (re-operation to change the implant), and worsening of arthritis on the outer side on X-ray.

What the researchers found

Put simply, across the studies available, patients whose MRI showed an outer-meniscus abnormality reported, on average, similar function and movement after medial UKA to those whose outer meniscus looked normal. The authors' own conclusion is deliberately limited: an outer-meniscus abnormality that causes no symptoms should not, on its own, automatically rule out a medial partial knee replacement in an otherwise suitable patient.

Limitations — why this is low-certainty evidence

The authors graded their own evidence using a standard system (GRADE). They rated the findings on function and range of motion as low certainty, and the findings on outer-knee pain, revision and arthritis progression as very low certainty. The main reasons:

• All observational studies: every included study was retrospective. Patients were not randomly assigned, and surgeons in those studies were already selecting who was suitable for a partial replacement, so the results describe selected patients rather than everyone with an abnormal outer meniscus.
• "Abnormality" meant different things: the studies defined the lateral meniscus finding in different ways — from internal wear signals, to definite tears, extrusion, calcification, or simply an "abnormal MRI". Results for one type may not apply to another.
• Symptoms often unclear: only two studies clearly described their patients as having no outer-knee symptoms; the rest did not report this reliably. No study gave usable data on people with a painful, catching or unstable outer meniscus tear.
• Not about root tears: the authors specifically say the results should not be applied to meniscus root tears, unstable or traumatic tears, or knees with advanced cartilage loss on the outer side.
• Small numbers for key outcomes: revisions were rare, so the review cannot say whether long-term implant survival differs. Most follow-up was short to medium term.
• Mostly one implant type: the function and movement results come from mobile-bearing implants only, so they may not carry over to other designs.
• Few studies: with fewer than ten studies per comparison, the authors could not formally check whether studies with unfavourable results had gone unpublished.

What this means for patients

If you have arthritis on the inner side of your knee and your MRI report mentions some wear or a tear in the outer meniscus, this review suggests that finding alone is not necessarily a reason to rule out a partial knee replacement — provided it is not causing symptoms and the outer side of the knee is otherwise healthy. It does not show that partial replacement is the right choice for any particular person, and it says nothing reassuring about outer-meniscus problems that do cause pain or locking.

Whether a partial or a total knee replacement fits you depends on the whole picture: where your pain is, how the outer and kneecap compartments look on standing X-rays and MRI, whether your ligaments are intact, how your knee moves on examination, and your own goals. An MRI finding is one piece of that picture, not the decision itself. It is reasonable to ask your surgeon what your scans show on the outer side of the knee and how that has been weighed.

How this relates to the approach in this practice

This practice offers both total and partial knee replacement. Partial (medial unicompartmental) replacement is considered for selected patients whose arthritis is limited mainly to the inner compartment, after examination, standing X-rays and, where helpful, MRI. Kinematic alignment is the technique used here for total knee replacement; neither total nor partial knee replacement in this practice uses robotic systems. The implants and selection criteria used in the studies in this review are not necessarily the same as those used here, and the review does not compare its results with this practice. Studies like this are part of the ongoing evidence we follow when discussing which operation, if any, suits an individual knee.