What a bone marrow lesion actually is

Reading 'bone marrow lesion' or 'bone marrow oedema' on an MRI report can be alarming — the phrase sounds serious, and most patients have never encountered it before. In practice, a BML is an area of abnormal signal detected in the subchondral bone: the layer of bone that sits directly beneath the cartilage surface. On fat-suppressed MRI sequences — such as STIR or proton-density fat-saturated images — this zone appears brighter than the surrounding bone, indicating that something has changed in its normal tissue composition.

The older label, 'bone marrow oedema', implied that the bright signal was simply caused by fluid accumulating in the bone, much like ankle swelling after a sprain. That framing has since been revised. Research showed that what the MRI is actually detecting is a combination of local inflammation, microdamage to the fine bony architecture, and repair activity — not straightforward fluid influx. The updated term, 'bone marrow lesion', better reflects that biological complexity, though 'oedema' still appears in many radiology reports because both terms remain in clinical circulation.

A BML is neither a fracture nor a tumour. It sits within a spectrum of subchondral bone stress responses — changes that can range from mild and transient to persistent and structurally significant depending on the underlying cause, its size, and its location within the knee.

How BMLs influence OA progression

A growing BML carries meaningful structural consequences — and the scale of that risk is now quantified. A deep-learning analysis of 3,869 knees without established osteoarthritis at baseline found that knees where BMLs progressed over four years were roughly three times more likely to develop measurable OA changes on X-ray compared with knees that stayed BML-free (hazard ratio 3.0), and twice as likely compared with knees where the BML regressed (HR 2.0). The direction of change matters as much as presence or absence.

Risk does not switch on at a threshold; it scales with magnitude. Larger BML volume increases were associated with proportionally greater hazard — for radiographic OA (HR up to 2.0 for the biggest increases) and for symptomatic OA (HR up to 1.7). This graded, dose-dependent pattern means tracking a BML's trajectory, not merely noting its existence, carries genuine prognostic value.

Location compounds the picture. The medial femoral and tibial subregions carried the highest structural risk in that cohort — the same compartments that absorb the greatest load in ordinary walking and standing.

A separate body of evidence, using correlation rather than hazard ratios, puts numbers on the cartilage side of that risk. Baseline BML volume in the tibia correlates with concurrent full-thickness cartilage loss (r = 0.63), while baseline femoral BML volume predicts how much cartilage is lost over the following 24 months (r = 0.48). In a multi-cohort study of 629 participants, a three-grade increase in maximum BML size was associated with an additional 0.306 mm of femorotibial cartilage thinning. Whether BMLs directly accelerate cartilage breakdown or largely reflect underlying joint stress — or both — remains an open question, but the two processes are linked. Equally important: BML regression is associated with substantially lower structural risk, which gives practical weight to load-reduction strategies aimed at settling the subchondral environment.

Why a BML does not always mean proportional pain

Patients sometimes arrive at a consultation confused in one of two directions: either their scan shows a BML yet they feel relatively little discomfort, or they are in considerable pain and assume the BML must be entirely to blame. The population data support both instincts — and neither.

In the 2025 Bunkyo Health Study, which followed 1,145 elderly participants, a BML on its own was not significantly associated with knee pain (odds ratio 1.32; 95% CI 0.95–1.83). The association crossed into statistical significance only when the BML coexisted with other subchondral changes: bone attrition raised the odds ratio to 2.22, and subchondral cysts to 1.79. In other words, the BML is part of a picture, not the picture itself.

This imaging–symptom mismatch is a well-recognised feature of knee osteoarthritis more broadly — structural changes on MRI and pain intensity correlate imperfectly across multiple tissue types, not only BMLs.

For the patient sitting with a scan report, the practical implication is this: neither the presence of a BML nor its absence settles the question of what is causing your pain. A specialist who weighs the BML alongside your symptom pattern, the compartment affected, whether bone attrition or cystic change is also present, and your clinical examination is far better placed to explain the relationship — and to decide whether further investigation or treatment is warranted.

BML grade and the risk of eventual surgery

Grade and compartment together determine how much weight a BML carries as a surgical signal. In a retrospective cohort of 1,011 knee OA patients followed over five years, high-grade BMLs — classified as grades 2 or 3 — in the medial and patellofemoral compartments were strongly associated with incident joint surgery. Lateral compartment BMLs, however, showed no such association, which means anatomic location is not incidental: the same grade in a different compartment can carry a materially different prognosis.

The flip side of that finding is equally informative. The cohort data showed high negative predictive value for BML-free knees — put plainly, patients whose MRI shows no significant BML face a low probability of requiring joint surgery within the next five years. Absence of a substantial BML is genuinely reassuring, not merely the absence of bad news.

For patients with moderate-to-high-grade medial or patellofemoral BMLs, the evidence supports treating this as a signal to bring forward monitoring and management review — not as a prediction that surgery is the next step. Many patients in this category are managed with load modification, physiotherapy, and injection-based options for extended periods. High grade indicates heightened vigilance, not inevitability.

BMLs and injection candidacy — what the evidence does and does not say

Three categories of treatment interact with BML findings in different ways — standard intra-articular injections, subchondral-targeted procedures, and systemic agents — and their evidence bases are unequal enough to treat separately.

For standard intra-articular options (hyaluronic acid, corticosteroid, and PRP), no randomised trial has yet tested whether BML grade or volume predicts who responds, so BML-specific candidacy thresholds remain undefined. The broader MSK literature suggests that advanced subchondral disease tends to reduce hyaluronic acid efficacy, though this has not been studied at the BML level specifically. None of these injections are contraindicated by a BML finding; they simply have not been evaluated with BML burden as a stratifying variable. The same is true for orthobiologics: BMLs harbour mesenchymal stem cell dysregulation in the subchondral zone, which provides a plausible biological rationale for interest in PRP and BMAC, but direct evidence that either agent modifies BML trajectory has not yet been established. Across all of these options, the pattern is the same — the evidence gap is real and consistent, not an oversight in any single trial.

Bisphosphonates follow a similar arc. A propensity-matched Osteoarthritis Initiative analysis found no statistically significant reduction in total BML volume over 12 months (p = 0.4). Women with pre-existing BMLs at baseline showed a trend towards volume decrease in bisphosphonate initiators (p = 0.03), a signal that requires confirmation before it can inform clinical decisions.

Subchondroplasty stands apart from all of these. Rather than an intra-articular injection, it is a percutaneous procedure that deposits calcium phosphate directly into the subchondral lesion — localised on pre-operative MRI and guided by intraoperative fluoroscopy. It targets the lesion itself rather than the joint space, making it mechanistically distinct, though its evidence base remains limited and it represents a specialist-led procedural pathway rather than a routine injection option.

What to do after a BML is found on your MRI

Finding a BML on your MRI report rarely requires urgent action — the more useful question is what the grade, compartment, and your current symptom burden, taken together, suggest about the right pace of response.

For a low-grade or incidental BML with minimal symptoms, the appropriate starting point is conservative management: load modification, targeted physiotherapy, and activity adjustment to reduce mechanical stress on the affected compartment. Many lesions remain stable or regress over this period without further intervention.

A moderate-to-high-grade BML in the medial or patellofemoral compartment — particularly alongside subchondral attrition or cystic change — justifies earlier MSK review rather than a watchful-waiting approach. The evidence covered above suggests this combination carries meaningful risk for both cartilage loss and, over time, surgical escalation.

When specialist assessment does occur, it typically covers clinical history, physical examination, and a full review of the MRI sequence — not just the one flagged line in the radiologist's report. Treatment follows the standard pathway from there: conservative care first, injection support where structural and symptom burden warrants it, and surgical consideration reserved for cases with progressive structural change or failed conservative management.

For general guidance on what to expect from an MSK assessment, NHS and NICE patient information resources offer a useful independent starting point.

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