Grade 3 or 4 on your MRI report — the short answer

Seeing 'Grade 3' or 'Grade 4' on a knee MRI report is understandably alarming — but those numbers describe what the cartilage looks like structurally, and what they do not tell you is what treatment you will need.

Grade 3 means the damage has eaten through more than half the full thickness of the cartilage. That is deep — significantly beyond a surface scratch — but the underlying bone plate is still intact. Grade 4 is the most severe category: the lesion has breached all the way through the cartilage and into the bone beneath it. Both sit at the serious end of a zero-to-four scale (the internationally used ICRS classification), and neither should be minimised.

What a grade alone cannot tell you is what happens next. Two people can receive the same grade on their report and end up on very different pathways. The size of the affected area, where in the knee it sits, and what symptoms it is causing all shape the clinical picture — and, ultimately, the options available.

Why cartilage is graded — and why it cannot heal itself

The reason grading matters at all comes down to biology. Articular cartilage — the smooth hyaline tissue that lines the ends of your knee bones — contains no blood vessels and no nerve endings. The absence of a blood supply means it cannot grow new tissue to fill a gap the way skin heals a cut; it has no mechanism to recruit the repair cells that other tissues rely on. The lack of nerve endings partly explains why damage can be painless in its early stages, even when the structural changes are already significant.

The ICRS scale reflects how far a lesion has penetrated that tissue. Grade 0 is a normal surface. Grade 1 is superficial softening or blistering with no significant depth. Grade 2 means damage extending less than halfway through the full cartilage thickness. At Grades 3 and 4 — covered in detail in the next section — the lesion has gone beyond the halfway point or through the bone plate entirely.

Because cartilage cannot close a defect spontaneously, a Grade 3 or 4 finding will not resolve on its own. Untreated, high-grade focal defects are a recognised risk factor for progressive osteoarthritis in the affected compartment — which is the biological basis for treating these findings as deserving specialist attention rather than watchful waiting.

What Grade 3 subgrades actually describe

To understand what separates 3A from 3D, it helps to picture cartilage not as a single slab but as three distinct zones stacked on top of one another: a surface layer that absorbs initial contact, a deeper structural layer that carries most of the load, and — sitting right at the base — a very thin calcified transition zone. This last layer is not cartilage and not quite bone; it is a mineralised boundary that anchors the cartilage firmly to the subchondral bone plate beneath it.

The three lettered subgrades A, B, and C describe how deeply the lesion has reached into those lower zones:

• Grade 3A — the damage has passed beyond the halfway point and extends through the deep structural layer, but stops just above the calcified boundary. The transition zone itself is still intact.
• Grade 3B — the lesion has reached the calcified layer. The anchor zone is threatened but not yet breached.
• Grade 3C — the damage has penetrated through the calcified layer, leaving the subchondral bone plate as the only remaining barrier.

Grade 3D is a distinct pattern. Rather than a clean downward lesion, the cartilage surface appears raised or blistered — but the blistering is caused by severe damage underneath. The surface looks partly intact; the tissue beneath it is not.

All four subtypes represent deep, structurally significant damage that cartilage cannot repair. Subgrade 3A is not a minor finding — it sits well beyond the halfway point of a tissue with no healing capacity, and all Grade 3 variants warrant specialist assessment.

Grade 4 — when damage reaches the bone

Grade 4 has a simpler structural definition than Grade 3: the lesion has gone all the way through the cartilage and through the subchondral bone plate beneath it — the thin, hard shell of bone that sits just below the cartilage surface — leaving the underlying bone directly exposed.

Bone is not engineered to act as a bearing surface. When it is exposed within the joint, the repeated loading of walking or exercise produces pain, and the hard surface accelerates wear on the cartilage of the opposing bone — the surface pressing against it with each step. Over time, this mechanical mismatch can drive progressive damage beyond the original defect site, which is part of why Grade 3–4 findings are linked to early osteoarthritis classification when left untreated.

The critical qualifier for patients reading a Grade 4 report, however, is the word focal. A contained, localised area of bare bone in a knee whose surrounding cartilage is otherwise intact is not the same as end-stage osteoarthritis affecting an entire joint compartment. The ICRS grade describes depth at one location; the overall joint picture is what determines the treatment pathway. A Grade 4 focal defect in an otherwise healthy knee remains a potential candidate for cartilage repair rather than joint replacement — the grade alone does not decide that question.

Focal defect versus osteoarthritis — why the difference changes everything

The clearest way to separate a focal cartilage defect from osteoarthritis is to look at what two different imaging modalities reveal. In established OA, a plain X-ray will typically show joint space narrowing, bony outgrowths (osteophytes) at the joint margins, and visible changes to the contour of the bone ends — all signs of compartment-wide surface deterioration. A focal Grade 3 or 4 cartilage defect, by contrast, will often produce a normal or near-normal plain X-ray, because the damage is confined to one area. MRI is the standard imaging that identifies it, which is one reason MRI has become the primary tool for cartilage assessment.

That contrast matters beyond radiology. Osteoarthritis is a diffuse process — the entire articular surface of a compartment breaks down progressively, and the mechanical environment of the joint is globally disrupted. A focal defect is a localised failure: one pothole in an otherwise sound road surface, with healthy cartilage on either side. That healthy surrounding tissue is what makes cartilage repair surgery feasible; a repair requires a stable biological foundation to integrate into.

Research criteria for early OA do include ICRS Grade 3–4 findings, but only when combined with widespread surrounding softening across at least one compartment. A single, well-contained defect in otherwise normal cartilage does not, by that definition, automatically satisfy the criterion.

Defect size is then the second key variable alongside grade. Whether the damaged area falls below or above approximately 2–4 cm² influences which repair strategies are clinically appropriate and how urgently specialist review is warranted — a question that requires both imaging and clinical examination together to answer.

Symptoms, what to do next, and when to seek specialist assessment

Pain and structural damage do not move in step. Some patients with confirmed Grade 4 lesions report mild discomfort that has been present for years; others with a Grade 3B defect find that loading the knee becomes quickly disabling. The symptoms most commonly associated with high-grade cartilage loss — pain on weight-bearing, a persistent background ache, swelling after activity, clicking, catching, or occasional locking — can all occur across a wide range of grades and defect sizes. This means neither the presence nor the absence of severe symptoms should be used to dismiss or confirm a Grade 3 or 4 finding without clinical assessment.

The MRI grade itself carries an important caveat: standard MRI sequences identify defects reliably, but the ICRS grade assigned from imaging can sometimes differ from what a surgeon records when looking directly at the cartilage surface during arthroscopy. If your symptoms feel markedly worse — or better — than the written grade suggests, that discrepancy is worth raising explicitly with your surgeon; arthroscopic assessment is the gold standard for confirming both grade and the true extent of the defect, and may be appropriate before committing to a treatment pathway.

For most patients, the initial response to a Grade 3 or 4 finding will include some combination of activity modification, physiotherapy to optimise loading patterns, and appropriate pain management. What these grades do not support is indefinite watchful waiting without specialist input: untreated high-grade defects tend to enlarge, and the earlier specialist review takes place the broader the range of repair options available.

A dedicated cartilage assessment — covering grade, defect size, location, age, activity level, and the overall condition of the joint — is what converts an MRI number into a structured treatment plan. The London Cartilage Clinic at Harley Street provides that kind of specialist assessment, and appointments can be arranged at londoncartilage.com.

1. [1] Articular cartilage damage. https://en.wikipedia.org/wiki/Articular_cartilage_damage https://en.wikipedia.org/wiki/Articular_cartilage_damage
2. [2] Hyaline cartilage. https://en.wikipedia.org/wiki/Hyaline_cartilage https://en.wikipedia.org/wiki/Hyaline_cartilage