A grade is a structural snapshot, not a verdict

An MRI report lands with a number on it — Grade 2, Grade 3 — and it is natural to read that number as a verdict. It is not. A cartilage grade is a structural descriptor: it tells the specialist how much of the cartilage has worn away and how deeply the damage extends, nothing more. It does not translate directly into how much pain you should be feeling, and it does not, by itself, determine whether surgery is appropriate.

This distinction matters because imaging and symptoms can diverge sharply. Grade 1 and Grade 2 findings — surface softening, early fissuring — are common incidental discoveries in people who have no knee pain at all, particularly from middle age onward. Conversely, a patient with a higher grade may function well with the right rehabilitation support. Neither scenario is unusual.

The grade is one piece of clinical information. A specialist will weigh it alongside the size of any defect, where it sits in the joint, your reported symptoms, how your knee functions on examination, and the arc of your history. That combination, not the number alone, shapes what happens next.

The two grading scales: Outerbridge and ICRS

Two grading frameworks dominate clinical practice: the Modified Outerbridge scale and the International Cartilage Repair Society (ICRS) scale. Both run from Grade 0 to Grade 4 and share the same structural logic — encountering either name in a report should not cause confusion.

Grade 0 is normal cartilage with uniform signal. Grade 1 is where the surface remains architecturally intact but internal signal change or softening has already appeared — the cartilage has begun to alter at a biochemical level without yet showing visible surface damage. Because standard morphological MRI sequences detect surface changes more reliably than internal softening, Grade 1 is the grade most likely to be missed or underreported on a routine scan.

The critical dividing line sits at 50% of cartilage thickness. Grade 2 describes partial damage shallower than that midpoint: surface fraying or focal fissures that have not yet reached halfway through. Grade 3 crosses the threshold — the damage extends deeper than 50% of the cartilage — but the underlying subchondral bone remains covered.

This is where the ICRS adds clinical detail the Outerbridge system lacks. Grade 3 is subclassified as 3A (above the calcified cartilage layer), 3B (reaching it), 3C (penetrating through it but stopping above subchondral bone), and 3D (surface blistering overlying a deeper lesion). These distinctions are not purely academic: the subtype informs whether marrow-stimulation techniques — which depend on access to the bone marrow beneath the defect — are technically viable.

Grade 4, representing full-thickness cartilage loss, is covered in the next section. In UK practice, Outerbridge terminology remains the norm in routine radiology reports; ICRS language appears more commonly in specialist cartilage centres and research settings.

Grades 0 to 4: what the scan is showing

Fat-saturated proton density (PD FS) sequences are the clinical workhorse for cartilage reporting: they create enough contrast between cartilage, synovial fluid, and bone for the radiologist to map surface integrity grade by grade.

On a PD FS image, Grade 0 cartilage appears as a smooth, uniformly bright band. Grade 1 shows a subtler picture — the surface remains intact and smooth, but a patch of altered signal appears within the cartilage substance, reflecting softening or early biochemical change. Standard morphological sequences often miss this entirely; the tissue looks near-normal on routine MRI. Quantitative techniques — T2 mapping and T1ρ relaxometry — can detect the underlying proteoglycan loss or collagen disorganisation before any crack is visible, which is why they are used in specialist monitoring contexts, particularly after ACL injury and in early osteoarthritis surveillance. They are not yet standard-issue in a routine knee MRI, and emerging methods such as ultra-short echo time (UTE) imaging remain research tools at this stage.

Grade 2 is where surface disruption becomes visible: fraying, small fissures, or blister-like changes appear on the PD FS scan, though the damage has not yet reached halfway through the cartilage depth. At Grade 3, the defect has crossed the 50% threshold, and the radiologist can often characterise how close it has come to the underlying calcified layer — the structural basis for the ICRS 3A–3D subtypes covered in the previous section.

Grade 4 looks different in kind, not just degree. Where the cartilage has worn away entirely, the subchondral bone sits exposed. The bone beneath frequently responds with oedema — visible on the MRI as a bright haze — indicating it is absorbing stress it was not designed to carry. Some reports also note subchondral cysts: small fluid-filled pockets within the bone itself, typically where repeated loading has begun to compromise the underlying structure. Both findings signal that the joint-surface disruption has extended well beyond the cartilage layer alone.

Report language in plain terms

MRI reports often include terms that sit alongside the grade number without explanation. A few appear frequently enough to be worth unpacking.

Chondromalacia is a softening term with a specific home: it refers to the patellar (kneecap) cartilage. Seeing it on a report does not mean every surface in the joint is damaged — it is a patellar label, not a general verdict on cartilage health.

Fibrillation describes what has happened to the cartilage surface at Grades 2 and 3: the once-smooth layer has broken down into a frayed, shredded texture — sometimes compared to crab meat. The bone is not yet exposed at this stage, but the protective surface has lost its integrity.

Bone marrow oedema and subchondral cysts, as described above in the context of Grade 3 and Grade 4 findings, appear in reports as secondary findings rather than separate diagnoses. Their presence tells the reporting radiologist — and the assessing clinician — how far the consequences of cartilage loss have reached into the underlying bone.

Loose bodies are fragments of cartilage or bone that have broken free and are now floating in the joint space. They may cause intermittent locking or a catching sensation — symptoms worth mentioning to a specialist even if the grade itself appears modest.

Why defect size matters as much as grade

The grade number answers one question — how deep is the damage? A second question runs alongside it in every treatment discussion: how wide is the defect?

Defect surface area, measured in square centimetres, carries roughly equal clinical weight to grade. A 1 cm² lesion and a 5 cm² lesion at the same grade sit in very different territory when repair options are considered. As a rough scale, 2 cm² is approximately the area of a small fingernail; 4 cm² is closer to a thumbnail.

The 2–4 cm² range marks a practical threshold. Defects below it are generally candidates for procedures such as microfracture, which encourages new tissue from the underlying bone. Larger defects — at or above 3 cm² — tend to favour options such as mosaicplasty or matrix-assisted cartilage implantation (MACI). The SUMMIT trial, which compared MACI against microfracture directly, found MACI produced meaningfully greater improvement in patients with defects of 3 cm² or more, a result that held at both two and five years of follow-up. That outcome is why the size figure in the radiologist's report carries direct clinical consequence.

Grade and area together are still only part of the picture. Where the defect sits — on a weight-bearing surface or away from it — matters considerably, as do patient age, activity demands, and lower-limb alignment. A specialist assessment is what converts those combined variables into a realistic set of options; the MRI report supplies the raw data, but the clinical decision-making begins in consultation.

What to do after getting your MRI result

The practical question after receiving an MRI result is not 'how bad is this?' but 'what happens next?' — and the answer depends on information the scan alone cannot supply.

For Grade 1 and 2 findings where symptoms are mild or manageable, the usual first step is conservative management: a structured physiotherapy programme, load modification, and monitored activity adjustment. Escalation at this stage is rarely indicated without a period of structured conservative care first.

Grade 3 or 4 findings in a younger or physically active patient, or any grade where conservative management has not produced meaningful improvement, are the clearer cases for specialist cartilage assessment. A specialist brings together the radiological grade, the defect's measured area, its location relative to load-bearing surfaces, clinical examination, and the patient's activity demands — converting those combined variables into a realistic set of options with specific trade-offs.

One principle worth carrying into that appointment: ask for both the grade and the defect size in square centimetres. A report may state one without the other, but — as the SUMMIT trial findings and the 2–4 cm² thresholds discussed above make clear — both figures directly shape which repair options are realistic. Arriving with that question already formed makes the clinical conversation considerably more productive.

Patients in London who need that level of specialist review can arrange a cartilage assessment at the London Cartilage Clinic, Harley Street, via londoncartilage.com.

1. [1] Chondromalacia patellae — Wikipedia. https://en.wikipedia.org/?curid=1944613 https://en.wikipedia.org/?curid=1944613
2. [2] Diagnostic Value of Pericruciate Fat Pad Measurement by MRI in Knee Cartilage Injury (2025). (2025). https://doi.org/10.12968/hmed.2024.0824 https://doi.org/10.12968/hmed.2024.0824
3. [3] Feasibility of in vivo quantitative UTE-MRI for detecting early cartilage degeneration (2024). (2024). https://doi.org/10.1186/s13244-024-01734-4 https://doi.org/10.1186/s13244-024-01734-4