What the grade number on your report is telling you

Receiving a knee MRI report with a cartilage grade written on it can feel alarming — particularly if the appointment to discuss it is still days away. The grade is, in fact, a standardised structural descriptor: it tells the radiologist's reader how deeply cartilage damage has penetrated through the cartilage layer, measured in four steps from completely normal (Grade 0) to full-thickness loss (Grade 4). It says nothing, directly, about the pain you are experiencing.

Two naming systems appear on UK radiology reports — the Modified Outerbridge scale and the International Cartilage Repair Society (ICRS) scale. Despite their different names, both run from Grade 0 to Grade 4 and use identical structural definitions at every tier. Seeing either label on your report means exactly the same thing, so the name itself is not a source of confusion worth dwelling on.

The number is one piece of information among several that a specialist weighs alongside defect size, location, your symptoms, and clinical examination. A grade alone does not determine your treatment pathway or prognosis.

Grade 0 to Grade 4 — what each stage describes

Each grade maps to a specific structural finding — here is what the radiologist is describing at every step on the scale.

Grade 0 — normal. The cartilage surface is smooth, the full thickness is preserved, and MRI signal is uniform throughout. No structural abnormality is present.

Grade 1 — biochemical change, surface intact. The cartilage architecture remains whole, but early internal softening or signal change has already begun beneath an outwardly intact surface. Standard proton-density fat-suppressed and T2 fat-suppressed sequences — used in the majority of routine clinical knee MRIs — detect surface damage reliably but are less sensitive to these pre-morphological changes. Grade 1 is therefore frequently absent from a routine report; this does not necessarily mean the change is absent, only that the sequence used may not have captured it.

Grade 2 — partial-thickness loss, less than half the cartilage depth. Surface fraying, fissuring, or blister-like swelling is now visible on imaging. The 50% depth threshold is the single most important structural dividing line on the entire scale: damage that has not yet reached halfway through the cartilage layer sits at Grade 2.

Grade 3 — partial-thickness loss, exceeding 50% depth. The cartilage is now deeply eroded but has not yet exposed the underlying bone. The ICRS system refines this stage into four sub-grades (3A to 3D), defined by how close the lesion has approached the calcified cartilage layer and subchondral bone — a distinction the specialist uses when planning further assessment or intervention. Radiologists sometimes describe a 'crab-meat appearance' at this stage, a descriptor patients may encounter in their report.

Grade 4 — full-thickness loss. The cartilage has been lost entirely, with the subchondral bone surface now directly exposed.

Why your grade and your pain level may not match

Cartilage has no nerve supply. That single anatomical fact explains most of the confusion patients feel when their grade does not match the pain they are experiencing — or when a relatively high grade produces surprisingly little discomfort.

Because cartilage cannot signal distress directly, pain associated with a chondral defect arises from surrounding structures: the highly innervated subchondral bone immediately below, the synovial lining, and the joint capsule. Of these, subchondral bone marrow lesions (BML) — visible on fat-suppressed MRI sequences as areas of high signal — are consistently identified in the literature as the primary driver of deep, aching, load-related knee pain in patients with cartilage damage. A patient carrying a Grade 3 finding with no accompanying BML may have considerably less pain than one whose Grade 2 lesion overlies an extensive marrow oedema signal.

The same logic applies in reverse. Some patients with Grade 3 changes manage well with structured physiotherapy and load management, with pain remaining modest; others with lower-grade findings report significant discomfort driven principally by synovial inflammation or soft-tissue involvement rather than cartilage loss itself. The grade describes structural depth of loss — it does not catalogue what else is happening inside the joint.

A specialist assessment draws these threads together: setting the grade and any BML or synovial signal against the pattern, timing, and behaviour of symptoms on loading and at rest — because those two sources of information are answering different questions.

Bone marrow lesions and other language in your report

The report language around bone is often what patients find most confusing — particularly when they expect to read about cartilage and instead encounter a separate paragraph describing changes in the bone itself.

Bone marrow lesions (BML) — appearing in reports variously as 'subchondral oedema', 'bone marrow signal change', 'T2 hyperintensity', or simply 'marrow oedema' — show up on fat-suppressed T2 sequences as bright, high-intensity areas in the bone immediately beneath the cartilage surface. This is a structurally separate finding from the cartilage grade: the grade describes the cartilage layer; BML describes what is happening in the supporting bone below it.

The two findings are clinically related, however. BML signals mechanical overload at that part of the joint, and in studies of knee osteoarthritis a progressive BML has been associated with a significantly elevated risk of further cartilage loss over time. This is part of why a Grade 3 or Grade 4 finding accompanied by an extensive BML signal may be more symptomatic — and carry more clinical weight — than the cartilage grade alone would suggest.

A specialist reads both together: the grade, the BML extent, the defect location, and the patient's actual symptom pattern each contribute to the overall picture, and none is assessed in isolation.

How a specialist reads beyond the grade

Two measurements on an MRI report carry more practical weight than many patients realise: the grade and the defect area, stated in square centimetres. A Grade 3 finding spanning less than 1 cm² in a non-weight-bearing zone has different implications from a Grade 2 lesion covering several square centimetres of the central weight-bearing surface of the medial femoral condyle — yet both might appear as similarly brief entries in the report text.

Defect area matters because it shapes which approaches are technically appropriate. A threshold of roughly 3 cm² features in the clinical planning literature as a point at which the range of suitable options changes — not because smaller defects are unimportant, but because different techniques are suited to different sizes. The grade alone does not encode this.

Location adds a further dimension. The central weight-bearing portion of the femoral condyle is under sustained compressive load with every step taken; the same grade in the same area at the trochlea or on the patella involves different mechanical forces and may behave differently over time. A specialist assessing an identical grade in two different sites may reach two materially different conclusions.

Clinical examination then sets all of this against the patient: how symptoms behave on loading versus at rest, activity level, age, and functional goals. Grade 1–2 findings in a mildly symptomatic patient often warrant structured monitoring rather than immediate intervention. A Grade 3–4 defect with well-localised, load-related pain in an active patient with a defined focal lesion triggers a different pathway — one that begins with conservative management and escalates depending on how the joint responds. That sequencing is why the specialist needs the full picture, not just the report.

When to seek a specialist assessment

Most Grade 1 and Grade 2 findings, in the absence of significant mechanical symptoms, are appropriately managed through a structured physiotherapy programme rather than immediate specialist referral. The grade alone rarely determines urgency.

Two patterns shift that calculation. If a Grade 2 or above finding accompanies persistent mechanical symptoms — locking, giving way, or swelling that returns reliably after activity — and these have not settled after a course of physiotherapy, a specialist assessment is the appropriate next step. If the report shows Grade 3 or Grade 4 changes and you are active, under 55–60, or have a defined focal defect, early review matters more acutely: the range of joint-preservation options available tends to narrow over time, and some approaches are only viable within a certain window.

Access is flexible. A GP referral is one route; self-referral to a specialist musculoskeletal clinic is another, particularly where a joint-preservation-focused opinion is the priority rather than a general orthopaedic review. At the London Cartilage Clinic, Professor Paul Y. F. Lee assesses focal cartilage findings against the full clinical picture — defect size, location, symptom pattern, and functional goals — rather than grade in isolation.

The MRI report is the starting point for that assessment, not its conclusion. Arriving with a clear account of when symptoms are worst, what aggravates and relieves them, and how function has changed over time gives the specialist the context the scan cannot supply.