Reading the grade on your report

'ICRS Grade 3 chondral defect, medial femoral condyle' — for most patients, that phrase lands without context. Here is what the number actually means.

The ICRS (International Cartilage Repair Society) scale runs from 0 to 4:

• Grade 0 — normal cartilage, healthy surface and signal
• Grade 1 — surface intact, but the cartilage has softened or shows early signal change within the tissue
• Grade 2 — surface broken, with damage extending less than halfway through the cartilage depth
• Grade 3 — damage runs deeper than half the cartilage thickness, but the underlying bone is still covered
• Grade 4 — full-thickness loss; bone is exposed

The step between Grade 2 and Grade 3 is the most clinically important on the scale. Research using electromechanical cartilage assessment at arthroscopy found a Pearson correlation of 0.9 between ICRS grade and cartilage stiffness, with a statistically significant jump between the Grade 0–2 group and the Grade 3–4 group (P < 0.0001) — confirming that this boundary reflects a genuine change in structural integrity, not just a label.

What the grade does not do is set a treatment pathway on its own. Defect area (measured in cm²), the patient's age and activity demands, the presence of bone marrow oedema, and the overall condition of the joint all bear on what comes next. The sections that follow explain how each of those factors is weighed.

ICRS and Outerbridge: two grading languages, one scale

Two different grading languages turn up in knee cartilage reports — and the first thing to know is that they describe the same spectrum.

The Outerbridge system dates from the 1960s, when it was devised for arthroscopy (keyhole surgery): a surgeon would grade cartilage directly as they looked inside the joint. The ICRS — International Cartilage Repair Society — system is now the international standard, used for both MRI and arthroscopic reporting. Most contemporary reports use ICRS; older documents and letters written after arthroscopy may still use Outerbridge language.

Both systems follow the same logical sequence: normal → surface softening → partial-thickness loss → deep partial-thickness loss → exposed bone. The grade numbers run 0 to 4 in both, and the overall arc is identical to the scale described above.

The one genuine difference is where each system places the dividing line between its middle grades. Outerbridge separates Grade II from Grade III by lesion diameter — the threshold is 1.5 cm. ICRS separates Grade 2 from Grade 3 by depth — specifically, whether the damage has passed the halfway point through the cartilage. A small but deep lesion and a wide but shallow one could therefore land on different grades depending on which system is used.

For MRI in particular, a modified version of the Outerbridge scale (using fat-saturated proton density sequences) maps almost identically to ICRS. If an older report uses Outerbridge terminology and a more recent scan uses ICRS, the grade numbers are directly comparable — no translation is needed.

What the MRI scanner is actually seeing

Cartilage's water content is the key to how MRI images it. Where tissue composition differs — in structure, density, or hydration — the scanner records a different signal, and radiologists read those contrasts to map the joint's condition.

On fat-saturated T2-weighted sequences (the standard sequences for cartilage assessment), healthy hyaline cartilage displays a distinctive three-zone pattern: the deep layer appears dark, reflecting tightly packed collagen fibres; the middle zone shows a brighter signal; and the superficial layer returns an intermediate signal. Radiologists may also report T2 relaxation values — a quantitative measure of how tissue responds to the magnetic pulse. Healthy cartilage typically falls between 30 and 40 milliseconds; values above roughly 45 ms suggest degeneration, as water content rises and collagen organisation breaks down.

Damage disrupts this layered pattern. Depending on grade, the report may describe signal heterogeneity (early change), fissuring, partial-thickness defects, or full-thickness loss. Bone marrow oedema — bright signal patches in the bone directly beneath the cartilage — is a common secondary finding at higher grades. It reflects the bone's response to increased stress through a thinning or absent cartilage layer and may independently contribute to pain, which is one reason surgeons weigh it alongside the cartilage grade itself.

One limitation is worth acknowledging: MRI sensitivity for the earliest changes (Grade 1) is relatively low. Where clinical suspicion remains high despite a near-normal scan, arthroscopy — direct visual inspection of the cartilage surface — remains the definitive grading method.

Why a high grade does not always mean high pain

Grade and symptoms do not run on the same scale. Roughly 5–10% of adults over 40 carry high-grade chondral lesions — ICRS Grade 3 or 4 — discovered incidentally on scans ordered for an unrelated reason, with no knee symptoms at all. The finding is structural; the absence of pain is real.

The relationship runs the other way too. Some patients present with significant, activity-limiting pain alongside Grade 1 or 2 changes — findings the scan classifies as mild or moderate. Others have lived functionally well for years with Grade 3 or 4 defects. Neither pattern is unusual.

This is why a consultant's assessment extends well beyond reading the report. Load-bearing tolerance, range of motion, joint-line tenderness, and mechanical symptoms such as locking or giving way tell a different story from the MRI — and often a more clinically relevant one. The grade locates a structural problem; physical examination begins to establish whether that problem is actually driving the patient's experience.

Receiving a Grade 3 or 4 report is a prompt for specialist assessment. It is not, on its own, a verdict that surgery is required or inevitable.

How grade and defect size shape the pathway forward

Two variables govern the treatment pathway once cartilage damage has been identified: the ICRS grade and the measured area of the defect in square centimetres. Grade sets the broad direction; defect area refines it.

Grades 0–2: conservative management first

At Grades 0 to 2, the standard first step is non-surgical. Physiotherapy — targeting load distribution, quadriceps strength, and movement patterns — forms the backbone of care. Where symptoms warrant, injection support such as corticosteroid, hyaluronic acid, or a biologic option may be considered alongside or after a course of rehabilitation. For most patients at this end of the scale, this pathway is sufficient.

Grades 3–4: surgical assessment, not automatic surgery

At Grades 3 and 4, a surgical assessment becomes appropriate — though even here, many patients initially pursue conservative care, and a specialist will weigh the full clinical picture before recommending an operation. When surgery is indicated, defect size becomes the primary decision variable.

For lesions smaller than roughly 2–4 cm², microfracture and mosaicplasty are both established options; for Grade 4 lesions specifically, mosaicplasty has demonstrated clinically meaningful functional gains (Tegner–Lysholm scores improved by 23–74 points in published series).

For larger lesions of 3 cm² or more, matrix-induced autologous chondrocyte implantation (MACI) showed superior KOOS pain and function outcomes over microfracture at both two and five years in the SUMMIT trial — making it the procedure most commonly considered for higher-volume defects.

Lesion location, patient age, activity goals, and joint alignment all factor into which option is appropriate, and none can be determined from the report number alone.

When to get a specialist opinion

Certain patterns make specialist cartilage assessment the clear next step — and recognising them early avoids unnecessary delay.

The clearest escalation triggers are: symptoms limiting daily activity or sport; MRI findings at Grade 3 or 4; mechanical symptoms such as locking, catching, or the knee giving way; or a course of structured conservative care that has not produced meaningful improvement after six to twelve weeks. Any one of these merits more than a GP-only review.

A specialist consultation combines detailed clinical history with physical examination — covering alignment, joint-line tenderness, patellar tracking, and range of motion — and a structured review of all imaging in that clinical context. At the London Cartilage Clinic, Professor Paul Y. F. Lee and the team take that integrated approach; a consultation can be arranged via londoncartilage.com.

Where imaging findings and clinical symptoms remain difficult to reconcile, arthroscopy may be recommended. It provides definitive cartilage grading and, where appropriate, allows for diagnostic and therapeutic work in a single procedure.

1. [1] Articular cartilage damage | Wikipedia. https://en.wikipedia.org/?curid=19057920 https://en.wikipedia.org/?curid=19057920
2. [2] Arthroscopic Electromechanical Assessment of Human Articular Cartilage Injury Correlates with ICRS Scores. (2023). https://doi.org/10.1177/19476035231216439 https://doi.org/10.1177/19476035231216439
3. [3] Assessment of the Correlation between Macroscopic ICRS Grading and Histopathological OARSI Scoring in Osteoarthritic Cartilage. (2026). https://doi.org/10.1177/19476035251393150 https://doi.org/10.1177/19476035251393150
4. [4] Hyaline cartilage | Wikipedia. https://en.wikipedia.org/?curid=1130627 https://en.wikipedia.org/?curid=1130627