The Cartilage Guide

Start here · Understand your injury

Your grade: what the number means, and what it doesn’t

The report says “grade III chondral lesion” and nothing else, and the number lands like a verdict. It isn’t one. Before the grade means anything you need one distinction the report may not spell out — and it is the distinction this whole site branches on.

The two injuries — the first question isn’t your grade

Focal defect

One bordered hole in otherwise healthy cartilage — a pothole. The surgical half of this guide exists for these.

→ Treatments & Surgery

Diffuse wear

Thinning across a surface — worn tread. The grade can read the same; the options are entirely different.

→ Exercise, injections, precautions

Two scales, one report

Your report will use one of two scales, and it may not say which. They look interchangeable. They are not — an Outerbridge II is not an ICRS 2, and no exact mapping between them exists. Both ledgers follow; when the report just says “grade,” ask which scale.

Outerbridge · 0–IVarthroscopic, 1961 — the one most US reports use

Five findings — not five steps on a ruler. The gaps are not equal, and the number alone does not predict your outcome.

0
Cartilage looks normal.does not mean: your pain has no source — plenty hurts on a grade-0 scope.
I
Softening or blistering; the surface is intact.does not mean: arthritis is coming — many grade-I knees never progress.
II
Partial-thickness fraying or fissuring.does not mean: twice as bad as grade I — the scale doesn't work like that.
III
Deep fissures reaching toward the bone.does not mean: surgery is inevitable — size and location decide more than depth.
IV
Full-thickness loss; exposed bone.does not mean: nothing can be done — this is the grade the comparison plate exists for.

The second scale is not a translation of the first. If your report is European, or from a surgical practice, it is likely this one.

ICRS · 0–4International Cartilage Repair Society, 2003
0
Normal.does not mean: nothing to watch — a normal surface can sit over abnormal bone.
1
Nearly normal — superficial softening or fissures.does not mean: the same thing as Outerbridge I — the scales don't map one-to-one.
2
Abnormal — defects less than half the cartilage depth.does not mean: half of grade 4.
3
Severely abnormal — more than half the depth.does not mean: a surgery verdict on its own.
4
Severely abnormal — through the cartilage into bone.does not mean: the end of the options.

How common is this

Very common in knees that reach arthroscopy. A multicenter review of 31,516 arthroscopies documented cartilage lesions in 63% of patients, and a single-center series of 25,124 found them in 60%. Both are arthroscopy-referred populations, so both overstate what is happening in the general public, and plenty of the lesions they counted were incidental and painless.

The more useful number is the rarer one. The classic focal defect in an otherwise healthy knee — the lesion the repair procedures on this site actually treat — was a small minority: roughly 7% of those under 40. Most cartilage damage found at arthroscopy is not the kind the comparison plate is about.

Where the number comes from

MRI reads one thing and arthroscopy another, and when they disagree the scope wins — it is the reference standard. What is worth knowing is the shape of the disagreement, which is not the one usually claimed.

A scan misses more than it mis-grades. On 1,260 cartilage surfaces, MRI sensitivity fell from 83% for grade IV to 68% for grade II and 8.8% for grade I, while specificity stayed above 99% throughout — so a scan that finds a high-grade lesion is trustworthy, and a scan that finds nothing mild tells you almost nothing. Where MRI does read low reliably is size: defects were larger at surgery in 74% of cases. One study looked specifically for a tendency to under-grade and reported that it “was not noticed.” Treat the number as imprecise rather than as reliably one grade low.

Size and location

These move the available options more than the grade does. A small, well-bordered grade IV can be more treatable than a large, poorly-placed II — which is also why the size finding above matters more than it first sounds: the variable a scan under-reads is the one that changes the plan.

Arthroscopy is the reference standard for size and grade; MRI is the reference for what sits underneath — bone edema, subchondral cysts. They are complements, not substitutes, and no imaging sequence yet closes the gap well enough to plan definitive surgery from a scan alone.

What your grade opens and closes

A focal grade III–IV defect is what the surgical procedures exist forthe comparison plate →
Diffuse grade II–III wear points to load management firstExercise & Rehab →
Whatever the grade: what not to do while you decidePrecautions →

Questions for your appointment

Those five facts together — not the grade alone — decide which procedures are realistic.

  • Which grading system does my report use — Outerbridge or ICRS?
  • What are the three numbers: grade (depth), size (cm²), and location?
  • Is this a focal defect or diffuse wear?
  • What are my alignment and meniscus status?

Sources

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