Understanding your injury: cartilage grading

Before comparing procedures, it pays to speak the language your surgeon and your operative report use. Articular cartilage is avascular, and its superficial layers have no nerve supply, so damage is graded by depth — depth predicts both symptoms and which repair options are on the table.

The two grading systems

Two systems dominate. Outerbridge, from a 1961 paper originally describing the patella at open surgery, grades I–IV: softening; fissuring over a small area; fissuring over a larger area; exposed bone. The ICRS classification (2003) refines this into grades 0–4 by depth relative to cartilage thickness and the subchondral bone, adds standardized lesion mapping, and includes a separate repair-tissue assessment — it is the language of every modern cartilage repair trial.

The catch: "grade 3" means different things in the two systems — large-area fissuring in Outerbridge, a defect deeper than 50% of cartilage thickness in ICRS. Ask which system your report uses. Neither was prospectively validated against outcomes before adoption, inter-observer agreement is imperfect, and mixed use of the two systems muddies comparisons across studies.

How common is this?

Very common in knees that reach arthroscopy. A multicenter review of 31,516 knee arthroscopies documented 53,569 cartilage lesions — 63% of patients — with grade IV damage concentrated on the medial femoral condyle; 36.6% of under-40s with grade IV lesions had no ligament or meniscus injury. A single-center series of 25,124 arthroscopies found chondral lesions in 60%.

Both numbers come from arthroscopy-referred populations, so they overstate prevalence in the general public, and many documented lesions are incidental and asymptomatic. The more useful figure 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% under age 40.

Defect vs diffuse wear: the fork in the road

This distinction matters more than the grade itself. A focal defect — a discrete pothole with healthy cartilage around it — has surgical repair options. Diffuse bipolar wear (kissing lesions on both sides of the joint, established osteoarthritis) mostly does not, short of realignment or joint replacement. If your imaging shows widespread thinning rather than a discrete lesion, the repair procedures in this section are largely not aimed at you.

Why the MRI is a floor, not a ceiling

In a study of 77 patients (92 defects) with preoperative MRI followed by arthroscopy, MRI underestimated defect area by about 70%; 74% of lesions were larger at surgery, by a mean of roughly 1 cm². The practical translation: the size quoted from your MRI report is a floor, not a ceiling, and the treatment plan can legitimately change once the surgeon actually looks.

Arthroscopy remains the reference standard for size and grade. MRI is the reference for what is underneath — bone edema, subchondral cysts. The two are complements, not substitutes. The study is single-center and predates some modern high-resolution cartilage sequences, so the gap may be narrower today, but the direction of the error is worth knowing.

Questions for your appointment

• Which grading system does my report use — Outerbridge or ICRS?
• What are the three numbers: grade (depth), size (cm²), and location?
• What are my alignment and meniscus status? Those five facts together, not the grade alone, drive which procedures are realistic.
• Is this a focal defect or diffuse wear?

The decisional risk

There is no intervention on this page, so the only "risk" is decisional: treating an MRI finding rather than a symptomatic, surgically confirmed lesion. Open questions in the field include whether quantitative MRI (T2 mapping, dGEMRIC) can ever close the gap with arthroscopic grading well enough to plan definitive surgery from imaging alone. For now, it cannot.