The two numbers that drive every treatment decision

If your scan report or clinic letter mentions an ICRS grade, it is answering one specific question: how deeply has the cartilage been damaged? The grade runs from 0 (normal, intact surface) to 4 (full-thickness loss with exposed subchondral bone). Standardised at the January 2000 ICRS Workshop in Schloss Münchenwiler, Switzerland, and validated in research cited more than 565 times, it remains the clinical benchmark for cartilage assessment.

Grade alone, however, is not enough to choose a treatment. Clinicians add a second number: the surface area of the defect, measured in square centimetres. Grade describes how deep the damage goes; area describes how wide it spreads. A 1 cm² Grade 3 lesion and a 6 cm² Grade 3 lesion carry identical depth classifications but sit in entirely different treatment categories.

Together, these two figures form a matrix that steers the decision between conservative management, minimally invasive repair, and surgical reconstruction. The sections that follow work through each zone of that matrix — and then address the additional factors, such as age, activity level, and bone involvement, that clinicians layer on top.

Grades 1 and 2: what conservative management looks like

Grade 1 and 2 findings mean the damage has not yet crossed the 50% depth threshold: the calcified cartilage layer and the bone beneath it remain structurally intact. For most patients, this is not a surgical situation.

The standard pathway at this stage is conservative. Physiotherapy is the cornerstone — targeted exercises to strengthen the muscles that support and offload the affected joint, combined with activity modification to avoid high-impact patterns that aggravate the surface. Where pain is significant, NSAIDs may be appropriate for short-term control. Injectable support such as hyaluronic acid (to restore lubrication) or platelet-rich plasma (PRP, to introduce growth factors) may be considered when symptoms have not settled with exercise-based management alone.

Arthroscopic chondroplasty — sometimes called debridement — is occasionally offered when a Grade 1–2 defect causes mechanical symptoms such as catching or locking. It works by smoothing or removing the unstable cartilage edges that provoke those sensations. It does not, however, regenerate cartilage: chondroplasty sits in the palliation category, managing symptoms rather than repairing tissue.

Grade 1–2 is not a crisis, but it is a signal worth acting on. Symptomatic lesions at this depth warrant monitoring to confirm that conservative measures are working and to catch any progression toward a deeper grade before the treatment options narrow.

Grades 3 and 4: when the damage reaches deeper layers

A Grade 3 finding is the point at which damage has broken through more than half of the cartilage's total depth. Grade 4 means it has gone all the way through, leaving bone directly exposed at the joint surface. This is the threshold where restoration — not symptom management alone — becomes the primary conversation.

Grade 3 itself divides into four subcategories (3A to 3D). The distinctions capture details relevant to operative planning: whether the damage has reached the calcified cartilage layer immediately below (3A), whether delamination has spread across the surface (3B), whether blistering has developed (3C), or whether full-depth involvement extends across the lesion floor (3D). For most patients, the subcategory will not change the headline picture, but it shapes which technique is most appropriate.

Grade 4 introduces a structurally different problem. When subchondral bone is damaged, cystic, or structurally compromised, techniques that address only the cartilage layer — including microfracture and autologous chondrocyte implantation (ACI) — cannot resolve the underlying deficit. Fresh osteochondral allograft (OCA) is the preferred option in these cases because it replaces both the cartilage surface and the bone beneath it in a single procedure.

Because cartilage has no direct blood supply, it cannot meaningfully repair itself — a fundamental constraint that distinguishes it from most other tissues. Symptomatic Grade 3–4 lesions tend to enlarge over time and carry a recognised risk of progression toward osteoarthritis without intervention, which grounds the case for active management rather than extended watchful waiting.

How defect size determines which technique fits

Surface area — measured in square centimetres — is the second axis on the treatment map, and the resulting ladder overlaps more than it might appear on paper.

Below roughly 2 cm² (approximately the area of a fingernail): bone marrow stimulation — microfracture or micro-drilling — is the standard minimally invasive first step for Grade 3–4 defects. Both techniques breach the subchondral bone to recruit progenitor cells into the defect. The fill produced is fibrocartilage rather than native hyaline cartilage, which is mechanically less resilient; durability under high activity load is a known limitation.

In the 1–4 cm² band, particularly for younger or higher-demand patients, osteochondral autograft transfer (OATS, or mosaicplasty) transplants intact hyaline cartilage-and-bone plugs from a low-load region of the same knee, restoring the structural surface directly rather than relying on fibrocartilage fill.

Above approximately 3 cm², MACI has strong trial support: the SUMMIT RCT demonstrated significantly better KOOS pain and function scores compared with microfracture at both two-year and five-year follow-up.

Above approximately 4 cm², autologous chondrocyte implantation (ACI) and its scaffold-based extension STACi, or fresh osteochondral allograft (OCA), are the primary reconstruction options. Matthews 2022 found comparable functional outcomes between ACI and OCA for comparable lesions.

These bands intentionally overlap — a defect of 3 cm², for instance, sits within both the OATS and MACI ranges simultaneously. The tiebreaker is usually patient age, activity demand, and the surgeon's assessment of each option's specific trade-offs: OATS involves a donor site in the same knee; MACI in its classic form requires a two-stage cell-harvesting process.

At the smaller end of this spectrum, injectable collagen scaffold options such as the ChondroFiller injection — delivered as an ultrasound-guided outpatient procedure — offer a single-stage, minimally invasive alternative pathway. The evidence base for this approach is still maturing, and it is not a direct substitute for the surgical techniques described above; its clinical differentiation lies in the outpatient delivery route and its applicability to cases where theatre-based intervention would be disproportionate.

The size thresholds quoted here should be treated as approximate. Published guidance varies by roughly 1–2 cm² depending on joint, patient profile, and source consulted; individual clinical assessment rather than area alone determines suitability.

Other factors that shift the decision beyond grade and size

The grade and size thresholds map out the terrain — but they do not determine which path is right for any individual patient. Two people with an identical ICRS score and defect area can reasonably be offered different treatments, and the reasons why illustrate why a scan report is never the final word.

Age and activity demand are the most immediate modifiers. A 28-year-old competitive athlete with a 2 cm² Grade 3 defect has different priorities — and different biological recovery capacity — than a 55-year-old who walks recreationally. The first may be directed toward a restoration technique capable of sustaining high joint loads over decades; the second may benefit from a less invasive approach matched to lower functional demands.

Joint alignment introduces a mechanical dimension. If the knee loads in varus (bow-legged) or valgus (knock-kneed), stress concentrates unevenly across the joint surface — directly where a defect sits. Repairing the cartilage without correcting that mechanical environment risks early failure; an osteotomy to realign the joint may be planned before or alongside the cartilage procedure.

Concomitant injuries add a further layer. Ligament instability or loss of meniscal support subjects any cartilage repair to abnormal forces from the outset. Addressing these — commonly in the same procedure — is a prerequisite rather than an optional extra.

A multimodal diagnostic framework, such as that proposed by Gherghel and colleagues in 2026, formalises how clinicians weigh these variables together: ICRS grade and defect area sit within a broader assessment that incorporates MRI findings, symptom burden, and full clinical history. The grade and size provide orientation; individual clinical assessment determines what to do with them.

Getting an accurate assessment and knowing your options in London

The decision to act on a Grade 3 or 4 finding is time-sensitive, and the practical question most patients reach after reading a scan report is simpler than the grade scale itself: where do I go from here?

The necessary next step is a specialist assessment that holds ICRS grade, defect area, and clinical picture together — alignment, activity demand, concomitant joint pathology, and symptom burden. A multimodal framework, such as that described by Gherghel and colleagues in 2026, formalises precisely this integration: the grade is a starting point; how it translates to your specific joint depends on factors only an in-person examination can assess.

For patients with eligible Grade 3–4 defects in the smaller size range, the ChondroFiller injection — an ultrasound-guided outpatient injectable collagen scaffold — offers a single-stage, non-surgical pathway. The scaffold recruits the patient's own progenitor cells to generate new cartilage matrix within the defect. In the UK, ChondroFiller is delivered under the Liquid Cartilage™ brand name: ChondroFiller is the CE-marked device; Liquid Cartilage™ is the service brand through which patients access it.

Liquid Cartilage™ is available at the London Cartilage Clinic on Harley Street, the UK's certified delivery centre for the treatment. Assessments can be booked at londoncartilage.com.