Why grade and defect size decide the treatment path

A consultant telling you that you have a cartilage defect raises an immediate practical question: does that mean an injection, an operation, or something more serious? The answer depends on two measurements, not one.

The first is depth — how far into the cartilage layer the damage extends. Clinicians express this using the ICRS (International Cartilage Repair Society) scale, which runs from Grade 0 (structurally normal) through to Grade 4, where damage has broken through to the underlying bone. The second is area — how many square centimetres of the joint surface are affected.

These two variables interact. A small Grade 3 lesion and a large Grade 3 lesion can be appropriate for entirely different procedures, because the biology of repair changes significantly once a defect exceeds certain size thresholds.

Together, grade and area place a patient on one of four broad pathway tiers: symptom management, injection or biologic support, cartilage restoration surgery, or joint replacement. Every non-replacement option on that ladder shares the same overarching goal — to delay or avoid total knee replacement, keeping the knee functional and as pain-free as possible for as long as the joint allows.

Neither grade nor area can be reliably estimated from symptoms alone. Accurate classification requires arthroscopic assessment or high-resolution MRI — the starting point before any treatment decision is made.

The ICRS grading scale — what each grade means

The ICRS scale gives surgeons a shared language for describing how far cartilage damage has penetrated — and gives patients a clearer way to understand what a grade means for their treatment options.

Grade 0 — structurally normal cartilage with no detectable lesion.

Grade 1 — the surface is disrupted: softening, swelling, or shallow fissures are visible, but the damage has not breached any significant depth. Many patients at Grade 1 have no symptoms, or only intermittent discomfort.

Grade 2 — damage extends into the cartilage but remains within the upper half of its thickness. The structural layer is compromised, though still present throughout.

Grade 3 — the surgical threshold. Damage now exceeds 50% of cartilage depth, and proximity to the underlying bone becomes clinically significant. Four subtypes reflect this progression: 3A stops above the calcified layer; 3B reaches it; 3C passes through it; 3D involves surface blistering without full penetration. This distinction matters because it influences not just whether surgery is appropriate but which type of restoration is technically viable.

Grade 4 — full-thickness loss with subchondral bone exposed. This is the 'bone-on-bone' scenario; when diffuse across a joint compartment, it represents a fundamentally different treatment pathway from the focal defects that cartilage repair addresses.

Grading is most precisely established during arthroscopy. The 2024 international consensus (Aman et al.) recommends that grade, lesion length, depth, and area all be formally documented at the time of surgery — findings that directly shape the treatment decision that follows.

Grade 1–2: what injections can and cannot do

For Grade 1 and Grade 2 lesions, and for early-to-moderate knee osteoarthritis, injections sit at the centre of the treatment tier — not because they repair damaged cartilage, but because they can meaningfully reduce pain and slow progression while the structural layer remains partly intact.

Hyaluronic acid (HA) viscosupplementation works by restoring the lubricating properties of synovial fluid, reducing friction across the joint surface. Benefits typically emerge at four to six weeks after treatment and, in suitable patients, can last up to six months. PRP (platelet-rich plasma) and BMAC (bone-marrow aspirate concentrate) bring a different mechanism — growth-factor release that may modulate local inflammation and support the joint environment — though evidence for any structural regenerative effect remains limited.

The biological ceiling matters here, and it should be stated plainly. No injection — including HA, PRP, corticosteroid, or any preparation described as stem-cell therapy — can rebuild articular cartilage once it has been lost. Injections at this grade are a symptom-management and joint-support strategy, not a restoration pathway.

For patients with a suitable focal defect who want to go beyond symptom management, a ChondroFiller injection — an injectable collagen scaffold placed under ultrasound guidance — offers a scaffold-based option that is distinct from standard HA or PRP. Its specific indications and mechanism are covered in the next section.

The appropriate expectation for Grade 1–2 injection treatment is reduced pain and preserved function, not structural recovery.

Grade 3 focal defects — how size determines which repair technique fits

Grade 3 changes the clinical calculus: depth has now exceeded half the cartilage thickness, and choosing a repair technique depends heavily on how large the lesion is. Size and grade work together to define what is technically viable at each tier.

Under 2 cm² — single-stage options

For smaller focal defects, two surgical approaches carry meaningful long-term evidence. OATS (osteochondral autograft transfer, sometimes performed as mosaicplasty) transplants a plug of healthy bone and cartilage from a low-load area of the same knee. A 10-year comparison by Gudas (2012) showed better clinical outcomes with OATS than with microfracture, and the technique is well-suited to defects in the 1–2 cm² range, with mosaic configurations extending to around 4 cm². AMIC (autologous matrix-induced chondrogenesis) pairs marrow stimulation with a collagen scaffold in a single stage — a technical bridge between older marrow-stimulation techniques and full cell-based repair.

For patients seeking a non-surgical route, a ChondroFiller injection — an injectable collagen scaffold placed under ultrasound guidance as an outpatient procedure — is clinically relevant for focal defects up to approximately 3 cm², with evaluation data supporting use up to 6 cm² in selected cases, and no general anaesthetic is required.

3 cm² and above — cell-based repair

When a defect reaches 3 cm² or larger, cell-based approaches become the preferred surgical option. Both MACI and ACI work by culturing the patient's own chondrocytes and returning them to the damaged site — MACI delivers cells on a collagen membrane; ACI uses a periosteal patch. The SUMMIT Phase 3 RCT demonstrated that MACI produces superior KOOS pain and function scores compared to microfracture at both 2 and 5 years for defects of 3 cm² or greater. For lesions above 4 cm², or where an earlier repair has failed, ACI is the established primary choice. The comparative picture between ACI and MACI in the 2–4 cm² range is less clear-cut in current evidence, and patient-specific factors guide that decision at consultation.

Microfracture — a declining baseline

Microfracture was historically the default for small full-thickness defects. Current evidence shows that the fibrocartilage it produces tends to break down within two to three years, and the procedure can damage the subchondral bone plate in ways that limit future repair options. It is no longer regarded as a first-line modern choice.

Grade 4 and diffuse loss — when joint replacement is the right path

The distinction that matters most at Grade 4 is not the grade itself — it is how much of the joint is involved.

A focal Grade 4 lesion, typically posttraumatic and confined to a single compartment, may still be amenable to surgical restoration. Fresh osteochondral allograft (OCA) is the primary option at this tier: donor bone and cartilage from a matched graft replaces the damaged area in one stage, with long-term follow-up data from Gross (2008) and Levy (2013) supporting durable outcomes in carefully selected patients. Where the defect is isolated, the goal remains joint preservation.

Diffuse Grade 4 — cartilage loss spreading across multiple compartments, with bone-on-bone contact through much of the joint surface — is a different clinical state entirely. No single graft, scaffold, or injection can address widespread structural loss of this kind, and the evidence base for any biological or restorative treatment does not extend to multi-compartmental disease.

Total knee replacement (TKR) is the appropriate, matched solution when cartilage loss is this extensive. It should be understood as a designed endpoint for that specific disease state, not as evidence that earlier treatment failed. For patients with end-stage disease isolated to one compartment, unicompartmental knee replacement may be a suitable alternative, preserving the unaffected areas of the joint.

The clinical boundary between a severe Grade 3D lesion and Grade 4 can be genuinely grey, and whether a defect is restorable or requires replacement is only fully answerable through arthroscopic assessment and individual clinical review.

Getting an accurate cartilage assessment

Grading a cartilage lesion accurately requires more than a plain X-ray. Standard X-rays show bone alignment but cannot reliably detect or characterise cartilage damage; MRI — ideally with dedicated cartilage sequences or T2 mapping to assess tissue quality and composition — is the non-invasive standard. Arthroscopy provides definitive confirmation of grade, lesion area, and boundary, and the 2024 international consensus (Aman et al.) is clear that ICRS grading should be documented arthroscopically alongside measurements of lesion length, depth, and area.

At Lincolnshire Knee, onMRI™ AI-driven cartilage segmentation and T2 mapping can support pre-arthroscopy lesion characterisation, giving clinicians a sharper picture before any surgical decision is made. Where biomechanical factors are relevant, MAI Motion® gait analysis can identify load-distribution patterns contributing to cartilage stress — useful both in treatment planning and in monitoring how joint mechanics change after any intervention.

Grade and defect size supply the framework; the individual clinical picture — age, activity level, compartment involvement, symptom burden, and prior treatment history — determines the actual pathway. No grading system replaces a structured clinical assessment.

Lincolnshire Knee is part of the MSK Doctors group and accepts patients without referral. Book an assessment at lincolnshireknee.co.uk.