What grade III and IV cartilage damage actually means

A diagnosis of grade III or IV cartilage damage confirms that the cartilage inside the knee has moved beyond ordinary wear into tissue that specialists classify as diseased — a distinction that matters because cartilage at this severity does not reliably repair itself.

Under the ICRS (International Cartilage Repair Society) grading system, grade III means more than half the cartilage depth has been lost. The damage may extend down to the calcified layer at the base of the cartilage, or present as surface blistering that conceals deeper structural disruption beneath. Grade IV is more severe still: the cartilage surface has been breached entirely, exposing the subchondral bone beneath it. Both grades sit at the diseased end of the chondropathy spectrum — grades 3 and 4 of 5 — where the body's self-repair capacity falls meaningfully short.

Typical symptoms include a persistent deep aching inside the knee, episodic catching or giving way, swelling after activity, and a progressive narrowing of what the joint will comfortably allow. These are not vague background discomforts; they usually correspond to a measurable structural lesion.

Grade III and IV chondral defects are often focal — concentrated lesions with identifiable borders — rather than the diffuse thinning seen in early osteoarthritis. That said, Kellgren-Lawrence grade III and IV osteoarthritis, which describes a broader pattern of joint wear visible on X-ray, can coexist or overlap clinically with focal lesions.

Lesion size — measured in square centimetres — matters as much as grade alone when weighing how to respond to the damage, and will form part of any thorough clinical assessment.

What ChondroFiller (Liquid Cartilage) is and how it works in the knee

ChondroFiller® — sold under the brand name Liquid Cartilage™ — is a CE-marked Class III medical device, placing it in the same regulatory category as joint implants and pacemakers. That classification means it has passed a rigorous European conformity assessment before being used in patients, which is a meaningful level of regulatory scrutiny for any injectable product.

At its core, ChondroFiller is an acellular scaffold: a purified Type I collagen gel derived from murine tissue. 'Acellular' simply means there are no donor cells or patient cells in the product itself — it contains only the structural collagen matrix. When it enters the warm environment of the knee joint, it polymerises within minutes, forming a viscoelastic gel that immediately provides a degree of mechanical cushioning over the damaged surface.

What happens next is the key to understanding why it differs from a conventional filler. The gel acts as a structured framework — closer to scaffolding than to plaster — that the body moves into rather than passively around. The collagen matrix creates chemical signals that draw the patient's own progenitor cells from the surrounding synovium and subchondral tissue. Those recruited cells migrate into the scaffold and differentiate, driving what is termed acellular matrix-induced chondrogenesis: cartilage repair initiated entirely by the body's own biology, guided by the scaffold's architecture. The aim is to support the body's own repair processes rather than to substitute for them.

Patients likely to benefit — and those who may not

Not every grade III or IV lesion points automatically to the same treatment, and candidacy for ChondroFiller depends on a combination of factors that a clinical assessment — including imaging — must confirm.

The clearest candidates are patients with a focal chondral defect up to 6 cm² in area, or with diffuse Kellgren-Lawrence grade III or IV osteoarthritis of the knee. That 6 cm² size limit is notably broader than microfracture, which is generally restricted to defects under 2–4 cm². For diffuse joint wear rather than a single focal lesion, the outpatient, ultrasound-guided injection pathway is typically the appropriate route — and that is the current service pathway at Lincolnshire Knee. Where a defect is very large, localised, and structurally unstable with clear borders of healthy surrounding cartilage, arthroscopic placement may be considered instead; a clinician will distinguish which scenario applies after imaging.

Candidacy also improves with a mechanically stable knee, a BMI within a reasonable range, and no active infection in or around the joint. Lesion size and stability drive the decision as much as grade alone — a fact worth discussing openly at assessment.

Factors that make ChondroFiller less suitable include end-stage osteoarthritis accompanied by significant malalignment (where a corrective osteotomy may need to come first), active joint infection, and confirmed allergy to collagen-derived products.

At Lincolnshire Knee, onMRI™ AI-assisted cartilage analysis can characterise defect size and depth at the assessment stage, helping to establish whether the injection pathway is appropriate before any treatment decision is made.

What the injection appointment involves

On the day of treatment, the appointment takes place in an outpatient clinic — there is no hospital admission, no theatre, and no surgical wound to manage.

Before the injection begins, the skin around the knee is cleaned and a local anaesthetic is applied to minimise discomfort. Mild sedation is an option for patients who prefer it, but most people tolerate the procedure with local anaesthesia alone. An ultrasound probe is used throughout to guide the needle to the precise location of the defect, confirming accurate placement of the collagen gel in real time.

The injection itself takes only a few minutes. Once inside the joint, ChondroFiller begins to polymerise at body temperature — the gel sets within the defect rather than dispersing freely through the joint fluid. Some patients notice a sense of fullness or mild pressure in the knee as this happens; that is a normal part of the process.

Afterwards, a degree of transient swelling or achiness over the first day or two is common as the scaffold settles. Most patients leave the clinic on the same day. Weight-bearing and rehabilitation guidance will be discussed at discharge — the recovery load is considerably lighter than that which follows any surgical procedure, with no wound care and no theatre-related restrictions.

Clinical outcomes: what the evidence shows

The most patient-meaningful figure from the available evidence is consistent across four European knee studies: ChondroFiller produces approximately 30 points of improvement in IKDC (International Knee Documentation Committee) scores over twelve months. That matters because the threshold at which a change in IKDC score becomes genuinely noticeable to a patient — the minimum clinically important difference — is 16.7 points. A 30-point gain roughly doubles that bar.

Durability data from the prospective PMCF study by Jerosch et al. suggests this improvement is sustained. At the three-year follow-up, the mean IKDC gain was 32.4 points, with patients reaching a mean functional score of 80 — indicating a return to reasonably active daily life for most participants.

What MRI shows

MRI provides an objective measure of how well the defect has filled and integrated. ChondroFiller is assessed using the MOCART score — essentially a 0–100 rating of repair tissue quality and fill level, where higher is better. Across European studies, MOCART scores reach 81.6 to 84.3, indicating that more than 80% of the defect is typically filled with tissue that integrates well with the surrounding cartilage. One study tracked this progression directly: MOCART rose from 65.3 at four weeks to 81.6 at twelve months, confirming that repair continues well after the injection rather than peaking early. Secondary MRI findings in some patients include reduced bone marrow oedema, diminished effusion, and visible widening of the joint space.

Safety and comparator context

The complication rate across the published data is approximately 0%, with a reoperation rate of 3–8%. For context, microfracture carries a reoperation rate of up to 41%, and ACI/MACI up to 37%, with complication rates reaching 17%.

One honest caveat applies to all of the above: the current evidence base draws predominantly from manufacturer-sponsored and European registry studies rather than large independent randomised controlled trials. The data are promising, but patients should understand they reflect a specific — if growing — evidence foundation.

Realistic expectations and taking the next step

The honest goal of ChondroFiller treatment is cartilage preservation and symptom control — not complete structural regeneration. For grade III and IV disease, that is a meaningful and realistic aim: slowing progression, improving daily function, and reducing the pain burden that pushes people towards joint replacement. Success is better measured by whether the knee allows more with less discomfort than by an MRI that looks entirely normal.

Knowing when the treatment has not worked is as important as knowing when it has. If symptoms fail to improve over three to six months, or if follow-up imaging at twelve months shows poor scaffold integration, a clinician will revisit the plan. Patients with significant malalignment — a varus or valgus deformity concentrating load on the damaged compartment — may need a corrective osteotomy alongside or before scaffold treatment. Those whose disease has progressed beyond what any scaffold can meaningfully support will be guided towards a knee replacement assessment. That is not a failure of the treatment; it is a natural progression of grade III/IV disease that appropriate monitoring catches in good time.

Long-term data on ChondroFiller beyond three years remains limited, so ongoing clinical review is appropriate regardless of which treatment pathway a patient pursues.

Before agreeing to any cartilage scaffold treatment — at any centre — it is reasonable to ask: what imaging was used to measure the defect, who will interpret the follow-up MRI, and what the plan is if improvement plateaus? Those questions apply equally everywhere.

Lincolnshire Knee is part of the MSK Doctors group, accepts patients without a GP referral, and offers a consultant-led assessment — including cartilage-specific MRI analysis where clinically indicated. Further information is available at lincolnshireknee.co.uk.