The damage grade that makes you a candidate

Most patients arrive at this question with a scan result in hand and a straightforward need: does the damage shown on that scan put them in range for ChondroFiller injection? The short answer is that the candidacy window opens at partial- to full-thickness cartilage loss — what clinicians classify as Grade III or Grade IV on the ICRS (Outerbridge) scale.

Grade III describes cartilage that has worn through more than half its depth, leaving a rough, softened surface but some tissue still present. Grade IV means the cartilage is gone all the way to bone. Below these grades — where fissuring is superficial and the remaining cartilage retains structural integrity — the scaffold is unlikely to offer meaningful additional support, and other management strategies are usually more appropriate. The threshold exists for sound biological reasons, not as arbitrary gatekeeping.

For patients with more widespread joint degeneration — Kellgren-Lawrence Grade III or IV osteoarthritis affecting broader areas of the joint surface — the injectable collagen scaffold operates differently, acting as a surface-coating cushion rather than a contained focal repair. This extends the candidacy window beyond discrete lesions without contradicting the core indication.

Critically, pain intensity alone does not determine grade. Some patients with severe cartilage loss report relatively mild symptoms; others with moderate loss are significantly disabled. A formal MRI assessment is non-negotiable before any candidacy decision is made, as it maps both the grade and the extent of damage with a precision that clinical examination and symptom scoring cannot replicate. Age is not a deciding factor; what matters is the MRI-confirmed pattern of damage.

Defect size: how much cartilage damage can be treated

Scaffold volume is matched precisely to the size of the affected area — so the first practical question is not just whether the damage qualifies, but how much of it there is.

The manufacturer's volumetric sizing guide works in three steps:

• 1.0 ml for defects up to 1.5 cm²
• 1.5 ml for defects up to 2.0 cm²
• 2.3 ml for defects up to 3.0 cm²

Think of this as a tailored fill rather than a standard dose: the consultant selects the volume that matches the defect's mapped area, ensuring the collagen scaffold can cover the damaged surface without excess. The practical ceiling for the ultrasound-guided injection pathway sits at approximately 3 cm².

Some clinical literature — including the manufacturer's Clinical Evaluation Report (CER v09, April 2025) — cites a broader ceiling of up to 6 cm². That figure applies to the arthroscopic surgical route, where the scaffold is placed into a dry, mechanically contained joint bed under direct vision. It does not apply to the injectable, outpatient pathway described here, and the two should not be conflated.

Within that up-to-3 cm² window, ChondroFiller occupies a useful position in the treatment spectrum. Microfracture is generally suited to smaller defects — typically under 2 cm² — and loses efficacy as lesion size increases. Cell-based procedures such as ACI and MACI, by contrast, are usually reserved for larger defects above 2–4 cm², where the cost and complexity of cell harvesting and re-implantation can be justified. ChondroFiller bridges that middle ground as a single-stage injectable option.

Where a patient has more than one focal defect within the same joint, it may be possible to address both at the same appointment, subject to the consultant's assessment of total defect area and joint condition.

Which joints can be treated

ChondroFiller is not a knee-only treatment. The scaffold is designed to gel in situ within any accessible articular surface, making it joint-agnostic in principle — though clinical volume is concentrated in a handful of joints where focal cartilage damage is most common.

The knee, hip, and ankle account for the majority of cases in published clinical data, reflecting the load-bearing demands placed on those joints and the frequency with which sports injuries, osteochondritis dissecans, and age-related wear affect them. Shoulder, elbow, wrist, and hand joints are also cited, particularly in the context of focal cartilage injuries sustained during sport.

Evidence for less common joints continues to emerge. A 2025 PubMed-indexed study by Demmer and colleagues (PMC12498443) confirmed the technical feasibility of ChondroFiller in residual wrist cartilage defects — specifically gaps of 0.5 mm to 2 mm — following surgical repair of distal radius fractures. This represents an expanding clinical footprint rather than routine practice, and should be understood as proof of concept rather than an established high-volume indication.

In every case, joint-level suitability is confirmed at assessment rather than assumed. The practical determinants are whether ultrasound-guided placement is technically achievable and whether the local biological environment within that joint can support matrix-induced chondrogenesis. A consultant review establishes both.

Structural conditions the joint must meet first

The scaffold can only do its job if the joint around it is mechanically sound. Cartilage repair through matrix-induced chondrogenesis depends on the local biological environment remaining intact — and that environment is undermined when the joint is structurally unstable, poorly aligned, or missing the support structures that distribute load across the articular surface.

Three mechanical factors are routinely assessed before ChondroFiller is considered appropriate:

• Ligament stability — untreated ligament laxity exposes a healing scaffold to abnormal shear forces. Where instability is identified, reconstruction or stabilisation is typically addressed before or alongside treatment.
• Meniscal integrity — significant meniscal deficiency alters load distribution, concentrating pressure on focal areas. Where meniscal tissue is substantially absent, this requires clinical consideration prior to proceeding.
• Limb alignment — malalignment greater than approximately 5° directs disproportionate load through the damaged compartment, and realignment may be indicated first.

These are assessment findings, not automatic disqualifiers. Identifying and managing them is part of protecting the biological investment the scaffold represents — correcting load and stability gives the patient's own recruited cells the best chance of maturing into durable repair tissue.

Prior conservative management — physiotherapy, anti-inflammatory medication, and in many cases earlier injection therapies — forms the expected baseline before a formal candidacy assessment is initiated. Biomechanical review sits alongside MRI findings in that assessment; where objective gait or loading analysis is relevant, MAI Motion® objective motion capture can quantify alignment and movement patterns that imaging alone may not capture.

When ChondroFiller is not suitable

Active infection in the joint is an absolute bar — not an administrative one. The collagen scaffold relies on the patient's own progenitor cells migrating into the matrix and differentiating into chondrocytes; an infected or acutely inflamed environment disrupts that process before it can begin. The same logic applies to crystal arthropathies such as gout: even between acute flares, the joint environment may be biologically hostile to scaffold integration.

Systemic inflammatory arthritis — rheumatoid arthritis being the most common example — poses a related problem. The underlying disease alters synovial biology in ways that compromise matrix-induced chondrogenesis, making reliable cell recruitment difficult to achieve regardless of how well symptoms are managed day to day.

Two further contraindications are standard across injectable biologics: a known allergy to collagen or murine-derived (mouse) proteins rules out treatment, since the scaffold is composed of acellular murine Type I collagen; and pregnancy or breastfeeding excludes patients as a precautionary measure.

Advanced bone-on-bone osteoarthritis requires a separate conversation. Where no viable biological environment remains in the joint, the surgical route is not appropriate — though the outpatient injection pathway for diffuse Kellgren-Lawrence Grade III–IV OA, discussed in relation to candidacy in the opening section, operates on a different basis and is reviewed individually.

Not every case fits cleanly within these boundaries. A patient with rheumatoid arthritis in sustained, well-documented remission, for instance, may sit in a genuine grey area — whether the joint biology has recovered sufficiently to support scaffold integration is a clinical judgement that an assessment consultation is designed to make.

Getting an assessment at MSK Doctors

Taken together, the preceding sections describe a fairly well-defined window: focal ICRS Grade III or IV cartilage loss, a structurally stable joint, no active infection or inflammatory arthritis, and a defect area within the scaffold's volumetric range. Patients who sit within that window — confirmed by MRI rather than symptom severity alone — are likely candidates; those who fall outside it will hear clearly at assessment why not, and what a more appropriate next step might be.

Confirming where a patient sits requires a consultant-led assessment, which at MSK Doctors does not depend on a GP referral or an NHS-style waiting list. For patients who have not yet been scanned, or whose existing images need updating, the Sleaford clinic (NG34) has an Open MRI scanner on site; existing scans from elsewhere can be reviewed at the same appointment. Patients who are more conveniently placed for Grantham (NG31) can be seen at the diagnostics centre there. London-based patients can access the same consultant-led pathway through the London Cartilage Clinic.

Appointments can be booked directly at mskdoctors.com — no referral needed.