The two criteria that open the door

Deciding whether a ChondroFiller injection is appropriate starts with a clinical question that surprises many patients: pain intensity is not the deciding factor. Severe, daily knee pain that has failed every pill and physiotherapy programme does not automatically make someone a candidate — and moderate, intermittent discomfort does not rule them out. Pain is a prompt to investigate, not a threshold to cross.

Two structured criteria must be satisfied before defect-specific assessment begins.

The first is documented failure of conservative management. This means a meaningful trial of physiotherapy, load modification, activity adjustment, and — where appropriate — injection therapies such as corticosteroid or hyaluronic acid. The rationale is straightforward: ChondroFiller injection targets structural cartilage damage, and that level of intervention is warranted only once simpler measures have demonstrably not held.

The second is MRI-confirmed cartilage damage. A clinical examination and patient history can suggest the diagnosis, but MRI is non-negotiable for mapping the defect — its location, depth, surrounding cartilage integrity, and any subchondral changes that would redirect management. Self-reported symptoms alone cannot provide this information.

Once both criteria are met, the assessment moves to defect-specific triage: whether the damage is a contained focal lesion or a pattern of diffuse joint-surface loss. Each of those presentations follows a different clinical track, covered in the next two sections.

Focal defect pathway: grade and size thresholds

The focal defect pathway applies when cartilage loss is graded Outerbridge III or IV — at least half the cartilage thickness gone (Grade III), or damage reaching the underlying subchondral bone (Grade IV) — and the lesion sits within a defined, bounded area rather than spreading diffusely across the joint surface.

Defect size is the central variable. The standard indication covers lesions up to 3 cm², with the CE-mark extending eligibility to 6 cm². That ceiling matters in clinical context: microfracture is generally limited to defects under 2–4 cm² and produces fibrocartilage rather than hyaline-like repair tissue. ACI and MACI can address comparable or larger areas but require two separate stages — a biopsy to harvest chondrocytes, then a return procedure for implantation. ChondroFiller injection achieves repair in a single outpatient visit under ultrasound guidance, with no general anaesthetic or operating theatre involved.

Containment is a practical prerequisite: the scaffold material needs a bounded cavity to gel into and hold position. Once that condition is met, the collagen sets within minutes and acts as a lattice for the patient's own progenitor cells, which migrate in from surrounding synovium and subchondral bone to build cartilage-like matrix over the following months — making the size and containment criteria directly relevant to how the treatment works, not just to whether it is indicated.

The pathway is not knee-specific. Focal Grade III–IV damage in the hip, ankle, shoulder, elbow, or wrist qualifies on the same structural criteria, provided the lesion is accessible to image-guided needle placement.

Diffuse OA and bone-on-bone: a separate eligibility track

Bone-on-bone status stops many patients from enquiring further — the assumption being that no remaining cartilage means no viable candidate. The diffuse OA pathway challenges that assumption directly.

Where cartilage loss is widespread rather than contained in a discrete lesion, Kellgren-Lawrence Grade III or IV disease — including presentations where joint-space narrowing has progressed to direct bone contact — qualifies under a separate clinical track. The mechanism shifts accordingly. Rather than filling a bounded void, the injectable collagen scaffold is applied as a surface coating across the articular face, restoring a protective viscoelastic layer over exposed bone and residual cartilage. Defect area is therefore less relevant than the stage and distribution of disease: the clinical question becomes whether a biological protective layer can be established across the affected surface, not whether a single cavity can be bridged.

Patients considering this pathway should weigh their expectations against the current evidence base. Published trial data supporting matrix-induced chondrogenesis — including the cohort outcomes discussed in the evidence section below — derive primarily from focal-defect populations; the diffuse OA track is clinically established through specialist practice and supported by the same scaffold mechanism, but has not yet been the subject of a large independent randomised trial. That honest qualification does not undermine the clinical rationale; it simply reflects where the evidence currently sits.

Age, BMI, and alignment in the eligibility assessment

Unlike ACI and MACI — where informal biological fitness thresholds mean age frequently enters the conversation as a limiting factor — no formal upper age limit applies to ChondroFiller injection under either clinical track. Older patients presenting with the same cartilage damage profile as younger ones are assessed against the same four clinical dimensions: joint mechanics, the biological environment (which includes age, BMI, and systemic health), tissue condition, and disease progression over time. Age feeds into the second of those dimensions as one input among several, not as a standalone gate.

Alignment and BMI receive equivalent treatment. Uncorrected severe limb malalignment is an absolute contraindication — the injectable scaffold depends on a stable biomechanical environment to gel, bond, and integrate correctly, and a chronically misloaded joint cannot reliably provide that. Moderate or correctable malalignment, however, is flagged as an assessment point rather than an automatic exclusion; where the malalignment can be addressed, it does not in itself bar candidacy. BMI is similarly weighed as a joint-load variable within the broader clinical picture, not used as a standalone cut-off.

The practical implication is that eligibility decisions here are individual rather than bracket-based — a meaningful distinction for patients who have previously been told they are 'too old' or 'not surgical candidates'.

When ChondroFiller injection is not the right option

ChondroFiller injection would not be suitable in a defined set of circumstances — recognising them avoids an unnecessary assessment journey for those affected.

Absolute contraindications

Four situations place treatment outside the suitable range. A known hypersensitivity to collagen materials or animal-derived biological components is a firm exclusion; patients with a documented allergy in this category should not proceed. Uncorrected severe limb malalignment or major ligament instability — covered in the section above — are also absolute contraindications, as both compromise the mechanical environment the scaffold depends on. Complete end-stage mechanical joint collapse, where no viable articular surface remains to support scaffold integration, likewise places a patient outside the indication. Finally, a joint location that is technically inaccessible to image-guided needle placement can render the injection impossible even when all other clinical criteria are met.

Systemic factors

Active immunosuppression and poorly controlled diabetes are the principal systemic exclusions. Both impair the biological conditions the scaffold relies on to recruit the patient's own progenitor cells — the mechanism that drives cartilage-like matrix formation after injection.

What the evidence shows and how to find out if you qualify

The outcome data available for ChondroFiller injection supports the eligibility framework described above, though with an important proviso about evidence quality.

In the Jerosch et al. post-market clinical follow-up study, patients showed a mean IKDC score improvement of 32.4 points — a gain that exceeded the established minimal clinically important difference of 16.7 points and was sustained, and marginally increased, at three-year follow-up, reaching a functional score of 80. Structural repair assessed by MRI produced MOCART scores of 81.6–84.3 across European cohort data, indicating more than 80% defect filling with good integration into surrounding native cartilage. The reoperation rate of approximately 3–8% compares favourably with microfracture, where rates of up to 41% have been reported, and with ACI/MACI at up to 37%.

One limitation deserves a plain statement: all cited trials to date are manufacturer- or clinic-sponsored, and no large independent randomised controlled trial has yet been published. The results are consistently positive across these studies, but independent confirmation at scale remains pending.

For patients who meet the criteria outlined in this article, the appropriate next step is a formal clinical assessment to map individual anatomy, damage grade, and joint mechanics. Liquid Cartilage™ is delivered in the UK at the London Cartilage Clinic on Harley Street, where Professor Paul Y. F. Lee leads ChondroFiller injection provision. Assessments can be booked at londoncartilage.com.