Focal or diffuse: which pathway applies to you

Whether ChondroFiller injection is the right path — and what that path looks like — depends on a single clinical distinction: is the cartilage damage focal and contained, or is it diffuse across the joint surface?

These are two separate candidacy tracks, and they work on different logic.

On the focal defect pathway, a discrete, bounded lesion has been identified on MRI. The defect's measured area is the central variable — it determines both whether ChondroFiller injection is indicated and which syringe volume the clinician selects. Depth of damage matters too: Grade III or IV loss on the Outerbridge scale is required, meaning the injury penetrates more than half the cartilage thickness or reaches subchondral bone. Size limits, grade criteria, and the integrity of surrounding tissue are all active decision points on this track.

On the diffuse osteoarthritis pathway, the clinical picture is different. Rather than targeting a contained defect, the injectable collagen scaffold is used to coat the articular surface as a viscoelastic cushion, providing a protective layer across more widespread joint-surface degeneration. Kellgren-Lawrence Grade III–IV disease — including presentations described as 'bone on bone' — can fall into this category. Here, defect area is a less relevant variable than the stage and distribution of joint disease. Some patients on this track have previously been told no restorative surgical option was viable.

Establishing which track applies is the first task of an assessment consultation. MRI is non-negotiable for that mapping: it confirms whether damage is focal or diffuse, defines the defect geometry on the focal pathway, and rules out features that would redirect care entirely.

How big a focal defect the ChondroFiller injection can fill

The CE-mark indication for ChondroFiller injection covers focal defects up to 6 cm² — a ceiling that is broader than it might first appear. Most lesions treated in clinical practice measure around 2.5 cm², so patients with moderately sized focal damage are well within the centre of the indicated range, not at its edge.

At the lower end, no fixed minimum applies. Published clinical data from Demmer (2025) documents ChondroFiller use in residual chondral defects measuring 0.5 to 2 millimetres in their largest linear dimension — a scale too small to express meaningfully in square centimetres — across 25 cases. This illustrates the full reach of the injectable approach, from sub-centimetre remnant damage up to the 6 cm² ceiling.

That upper ceiling sits meaningfully above where conventional alternatives run out of options. Microfracture and mosaicplasty are generally confined to lesions below 2–4 cm². MACI can treat larger defects, but requires a two-stage procedure, healthy perilesional tissue, and a longer recovery pathway. The SUMMIT trial identified ≥3 cm² as the threshold at which MACI consistently outperforms microfracture for pain and function — precisely the intermediate zone where a single-stage, outpatient ChondroFiller injection may offer a more practical route for suitable patients.

Defects between 3 and 6 cm² remain within the indicated scope, though this upper portion of the range carries a thinner evidence base, and clinical technique may need to be adapted accordingly. Individual assessment — including MRI defect mapping — remains the definitive step for any lesion approaching that upper boundary.

How defect area determines which syringe volume is selected

The three syringe volumes Meidrix Biomedicals supplies are each calibrated to a specific defect area:

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

This ladder makes the MRI defect measurement a direct clinical input: the area recorded on imaging determines which syringe is selected on the day of treatment. Undersizing the volume risks incomplete scaffold coverage of the defect floor, which is why accurate mapping at the pre-treatment stage carries real procedural weight.

For lesions between 3 cm² and the 6 cm² upper ceiling, the published dosing guidance does not specify a standard volume protocol — adapted technique or combined volumes may apply, and this remains a practical gap in the available literature. Individual clinical judgement guides those cases.

Why lesion depth matters as much as area

Depth determines biological viability. The collagen scaffold works by drawing mesenchymal progenitor cells upward from the deep layers of tissue — subchondral bone and the cartilage bed immediately above it. A lesion that is shallow, with most of the cartilage thickness intact, sits too far from that cellular reservoir for the scaffold to recruit effectively. That is why superficial damage does not qualify, not as an arbitrary rule but as a consequence of how the repair process works.

The entry threshold for the focal repair pathway is Outerbridge Grade III (damage extending through more than half the cartilage thickness) or Grade IV (full-thickness loss, reaching bone). Grades I and II — surface softening and partial-depth fraying — fall below the biological minimum.

Grade is determined on MRI. Patients do not need to know their own grade before making an enquiry; a consultant review interprets the imaging and confirms whether the depth criterion is met.

On the diffuse osteoarthritis track, the depth-threshold logic differs, for the reasons set out in the opening section — the scaffold's role there is surface-level rather than deep-defect recruitment, so disease-stage rather than lesion depth becomes the relevant measure.

Factors that rule out candidacy or require correction first

Not every patient with cartilage damage is ready for ChondroFiller injection today — but the distinction between a permanent exclusion and a correctable prerequisite matters considerably for how assessment is framed.

The one categorical exclusion within the focal repair pathway is widespread, multi-compartment osteoarthritis. When degeneration has spread across the joint rather than remaining contained, the perilesional tissue that the collagen scaffold depends on for cell recruitment is no longer viable. That biological prerequisite cannot be manufactured around, and it is the reason diffuse joint disease disqualifies patients from the focal repair track — not as an arbitrary rule, but as a consequence of the scaffold's repair mechanism.

Mechanical factors, by contrast, are often correctable. Untreated ligament laxity, significant malalignment, or a meniscal deficit driving abnormal load through the joint must be addressed before ChondroFiller injection is appropriate — the scaffold cannot integrate reliably in a mechanically unstable environment. Osteotomy, ligament repair, or meniscal surgery may therefore precede cartilage treatment rather than replace it; these are preliminary steps, not permanent disqualifications.

Age is not an exclusion criterion. Candidacy depends on the damage pattern, not the patient's year of birth.

MRI remains essential: candidacy cannot be confirmed — and the appropriate syringe volume cannot be determined — without clear defect mapping prior to treatment.

Outcomes at one to five years and next steps at LCC

Across four knee studies, ChondroFiller injection consistently improves IKDC functional scores by approximately 30 points — above the 16.7-point threshold recognised as the minimum clinically important difference. The Jerosch et al. post-market clinical follow-up study recorded a mean gain of 32.4 IKDC points, with patients reaching a functional score of 80, sustained and slightly increased at the three-year mark.

MRI-based MOCART scoring progressed from a mean of 65.3 at four weeks to 81.6 at one year — indicating greater than 80% defect fill, as the scaffold helps the body build replacement tissue progressively rather than supplying the tissue itself. Across knee, hip, and small-joint applications, 70–85% of patients achieve meaningful symptom relief at three to five years, with a near-zero complication rate and a reoperation rate of 3–8%.

One honest limitation applies: direct evidence comparing outcomes for defects below 2 cm² against those in the 2–6 cm² range is limited. Patients with larger defects within the permitted range should expect individual counselling rather than a size-specific outcome prediction.

At the London Cartilage Clinic, assessment covers MRI review, defect mapping, and a structured discussion of which pathway — focal scaffold repair or diffuse-OA cushioning — the imaging actually supports. That determination, not the defect measurement in isolation, is what shapes whether treatment proceeds and on what terms.