Two routes to candidacy — focal defect and diffuse OA

Suitability for ChondroFiller injection is not a single yes-or-no gate — it is a structured clinical assessment that routes patients down one of two distinct pathways, each with its own logic.

The focal defect pathway applies when MRI identifies a discrete, bounded area of cartilage damage. Think of it as filling a pothole: the injectable collagen scaffold is placed into the lesion under ultrasound guidance as an outpatient procedure, where it gels in situ and recruits the body's own progenitor cells to support repair. Classical criteria — defect depth and surface area — are the primary variables on this track.

The diffuse osteoarthritis pathway is different in kind. Here, the scaffold is not filling a single bounded lesion; it coats the broader articular surface as a viscoelastic cushion across a joint affected by Kellgren-Lawrence Grade III or IV disease — including presentations commonly described as 'bone on bone'. Defect area is a less determinative variable. This pathway matters because it opens assessment to patients who have already been told that surgical cartilage repair is not an option for them.

Which pathway applies depends on individual MRI findings, not on a single shared criterion. Mapping that distinction accurately is the starting point for every candidacy assessment — and the sections below work through each eligibility layer in turn.

The two non-negotiable gateway criteria

Before defect size, joint alignment, or disease stage enters the picture, two criteria must be met — and both apply equally to the focal defect and diffuse osteoarthritis routes described above.

1. Prior conservative management has been tried and has not provided sufficient relief. This typically means a structured course of physiotherapy, meaningful load modification, and at least one relevant injection therapy — such as a corticosteroid or hyaluronic acid — has already been completed. The threshold is not a set number of weeks; it reflects whether the clinical team can reasonably conclude that non-invasive measures alone will not restore acceptable function.

2. MRI has confirmed structural cartilage damage. Imaging evidence of the damage itself — not the patient's reported pain level — is what drives candidacy. This matters in practice: some patients have learned to manage around their symptoms and present with lower pain scores than the underlying structural picture might suggest. Others are told their pain 'isn't bad enough' to qualify for anything, when in fact an MRI would reveal significant structural change. ChondroFiller assessment sidesteps that trap by anchoring eligibility to objective imaging findings rather than symptom thresholds.

Neither gate can be skipped; both must be satisfied before defect-specific or pathway-specific criteria become relevant.

Defect size and depth on the focal pathway

Two numbers define the focal pathway: 6 cm² and Grade III.

The CE-mark indication for ChondroFiller covers focal articular cartilage defects up to 6 cm² in surface area — a ceiling that is meaningfully more permissive than most surgical alternatives. Microfracture is generally considered suitable only for defects smaller than 2–4 cm²; autologous chondrocyte implantation (ACI) and its matrix-assisted variant MACI require staged operative procedures, typically including an initial arthroscopic biopsy visit months before the repair itself. ChondroFiller, placed as an injectable collagen scaffold under ultrasound guidance in a single outpatient appointment, bypasses both the size restriction and the staged surgical pathway entirely.

On depth, the minimum threshold is Outerbridge or ICRS Grade III — damage penetrating more than half the cartilage thickness — or Grade IV, where the full cartilage layer is lost and the underlying subchondral bone is exposed. Superficial surface wear that falls below this threshold does not meet the focal pathway criteria; the structural signal must be substantial enough that MRI confirms it.

The acellular mechanism is directly relevant here. Because ChondroFiller works by matrix-induced chondrogenesis — recruiting the patient's own progenitor cells from the synovium and subchondral bone into the scaffold — there is no requirement to harvest or culture the patient's chondrocytes. The cell-viability constraint that limits ACI candidacy to younger patients with sufficient healthy donor cartilage does not apply.

In published series, the focal pathway produces approximately 30-point IKDC score improvements, MOCART scores in the range of 70–87, and a reoperation rate of 3–8% — outcomes that contextualise the 6 cm² indication as both clinically meaningful and technically achievable without open surgery.

Joint alignment, stability, and meniscal status

The defect itself accounts for only part of the clinical picture. Before ChondroFiller candidacy can be confirmed, the mechanical environment of the joint must be assessed — because scaffold integration depends on loading conditions as much as on the biology of the cartilage lesion.

Limb malalignment is the most straightforward of these criteria: if the leg deviates more than 5° from neutral alignment, the angular stress concentrates load onto the treated compartment in a way that prevents the collagen scaffold from integrating and maturing. This is an absolute contraindication, not a manageable risk factor. The principle underpinning it is not unique to ChondroFiller — a 2026 framework paper on focal cartilage defect management established alignment, meniscal integrity, and ligamentous stability as a mandatory first layer of assessment before any repair technique is selected, noting that long-term outcomes correlate more strongly with the mechanical environment than with the complexity of the repair material itself.

Major ligament instability carries the same logic: in a joint where shear forces are uncontrolled, the scaffold is exposed to loading patterns it cannot withstand before integration is complete. Severe meniscal absence that overloads the target compartment creates a comparable problem and is similarly disqualifying unless surgically addressed beforehand.

Crucially, these criteria are sequential rather than permanently exclusionary. Malalignment within 5°, or alignment already corrected by a prior procedure, does not bar candidacy — it simply means the assessment confirms the mechanical environment is sound before the injection is planned. For patients who have been told they 'need an osteotomy first', this framing is important: correction of alignment or instability may be part of the pathway to ChondroFiller, not a reason to rule it out.

Age, activity profile, and return-to-sport considerations

Age is rarely a barrier. Patients in their sixties and seventies are routinely assessed for ChondroFiller injection — because the scaffold's approach does not depend on the patient being young or surgically fit. Where cell-based therapies require a viable donor-cartilage population, a constraint that disadvantages older patients, ChondroFiller's acellular design sidesteps that requirement entirely. The relevant question is not chronological age but whether the joint environment meets the mechanical and structural criteria already described.

Activity profile is similarly broad. Sports-related and overuse wear, post-traumatic cartilage damage from injury or previous surgery, age-related degeneration, and chronic synovial inflammation all fall within the recognised candidate range. No consistently reported numerical threshold — no BMI cutoff or minimum functional score — gates eligibility on activity level in the available evidence. In practice, a competitive recreational runner in their fifties would discuss the intensity and frequency of loading during the post-injection period; a less active patient in their seventies would have a different conversation centred on return to comfortable daily function. The consultation shapes the assessment around the individual's joint demands rather than applying a fixed rule.

That distinction carries most weight for higher-activity patients when planning the post-injection period. Biomechanical in-vitro data indicate that the collagen scaffold has initial mechanical instability after placement: full weight-bearing should be delayed until stable integration is achieved, and return-to-sport timelines require specific guidance at the point of assessment. This is a practical scheduling consideration for active patients — not a disqualifying factor.

Absolute contraindications and how suitability is confirmed

Certain presentations rule out ChondroFiller injection regardless of defect size or pathway. The absolute contraindications are:

• Known allergy to Type I collagen or murine (mouse or rat)-derived proteins
• Active localised joint infection or systemic infection
• Active inflammatory arthritis — rheumatoid or psoriatic — where immune-mediated joint inflammation disrupts the local environment the scaffold depends on
• Active crystal arthropathy: gout or calcium pyrophosphate deposition disease (CPPD) during a flare
• Active malignancy
• Pregnancy or breastfeeding

Several of these are time-limited rather than permanent. Once an acute infection or crystal flare resolves, candidacy can be reassessed. The mechanical contraindications — malalignment exceeding 5°, untreated ligament instability, and severe meniscal absence — discussed in the preceding section — are similarly correctable for some patients before treatment is planned.

One prognostic caveat applies specifically on the diffuse osteoarthritis pathway. A prospective hip cohort study of 26 patients followed for three to five years found that participants with pre-existing Tönnis Grade 2–3 osteoarthritis had poor outcomes despite treatment, even when the procedural criteria were otherwise met. Advanced background OA does not automatically exclude a patient from the diffuse pathway, but it is a meaningful modifier that the treating clinician will weigh when counselling on realistic expectations.

No single criterion settles the question in isolation. Candidacy emerges from the intersection of MRI findings, mechanical assessment, disease stage, and individual joint demands — which is precisely why a structured clinical consultation is the necessary next step. Patients who want to understand where they stand can request an assessment at the London Cartilage Clinic's Harley Street practice via londoncartilage.com.

1. [1] Arthroscopic utilization of ChondroFiller gel for the treatment of hip articular cartilage defects: a cohort study with 12- to 60-month follow-up. (2021). https://doi.org/10.1093/jhps/hnab002 https://doi.org/10.1093/jhps/hnab002
2. [2] Cartilage reconstruction using Chondrofiller in intra-articular distal radius fractures. (2025). https://doi.org/10.1186/s42836-025-00333-y https://doi.org/10.1186/s42836-025-00333-y
3. [3] An Age-Aware, Mechanics-First Framework for Focal Cartilage Defects in Young Active Patients. (2026). https://doi.org/10.54254/2753-8818/2026.35509 https://doi.org/10.54254/2753-8818/2026.35509
4. [4] Influence of cartilage defects and a collagen gel on integrity of corresponding intact cartilage: a biomechanical in-vitro study. (2024). https://doi.org/10.1007/s00402-024-05530-z https://doi.org/10.1007/s00402-024-05530-z