The short answer: cartilage grade, joint stability, and biological readiness

For most patients who arrive having heard the treatment name, the practical question is: does my joint actually meet the threshold for ChondroFiller injection?

Candidacy rests on three gates. First, the cartilage damage must be severe enough — grade III or IV on the Outerbridge (ICRS) scale, meaning partial- to full-thickness loss. Minor surface changes at grade I or early grade II do not meet this threshold. Second, the joint must be mechanically sound: stable ligaments, acceptable alignment, and an intact subchondral base. Third, the patient's biological environment must be capable of supporting repair — active infection, uncontrolled systemic disease, or end-stage diffuse degeneration each close this gate.

Passing all three opens candidacy. Failing any one changes the pathway — sometimes to a different treatment entirely, sometimes to a preparatory step first.

ChondroFiller works as an acellular collagen scaffold: placed under ultrasound guidance in an outpatient appointment, it gels in situ and triggers matrix-induced chondrogenesis, drawing on the joint's own progenitor cells to drive repair. This is not a surgical procedure. On the injectable pathway there is no fixed defect-size ceiling, which means it can address both focal lesions and more diffuse articular wear — a broader window than the arthroscopic form of the device, which targets contained lesions up to 6 cm².

The cartilage damage threshold — what grade qualifies

Cartilage grading uses a scale of I to IV, where grade I describes superficial softening or surface fibrillation and grade IV means bone is exposed beneath. Grades III and IV — partial- to full-thickness loss — mark the structural threshold at which ChondroFiller's scaffold has both a meaningful deficit to occupy and enough biological stimulus from surrounding tissue to initiate matrix-induced chondrogenesis. At grades I and early II, the surface disruption has not overwhelmed the joint's own repair capacity; the scaffold mechanism cannot add to what the tissue can still manage without it.

For patients with osteoarthritis rather than a single focal lesion, a second scale becomes relevant. Kellgren-Lawrence (KL) grading reads joint-wide changes — space narrowing, sclerosis, osteophytes — from a weight-bearing X-ray, whereas Outerbridge/ICRS grading describes the cartilage surface itself. The two appear on the same report because they measure different things: one captures the articular surface, the other the whole joint picture. In clinical practice, Kellgren-Lawrence grade III–IV represents the most common real-world indication for the injectable pathway. Diffuse, end-stage generalised osteoarthritis sits outside the treatable window, however, because the surrounding tissue environment the scaffold depends on — capable of recruiting and sustaining progenitor cells — is no longer reliably intact at that stage.

Within the qualifying range, the damage type is flexible. Focal post-traumatic chondral lesions, osteochondritis dissecans (OCD), and cartilage loss arising after meniscal tears or ligament injuries are all recognised presentations, provided the underlying mechanics have been addressed. The injectable pathway also carries no upper defect-size limit, making it relevant for patients whose wear extends beyond a single contained patch.

Age is not a barrier — why older patients are routinely assessed

Many patients assume cartilage treatments are reserved for younger joints — an assumption ChondroFiller's published criteria directly contradict. There is no fixed upper age limit; active people in their 60s, 70s and beyond are routinely assessed at MSK Doctors consultations, and the treatment is positioned explicitly as a joint-preservation step before total replacement becomes necessary.

This sets ChondroFiller apart from cell-based procedures such as ACI and MACI, which rely on harvesting and expanding a patient's own cartilage cells and are typically restricted to younger adults.

Biological readiness matters more than the number of years on a birth certificate. What the scaffold requires is a joint environment capable of recruiting progenitor cells — and that depends on metabolic health, absence of inflammatory disease, and adequate surrounding tissue, not on chronological age. The published hip cohort study (Mazek, PMC8460160) reinforces this: among the patients followed at three to five years, 17 of 21 achieved good or excellent results. The distinguishing factor in poor outcomes was pre-existing Tönnis grade 2–3 osteoarthritis — a measure of joint-wide degeneration — rather than patient age.

Published data do not yet characterise precisely how healing rates vary across the upper age range, and candidacy at that end of the spectrum is assessed individually. A consultant-led evaluation weighs these biological factors case by case rather than applying a blanket age ceiling.

Mechanical prerequisites — why joint stability and alignment must come first

The scaffold's repair mechanism — matrix-induced chondrogenesis — depends on consistent, even loading across the treated surface. If the joint is unstable or misaligned, abnormal forces are transmitted through that surface with every step, and the cellular environment the scaffold is trying to establish cannot hold. This is why mechanical prerequisites are assessed before cartilage grade, not after it.

Ligament integrity comes first. Untreated laxity in the ACL or PCL allows abnormal translational movement in the joint; the scaffold cannot consolidate under those shear forces and must be addressed before or alongside treatment.

Angular malalignment of more than approximately 5° creates asymmetric loading across the compartments. Even a structurally sound scaffold placed in a varus or valgus knee will be overloaded on one side and underloaded on the other, both of which prevent the repair process from taking hold.

Meniscal deficits carry a similar mechanical consequence. The meniscus distributes load across the tibial plateau; where significant meniscal tissue is absent or irreparably damaged, focal stress concentrations can overwhelm any regenerative intervention placed beneath them.

Subchondral bone integrity is a fourth requirement. The scaffold recruits progenitor cells that migrate upward from subchondral bone, so that underlying layer must be structurally sound enough to serve as a source bed.

None of these findings automatically closes the door on treatment. Many can be corrected — or planned concurrently — before the injection appointment, and the consultant assessment stage exists specifically to map out that sequence.

Contraindications — when ChondroFiller injection is not appropriate

Not every patient with grade III or IV cartilage damage will be suitable, and understanding why helps frame the right questions for a consultant assessment.

Absolute exclusions

Some conditions rule out ChondroFiller injection regardless of cartilage grade or joint condition:

• End-stage diffuse generalised osteoarthritis — the most common clinical exclusion in practice. The scaffold recruits progenitor cells from surrounding tissue; where multi-compartmental degeneration has destroyed that biological environment, the repair mechanism has nothing to build from. The hip cohort study (Mazek, PMC8460160) confirmed this directly: patients with Tönnis grade 2–3 osteoarthritis had poor results.
• Active joint or local infection
• Known allergy to collagen or rat protein (the scaffold is derived from veterinary-monitored rats)
• Active malignancy
• Haematopoietic disorders
• Severe metabolic disease
• Pregnancy or breastfeeding
• Significant bleeding risk or anticoagulation that cannot be safely paused

This list covers the principal absolute exclusions; a treating consultant reviews the full medical history before confirming candidacy.

Relative exclusions — assessment required

Active smoking, uncontrolled diabetes, rheumatoid arthritis, gout, and lupus each impair the host healing response the scaffold depends on — the body's ability to mobilise and sustain progenitor-cell activity. Patients in these groups are not automatically excluded, but candidacy requires specialist review of disease-control status and overall biological readiness. A well-controlled inflammatory arthropathy, for instance, presents a different risk profile from an active flare.

If any of these factors apply, raising them at the initial assessment allows the consultant to weigh risk and benefit with the full clinical picture in view.

How candidacy is confirmed — the assessment process

Joint pain, stiffness, and a sense of instability are what bring most patients to seek advice — but those symptoms alone cannot tell you which grade your cartilage damage has reached, how contained the lesion is, or whether the joint's mechanical environment can support treatment. That gap between what a patient feels and what a clinician needs to know is precisely why formal imaging is the starting point, not an optional add-on.

MRI is the primary tool for characterising cartilage damage. It reveals defect grade and depth, distinguishes focal contained lesions from diffuse multi-surface wear, and identifies whether the subchondral bone beneath is structurally intact — all of which bear directly on whether the injectable scaffold pathway is appropriate and which delivery approach best fits.

Weight-bearing X-rays add a mechanical dimension. Images taken under load expose angular malalignment, joint-space narrowing across compartments, and overall osteoarthritis staging in a way that non-weight-bearing scans can miss. They also confirm whether the broader joint architecture is within the range where regenerative treatment is likely to hold.

Clinical scoring — standardised functional questionnaires and a consultant-led physical examination — completes the picture by mapping symptom burden and joint behaviour to the imaging findings.

The character of the damage, not its severity on a symptom scale, is what ultimately separates candidates for the injectable pathway from those better served by a different approach. Focal, contained damage in a mechanically stable joint points in one direction; diffuse multi-compartmental change points in another. That distinction emerges only from structured assessment — which is why self-referral checklists and online symptom tools are a poor substitute for a consultant review with imaging in hand.