Who this injection is designed for

Most patients arrive at a cartilage consultation expecting to be told their defect is either small enough for surgery or too far gone for anything other than a joint replacement. ChondroFiller® operates outside that binary — it is an ultrasound-guided injectable collagen scaffold placed during an outpatient appointment, and its eligibility framework is genuinely different from the size-threshold rules that govern surgical cartilage repair.

The practical result is a wider candidate pool. Patients with early focal lesions, those with more diffuse joint wear, and even patients clinically described as 'bone on bone' are not automatically excluded. What determines suitability is a structured assessment across four dimensions: the mechanics of the joint (physics), the biological environment inside it (chemistry), the condition of the tissue itself (biology), and how far wear has progressed over time (timing). Together these lenses route each patient to one of four care pathways rather than delivering a simple yes-or-no verdict.

The sections below examine how defect characteristics, joint condition, and activity profile each feed into that assessment.

Why defect size works differently here

For most surgical cartilage repair techniques, the area of the defect is a formal gate. Microfracture and mosaicplasty are generally suited to lesions below approximately 2–4 cm²; for defects at or above 3 cm², scaffold-assisted procedures such as MACI show superior outcomes in published trials, but they carry their own dimensional and tissue-quality requirements. If a defect falls outside these windows, patients are often told that a particular procedure is simply 'not indicated' for them.

ChondroFiller® sidesteps that framing because of how it works. Rather than requiring the body to grow new tissue across a defined defect boundary, the injectable collagen scaffold is delivered under ultrasound guidance and coats the cartilage surface across the whole joint. It provides a three-dimensional matrix that the body's own cells migrate into — so there is no fixed gap that needs to be bridged before the scaffold can function. No formal upper defect-size limit has been published for the injection pathway, and patients with clinically advanced cartilage loss are not automatically excluded on size grounds alone.

Defect volume does influence how much product is needed. Single-compartment lesions typically require less material than larger or multi-compartment presentations, where additional doses may be used — but this is a dosing consideration, not a candidacy exclusion. Where a defect is judged too large or structurally complex even for a dose-adjusted injection, a surgical escalation pathway exists that pairs the same collagen scaffold with cell-based biological support; that route carries its own separate eligibility criteria and is distinct from the outpatient injection arm.

Joint stability and which conditions qualify

The injection pathway does not carry a formal ligamentous stability grade, axial alignment requirement, or minimum residual bone-stock threshold as a published entry criterion. That distinction matters practically: it means the assessment question is not 'does this joint pass a structural checklist?' but rather 'does the clinical picture, taken as a whole, support a scaffold injection?'

Several mechanically complex joint states appear in published indication lists for the injection — osteochondritis dissecans (OCD), femoroacetabular impingement (FAI), post-traumatic chondral and osteochondral knee defects, and ankle chondral lesions. These presentations involve bony geometry changes, impingement, or traumatic disruption, yet none categorically excludes a patient from the injection pathway. The scaffold coats the whole joint surface under ultrasound guidance and does not depend on the perilesional cartilage being intact — unlike surgical scaffold implantation, which requires surrounding cartilage to be reasonably preserved before a cell-supported procedure is considered viable. That perilesional-tissue requirement belongs to the surgical escalation route, not the injection arm.

The clearest trigger for the surgical pathway rather than the injection is a defect judged too structurally complex for a stand-alone scaffold dose — not instability per se, but a combination of defect depth, bone involvement, or inaccessibility that the injection alone cannot adequately address.

Patients with clinically advanced wear, including those sometimes described as 'bone on bone,' may still be assessed for the injection. Imaging review is part of individual candidacy evaluation, and outcome expectations are shaped by the overall joint environment at that point.

Activity level and which patient profile fits each pathway

What activity level does influence is pathway assignment, not candidacy itself. No published Tegner score, minimum weekly exercise volume, or maximum activity ceiling has been established as an entry requirement for the injection; the framework is explicitly designed to accommodate patients at opposite ends of the activity spectrum.

Instead, candidates are directed into one of four clinical pathways — Prevention, Regeneration, Combination, and Support — based on where their joint and lifestyle sit on the disease and activity continuum.

• Prevention is oriented towards patients with early-stage cartilage changes or identifiable risk factors who want to slow further deterioration rather than address established damage.
• Regeneration centres on patients with focal or osteochondral defects where the primary goal is scaffold-supported tissue repair.
• Combination applies where more than one compartment is affected, or where a mixed strategy — addressing both structural defects and the broader joint environment — is indicated.
• Support is typically considered for patients with more advanced or diffuse cartilage loss, where the aim shifts towards protecting what remains and managing symptoms rather than focal regeneration.

An active recreational runner with an early focal knee lesion and a less active patient with broader degenerative change may both be suitable candidates, but they are likely to be guided towards different pathways. The assessment weighs biomechanics, biological joint environment, tissue condition, and timing of presentation — with individual imaging and treatment goals shaping the final recommendation.

What a candidacy assessment actually looks at

Physics looks at how load moves through the joint — whether weight-bearing alignment, impingement, or muscular imbalance concentrates stress on the affected cartilage surface. If biomechanical factors are likely to compromise the scaffold's environment, addressing them alongside the injection may form part of the overall plan.

Chemistry considers the inflammatory state of the joint at the time of assessment. A joint that is highly inflamed may limit how effectively the collagen scaffold supports cell migration into its structure, making the biological climate relevant both to timing and to preparation.

Biology examines tissue quality directly: cartilage thickness, subchondral bone integrity, and the regenerative capacity of the tissue surrounding the defect. This is where MRI plays its most important role — detailed imaging maps defect pattern and depth, and reveals the condition of adjacent structures that clinical examination alone cannot fully characterise.

Timing addresses disease stage. Earlier intervention, before widespread subchondral change or secondary muscle deconditioning sets in, may create a more favourable environment for scaffold integration. That does not mean advanced-stage patients are excluded — the four treatment pathways exist precisely because patients arrive at every point on that continuum — but the assessment considers what realistic outcomes look like at each stage.

Some of these parameters are only determinable once imaging has been reviewed alongside a physical examination. A clinical consultation, rather than a self-assessment checklist, is where candidacy is properly established.

When other routes may be more appropriate

Not every presentation is suited to an injectable scaffold, and honest candidacy assessment includes recognising those situations.

Defects with significant bone involvement — deep osteochondral lesions where the subchondral plate is substantially disrupted — may fall into the category judged too complex for a stand-alone injection, and surgical cartilage restoration is the more appropriate route in those cases. Patients requiring concurrent structural correction, such as a realignment osteotomy for pronounced axial malalignment, may need that addressed before or alongside any injection pathway; the scaffold works within a mechanical environment, and if that environment is severely compromised, the clinical plan needs to reflect it. Active joint infection, certain systemic inflammatory conditions, and known contraindications to collagen-based materials would also preclude the injection — though these are confirmed through clinical assessment rather than self-screening.

Where the treatment goal is purely palliative — managing pain without an aim to restore tissue — alternatives such as hyaluronic acid viscosupplementation, polyacrylamide hydrogel, or corticosteroid each have a legitimate role. These are different mechanisms serving a different clinical purpose, and in some situations they represent the more appropriate first step.

In broad terms, the injection pathway tends to fit patients who have identifiable focal or diffuse cartilage change, a joint environment that can support scaffold integration, and a goal of preservation or regeneration rather than symptom masking alone. Patients whose joint state, biology, or treatment objectives fall outside that profile are better served by a different route — and knowing that distinction early is the point of a structured assessment. To find out which pathway applies to your situation, complete the eligibility assessment at amsk.co.uk.