Why defect size is rarely the deciding factor

Patients researching cartilage treatment frequently arrive at consultation having memorised defect-size thresholds — 2 cm², 4 cm², 6 cm² — and convinced themselves they are beyond what any injection can help. For ChondroFiller as an outpatient ultrasound-guided injection, that concern is largely misplaced. The injection pathway carries no upper limit on defect area and no upper age limit; the collagen scaffold can be placed to cover a single focal lesion or a broadly worn articular surface within a single clinic appointment.

Defect area alone, therefore, is not what closes the door for most patients.

The real candidacy screen focuses on the mechanical environment of the joint. Three conditions need to be in place before the scaffold can support the body's own repair processes effectively: the joint must be mechanically stable, the coronal alignment must be acceptable, and the load distribution across compartments must not be driven by a severely deficient meniscus. When any of these conditions is absent, the underlying mechanical problem tends to undermine regeneration regardless of which product is used — and addressing it first becomes the priority. The sections below examine each of those three factors and explain where they redirect the treatment pathway.

The three joint conditions that must be in place

Ligament integrity comes first in the assessment — and with good reason. When the stabilising ligaments of the knee are lax or disrupted, the forces crossing the joint during movement become unpredictable. Load transfer across a repair site that shifts position with each step makes it very difficult for a collagen scaffold to bed in and for the body's progenitor cells to differentiate in a stable environment. The London Cartilage Clinic's suitability guidance identifies untreated mechanical instability as the one factor that most consistently undermines ChondroFiller outcomes — a finding echoed in the broader cartilage-repair literature. Because patients frequently self-refer having noticed pain and stiffness without recognising that a previous ligament injury is still functionally unresolved, this is the highest-priority check at the point of assessment.

Coronal malalignment — a varus ('bow-legged') or valgus ('knock-kneed') deformity of the mechanical axis — creates a second, distinct mechanical problem. A joint loaded asymmetrically does not spread force evenly across the articular surface; instead, it concentrates it on whichever compartment sits closest to the axis of load. Placing a scaffold into a zone of chronic overload and expecting regeneration to proceed is, in biomechanical terms, counterproductive: the repair tissue is subject to the same abnormal stress that degraded the cartilage in the first place.

A severely deficient meniscus produces a similar outcome through a different route. The menisci act as load-distributing shock absorbers; when one is substantially absent, compartmental contact pressure rises sharply. That altered load environment prevents the scaffold from functioning within the mechanical conditions it requires. Meniscal allograft transplantation to restore load distribution is the standard redirect in this situation.

None of these three findings closes the door on cartilage regeneration permanently. Each one identifies a mechanical problem that should be resolved — through ligament reconstruction, realignment osteotomy, or meniscal restoration — before or alongside cartilage treatment, so that the repair proceeds in an environment that gives it a realistic chance of success.

Malalignment: the osteotomy pathway that comes first

Correcting the load axis is a staging step, not a detour — and understanding what that means in practice helps patients make sense of a pathway that may involve more than one procedure.

In a varus (bow-legged) knee, the mechanical axis runs through or medial to the inner compartment, concentrating load on cartilage that is already damaged. The standard correction is a high tibial osteotomy (HTO), typically a medial opening-wedge technique, which shifts the axis laterally and redistributes force. For valgus (knock-kneed) malalignment, a distal femoral osteotomy (DFO) achieves the equivalent correction at the femur. In both cases the logic is the same: cartilage treatment placed into a joint that continues to overload one compartment will fail for the same mechanical reason the original cartilage degraded.

Where no chondrocyte cultivation is required — as is the case with injectable scaffold approaches — a combined single-stage procedure, osteotomy and cartilage treatment together, is generally preferred over two separate operations and recovery periods.

One scenario redirects the pathway away from osteotomy altogether. Before any realignment surgery, arthroscopic assessment of the opposite compartment is recommended; if the cartilage there is severely compromised, transferring load to it via osteotomy would simply move the problem. In that situation, partial or total arthroplasty becomes the more appropriate option.

The precise mechanical-axis angle at which realignment becomes mandatory for the injection pathway has not been separately established — the thresholds used come from the broader surgical cartilage-repair literature — so the treating consultant weighs degree of deformity against the patient's symptom pattern, activity level, and compartmental loading on a case-by-case basis.

Absolute contraindications — when the pathway closes entirely

Three categories sit outside the staging logic covered in the previous two sections. These are not 'address first and return' findings — they are categorical redirections where ChondroFiller injection is not appropriate at any stage.

Murine collagen hypersensitivity. The scaffold is derived from Type I murine collagen. A confirmed allergy or hypersensitivity to animal-derived collagen is an absolute hard stop: no delivery route or timing adjustment changes the material the patient would be exposed to.

Active inflammatory or rheumatoid arthritis. A joint under sustained systemic inflammatory attack cannot provide the repair environment a collagen scaffold requires. The inflammatory milieu degrades implanted matrix and prevents the differentiation of progenitor cells into stable chondrocyte-like tissue. This contraindication applies across scaffold-based cartilage repair broadly, not only to ChondroFiller.

Terminal end-stage osteoarthritis. ChondroFiller is a joint-preservation therapy. When articular surfaces are so extensively degraded that no preservation option remains viable, the appropriate redirect is total or unicompartmental knee replacement.

Most patients researching ChondroFiller will not meet any of these three criteria. They are distinct from the mechanical-environment findings discussed earlier — those are staging conditions; these close the joint-preservation pathway entirely.

Where defect size still shapes the choice of technique

The broader cartilage-repair literature organises treatment options around defect area — a useful framework for placing any individual technique in context, and for understanding where ChondroFiller's injectable form fits within it.

For lesions roughly below 2 cm², microfracture and mosaicplasty have long clinical track records and remain well-supported first-line options. Above approximately 4 cm², autologous chondrocyte implantation (ACI) and its matrix-assisted variant MACI become the preferred approach for focal defects; the SUMMIT trial confirmed MACI's superiority over microfracture for lesions of 3 cm² or more at both two and five years of follow-up. The surgical delivery form of ChondroFiller bridges the 2–6 cm² focal-defect window as a single-stage procedure — though that is a distinct route from the outpatient injection pathway.

For diffuse or advanced-grade wear, where containment walls cannot be assumed, none of the above surgical approaches are appropriate — yet the injectable form remains a candidate precisely because it does not depend on a bordered defect. Advanced OA that closes the surgical cartilage-repair window does not automatically close the injection route.

What this landscape illustrates is that defect area is one variable, not the variable. Lesion morphology, depth, the integrity of surrounding cartilage, and the mechanical environment — the ligament, alignment, and meniscal factors addressed earlier — all shape which pathway is realistic. A well-contained 3 cm² lesion in a stable, well-aligned joint occupies a fundamentally different position from the same area of damage alongside compartmental overload or widespread wear.

Redirected, not refused — what the next step usually looks like

For most patients who leave an initial assessment without a ChondroFiller injection booked, the message is not a closed door — it is a sequenced plan. The mechanical conditions that currently make the joint unsuitable are, in the majority of cases, the same conditions that can be corrected: a misaligned axis, an unstable ligament, a compromised meniscus. Once the joint environment is restored, candidacy is reassessed with fresh eyes and a meaningful prospect of proceeding.

The minority who reach a genuine absolute stop — active inflammatory arthritis, confirmed murine collagen allergy, or articular surfaces too far degraded for any preservation option — receive that clarity at the same appointment, alongside a clear steer toward the appropriate alternative pathway rather than an indefinite wait.

That distinction — redirected versus refused — is precisely what a structured first consultation is designed to establish. At MSK Doctors, the assessment may include imaging review alongside biomechanical evaluation using MAI Motion®, the clinic's AI-powered markerless motion-capture system, which can surface loading asymmetries and alignment findings that static MRI does not always capture. Patients do not need a GP referral; appointments can be booked directly at mskdoctors.com.