Why lesion size is the first number your surgeon looks at

When an MRI report lands on a surgeon's desk describing a cartilage defect on the medial femoral condyle, the first figure they reach for is area — measured in square centimetres. It is a practical unit: one cm² is roughly the size of a 1 cm × 1 cm square, about the area of a small fingernail. A defect of 3 cm² is approximately the size of a postage stamp. These may sound small, but in a joint that bears several times body weight with every step, even a few square centimetres of missing cartilage can cause significant pain, swelling, and mechanical symptoms.

The medial femoral condyle — the inner rounded end of the thighbone at the knee — is the most commonly affected site for focal cartilage defects, and the one most thoroughly studied in clinical trials. This makes it the reference location for most published size thresholds.

Size matters because articular cartilage has no meaningful capacity for self-repair. Unlike bone, it carries no blood supply and contains too few cells to mount a regenerative response on its own. When simpler techniques such as bone marrow stimulation are used, the repair tissue that forms is fibrocartilage — a mechanically inferior substitute that tends to break down over time, particularly across larger defects. Matching the technique to the defect size is therefore not a matter of surgical preference; it reflects the biological reality that larger areas of damage need more sophisticated regenerative approaches to produce durable results.

Lesion size is the starting number, not the only number. Depth of damage, whether the underlying bone is involved, the patient's age, activity demands, and any co-existing pathology — ligament injury, meniscal damage, or malalignment — are all read alongside it. The size thresholds discussed in the following sections should be understood in that context.

The size thresholds that shape each treatment decision

Four size bands, roughly ordered by increasing biological complexity, map most published guidance on MFC cartilage defects.

Very small defects (≤1 cm²)

At this scale, conservative management — physiotherapy, load modification, and anti-inflammatory support — is usually the first approach, particularly where symptoms are mild. For well-localised full-thickness lesions, fixation of a detached fragment or microfracture may be appropriate. Dr LaPrade's published guidance specifies that microfracture is best indicated for lesions of 1 cm or less in diameter with a normal underlying bone surface.

Small-to-moderate defects (roughly 1–3 cm²)

A published review by Husen (2022) identifies 1.5 cm² as a practical inflection point: below it, fixation or conservative strategies tend to dominate; above it, restorative or cell-based surgery is generally preferred. Within the broader 1–3 cm² band, both mosaicplasty (osteochondral autograft transfer) and microfracture are accepted options, with mosaicplasty demonstrating superior long-term clinical scores in direct comparisons.

Larger defects (≥3 cm²)

The SUMMIT randomised controlled trial — the strongest prospective evidence in this area — found that MACI (matrix-induced autologous chondrocyte implantation) produced significantly better KOOS pain and function scores than microfracture at both two and five years in lesions of 3 cm² or more. At this size, marrow stimulation alone is unlikely to produce a durable repair across the full defect area.

Very large or bone-involving defects (>4 cm²)

When the defect exceeds approximately 4 cm², or when the underlying subchondral bone is substantially compromised, osteochondral allograft transplantation (OCA) — donor tissue that restores both cartilage and bone — becomes the preferred reconstruction approach.

The precise upper boundary for mosaicplasty versus cell-based therapy varies between 2 cm² and 4 cm² across published guidelines. This reflects a practical constraint — the volume of healthy donor cartilage that can safely be harvested from the patient's own knee — rather than a strict biological threshold.

How depth changes what size alone cannot tell you

Area and depth are both reported on an MRI or at arthroscopy, and the two metrics tell different parts of the story. The International Cartilage Repair Society (ICRS) grading system classifies how deeply a lesion penetrates through the cartilage layer. The clinically significant contrast is between partial-thickness and full-thickness damage.

Grade 2 lesions affect less than half the cartilage thickness. Grade 3 lesions go deeper — beyond 50% of the depth, sometimes reaching the calcified layer at the cartilage-bone interface. Grade 4 is full-thickness loss, with the underlying subchondral bone directly exposed.

Why does exposed bone change the picture? Bare subchondral bone alters the local mechanical environment, concentrating load onto the cartilage around the defect rim and accelerating its wear. There is also a biological shift: when the bone surface is breached, bleeding from the subchondral layer can release bone marrow progenitor cells into the defect — a limited natural response that surgical techniques such as microfracture are designed to amplify. When both cartilage and a meaningful depth of bone are lost together (an osteochondral lesion), the reconstruction approach may need to address the bony bed as well as the surface, which is part of the rationale for osteochondral allograft in deep or large combined defects.

The practical upshot for patients: a sizeable Grade 2 lesion may still be managed conservatively if symptoms are mild, whereas a moderate-sized Grade 4 lesion with exposed bone tends to reach the threshold for surgical discussion sooner. MRI reports and arthroscopy findings typically state both area and ICRS grade; if yours does not, asking your specialist to explain both gives a clearer picture of where your defect sits on the treatment pathway.

Patient factors that refine the decision beyond the scan

Scan findings tell a specialist where the damage is and how deep it runs — but two patients with an identical MRI report may leave clinic with different treatment plans. That is not inconsistency; it reflects the several clinical variables a surgeon weighs alongside area and depth.

Age and activity level matter because a restorative procedure needs to outlast the patient's functional demands. Younger, more active individuals face decades of loading on the repaired surface, which tends to shift the recommendation towards techniques that more closely approximate hyaline cartilage rather than the fibrocartilage that marrow stimulation produces.

Concomitant knee pathology is arguably the most important contextual factor. An ACL-deficient knee, significant meniscal damage, or a varus or valgus malalignment all place abnormal mechanical stress on the medial femoral condyle. Carrying out cartilage repair without addressing these co-existing problems risks the repair failing prematurely; a specialist will typically plan to treat them in combination or in sequence.

Subchondral bone quality also influences technique selection. Where bone stock is poor or the bony bed is compromised, marrow-stimulation approaches may be insufficient on their own, and bone grafting alongside cartilage work may be required.

Debridement (chondroplasty) can reduce symptoms by smoothing unstable cartilage edges, and it is used to prepare a defect site for other procedures. It offers no regenerative benefit, however, and is not recommended as a standalone long-term strategy for an established chondral defect.

The decision that follows a scan integrates imaging, a functional and activity assessment, patient goals, and the treating clinician's experience — which is why a specialist consultation, rather than a size threshold alone, determines the recommended pathway.

Where the ChondroFiller injection fits in the size-treatment framework

The ChondroFiller injection occupies a distinct place in the treatment spectrum — not a surgical alternative to the options described earlier, but a categorically different approach that sits at a different stage in the care pathway.

As an injectable collagen scaffold, it gels within the defect following ultrasound-guided placement and provides a three-dimensional matrix that recruits the patient's own progenitor cells to produce new cartilage tissue — a process known as matrix-induced chondrogenesis. No cells are harvested beforehand and no second procedure is required. The treatment is delivered as an outpatient appointment, not in an operating theatre.

In terms of the size framework set out in the preceding sections, the ChondroFiller injection is most relevant to focal, cartilage-level defects in the small-to-medium range — broadly the territory where marrow stimulation or mosaicplasty would ordinarily be considered, from roughly 1 cm² to the lower end of the larger-lesion band. It is not configured for very large defects requiring osteochondral allograft reconstruction, nor for lesions with significant bony involvement where the subchondral bed itself needs rebuilding. Whether a specific defect's area, depth grade, and surrounding joint condition make it suitable is confirmed through specialist assessment — the scaffold approach works best when the surrounding cartilage and bone architecture can support it.

This positions the ChondroFiller injection as a regenerative option relevant to patients who wish to explore a minimally invasive route before committing to major cartilage surgery, or where the recovery demands of a theatre-based procedure are not feasible. The mechanism differs categorically from MACI and ACI, both of which require cell harvesting and — in the case of traditional ACI — a two-stage operation; it also differs from microfracture, which amplifies a bleeding response rather than delivering a scaffold. ChondroFiller injection is available at the London Cartilage Clinic on Harley Street, where this type of specialist assessment takes place.

Getting a specialist assessment at the London Cartilage Clinic

Before any treatment decision is made, a specialist consultation is needed — particularly given how closely area, depth, and patient context interact to shape the final plan. At a first appointment, the clinician will review existing MRI imaging (bring the report and disc if you have them), take a detailed history of symptoms and activity demands, and carry out a clinical examination. The full treatment spectrum relevant to the defect's size and ICRS grade is then discussed, from conservative physiotherapy through to the ChondroFiller injection and, where indicated, surgical options — so the visit ends with a clear, personalised picture of where the defect sits on the decision pathway rather than a predetermined recommendation.

A useful frame for that conversation: know the reported defect area in cm², the ICRS grade if recorded, and what activities are currently limited. That detail lets the clinician move quickly to the decision-relevant part of the assessment.

The ChondroFiller injection is delivered in the UK at the London Cartilage Clinic on Harley Street, where Professor Paul Y. F. Lee leads the service. To book an assessment, visit londoncartilage.com.