What your MRI result is actually telling you

The report lands in your inbox and the phrase "focal chondral defect" appears alongside a grade and a measurement in square centimetres. Those two numbers do different jobs, and understanding the difference is the starting point for everything that follows.

Grade describes the depth of the damage using the Outerbridge or ICRS classification. Grade 1 is surface softening with no visible loss of thickness; Grade 2 is surface fraying that stays within the upper half of the cartilage; Grade 3 is a partial-thickness loss that goes deeper than 50% but stops short of the bone beneath; Grade 4 means the cartilage is gone and the subchondral bone is exposed. Grades 1 and 2 are generally managed conservatively — physiotherapy, anti-inflammatory medication, and monitored over time. Grades 3 and 4 are where surgical options enter the conversation.

Area, measured in cm², is the single most influential variable when clinicians weigh one intervention against another. Even within the same grade, a defect of 1.5 cm² and one of 5 cm² lead to very different treatment discussions — a distinction grading alone cannot capture.

The same scan may also reveal bone marrow oedema beneath the defect, meniscal changes, or alignment irregularities, each of which adds a further layer. A 3-Tesla MRI gives the clearest picture of all these factors together. What the grade and area figures ultimately tell you is not a verdict — they are the clinical language that makes a precise, personalised treatment plan possible.

Small defects under 2 cm²: non-surgical care is usually the right first step

For many patients with small defects — roughly under 1.5–2 cm² and graded 1 or 2 on either the Outerbridge or ICRS scale — the immediate answer is no, surgery is not necessarily required. The first step is conservative management, and for a significant proportion of people it remains the appropriate approach for some time.

Structured physiotherapy is the cornerstone: targeted exercises to offload the damaged area, strengthen the surrounding musculature, and improve joint mechanics. Load management, activity modification, and anti-inflammatory medication (NSAIDs where appropriate) sit alongside it.

Intra-articular injections offer a further layer of support. Hyaluronic acid and PRP are established options for symptom management. An injectable collagen scaffold — such as ChondroFiller, delivered as an ultrasound-guided outpatient injection at the London Cartilage Clinic — works differently: the scaffold gels within the defect and is thought to recruit the patient's own progenitor cells through matrix-induced chondrogenesis, supporting tissue repair rather than purely managing symptoms. This distinction matters because most conservative measures address pain and function without structurally restoring the cartilage surface.

For small, contained full-thickness defects in lower-demand patients, marrow stimulation (microfracture) remains an operative option. Its outcomes at this size are more predictable than in larger defects, though the repair tissue produced is fibrocartilage rather than native hyaline cartilage.

One exception applies regardless of grade or area: an unstable chondral fragment — one that catches, locks, or has partially detached — generally warrants prompt assessment and possible stabilisation irrespective of how modest the measurement appears on the scan.

Medium defects (2–4 cm²): the size range where treatment paths diverge most

The 2–4 cm² bracket carries more clinical decision weight than any other size range. It is where microfracture's usefulness progressively declines, where structural and cell-based alternatives come into sharper focus, and where individual patient factors — age, activity demand, the presence or absence of subchondral bone loss — have the greatest bearing on the final choice.

Microfracture remains acceptable at the lower end of this range. Its outcomes are more predictable for smaller, contained defects, but for defects approaching or exceeding 3 cm² in younger, high-demand patients the evidence increasingly disfavours it; repair quality deteriorates as defect area grows.

Mosaicplasty (osteochondral autograft transplant) counters that limitation by transferring cylindrical plugs of native bone and cartilage from a donor site within the same knee. Long-term clinical scores favour it over microfracture, but donor-site morbidity is a genuine trade-off, and the total graft area that can realistically be harvested constrains its use to the smaller end of this bracket.

MACI carries the strongest trial support for defects at the upper end of the range. The SUMMIT RCT — the landmark comparative study — found that for defects of 3 cm² or greater, MACI produced superior KOOS pain and function scores at both 2 and 5 years compared with microfracture. MACI is a two-stage, theatre-based surgical procedure: cells are harvested in one operation and reimplanted in a second.

For purely chondral defects within this range — no subchondral bone involvement — an injectable collagen scaffold such as ChondroFiller offers a different route. Delivered as an ultrasound-guided outpatient injection at the London Cartilage Clinic, it recruits the patient's own progenitor cells through matrix-induced chondrogenesis without requiring a theatre booking or a staged cell harvest. MACI and ChondroFiller work through related biological logic but differ fundamentally in delivery route, staging, and clinical setting — precisely the kind of distinction that makes this size range the most complex bracket in the entire treatment algorithm.

Large defects over 4 cm²: when structural restoration becomes the priority

Once a defect crosses roughly 4 cm², the clinical calculus shifts decisively toward structural restoration. At this scale, osteochondral allograft transplantation (OCA) is the established benchmark — a surgical procedure that replaces both the damaged cartilage and the underlying subchondral bone in a single stage using donor tissue. The distinction from cell-based approaches matters here: when MRI confirms bone involvement alongside a large surface defect, the structural unit has to be rebuilt, not just the cartilage layer above it.

MACI remains a valid option for large defects that are purely chondral — where the bone beneath is structurally intact. It cannot, however, address subchondral compromise, which is why imaging interpretation of depth, not area alone, determines which category a patient falls into. That reading requires specialist assessment; a cm² measurement on its own is not a complete picture.

OCA carries its own weight. It is a major surgical procedure with meaningful recovery demands, and outcomes depend partly on graft availability and tissue matching — factors that can affect timing as well as planning. Long-term durability data support its use, though head-to-head RCT comparisons across all four major modalities at ten years or beyond are still absent from the published literature, and the exact cm² threshold separating OAT from OCA varies between centres.

An injectable collagen scaffold such as ChondroFiller is not the indicated primary treatment for defects in this bracket, particularly where bone loss is confirmed. That limitation reflects biology: when the subchondral architecture is compromised, structural grafting is what the clinical situation calls for — not a scaffold operating at the cartilage surface alone.

Factors that can shift you into a different treatment bracket

Knowing your defect's area is the starting point, not the full story. Three categories of finding can each push a patient into a different treatment bracket — regardless of where the cm² measurement falls.

The bone beneath the cartilage. When MRI shows that damage extends into the subchondral bone — not just the cartilage surface above — the plan changes substantially. Bone involvement requires structural replacement: an osteochondral autograft (OAT) or allograft (OCA) that restores both layers together. Cell-based approaches and injectable collagen scaffolds work at the cartilage level and cannot rebuild a compromised bony foundation.

Joint location. The patellofemoral joint — the kneecap compartment — follows its own rules. Conformity challenges make osteochondral allograft less reliable there; cell-based modalities and injectable scaffold approaches are generally preferred regardless of lesion size.

The mechanical environment. Varus or valgus malalignment concentrates load unevenly; without corrective osteotomy, any cartilage repair works against persistent mechanical stress. Meniscal deficiency and ligamentous instability act the same way — they must be addressed alongside the cartilage repair, not deferred until after it.

Patient factors — age, activity level, and BMI — then calibrate which bracket's evidence most closely applies. The clinical logic running through all of these variables is consistent: cartilage restoration in a mechanically hostile environment underperforms regardless of which technique is used.

Getting a specialist assessment and what happens next

Confirming which treatment bracket applies requires more than a cm² figure from a radiology report. A specialist consultation brings together clinical examination, MRI review, and a full history — including prior treatments and mechanical factors — to establish grade, bone status, and joint environment before any recommendation is made. Patients attending an initial assessment should bring their MRI report and imaging, as the clinical question being answered is whether bone is involved, whether the joint environment is mechanical sound, and whether the defect is contained.

For patients whose assessment confirms a small-to-medium, chondral-only defect in a well-aligned knee, an ultrasound-guided ChondroFiller injection offers an outpatient route that does not require a theatre booking or surgical recovery. The injectable collagen scaffold is placed under image guidance and works by providing a matrix into which the patient's own progenitor cells can migrate and begin building new tissue. Placement precision matters — the technique is sensitive enough that outcomes are meaningfully influenced by how accurately the scaffold is delivered.

In the UK, Liquid Cartilage™ — the clinical brand for the ChondroFiller injection pathway — is delivered at the London Cartilage Clinic on Harley Street, where Professor Paul Y. F. Lee leads the service. Patients seeking an assessment can book through londoncartilage.com.