What a focal cartilage defect actually is

Inside the knee, the ends of the femur, tibia, and patella are coated in a smooth, pearlescent layer called articular — or hyaline — cartilage. Roughly two to four millimetres thick, it acts as both a shock absorber and a near-frictionless bearing surface, allowing the joint to move through its full range under load without the bones grinding together.

A focal chondral defect is a contained, localised area where that surface is missing or damaged — a patch, not a generalised thinning across the joint. The 'tile missing from the floor' analogy captures this well: the surrounding cartilage may look and function normally, but the gap changes how load spreads across the joint surface, concentrating mechanical stress at the defect edges and accelerating degeneration of the cartilage immediately surrounding it.

It is worth distinguishing this clearly from osteoarthritis. OA involves diffuse cartilage loss across an entire compartment of the knee — a widespread wearing away rather than a localised hole. A focal defect is spatially limited, though without treatment it can set off the progressive joint-wide deterioration that characterises OA.

Defects also vary in how deeply they penetrate the cartilage layer. Some affect only the surface — producing softening or a fine, frayed texture — while others extend partially or fully through the cartilage to expose the subchondral bone beneath. That depth is one of the most clinically important features of any defect.

Why cartilage cannot repair itself

Unlike skin or muscle, cartilage has no plumbing. It contains no blood vessels, receiving its nutrients instead from the synovial fluid that bathes the joint. That arrangement works well for a tissue under near-constant compression — blood vessels would be crushed with every step — but it has a critical consequence: when damage occurs, the repair cells, growth factors, and inflammatory signals that the body normally dispatches through the bloodstream never arrive at the defect site. The injury is, in effect, invisible to the body's standard healing machinery.

The cells that make and maintain cartilage — chondrocytes — compound the problem. Embedded within a dense matrix of collagen and proteoglycans, they have very little capacity to migrate towards a wound edge. In most other tissues, cells can travel to a site of injury and begin rebuilding; chondrocytes largely stay where they are. The result is that the defect cannot fill itself in, even partially, even over years.

Cartilage is also entirely without nerve endings. This has a practical implication that patients often find counterintuitive: the cartilage itself cannot hurt. Pain is generated by the subchondral bone beneath the defect when it becomes exposed or inflamed, and by the synovium and surrounding soft tissues as they react to debris and altered mechanical loading. Because those secondary structures are what produce symptoms, pain is an unreliable guide to defect size or severity. A small but deep defect reaching subchondral bone may be acutely painful; a broader partial-thickness lesion may produce almost no discomfort at all. This mismatch is one reason why imaging can reveal findings that surprise even patients who felt they had 'mild' symptoms.

It is worth being precise about what 'cannot heal' means in practice. Very shallow, partial-thickness defects — Grade I or early Grade II — may partially stabilise over time, particularly in younger patients with well-aligned joints and low inflammatory burden. What does not happen, at any grade, is spontaneous filling of the defect with healthy hyaline cartilage. The biology for that process simply does not exist in the adult knee.

Symptoms — and why some defects go unnoticed

Discovering a cartilage defect on an MRI report can feel alarming — but context matters. Chondral lesions are found incidentally in 11 to 34.6% of routine knee arthroscopies, often in joints that were causing no significant trouble at the time. Roughly 5 to 10% of adults over 40 have one, and a meaningful proportion cause no symptoms whatsoever at the point of discovery.

When a defect does become symptomatic, the presentation tends to follow a recognisable pattern: a sharp or aching pain during weight-bearing activity, recurrent swelling (effusion) that settles with rest then returns with use, and stiffness that is worst after a period of sitting. If a fragment of cartilage has broken free and is moving within the joint, catching or locking may occur — a more acute symptom that typically prompts earlier assessment.

None of these features, however, are unique to a cartilage defect. The same picture can arise from a meniscal tear, patellofemoral pain, or early compartmental OA, which is why a clinical examination and targeted imaging are needed to distinguish between them — a symptom list alone cannot do that reliably.

Being asymptomatic is not the same as being safe from long-term progression. Defects that cause no pain today may still slowly enlarge, particularly if the underlying risk factors — malalignment, excess load, or joint instability — remain unaddressed.

Natural history: who stays stable and who progresses

The question patients most often want answered is a simple one: will mine get worse? The honest answer is that it depends — and the evidence reflects that complexity.

Small, superficial lesions that have not yet penetrated deeply into the cartilage may remain stable for years or even decades in younger patients with good joint alignment and no associated instability. A prospective study published in JAMA Internal Medicine in 2006 and a separate analysis in Osteoarthritis and Cartilage in 2008 both established that defect behaviour is highly variable and difficult to predict from imaging alone. Neither identified a reliable threshold at which stability could be assumed.

Deeper defects tell a different story. Those penetrating more than half the cartilage thickness — or reaching the underlying bone entirely (the most severe end of the clinical grading scale, explained in the next section) — act as mechanical stress risers, concentrating abnormal load on the cartilage at the defect margins. Research published in 2019 using a porcine joint model demonstrated progressive loss of sulfated glycosaminoglycans — the structural molecules responsible for cartilage's shock-absorbing capacity — along those edges, with degeneration continuing even under reduced loading conditions. Relative rest may ease symptoms; it does not halt the underlying biological process.

Several factors consistently predict faster progression: older age, varus or valgus malalignment (a bowlegged or knock-kneed alignment that concentrates load unevenly across the joint), elevated BMI, concurrent ligament instability — in particular ACL deficiency — and a defect that is larger in surface area or greater in depth.

No imaging tool currently provides a reliable way to predict an individual patient's trajectory from a scan alone. This is an acknowledged gap in the evidence, and it is one reason why specialist assessment carries more value than extended observation without a clear plan.

How a defect is assessed and graded

Assessment follows a structured sequence, beginning well before any scan is ordered. A consultant will first establish the mechanism — whether symptoms followed an acute twist or blow, or developed gradually — alongside the precise location of pain, the presence of mechanical symptoms such as catching, and a standing alignment check. That clinical picture guides both the choice of imaging and the interpretation of its results.

MRI is the standard first-line investigation. It is sensitive enough to detect cartilage thinning, subchondral bone oedema, loose bodies, and concurrent meniscal or ligament pathology — all of which influence the management plan. What it cannot do is tell the whole story. MRI findings carry meaning only when read alongside symptoms: cartilage changes visible on a scan are common in asymptomatic knees, and an incidental finding on imaging is not, by itself, a treatment indication.

When the clinical picture and MRI point strongly towards a significant defect, arthroscopy provides the definitive answer. Direct visualisation allows the defect to be graded accurately using the ICRS/Outerbridge scale, which runs from Grade I (surface softening, cartilage still intact) through to Grade IV (full-thickness loss with subchondral bone exposed). The practical value of arthroscopy extends beyond diagnosis: if a defect is confirmed, appropriate treatment — such as microfracture or a cartilage repair procedure — can often be performed at the same sitting, sparing the patient a second procedure.

The realistic path forward from here

For most people with a confirmed focal defect, the next step is not an operation.

Conservative management — load modification, physiotherapy-guided strengthening of the quadriceps and surrounding hip musculature, and attention to modifiable risk factors such as BMI and joint malalignment — is the appropriate starting point. The goal is to reduce compressive force at the defect site, ease mechanical loading at its margins, and keep the joint as functional as possible while a longer-term plan is established.

Injections such as PRP or hyaluronic acid can help manage pain and inflammation during this phase. They do not repair the structural defect and are not a substitute for a clear pathway, but they may provide useful symptomatic support while conservative measures are given adequate time to work.

When conservative management fails to control symptoms, mechanical features such as locking or giving way appear, or defect characteristics suggest significant progression risk — typically Grade III–IV depth or a surface area above 2 cm² — surgical options become relevant. These include microfracture, osteochondral autograft transplantation, and matrix-assisted cell implantation techniques. The appropriate choice depends on defect size and location, overall joint health, and the patient's activity goals; the detail of each belongs to the cartilage repair discussion rather than here.

A concern that arises regularly is whether being in one's fifties makes surgery unrealistic. A 2025 study following 217 patients over a mean of 6.8 years found that those aged 50–69 achieved comparable pain and functional outcomes to patients under 50 after cartilage repair — age alone is not a reason to close off that conversation.

Specialist assessment is the right next move when pain persists or worsens despite conservative care, mechanical symptoms develop, or an MRI has identified a significant defect without a formal management plan. Professor Paul Y. F. Lee provides this assessment at London Cartilage Clinic; a first consultation can be arranged at londoncartilage.com.

1. [1] Hyaline Cartilage – Wikipedia. https://en.wikipedia.org/?curid=1130627 https://en.wikipedia.org/?curid=1130627
2. [2] Articular Cartilage Repair Biomaterials: Strategies and Applications (2024). (2024). https://doi.org/10.1016/j.mtbio.2024.100948 https://doi.org/10.1016/j.mtbio.2024.100948
3. [3] Isolated Osteochondral Autograft Transplantation for a Focal Chondral Defect of the Patella (2025). (2025). https://doi.org/10.1016/j.eats.2025.103673 https://doi.org/10.1016/j.eats.2025.103673
4. [4] Assessment and Prevention of Cartilage Degeneration Surrounding a Focal Chondral Defect in the Porcine Model. (2019). https://doi.org/10.1016/j.bbrc.2019.05.034 https://doi.org/10.1016/j.bbrc.2019.05.034
5. [5] Patients Aged 50–69 Show Comparable Outcomes to Those Under 50 After AMIC for Focal Chondral Defects (BJJ, 2025). (2025). https://doi.org/10.1302/0301-620X.107B10.BJJ-2024-1397.R2 https://doi.org/10.1302/0301-620X.107B10.BJJ-2024-1397.R2