What a focal chondral defect actually is

"I've been told I have a cartilage defect — what does that actually mean?" It means there is a localised patch of damage on one of the smooth surfaces lining your knee joint — not the widespread thinning seen in osteoarthritis, but something closer to a pothole in an otherwise intact road. An FCD sits on a joint surface that is, by definition, largely healthy around it; that is what sets it apart from generalised joint wear.

Articular cartilage has no blood vessels and no nerve fibres of its own. This produces a counterintuitive finding that clinicians encounter regularly: a relatively small defect can cause sharp, debilitating pain, while a larger one may barely register — sometimes turning up incidentally when a scan is requested for a ligament or meniscal problem. Symptom severity alone cannot indicate how significant the damage is, which is precisely why a structured assessment is needed rather than acting on imaging alone.

Most focal chondral defects arise from a single acute event — a twisting fall, a direct blow to the knee, or a collision during sport — or from sustained mechanical loading on a joint that is misaligned or has already been stressed by a previous injury. A small number are identified without any obvious precipitating cause.

Because cartilage cannot repair itself, an untreated defect can slowly enlarge and begin to alter the joint environment in ways that accelerate broader wear. Injuries larger than 1 cm, in particular, may worsen over time. Early, accurate assessment is the starting point for preventing that progression.

Symptoms that prompt investigation

Several symptom patterns regularly prompt a specialist to consider a focal chondral defect — and recognising them can help you understand why you have been referred for further assessment.

The most common complaint is pain that is clearly tied to load: walking on hard ground, climbing or descending stairs, squatting, or kneeling. The knee may also produce an effusion — a warm, puffy swelling that builds over hours after activity rather than appearing immediately. Alongside pain and swelling, many patients describe mechanical sensations: a click, a catch, or an intermittent locking feeling as the joint moves through its range. These sensations arise because a damaged cartilage surface disrupts the otherwise smooth gliding between the femur and tibia or patella.

Reduced range of motion is noted carefully at the first clinical assessment. An arc of movement below 90° before any treatment has begun is a recognised negative prognostic indicator — it suggests the joint may already be responding to sustained irritation — and is formally documented because it shapes subsequent decisions.

How long symptoms have been present, and how they began, matters as much as the symptoms themselves. A sharp onset after a fall or collision points towards an acute cartilage injury, often in an otherwise healthy joint. A gradual build-up over months, sometimes linked to a previous operation or alignment problem, suggests a more chronic degenerative process. These two trajectories tend to lead to different assessment priorities and different treatment conversations.

Importantly, none of these symptoms can confirm a cartilage defect on their own, nor can they indicate its size or depth. Their pattern does something more specific: it directs the clinician towards the right examination tests and imaging sequences to find out what is actually happening beneath the joint surface.

What the clinical examination covers

The consultation begins with the clinician looking at the knee before touching it — observing how the joint sits, whether there is visible swelling or muscle wasting around the thigh, and how weight is distributed when standing. Palpation then maps where the tenderness actually lives. Pressing along the inner (medial) joint line, the outer (lateral) line, or behind the kneecap narrows the likely site of the defect before any imaging is requested.

Flexion and extension are measured formally, not estimated. Because restricted movement below a 90° arc has already been flagged as a planning concern, having a recorded baseline allows the team to track change over time and guides decisions about what treatment can realistically be offered at this stage.

The part of the examination that often surprises patients is the ligament screen. Testing the ACL with the Lachman and pivot-shift manoeuvres, the PCL with a posterior drawer, and the posterolateral corner with a dial test is standard practice — not because the clinician suspects a ligament problem, but because cartilage damage and ligament or meniscal injury frequently occur together. An unstable knee places repeated abnormal load across a cartilage repair, undermining it regardless of how well the repair itself is performed. If instability is found, it must be addressed as part of the overall treatment plan, not treated separately as an afterthought.

What examination cannot do is confirm how deep the defect is or measure its area precisely. Its role is to build a clinical picture that directs the right imaging sequences — that step comes next.

MRI and what it can — and cannot — show

Imaging follows a clear hierarchy. Plain X-ray and CT are rarely useful for focal chondral defects: cartilage does not show on X-ray, and CT adds little except in cases of severe bony disease or when a surgeon needs detailed bone-stock mapping before a procedure. For most patients with a suspected focal defect, neither modality is the investigation that matters.

MRI is the first-line non-invasive tool. A 3T scanner improves sensitivity over the standard 1.5T — articular cartilage is thin, and higher field strength better resolves the subtle signal differences between healthy and damaged tissue. Standard morphological sequences show the physical shape of the cartilage: visible fissures, gaps, or areas of thinning.

What standard MRI cannot reliably show is, clinically, the more important point. Grade I lesions — soft and probe-detectable but morphologically intact — may appear entirely normal on even a good-quality scan. A negative or equivocal result therefore does not exclude a clinically significant defect; this is one reason arthroscopy retains its position as the definitive investigation when imaging is inconclusive.

Beyond morphology, specialist sequences — T2 mapping, T1ρ, dGEMRIC, sodium imaging, and diffusion-weighted imaging — can detect early biochemical changes in the cartilage matrix before any structural loss is visible, offering a pre-morphological detection window that standard sequences miss entirely. These techniques remain an active area of research and are not yet integrated into routine clinical grading systems.

Returning to what current grading tools do capture, the AMADEUS score (Area Measurement And DEpth and Underlying Structures) quantifies defect area, depth, and underlying bone condition from MRI. It shows moderate correlation with arthroscopic ICRS grading — a sensitivity of 0.70 — and reasonable interrater agreement (ICC 0.75 for the total score). Intrarater reliability is weaker, and AMADEUS scores do not predict patient-reported outcomes after cartilage repair procedures. It functions as a useful pre-operative planning reference, not an outcome-forecasting tool.

How cartilage damage is graded

If your clinical letter mentions 'Outerbridge Grade III' or 'ICRS Grade 3b', you are looking at a standardised depth score — one half of the picture the clinical team uses to plan treatment. Understanding what each grade means makes the conversation with your consultant considerably easier to follow.

The Outerbridge scale

Originally developed in 1961 for cartilage changes on the back of the kneecap and extended to the whole knee joint in 1989, Outerbridge remains the most widely used arthroscopic grading system:

• Grade 0 — normal cartilage
• Grade I — softening and slight swelling of the surface; the cartilage looks intact but yields slightly under a probe
• Grade II — partial-thickness fissures or cracks measuring 1.5 cm or less in diameter
• Grade III — deeper fissures extending more than 1.5 cm, reaching but not breaking through the hard bone beneath the cartilage
• Grade IV — full-thickness loss with exposed bone

The ICRS scale and why Grade 3 has sub-grades

The International Cartilage Repair Society (ICRS) system follows the same broad structure but adds precision at the most surgically consequential level:

• Grade 3a — defect deeper than 50% of cartilage thickness, still above the calcified lower layer
• Grade 3b — reaching the calcified layer
• Grade 3c — penetrating through the calcified layer but not yet through subchondral bone
• Grade 3d — surface blisters with deep underlying involvement

This granularity matters because the calcified layer and subchondral bone behave differently under repair conditions; a 3c lesion presents a different technical challenge than a 3a, even though both sit within 'Grade 3'.

What grade does not tell you

Depth is one axis of severity. Defect area — measured in square centimetres — is the second and equally important variable. A Grade 3a lesion spanning 5 cm² carries different implications than one of 1 cm², and treatment selection depends on both measurements together.

One further point worth noting: a Grade I lesion produces no visible structural change on MRI. Only direct tactile probing during arthroscopy can confirm that softening is present. This is why a normal-looking scan does not always end the investigation when symptoms remain unexplained.

What the findings mean for treatment planning

Knowing a defect's grade and area is the starting point, not the end point, of treatment planning. The full picture also weighs the patient's age and physical demands, the alignment of the leg, and any concurrent damage — ligamentous instability or a meniscal tear that co-exists with a chondral lesion will need addressing alongside it, not after.

Conservative care as the default first step

For most patients, structured physiotherapy — targeted muscle conditioning, load modification, and appropriate analgesia — is the right first step regardless of grade. Symptoms and function, not imaging findings alone, determine when escalation is warranted. A Grade 3a lesion that responds well to load management and exercise may not need further intervention at all; a Grade 2 lesion that does not may require it sooner than expected.

The biological tier

When conservative measures prove insufficient, injectable biological options occupy the space between physiotherapy and theatre-based repair. Injectable collagen scaffolds — including ChondroFiller, available as an ultrasound-guided outpatient injection through the London Cartilage Clinic on Harley Street — are designed for focal, contained defects where the aim is matrix-induced chondrogenesis rather than symptom management alone. Suitability depends on whether the defect is sufficiently focal, whether the joint environment is mechanically stable enough to support repair, and whether symptoms justify the step.

How defect area shifts the surgical options

The area thresholds discussed in the previous section carry most weight here. Marrow-stimulation techniques suit smaller, contained defects; at the ≥3 cm² threshold, SUMMIT trial data support scaffold- and cell-based approaches over microfracture on KOOS pain and function outcomes at two and five years.

When complexity changes the calculation

Grade IV lesions with exposed subchondral bone, or any significant defect in a joint with marked malalignment, belong to a different category. A patient with a Grade IV medial femoral condyle lesion and five degrees of varus loading presents a mechanical problem that cartilage repair alone cannot solve — osteotomy to correct alignment before any repair proceeds may be a prerequisite, not an option. In these cases, grade and area are necessary inputs but insufficient guides; specialist assessment is needed to map out the sequencing and what each stage can realistically achieve.

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