What osteochondritis dissecans actually is

Osteochondritis dissecans — or OCD — is not, despite its name, an inflammatory condition. The term is a historical accident; what the condition actually describes is a focal patch of subchondral bone (the dense bone sitting just beneath the cartilage surface) that begins to lose its blood supply and, over time, separates from the surrounding healthy tissue. Think of it as a small area of bone quietly losing its foothold beneath an otherwise intact joint surface.

The articular cartilage above that patch depends on the underlying bone for structural support. As the bony island weakens, the overlying cartilage is progressively undermined. Left unmanaged, the lesion can progress from a stable but vulnerable area of bone to a fragment that is partially or fully detached — eventually becoming a loose body inside the joint. At that point, mechanical symptoms emerge, cartilage wear accelerates, and the risk of early osteoarthritis rises.

Cause is multifactorial. Genetic predisposition, localised vascular insufficiency, and repetitive microtrauma each contribute — which is why OCD is most often seen in physically active adolescents and young adults. The lesion most commonly develops on the lateral face of the medial femoral condyle; the patella and trochlea are affected less frequently.

How serious the condition proves to be depends heavily on the patient's age and how stable the lesion is at the time of diagnosis — a point that shapes every decision discussed in later sections.

Symptoms to recognise — and the ones that mean act now

An aching knee that flares during sport and settles with rest is easy to dismiss — a growth pain, a pulled muscle, the price of a hard training week. For many patients with OCD, this is precisely how the condition announces itself: a dull, poorly localised discomfort around the front or inner side of the knee, accompanied by mild swelling after activity and stiffness that eases once warmed up. Symptoms at this stable stage come and go, which is part of why a formal diagnosis is often delayed by months.

The picture changes when the underlying bone fragment begins to loosen. At that point, the joint starts to behave unpredictably — a sudden catching sensation mid-stride, an audible click, or a knee that locks momentarily and then releases. Giving-way episodes, where the leg feels briefly unreliable underfoot, are also reported. These are mechanical symptoms, and they matter: they suggest the lesion is no longer stable and that a fragment may be partially or fully free within the joint. They are the signal to seek specialist assessment promptly, not to observe and wait.

Some patients also notice they have unconsciously begun walking with the affected foot turned outward — an antalgic adaptation that offloads the most vulnerable part of the condyle.

GPs sometimes perform Wilson's test (pressing the knee into internal rotation at around 30° of flexion to reproduce pain that then eases with external rotation), which targets the classic medial condyle site. It can be a useful pointer, but it is negative in a substantial proportion of confirmed OCD cases, so a normal result should not be taken as reassurance if symptoms persist.

How OCD is staged: X-ray first, then MRI

Your GP or first-contact clinician will usually arrange plain X-rays first — and this is exactly right. A standard set for suspected knee OCD includes weight-bearing anteroposterior and lateral views, along with a tunnel or notch view taken with the knee flexed. That last projection is worth knowing about: the tunnel view opens up the posterior aspect of the femoral condyles — the zone where OCD most commonly develops — which a straight-on AP image can obscure. If the patella or trochlea may be involved, a skyline (sunrise) view is added.

X-rays are a sound starting point, but they have real limits. In early OCD, the images can appear completely normal, and even when a lesion is visible, plain films cannot reliably show whether it is stable or beginning to loosen. A normal X-ray does not exclude the diagnosis.

That is where MRI earns its place as the essential next step. What MRI adds is detail about the interface between the fragment and the surrounding bone — specifically, whether joint fluid is seeping behind the lesion, whether the overlying cartilage has been disrupted, and whether any cysts have formed beneath the fragment. Taken together, these findings allow a radiologist and consultant to classify the lesion as stable or unstable. Stable means the fragment still has a viable connection to its bed; unstable means that connection is compromised and the fragment risks becoming fully detached.

Two validated grading systems — the five-stage Hefti classification and the simpler three-grade Kocher system — give consultants a common language for this assessment, and both have shown comparable reliability in studies. The patient does not need to know the staging number; what matters is that MRI gives the specialist the information needed to decide whether conservative management is appropriate or whether a surgical opinion is required.

In a small proportion of cases, MRI findings remain ambiguous. When that happens, a specialist may recommend direct arthroscopic inspection of the joint — the only way to examine the cartilage surface and probe the fragment's actual stability in real time.

Why skeletal maturity is the single most important factor

Whether your growth plates are still open or have already closed is — more than lesion size, more than symptom severity — the single biggest factor in how your OCD is likely to behave and how urgently you need a specialist.

In juvenile OCD (JOCD), the bone has not yet finished growing. Open growth plates bring a genuine biological capacity for healing: stable lesions in skeletally immature patients can, with structured conservative management, resolve without surgery. That healing window is real, and it is worth protecting.

Once the growth plates have fused — meaning the bones have finished growing, a state called skeletal maturity — that window closes. Adult OCD (AOCD) has very little intrinsic capacity to repair itself. Even a lesion that appears relatively stable on MRI is unlikely to resolve with physiotherapy alone, which is why any skeletally mature patient with a confirmed OCD lesion should be assessed by a cartilage specialist promptly, rather than spending months on conservative measures first.

Importantly, skeletal maturity is confirmed on imaging — not assumed from a patient's age. A 16-year-old may still have open physes; a 15-year-old may already be skeletally mature. Chronological age is a guide, not a verdict.

The 2022 ROCK group definition of OCD — anchoring the condition in subchondral bone pathology — reinforces this principle: it is the biology of the bone, not the loudness of the symptoms, that should drive the escalation decision.

Conservative management for stable juvenile OCD

For stable JOCD, the first-line approach is structured and deliberate: six to eight weeks of activity modification combined with a supervised physiotherapy programme. The aim during this period is to offload the affected condyle — reducing the mechanical stress that prevents the subchondral bone from consolidating — while keeping the surrounding muscles strong enough to protect the joint. Physiotherapy at this stage is about load management and strength maintenance, not joint mobilisation; the priority is to let the lesion heal, not to push range of motion.

This is a genuine commitment. If symptoms persist or initial restriction proves insufficient, the programme may extend to three to six months of restricted weight-bearing — sometimes involving crutches or a complete break from impact sport. Progress is measured in imaging findings as well as symptoms. Feeling better is encouraging; what matters clinically is whether the lesion is consolidating on follow-up MRI. Symptom improvement alone is not a reliable proxy for bone healing, which is why imaging review within the treatment window is essential.

It is worth being clear about who this pathway applies to: stable lesions in skeletally immature patients. If symptoms worsen during this period, mechanical signs appear, or repeat imaging at the three-to-six-month mark fails to show healing progress, the picture shifts — and that shift is the starting point for the next section.

Five triggers for referral to a cartilage specialist

Five questions offer a reliable framework for deciding when a cartilage specialist — rather than continued physiotherapy or GP-led care — should be involved.

1. Is the patient skeletally mature? Adult OCD has negligible intrinsic healing capacity. Any confirmed OCD lesion in a patient with closed growth plates warrants early specialist referral, regardless of how mild symptoms currently appear.

2. Are mechanical symptoms present? Sudden locking, catching, or a knee that gives way at any age points to a loose body or an unstable cartilage flap. These symptoms should prompt specialist review rather than continuation of conservative treatment.

3. What does MRI staging show? Hefti Stage III–V or Kocher Grade 3 indicates the lesion has lost the structural integrity required for conservative healing. Fragment instability at this level is an indication for surgical or procedural assessment.

4. Is the lesion large in a young patient? In skeletally immature patients, lesions exceeding 2 cm in length or covering more than 6% of the femoral condyle surface area exceed the threshold at which natural healing reliably occurs — specialist input is warranted even in a still-growing skeleton.

5. Has conservative management stalled? No confirmed healing on follow-up imaging after three to six months of structured non-operative treatment is a clear signal that continuing the same approach is unlikely to change the outcome.

What specialist assessment typically involves

A cartilage specialist will review the clinical history, examine the knee, and reassess existing imaging. Where MRI findings are equivocal, arthroscopy remains the gold standard for confirming fragment stability. Treatment options discussed typically include subchondral drilling, fragment fixation, microfracture, and osteochondral grafting, depending on lesion size, patient age, and stability grade. For suitable focal lesions, an injectable collagen scaffold such as ChondroFiller may also be considered as a regenerative pathway. In London, this specialist pathway is available at the London Cartilage Clinic on Harley Street (londoncartilage.com).

The consistent finding across OCD research is that timing changes outcomes. A lesion staged correctly and assessed early — with growth-plate status and MRI instability graded before fragmentation occurs — carries a meaningfully better prognosis than one that reaches displacement before specialist options are explored.

1. [1] Osteochondritis dissecans — Wikipedia. https://en.wikipedia.org/?curid=3762029 https://en.wikipedia.org/?curid=3762029