What OCD of the knee feels like

A persistent dull ache around the front or inner side of the knee — worse during sport, easier after a day's rest — is easy to dismiss as growing pains or a minor strain. In some adolescents, however, that ache is the first sign of osteochondritis dissecans (OCD): a condition in which a segment of bone just beneath the articular cartilage of the femoral condyle loses its blood supply, softens, and may eventually separate from the surrounding bone.

OCD most commonly appears in young people aged 10–15, typically active adolescents in the middle of a growth spurt. It is less common in skeletally mature adults, though adults can and do develop it. Around 70% of knee lesions arise on the posterolateral aspect of the medial femoral condyle — the inner rounded end of the thigh bone — making that the characteristic site to investigate.

The symptom picture shifts clearly as the condition progresses. In early disease, the affected bone is still intact and the overlying cartilage unbroken, so the only complaint is usually an activity-related ache that settles with rest. As the fragment becomes less stable or begins to shift, mechanical symptoms emerge: a sense of catching or locking mid-movement, intermittent swelling, and sometimes a feeling that the knee gives way. These later features suggest the fragment may no longer be sitting securely in its bed.

No single cause explains OCD. Repetitive microtrauma, localised vascular insufficiency, and hereditary susceptibility have all been implicated, but none has been confirmed as the definitive trigger. Patients should not attribute the condition to one specific incident or sport — it is more likely the result of several overlapping factors.

How OCD is diagnosed in clinic

During a first clinic assessment for suspected OCD, the clinician works through a short but deliberate sequence of examination steps before ordering any scans.

A simple check for joint effusion — fluid inside the knee — comes first, followed by careful palpation along the femoral condyle to find whether there is a localised tender spot. When the knee is bent to around 90° and the lower leg is rotated inward, then slowly straightened, many patients with OCD feel a familiar ache reproduced somewhere between 90° and 30° of flexion; rotating the leg outward at that point relieves it. This is Wilson's test. It is a useful screen, though not infallible — a negative result does not firmly exclude OCD, and the clinician weighs it alongside the overall clinical picture.

Imaging begins with plain X-rays rather than MRI. Three views are standard: a weight-bearing anteroposterior (AP), a lateral, and a notch (tunnel) view — the last taken with the knee in partial flexion to open up the posterior aspect of the condyles where OCD most often arises. The notch view can reveal a subchondral defect or crescentic lucency that a standard AP would miss entirely.

That said, plain X-rays are normal in a meaningful proportion of early or purely cartilaginous lesions. A GP's X-ray that looks unremarkable does not therefore rule the diagnosis out. If the clinical history and examination raise a reasonable suspicion — activity-related aching in an active teenager, localised condylar tenderness, a positive Wilson's test — MRI is almost always the appropriate next step, and the reasons for that are covered in the next section.

Why MRI stability matters more than lesion size

"Unstable" on an MRI report has a specific structural meaning — and it is that single word, more than the lesion's size, that determines whether conservative management or surgery is appropriate.

MRI serves three purposes in OCD: confirming the exact lesion dimensions, characterising the health of the subchondral bone beneath the cartilage surface, and — most critically — establishing whether the fragment is stable or beginning to loosen. Of the three, stability is the decisive variable.

Four MRI features signal instability. A T2 high-signal rim — a halo of bright fluid signal immediately behind the fragment — suggests the bond between the fragment and its underlying bone has been compromised. Breach of the overlying articular cartilage indicates the joint surface itself is no longer intact. Perilesional cysts within the subchondral bone point to progressive osteochondral damage. Fluid tracking completely around the fragment on fluid-sensitive sequences is the most definitive sign that separation has begun.

The Clanton & DeLee types

• Type I — depressed osteochondral fracture; cartilage intact; stable
• Type II — fragment retains an osseous bridge to bone beneath; stable
• Type III — detached but non-displaced fragment; unstable
• Type IV — completely displaced fragment; loose body; unstable

Types III and IV indicate a surgical pathway regardless of how small the fragment is. A lesion can be modest in area yet classified as Type III if the MRI signals confirm detachment — size alone does not govern the decision.

A separate Brittberg grading system (Grades I–IV by cartilage depth) becomes relevant only later, at the point of surgical planning, to guide the choice of cartilage restoration technique. It is not the tool used to decide whether to operate.

Finally, OCD lesions can consolidate or progress over time, which is why a single MRI at presentation is never treated as the definitive final word.

Conservative management and physiotherapy's role

Three months of structured activity restriction, protected weight-bearing, and supervised physiotherapy is what non-operative management actually looks like — not prolonged bed rest, and not simply waiting to see what happens.

For stable lesions (Types I and II) in younger patients whose growth plates are still open, this conservative window is a genuine opportunity. The three factors that most reliably predict spontaneous healing are open physes, a small lesion size, and MRI-confirmed stability — and when all three are present together, many lesions do consolidate without surgical input.

Physiotherapy's role within that window is supportive rather than curative. It maintains quadriceps strength, preserves proprioception, and keeps the knee functioning under controlled, reduced load during a period when normal training is suspended. It cannot drive bone revascularisation or encourage a fragment to reattach — those processes depend on biological conditions in the subchondral bone itself, which physio cannot directly influence.

Progress is tracked with repeat imaging at three and six months — typically a further MRI, or plain X-rays if earlier scans were diagnostic. Clinical improvement on its own is not sufficient confirmation of healing; the bone-side response needs to be seen on imaging before the activity programme is safely advanced.

For adults with closed physes, the same three-to-six-month framework applies, but the spontaneous healing capacity is substantially lower. The monitoring window is tighter, and the threshold for moving to an orthopaedic review is correspondingly earlier.

Four signs OCD needs more than physiotherapy

Four specific circumstances should prompt referral to an orthopaedic specialist rather than extending the physiotherapy window further.

1. No improvement after 3–6 months of structured conservative management. If serial imaging at the three- or six-month mark shows no radiological consolidation — or clinical symptoms remain unchanged — continuing the same approach is unlikely to produce a different result. This is the clearest and most common trigger for escalation.

2. An unstable lesion (Type III or IV) at any point of presentation. MRI signs of instability — particularly fluid tracking around a detached fragment — indicate that the conditions for spontaneous healing are not present, regardless of how tolerable the symptoms feel day to day. Waiting in this situation risks further separation and loss of a fragment that might otherwise be salvageable. Unstable lesions warrant orthopaedic referral promptly, not after a trial of conservative care.

3. Skeletal maturity (closed physes). The biological capacity for spontaneous bone healing is substantially lower once the growth plates have closed. Adult-onset OCD is considerably less likely to resolve without surgical intervention, so the threshold for early specialist review should be lower from the outset — not the same as for a thirteen-year-old with open physes.

4. Worsening symptoms or imaging progression during monitoring. Clinical deterioration — new mechanical symptoms such as catching or locking, a recurrent effusion, or a lesion visibly enlarging on repeat imaging — overrides the remaining conservative window. Untreated or inadequately managed OCD that progresses to a displaced intra-articular loose body carries a recognised risk of secondary osteoarthritis; acting on deterioration rather than waiting for the six-month mark is the appropriate response.

What specialist treatment looks like at escalation

Referral to an orthopaedic specialist is a pathway-clarification appointment — one that maps options according to what the surgeon finds on re-examination and imaging, not a commitment to theatre.

The surgical approach follows fragment viability. For stable lesions that have not consolidated after conservative management, arthroscopic drilling creates micro-channels in the subchondral bone to stimulate revascularisation. Where a fragment is unstable but the bone remains viable, fixation with screws or absorbable pins offers the best chance of preserving the patient's own tissue. When a fragment is non-viable or already loose within the joint, removal comes first, followed by cartilage restoration matched to defect size: microfracture or mosaicplasty for defects below roughly 2–4 cm², ACI or MACI for defects at 3 cm² or above. SUMMIT trial data place ACI/MACI ahead of microfracture on KOOS pain and function scores at both two and five years.

Drilling is established practice for stable lesions failing physiotherapy, though direct trial evidence comparing it against continued observation at three months is absent; the escalation decision rests on clinical trajectory and individual presentation.

For patients left with a residual cartilage defect where fragment instability has already been resolved, an ultrasound-guided injectable collagen scaffold (ChondroFiller injection) is an outpatient alternative to further cartilage surgery in appropriately selected cases. The scaffold gels within the defect and provides a matrix for the body's own progenitor cells to support cartilage regeneration — a different mechanism from fixation or restoration, suited to a different stage of the problem. Long-term outcome data beyond five years remain limited for all newer cartilage restoration approaches in this population.

Which combination of these options fits a specific presentation is what specialist assessment determines — in London, that assessment is available at the London Cartilage Clinic on Harley Street.

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