What OCD of the knee actually is

A teenage runner develops a nagging ache deep inside the knee after training sessions. At rest it fades; on the pitch it returns. When the MRI comes back, the report names osteochondritis dissecans — a phrase that sounds alarming but describes something precise and, in the right circumstances, treatable without surgery.

OCD is a focal condition in which a segment of subchondral bone loses its blood supply, weakens, and risks separating from the joint surface it supports. As the underlying bone softens, the overlying articular cartilage — which has no blood supply of its own and depends on the bone beneath it — becomes vulnerable to cracking and, eventually, detachment. The most common site is the medial femoral condyle, the inner curve of the thigh bone where load concentrates with every step. Around 1 in 1,000 children are affected, predominantly those in regular competitive sport.

The single most important variable in predicting whether the condition will resolve without surgery is not age in years — it is growth plate status. When the growth plates are still open (visible on plain X-ray as radiolucent lines at the bone ends), the lesion is classified as juvenile OCD. The biological activity around open physis appears to support healing capacity in ways that closed-plate bone does not. Once the growth plates have fused, the lesion is classified as adult OCD, and conservative resolution becomes uncommon.

It is worth noting that despite its name, inflammation is not the primary driver here — 'osteochondritis' is a historical label that has persisted in clinical use rather than a description of what is actually happening in the tissue.

How OCD is diagnosed

Symptoms typically begin insidiously — a dull ache behind or inside the knee that surfaces during activity and settles with rest. Localised tenderness over the affected condyle, often elicited only with the knee flexed, is a common early finding. As the lesion progresses and the fragment becomes less secure, the pattern shifts: swelling persists after exercise, and the joint may catch, click, or lock briefly — signs that the fragment has begun to move within the joint or has separated.

When these symptoms prompt referral, imaging follows a two-step sequence. Plain radiographs — AP, lateral, and tunnel (notch) projections — are obtained first to identify bony abnormalities and screen for other diagnoses. Because X-ray cannot assess the overlying cartilage, MRI follows whenever OCD is suspected.

MRI is where the management-defining question is answered: is the lesion stable or unstable? A stable lesion has intact overlying cartilage with no fluid tracking behind the fragment. An unstable lesion — the finding most directly associated with escalation to surgery — shows a rim of high-signal fluid undercutting the bony fragment, a breach in the cartilage surface, or partial to complete detachment from the parent bone.

One important caveat applies in younger patients. A 2025 study found that the MRI signal-intensity thresholds used to predict surgical need in adults do not translate reliably to skeletally immature patients. Interpreting a scan in context — weighing image findings alongside growth plate status, symptom severity, and clinical examination — is therefore essential. No single imaging finding should be read as a standalone verdict.

What conservative treatment involves

Conservative management is more demanding than the label suggests. For patients who qualify — principally those with a stable lesion and open growth plates — it requires genuine activity restriction followed by a structured, active rehabilitation programme.

Phase one: protection (approximately months 0–3)

The first priority is removing mechanical load from the affected bone. High-impact activities — running, jumping, pivoting — stop entirely. Depending on symptom severity, crutches or an unloading brace may be used to further reduce force through the joint, giving the weakened subchondral bone a chance to stabilise.

Phase two: rehabilitation

The move into rehabilitation is not automatic. A follow-up MRI at around 12 weeks is commonly used to assess whether early healing is under way before any increase in loading is approved. When the imaging shows a positive response and symptoms are settling, supervised physiotherapy begins: progressive quadriceps strengthening, restoration of joint mobility, and a graded return to activity. This phase is active, not passive — the quality of the rehabilitation work matters to the outcome.

In skeletally immature patients, conservative management achieves healing in approximately 50–60% of cases; when it works, recovery typically takes between 6 and 18 months. For patients with closed growth plates — as covered in the earlier section — conservative resolution is unusual, and the threshold for surgical consideration arrives sooner.

How long to persist before seeking a surgical opinion

For adults, six months of appropriately conducted conservative care is the point at which ongoing non-operative management needs a clear justification: imaging evidence that the lesion is healing. Without it, surgical referral should follow. In children and adolescents, the juvenile skeleton's greater capacity for repair makes a longer window — up to six to twelve months — reasonable, provided the clinical picture remains stable.

The rationale is not arbitrary. The six-month threshold reflects the period by which a healing response, if it is going to occur, should be visible on MRI. The absence of symptoms alone is not a reliable indicator that repair is occurring; a lesion can feel manageable while remaining biologically unchanged or silently progressing. Imaging evidence of healing is the meaningful benchmark.

Certain findings override the timeline entirely and warrant earlier escalation regardless of how many weeks have passed:

• Symptoms that worsen during the protection phase rather than settling
• Development of mechanical symptoms — locking, catching, or a sudden effusion — signalling fragment movement within the joint
• MRI confirmation of instability (fluid undercutting the fragment, cartilage breach) at any point before the six-month mark

No randomised controlled trials define these thresholds precisely; they represent consensus across multiple specialist reviews rather than fixed protocol. What they share is an underlying principle: conservative care is time-limited and must be actively monitored, not passively continued.

A specialist should be involved from the outset rather than only at the point of escalation. Monitoring the imaging response, recognising early warning signs, and timing the decision to advance are clinical judgements that require orthopaedic input throughout the conservative phase.

When surgery is the right next step

Surgery for OCD is not a fallback — for certain lesion types it is the expected pathway from the outset, and a time-bounded option for others once conservative care has run its course.

A specialist will consider escalation in five situations:

• Conservative treatment has run its course without imaging evidence of healing at the age-appropriate threshold
• MRI confirms instability — a fluid rim undercutting the fragment, cartilage breach, or frank detachment
• Mechanical symptoms (locking, catching) indicate the fragment is mobile within the joint
• Lesion diameter exceeds approximately 1–2 cm — a consensus estimate rather than a figure derived from comparative trials, and always weighed alongside fragment stability and patient age
• The patient has closed growth plates (as covered in earlier sections)

The choice of surgical technique follows the lesion, not a fixed algorithm. Stable but unhealed lesions may be treated with subchondral drilling to stimulate biological repair. Unstable but structurally intact fragments are candidates for internal fixation with bioabsorbable or metal headless screws — a 2024 retrospective review found statistically significant functional improvements and a 12% reoperation rate at mean 6.4-year follow-up in grade I–II lesions that had failed at least six months of conservative care. Where the fragment cannot be salvaged, osteochondral restoration procedures such as OATS or ACI address the defect.

The surgical goal throughout is joint preservation. OCD treated at this stage does not progress to joint replacement.

What comes after OCD — the cartilage repair pathway

Residual cartilage gaps are a realistic outcome even after well-managed OCD — either because the original lesion left a focal defect that partial healing could not fully fill, or because surgical stabilisation, while successful, preserved structure without fully restoring the articular surface. For these patients, the clinical question shifts from managing OCD to managing a focal osteochondral defect.

The cartilage repair pathway at that stage includes marrow-stimulation techniques, osteochondral grafting, and — for smaller, accessible defects with adequate subchondral bone support — injectable scaffold approaches. One such option is the ChondroFiller injection: an acellular collagen matrix placed under ultrasound guidance directly into the defect, where it gels in situ and supports the patient's own progenitor cells in building new cartilage tissue. It is an outpatient treatment, not a theatre procedure.

Not every residual OCD defect is a candidate for this route. Defect size, depth, bone quality, and the condition of the surrounding cartilage all determine which approach is appropriate — a specialist assessment reviewing current imaging is the necessary starting point.

Patients in London pursuing that assessment can access it through the London Cartilage Clinic on Harley Street, the UK centre for ChondroFiller injection. Appointments are arranged at londoncartilage.com.

1. [1] Bioabsorbable Screw Fixation for Stable OCD Lesions After Failed Conservative Treatment (ASMR, 2024). (2024). https://doi.org/10.1016/j.asmr.2023.100863 https://doi.org/10.1016/j.asmr.2023.100863
2. [2] MRI Prediction of Surgical Treatment for Juvenile OCD (2025). (2025). https://doi.org/10.18060/29090 https://doi.org/10.18060/29090
3. [3] Osteochondritis Dissecans – Knee MRI (Radiopaedia, 2021). (2021). https://doi.org/10.53347/rid-85560 https://doi.org/10.53347/rid-85560