What osteochondritis dissecans of the knee actually is

Osteochondritis dissecans (OCD) is not wear-and-tear arthritis — it is a focal problem affecting a small, specific segment of bone just beneath the joint surface. In OCD, that segment loses its blood supply, softens, and begins to separate from the surrounding healthy bone. The articular cartilage sitting on top becomes unstable, and in more advanced cases the fragment can detach entirely and float loose within the joint.

In the knee, the medial femoral condyle — the inner curve of the thigh bone — is by far the most commonly affected site. The condition predominantly develops in active young people between roughly ten and twenty years of age, often linked to repetitive loading through sport. A familial predisposition has been documented, so a family history of the condition is worth establishing at the first consultation.

What sets OCD apart from general joint degeneration is that the surrounding cartilage and the rest of the joint are typically healthy. That distinction matters clinically: because the damage is focal rather than diffuse, cartilage preservation is genuinely achievable — but only if the lesion is caught and graded at the right stage. How stable the fragment is at the time of diagnosis is the factor that drives every subsequent treatment decision.

Symptoms that point to OCD

For many patients, OCD begins with months of medial knee pain that surfaces during sport and eases with rest — no clear injury event, no swelling, nothing alarming enough to prompt a clinic visit. The discomfort is typically dull and activity-related, worsened by running, jumping, or cutting movements, and settles well enough between sessions to be dismissed as an overuse niggle.

As the lesion progresses and the bone fragment begins to lose its hold, the character of the symptoms changes. Joint effusion — persistent background swelling — may develop, and the knee can start to catch or lock during movement. These mechanical features reflect a fragment that is unstable or has partially displaced, and they carry a different clinical weight to the earlier ache.

Clinicians may elicit Wilson's sign during examination — pain reproduced on internal rotation of the tibia as the knee extends — a classic finding for medial condyle OCD, though it is not universally present and its absence does not rule the condition out.

That shift from a dull ache to catching or locking is the prompt to act promptly. It does not, on its own, confirm how severe the lesion has become: OCD at different grades can overlap considerably in how it feels day to day. Plain X-ray followed by MRI is always required to define what is actually happening in the joint — symptoms are the prompt, not the verdict.

The diagnosis pathway: from X-ray to MRI to arthroscopy

Investigation follows a clear sequence. When OCD is suspected, the starting point is a plain X-ray. The standard anteroposterior and lateral views are supplemented by the tunnel (notch) view, which positions the knee in partial flexion and brings the posterior femoral condyles into profile — the projection most likely to reveal the characteristic subchondral radiolucency with a sclerotic border that marks an OCD lesion. Radiographic grading systems exist, notably that of Cahill and Berg, but MRI has largely superseded plain-film classification for staging purposes.

MRI is the gold-standard modality for staging OCD. Where X-ray confirms a lesion is present, MRI defines its clinical significance: the integrity of the overlying cartilage surface, the size of the affected area, the degree of subchondral oedema, and — critically — whether a fluid-signal rim is forming at the interface between the fragment and the parent bone. That rim is the radiological marker of a fragment beginning to lose its attachment.

The De Smet criteria formalise the MRI signs of instability. A high-signal rim of 5 mm or more on T2 sequences, breach of the articular cartilage surface, fluid-filled cysts beneath the fragment, or a homogeneous bright signal at the fragment margin each indicate that the lesion may be progressing towards displacement.

When MRI findings are equivocal — and they sometimes are — arthroscopic probing provides the definitive answer. A fragment that feels soft, springy, or hinged under probing is classified as unstable regardless of what MRI suggested; in cases of disagreement, arthroscopy is the arbiter. That relationship between the two modalities also reflects a broader clinical reality: the scan informs decision-making when read alongside history and examination rather than in isolation — which is precisely why equivocal MRI findings prompt direct assessment rather than immediate treatment.

ICRS grading: what the four stability grades mean

Four grades run from intact-but-troubled to fully displaced, and each carries a different set of clinical implications.

Grade I describes a stable lesion with intact articular cartilage. The subchondral bone shows signal change on MRI, but the surface above it is unbroken and the fragment does not move on probing. Treatment direction is conservative or, if that fails, minimally invasive drilling to encourage healing — no structural repair is yet required.

Grade II is a watershed. The cartilage surface has been breached, but the fragment remains stable on probing. At this point the consultant faces a genuine decision: the lesion has crossed the threshold of purely conservative management, yet the native tissue may still be salvageable without fixation. Patient age and skeletal maturity weigh heavily here.

Grade III means the fragment has lost continuity with the parent bone but remains in situ. Because the native osteochondral unit is still present and potentially viable, surgical fixation — using compression screws or bone pegs — is generally indicated. The goal is to reattach what is still there rather than replace it.

Grade IV describes a displaced loose body or a defect left behind after an irreparable fragment has been removed. Fixation is no longer an option; the treatment goal shifts to cartilage restoration. The specific approach depends largely on defect size, which MRI quantifies accurately and which is examined in detail in the section on surgical options below.

Size is a meaningful secondary variable at every grade but becomes the decisive factor at Grade IV. A lesion measuring below approximately 2 cm² sits in a different treatment bracket to one exceeding 2–4 cm², and the restoration strategies available differ materially between those ranges.

Grade and lesion size together tell only part of the story. The other variable — skeletal maturity — governs how aggressively the clinician intervenes even when the grade alone might suggest surgery.

Why skeletal maturity changes the prognosis

The age at which OCD develops — specifically, whether the growth plates are still open — is the single variable that most reliably predicts what happens next. Grade, lesion size, and MRI signal all feed into the management decision, but skeletal maturity determines the biological context in which those findings sit.

In skeletally immature patients — adolescents whose physis is still active — the subchondral bone retains a capacity for spontaneous repair that adult bone does not. Hughes et al., cited in the 2021 Cartilage Book, found that approximately 95% of juvenile OCD lesions with intact overlying cartilage improved with conservative management, even when MRI showed subchondral change beneath the surface. That figure applies to a specific subgroup: open physis, stable grade, intact articular surface. Conservative management in practice means sport restriction, joint unloading (sometimes with partial weight-bearing), and serial MRI monitoring to confirm the fragment is not progressing. The biological rationale is straightforward — growth-plate activity supports the vascular ingrowth that allows subchondral healing to occur.

Once the skeleton matures, that repair window closes. In adults, evidence consistently shows poor outcomes with conservative treatment alone, and surgical intervention is generally recommended regardless of stability grade. Waiting carries a different risk calculus: without the vascular and cellular support that an active physis provides, a lesion is unlikely to consolidate on its own, and delay may allow a stable fragment to progress to displacement.

Which surgical approach is appropriate depends on both grade and the size of any resulting defect — the subject of the following section.

What your grade means for cartilage repair options

Treatment direction follows the grading hierarchy, though the final pathway always depends on the specialist's assessment of grade, maturity, defect geometry, and individual patient factors together.

Grades I–II: conservative care first, drilling if needed

Stable lesions in skeletally immature patients are managed conservatively: sport restriction, load modification, and serial imaging to confirm the fragment is progressing toward healing. Where no improvement is seen after three to six months, transarticular or retrograde drilling is the first surgical option — creating channels that promote vascular ingrowth and subchondral consolidation without sacrificing any cartilage surface.

Grade III: preserve the native fragment

An in-situ but unstable fragment shifts the goal toward fixation rather than replacement. Compression screws, headless screws, or bone pegs are used to reattach the fragment to parent bone and restore the osteochondral unit. Viability of the fragment — assessed directly at arthroscopy — determines whether fixation is biomechanically worthwhile; when the tissue is viable, preserving the native surface is strongly preferred over any reconstructive alternative.

Grade IV and failed fixation: restoration scaled to defect size

Once a fragment has displaced or cannot be salvaged, restoring the articular surface becomes the operative goal. Defect size is the governing variable. Lesions below approximately 2 cm² are candidates for marrow-stimulation techniques such as microfracture. Larger defects — above 2–4 cm² — generally require structural reconstruction: osteochondral autograft transfer (OATS), fresh osteochondral allograft, or autologous chondrocyte implantation (ACI/MACI). For focal defects at the lower end of the size range, an injectable collagen scaffold (ChondroFiller injection, available at the London Cartilage Clinic on Harley Street) provides an ultrasound-guided, outpatient route that sits outside the operative-theatre pathway.

Pre-operative assessment

Before any intervention is confirmed, the clinician will evaluate range of motion — below 90° raises arthrofibrosis risk — alongside limb alignment and ligament integrity. Systemic laxity conditions, including Ehlers-Danlos syndrome, may modify or preclude certain repair strategies.

The grade-and-maturity matrix means two patients presenting with identical MRI findings can reasonably be directed along entirely different pathways — one managed conservatively, the other directed to structural reconstruction. Knowing one's grade is the starting point; the full clinical picture is what determines where on that ladder a patient actually stands.

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