The bone layer that changes everything

If your scan report mentions an 'OCD lesion', you may have been told it is cartilage damage — and that is only partly right. Osteochondritis dissecans (OCD) of the talus begins not at the cartilage surface but in the bone beneath it. Understanding that distinction matters because it determines what any effective treatment actually has to do.

The subchondral bone is the dense layer that sits immediately below the joint's articular cartilage. It acts as a mechanical foundation and supplies nutrients upward into the cartilage above. In OCD, disrupted blood supply causes a segment of this bone to die and begin to crack. The cartilage overlying that segment loses its support and eventually cracks too — but it is a secondary casualty, not the origin of the problem.

A pure chondral defect is different in a fundamental way: the bone bed is intact, and only the cartilage surface has been damaged, typically by a single traumatic episode or focal wear. In talar OCD, the bone fails first. Cartilage involvement follows from that bony failure, which is why imaging — particularly MRI — often reveals bone oedema, fragment instability, or cystic change that a cartilage-only scan finding would not show.

The practical consequence is straightforward: if the subchondral bone is the origin, then repairing or resurfacing the cartilage alone is unlikely to be sufficient. Any treatment plan needs to address the bone layer. That bone-first pathology is what makes talar OCD a distinct clinical entity — and it shapes every decision that follows.

Why talar OCD forms — and where it tends to appear

Where the lesion sits within the talar dome tells a great deal about how it got there.

Lateral lesions tend to be shallow and wafer-shaped, sitting on the outer edge of the dome. The most common cause is an acute inversion sprain — the foot rolls inward, the talus is forced against the fibula, and a thin sliver of bone is sheared away. If you have a sprain in your injury history and a lateral OCD, the two events are almost certainly connected.

Medial lesions are typically deeper and cup-shaped, occupying the inner dome. A single dramatic injury is less often the culprit here; instead, years of repetitive loading and microtrauma gradually compromise the local blood supply. Symptoms tend to emerge more slowly, without a clear 'it started when I twisted my ankle' moment.

Age shapes the likely course significantly. In children and adolescents with open growth plates, the subchondral bone retains enough healing capacity that many lesions resolve without intervention. Once growth plates have closed, spontaneous healing becomes far less reliable, and a fragment that appeared stable on early imaging is more prone to loosening over time — or detaching entirely to become a loose body within the joint.

Talar OCD is also distinct from two conditions it is frequently confused with: diffuse ankle osteoarthritis, which affects the whole joint surface rather than one focal area, and ligamentous instability from recurrent sprains. In practice all three can co-exist, but each follows a different treatment logic.

How talar OCD is graded — and why imaging matters

Clinicians describing a talar OCD lesion will almost always reach for the same shorthand: a Berndt-Harty stage. Originally described in 1959 — and now largely encountered through later clinical reviews and syntheses rather than the primary paper itself — the system divides lesion severity into four stages that map directly onto how much the fragment has moved:

• Stage I — the subchondral bone has been compressed but remains in position; no crack visible on plain film.
• Stage II — the fragment has begun to separate and lift at its edges, but is still partially attached.
• Stage III — detachment is complete, yet the fragment sits within its bed, held in place rather than floating free.
• Stage IV — the fragment has displaced into the joint, becoming a loose body that can catch and lock the ankle.

Modern usage adds a Stage V variant to capture lesions where the subchondral bone has liquefied into a cyst — a pattern the original plain-radiograph system was never designed to detect.

That limitation matters clinically. A standard plain X-ray will often look entirely normal in Stage I and Stage II disease; if symptoms persist despite a normal ankle film, that does not rule out OCD. MRI is now the standard next step — it detects bone oedema, haemorrhage, and cartilage integrity in a single examination, making early-stage lesions visible that plain radiographs would miss altogether. Once a lesion is confirmed and surgery is being considered, CT takes over, giving the precise bony geometry — cyst volume, fragment shape, subchondral plate thickness — needed to plan the approach.

Stage is not an academic label. It directly shapes the treatment conversation: lower stages, particularly in an acute setting, may be managed conservatively; a displaced Stage IV fragment in an adult is unlikely to resolve without intervention.

The treatment pathway from conservative care to surgery

For most patients, the treatment conversation starts not in the operating theatre but with a trial of conservative management. Stages I and II non-displaced lesions — particularly in an acute presentation — are worth treating conservatively first: restricted weight-bearing in a walking boot, physiotherapy to offload the ankle and restore neuromuscular control, and activity modification. Published series suggest roughly half of acute non-displaced lesions settle with this approach, making it a genuine first-line option rather than a box-ticking exercise before surgery.

When six to twelve weeks of conservative care fails to resolve symptoms, or when staging is III or IV at presentation, the decision shifts toward surgery. For smaller lesions, arthroscopic bone marrow stimulation — typically microfracture — is usually the first step, perforating the subchondral plate to release marrow elements that form reparative tissue. Lesion size is the critical prognostic variable here: below approximately 15 mm average diameter, outcomes are consistently good; published cohort data show success rates falling sharply at or above that threshold.

Where a significant fragment remains structurally intact, fixation is preferred over removal — cortical bone pegs or low-profile screws hold the fragment in place to allow biological healing. A distinct situation calls for retrograde drilling: when the cartilage surface above a subchondral cyst is still intact, drilling from below targets the diseased bone without sacrificing the cartilage layer above — an approach validated in a 2023 systematic review by Artioli and colleagues.

Posteromedial lesions can sit beyond the reach of standard arthroscopic portals. Access in these cases often requires a chevron-type medial malleolar osteotomy — a temporary division of the inner ankle bone to open the exposure — which patients should understand is part of the planned procedure, not an intraoperative complication.

Larger or persistently symptomatic lesions prompt consideration of regenerative resurfacing: AMIC, autologous chondrocyte implantation, or osteochondral autograft transfer, each selected on lesion area and subchondral architecture. Injectable adjuncts such as PRP and hyaluronate show some symptomatic benefit in the literature but have not yet been established as a standalone alternative to surgery for structurally significant OCD.

Why lesion size is the single most important surgical decision

Fifteen millimetres is a small number with large consequences. Published cohort data from a series of 105 osteochondral lesions found that below roughly 15 mm average diameter — corresponding to approximately 150 mm² on MRI cross-section, corroborated in a separate 168-lesion series — bone marrow stimulation achieved near-universal success. Once a lesion reached or exceeded that threshold, only around 3% of cases succeeded with the same technique.

This is worth sitting with for a moment, because it shifts how patients should understand a microfracture recommendation. A lesion that sits above the size threshold is not a harder version of a small lesion — it is a categorically different surgical problem. The technique itself becomes unreliable, not because of anything specific to the patient's biology, but because the reparative tissue that microfracture generates cannot adequately fill a larger bony and cartilage void.

For lesions that cross this threshold, the treatment ladder steps up to regenerative resurfacing: AMIC, autologous chondrocyte implantation or MACI, and osteochondral autograft transfer (OATS or mosaicplasty) are the established surgical routes. Where lesion profile and subchondral bone quality are suitable, an injectable collagen scaffold — such as a ChondroFiller injection delivered under ultrasound guidance as an outpatient procedure — may also be considered as part of the regenerative pathway.

Size is rarely assessed from one scan alone. MRI defines cartilage involvement and bone oedema; CT maps the bony geometry and cyst architecture. Together they give the consultant the full picture — and explain why two patients with ostensibly similar ankle symptoms can need very different interventions.

Getting an accurate assessment for talar OCD in London

One clinical reality that tends to surprise patients: a talar OCD lesion visible on MRI is not automatically the reason the ankle hurts. Scan-detected lesions can be structurally present but clinically silent, and the specialist's job at assessment is to correlate imaging with symptoms and clinical examination — not to treat a report in isolation.

That correlation depends on detail no scan can supply. How the joint has been loaded since symptoms began matters considerably: a patient who has continued to play sport on an untreated Stage II lesion is biomechanically — and prognostically — in a different position from someone who presented early and protected the ankle. The mechanism of onset is equally informative: a single acute sprain, a pattern of repetitive loading, and no identifiable event each suggest different underlying processes and point toward different management priorities. Bringing that history to a first appointment — along with imaging on disc where possible — gives the clinician the context to judge what the scan is actually showing, rather than what it appears to show in isolation.

For patients whose assessment points toward a regenerative approach, Liquid Cartilage™ is available at the London Cartilage Clinic on Harley Street as an ultrasound-guided outpatient ChondroFiller injection. Suitability depends on lesion profile, staging, and subchondral bone quality — all determined at assessment. Book via londoncartilage.com to discuss imaging findings and next steps with the team.

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