What the preservation window actually means

'Has this gone too far, or is there still something that can help the joint itself?' That question sits at the centre of every ankle OA consultation in which the patient is relatively young, still active, and trying to avoid fusion or replacement.

The preservation window is the period during which the ankle joint retains enough surviving cartilage, structural integrity, and mechanical correctability to respond to treatments that work with the native joint rather than replacing it. It is not a fixed point in time — it is a clinical state defined by what the joint still has, not only by how long symptoms have been present.

Once bone-on-bone contact is complete across the full tibiotalar surface, that window closes. At that point, the clinical goal shifts to reliable pain relief through arthrodesis or total ankle replacement — both effective procedures, but fundamentally different in what they offer a patient in their forties or fifties with decades of activity still ahead.

Identifying where a given ankle sits requires combining imaging, mechanical assessment, and symptom severity. No single test settles it.

How ankle OA differs from hip and knee arthritis

Unlike hip or knee arthritis, ankle OA is rarely something that simply develops with age. Over 75–80% of cases are post-traumatic — the consequence of a previous fracture, a poorly healed ligament injury, or years of abnormal loading from chronic instability. Primary wear-and-tear accounts for fewer than 10% of ankle presentations.

This origin changes both who develops the condition and how it behaves. Post-traumatic ankle OA tends to arrive roughly 14 years earlier than the idiopathic form, often affecting people in their forties or early fifties rather than their sixties or seventies. A working-age adult carrying an old ankle fracture or persistent ligament laxity is therefore the typical patient for whom the preservation window is most clinically relevant — and most urgent.

The mechanical basis of the condition carries a practical implication. If abnormal joint loading — from malunited bone, a lax lateral ligament, or a shifted talus — is driving cartilage loss, then correcting that loading has a clear biological rationale. Structural correction does not simply manage symptoms; it may address the underlying mechanism of progression.

Patients without any history of trauma or instability should mention this to their clinician. Primary ankle OA follows the same broad pathway, but the absence of a correctible mechanical cause changes which preservation options are most applicable.

The imaging thresholds that define the window

Weight-bearing radiographs — not resting scans — are the standard starting point for staging ankle OA. Films taken without load systematically underestimate how much joint space has been lost, so the distinction is clinically important.

The Takakura classification is the most used framework for interpreting these images. It runs from Stage I (early sclerosis and marginal osteophytes, joint space intact) through Stage II (medial joint space narrowing, but no bone-on-bone contact). Both stages fall clearly inside the preservation window: cartilage is still present across the tibiotalar surface, and structural intervention has a sound biological rationale.

Stage IIIA is genuinely uncertain territory. Medial subchondral bone contact has occurred, but some joint space persists laterally. Certain preservation procedures — particularly supramalleolar osteotomy in a mechanically suitable patient — can still be appropriate here, though guidelines do not uniformly agree on the precise threshold.

Stage IIIB, where bone contact extends across the talar dome roof, marks the radiographic line at which the 2022 Herrera-Pérez review recommends switching to joint-sacrificing procedures, such as arthrodesis or total ankle replacement. Stage IV, with full pan-tibiotalar contact, is unequivocally beyond preservation.

A practical clinical proxy runs alongside the staging system: at least 50% of the tibiotalar joint space should remain on a weight-bearing film for preservation to be considered.

MRI cartilage mapping can detect early cartilage loss before it appears on plain radiograph, and where an X-ray report is equivocal it adds useful information. Its routine role in preservation decision-making, however, is not yet standardised.

Why malalignment and ligament laxity determine preservation success

A Stage II radiograph does not, by itself, make someone a preservation candidate. The angle at which the ankle sits — and whether that angle can be corrected — carries just as much weight as the imaging stage.

The reason lies in joint mechanics. Even a 1 mm lateral shift of the talus reduces the tibiotalar contact area by roughly 42%, concentrating load onto a smaller patch of cartilage and accelerating its breakdown. Where that shift is correctable, realigning the joint redistributes stress onto surviving cartilage and gives it a viable mechanical environment. Where it cannot be corrected — because the deformity is fixed or the hindfoot cannot be brought to a neutral position — that mechanical rationale disappears, regardless of what the X-ray stage shows.

Correctable varus or valgus tilt with at least 50% cartilage remaining is the core indication for supramalleolar osteotomy and similar realignment procedures. Once the tilt becomes irreducible, the biomechanical case for preservation fails.

Ligament laxity is a related but separately assessed factor. Chronic lateral ligament laxity perpetuates abnormal loading even after structural correction has been achieved; an unaddressed lax ankle will continue driving cartilage loss along the same mechanical pathway that caused the original damage. This does not automatically rule out preservation — laxity can often be addressed as part of the same treatment plan — but it must be formally evaluated beforehand.

None of this can be determined from a plain X-ray alone. Stress views, clinical examination of ligament stability, and a specialist's assessment of hindfoot alignment are all part of this mechanical layer of the decision.

Clinical signs the preservation window has closed

The transition from a preservable joint to one that is not is often felt before it is fully confirmed on imaging.

Before surgical preservation is considered, a structured conservative trial is typically required: three to six months of activity modification, bracing, physiotherapy, and appropriate analgesia. Failure to improve over that period is a prerequisite signal, not an optional data point. The components of that foundation are covered in the section that follows; the point here is that it forms the first gate in the assessment.

The structural endpoint is complete bone-on-bone contact on weight-bearing imaging — Takakura Stage IIIB or IV — but this becomes definitive only when symptoms match it. Pain at rest while sitting, pain that persists through a short flat walk despite modified footwear, and a noticeably altered gait — shortened stride, weight shifted to the outer foot to offload the joint — are the behavioural signs that functional reserve is exhausted.

Comorbidities add further complexity. High BMI, inflammatory arthropathy, and poor bone quality all modify eligibility, though no validated composite decision tool currently brings all of these variables into a single threshold.

The clearest indicator is not any one finding in isolation but their convergence: advanced imaging stage, irreducible malalignment, and persistent severe symptoms despite sustained conservative management. A patient still walking to work whose pain settles with rest occupies a meaningfully different position from one with constant rest pain and a compensatory limp — and that distinction is precisely what specialist assessment is designed to resolve.

Treatment options while the ankle is still preservable

Conservative management comes first. Three to six months of physiotherapy, bracing, appropriate analgesia, and activity modification are required before escalation is considered — and failure to improve over that period is itself a clinical signal, not simply a starting point.

Within the preservable window, several further options exist, each suited to a different point on the severity spectrum:

• Arthroscopic debridement suits Stage I–II ankles presenting with anterior impingement. It removes osteophytes, loose bodies, and inflamed synovial tissue, but does not restore cartilage and cannot substitute for structural correction where malalignment is present.
• Ankle distraction arthroplasty reduces compressive tibiotalar load mechanically, allowing some biological recovery of the cartilage surface. It is appropriate for mild-to-moderate OA without complete cartilage loss.
• Focal osteochondral lesion repair addresses discrete cartilage defects within the broader preservable stage. Options include bone marrow stimulation and injectable collagen scaffold approaches such as ChondroFiller injection, delivered via ultrasound-guided outpatient placement. Non-operative management succeeds in roughly 50% of osteochondral lesion cases; injection or surgical routes are selected by lesion size and overall OA grade.
• Hyaluronic acid and PRP injections offer symptomatic support in early-to-moderate disease. Evidence for both continues to mature, and neither constitutes tissue restoration.

The option chosen depends on which problem dominates: degree of cartilage loss, whether malalignment is correctable, and how the joint actually loads under bodyweight. These tiers are not mutually exclusive — a realignment procedure may be combined with focal lesion treatment, for instance — and the right sequence is established through specialist assessment rather than from imaging in isolation.

Specialist assessment

Liquid Cartilage™ is delivered in the UK at the London Cartilage Clinic on Harley Street. Patients wishing to establish whether their ankle remains within the preservation window can book an assessment at londoncartilage.com.