Who reaches ACI candidacy at the ankle

The practical question most patients bring to a first ankle consultation is whether their lesion is large or complex enough to warrant a cell-based repair rather than a simpler procedure. For osteochondral lesions of the talus (OLTs), two signals tend to tip the answer toward ACI.

First, the typical profile. OLTs cluster in young, active adults aged 20–40 with a history of ankle injury — up to 50% of ankle sprains and over 70% of ankle fractures are associated with OLT formation. Conservative management (bracing, NSAIDs, protected weight-bearing) resolves symptoms in roughly half of acute, non-displaced cases; it is the persistently symptomatic or displaced lesions that reach surgical consideration.

Among surgical options, defect size is the primary decision fork. Smaller focal lesions lend themselves to OATS — transferring osteochondral plugs from the non-weight-bearing knee — or to marrow-stimulation techniques. Once the defect is large or geometrically irregular, the finite supply of donor plugs constrains what OATS can cover, and ACI becomes the appropriate tier. A second candidacy route is prior treatment failure: if microfracture or an earlier OATS procedure has not held, ACI is typically the next step because it replenishes cells rather than repeating plug geometry, and it carries no stated upper size limit.

No universal centimetre-squared cut-off separates these options; clinicians weigh defect size, shape, cystic involvement, and treatment history together. In practice, lesions broadly in the range of 2 cm² and above — or those that have failed a prior cartilage procedure — are where the ACI conversation most commonly begins.

ACI vs OATS: why the same joint needs different techniques

OATS and ACI both aim to restore durable hyaline-like cartilage to the talar surface, but they achieve that goal by entirely different means — and that difference is why defect geometry largely determines which is chosen.

OATS works by harvesting cylindrical osteochondral plugs from the non-weight-bearing zone of the patient's knee and pressing them into the talar defect. The bone component integrates with host bone; the cartilage cap provides an immediate hyaline surface in a single operative session. Where a single plug falls short, multiple plugs can be arranged in a mosaic pattern. The constraint is anatomical: donor area from the knee is finite, making OATS well-suited to defects of roughly 1–2 cm², with mosaicplasty extending coverage to around 4 cm².

ACI removes that geometry constraint entirely. Chondrocytes are harvested arthroscopically, cultured in a laboratory for four to six weeks, then reimplanted as an expanded cell population — under a periosteal flap in original ACI, or within a collagen membrane in the more common MACI evolution. Because coverage depends on cell number rather than plug supply, large, irregular, cystic, or previously operated defects remain treatable.

Unlike either technique, microfracture — historically used for smaller defects — generates fibrocartilage rather than true hyaline tissue. Fibrocartilage is known to break down within two to three years and can compromise the subchondral bone plate, narrowing future repair options; it is no longer considered a modern first-line choice.

The core procedural trade-off is clear: OATS is single-stage but supply-limited; traditional ACI requires two surgeries separated by a laboratory phase. STACi, a next-generation single-stage variant, brings the cell laboratory into the operating theatre and eliminates the second procedure — though as an emerging approach, its long-term evidence base remains limited.

What the ACI procedure involves at the talus

Two operative visits structure the traditional pathway, but talar anatomy introduces a layer of surgical complexity rarely reflected in the knee-based ACI literature from which most patient expectations are drawn.

Stage one is an arthroscopic session. A sample of healthy cartilage is taken from a low-load area of the ankle — and notably, Giannini et al. (2005) demonstrated that the detached osteochondral fragment itself can sometimes supply the donor cells, sparing patients a separate biopsy site. Where ankle tissue is unsuitable, a harvest from the knee provides the cell source. Those chondrocytes are then sent to an accredited laboratory for expansion over the following weeks before reimplantation.

Stage two returns the expanded cell population to the defect: beneath a periosteal patch in first-generation ACI, or seeded onto a collagen membrane in the MACI evolution, which has largely replaced the periosteal technique at most specialist centres owing to a simpler technical profile.

What makes ankle ACI distinctly demanding is access. Posteromedial talar lesions — among the most commonly encountered sites — frequently cannot be reached without a medial malleolar osteotomy, typically of the chevron type. The tibia is partially cut to open the joint, the cartilage repair is performed, and the osteotomy is then fixed with screws. Healing of the bone cut runs alongside cartilage recovery, meaning two separate biological processes govern the overall timeline rather than one.

STACi compresses this into a single operative session by running cell isolation intraoperatively, combining harvested chondrocytes with bone marrow mesenchymal stem cells, and seeding them into a 3D collagen scaffold before implantation. It is a next-generation approach; early clinical results are promising, but its long-term evidence base is still accumulating.

Surgical access: why ankle ACI is more complex than knee ACI

The osteotomy step covered in the previous section is not a minor addition to an otherwise standard cartilage procedure — it reframes the surgical skill set the operation demands.

Knee cartilage repair, even at the level of ACI or MACI, takes place within a joint that offers generous arthroscopic access; most lesions can be reached and prepared without cutting bone. Talar repair requires the surgeon to pursue two concurrent technical objectives: precise cartilage implantation within a smaller, more constrained joint, and stable fixation of the malleolar osteotomy. Each carries its own post-operative biology, and each can influence the other's outcome. Incomplete osteotomy healing, for instance, may prolong protected weight-bearing beyond what the cartilage repair alone would have required, extending the overall rehabilitation calendar as a consequence.

This procedural layering also has direct implications for interpreting published data. Ankle-specific ACI series are fewer in number, smaller in patient volume, and shorter in follow-up than the knee literature that dominates most outcome discussions. Where evidence is thinner, individual surgeon experience and caseload carry proportionally more weight in predicting results.

For patients weighing their options, familiarity with both talar cartilage repair and osteotomy management is therefore a meaningful criterion when selecting a specialist — not a secondary one.

Outcomes and evidence: what the data shows — and where gaps remain

Published evidence for ACI at the talus is genuinely encouraging at medium term — but it is not as deep as the knee literature, and that distinction matters when a patient is weighing surgical options.

The most cited clinical anchor is Giannini et al. (2005), which demonstrated meaningful functional improvement in an ankle ACI cohort and confirmed that the detached osteochondral fragment can supply chondrocyte cells directly, reducing the biopsy burden for some patients. For matrix-augmented approaches, Weigelt et al. (2019) reported clinical and radiological outcomes at two to eight years for AMIC applied to talar osteochondral lesions, with results supporting its use as a reparative option across that follow-up horizon.

What the ankle-specific literature does not yet offer is large-scale, long-term randomised controlled trial data comparable to knee ACI series extending beyond nine or ten years. That gap should be stated plainly: it is not a reason to avoid ACI where the clinical picture supports it, but it does mean outcome projections carry more uncertainty than at the knee, and individual surgeon caseload consequently carries more weight in predicting results.

ACI is also not the only evidence-based surgical route for talar lesions. Arthroscopic LDFF — lift, drill, fill, fix — remains a recognised primary option for smaller, well-contained defects, with published support for its use in that setting. The surgical assessment determines which approach fits which defect profile.

At the five-to-eight-year horizon captured by the available series, durable functional improvement appears achievable for patients whose defect size, prior treatment history, and activity level align with ACI's indications. That conditional is the operative one: the consistency of outcomes in published cohorts tracks closely with the precision of patient selection.

Finding the right specialist for ankle ACI in the UK

Ankle ACI sits at the intersection of two specialist skill sets: cartilage restoration and ankle and hindfoot surgery. Not every surgeon who performs ACI at the knee operates routinely at the talus, and — as the complexity discussed in earlier sections illustrates — the two settings are not interchangeable. Patients are better served by a clinician who can offer, and independently evaluate, the full range of ankle cartilage repair options: OATS, AMIC, ACI or MACI, and newer single-stage variants where available. A practice constrained to one technique is, by definition, constrained in its recommendations.

Referral routes include a GP referral to an orthopaedic foot-and-ankle specialist, direct self-referral to a cartilage or ankle surgeon, or an initial assessment with a sports-medicine physician who can direct onwards once the nature of the lesion is established.

Search MSK lists orthopaedic and sports-medicine specialists across the UK with experience in ankle cartilage conditions — the region and specialty filters are a practical starting point for identifying a clinician whose caseload genuinely includes talar repair.