The size threshold that determines your best repair option

Hearing that you need a more involved operation than you anticipated — particularly when you were expecting a simple keyhole procedure — usually comes down to one key measurement: how large the damaged patch of cartilage actually is. For talar osteochondral lesions (OLTs), lesion size is the primary branching point in clinical decision-making, and it shapes the recommendation more than almost any other single factor.

For small defects — roughly under 60 mm², about the area of a small fingernail — bone marrow stimulation (BMS, including microfracture) and arthroscopic debridement produce outcomes comparable to more complex procedures. In this range, the simpler first-line approach is entirely reasonable.

Once a lesion exceeds that threshold and approaches 150 mm² (a diameter of around 15 mm — roughly the size of a small coin), the picture changes sharply. Research by Chuckpaiwong and colleagues (2008) found virtually no treatment failures for lesions below 15 mm in average diameter, but only around 3% success when lesions reached or exceeded that size. A separate analysis by Choi and colleagues (2009) placed a comparable cut-off at 150 mm² on MRI measurement. At this point, marrow stimulation reliably fails, and osteochondral autograft transfer (OATS/mosaicplasty) becomes the primary escalation.

Size is not the only trigger, however. Non-containable lesions at the 'shoulder' of the talar dome, cases where an earlier BMS procedure has not held, and defects involving subchondral bone cysts — where damage extends below the cartilage layer — all shift the clinical balance toward OATS regardless of precise area measurements. In these situations, a procedure that addresses both the cartilage surface and the underlying bone simultaneously offers a more complete repair.

Why hyaline cartilage plugs outlast fibrocartilage repair

The reason size matters so much — as outlined above — comes down to what each technique actually deposits in the defect.

Bone marrow stimulation works by breaching the bone beneath the damaged cartilage, releasing stem cells that migrate up and fill the cavity. The tissue they produce is fibrocartilage: a patching material that initially looks adequate but is structurally weaker than the cartilage lining a healthy joint. Think of it as a temporary filler rather than a like-for-like replacement — it handles light loading reasonably well, but under the repetitive demands of an active ankle it tends to soften and break down, typically within two to three years. For small defects in less mechanically stressed situations, this trade-off may be acceptable.

OATS takes a different approach entirely. Rather than stimulating the body to generate a substitute, it transfers intact plugs of bone and cartilage from a donor site — usually the knee, or in some cases the lateral talar facet — into the defect. Because those plugs carry the original hyaline cartilage surface, the repair tissue is the same material that lines healthy joints: denser, better organised, and built to distribute load across decades rather than years. The 2021 Cartilage Book notes that this shift in tissue quality underpins the clinical preference for OATS in active or younger patients with larger defects, and helps explain the measurable decline in microfracture use for this patient group.

For revision cases — where an earlier marrow-stimulation attempt has already left fibrocartilage that has partially failed — the distinction is especially relevant. Repeating the same technique is unlikely to improve on an already degraded repair bed; restoring a hyaline surface becomes the more defensible option.

What the outcome data shows

The longest talar-specific dataset published to date tracked 19 patients for more than ten years after mosaicplasty. Every transplanted plug had incorporated into the surrounding bone, roughly nine in ten patients reported satisfaction or a neutral outcome, and more than half showed no worsening of osteoarthritis grade across the decade — a meaningful signal of durability for a procedure most often carried out in younger, active adults.

Shorter-term studies add functional detail. A 32-patient prospective study of mosaicplasty performed without removing tissue from the recipient site recorded an average gain of roughly 26 points on the AOFAS functional scale, while pain scores (VAS) fell from 6.0 to 1.6; MRI confirmed good plug integrity in every patient at a mean follow-up of just under three years. Pre-existing osteoarthritis was the main factor associated with a smaller functional gain.

A 19-patient series in which OATS was combined with post-operative PRP and hyaluronic acid injections reported good-to-excellent functional outcomes in 94% of patients, with no long-term donor-site morbidity at mean three-year follow-up. A separate 21-patient case series of grade 3 or higher talar lesions — using a knee donor site — recorded no graft failures and no major complications, with the best results in younger patients and those with acute sports-related injury.

No head-to-head randomised trial has yet compared OATS directly with cell-based alternatives such as ACI or particulated juvenile allograft (DeNovo NT) for ankle lesions. The cohort evidence is encouraging, but cross-study comparisons should be read with appropriate caution until controlled trials are available.

Which patients get the best results from mosaicplasty

Outcomes from mosaicplasty cluster most reliably in a recognisable patient profile: younger adults — typically those in their twenties or thirties — with a clearly defined traumatic event (a sports injury or ankle fracture) and a large focal defect that has not responded to conservative care. In published case series, this group consistently shows the most complete functional recovery and the lowest rates of residual pain.

Two factors reliably reduce that expected gain. Pre-existing osteoarthritis across the ankle joint — rather than a focal defect alone — limits how much improvement the procedure can deliver; OATS restores the damaged patch of cartilage but cannot reverse joint-wide degeneration. The second, less intuitive, modifier is lower-limb alignment. A study of 44 patients found that those with knee-origin varus deformity reported pain scores nearly three times higher than neutrally aligned patients after mosaicplasty (VAS 4.4 versus 1.5), even though MRI showed no difference in cartilage repair quality between the two groups. The cartilage healed — but uneven load distribution across the joint kept pain elevated. Patients with varus alignment should ask their surgeon whether corrective realignment needs to be assessed alongside or before the procedure.

In skeletally immature patients, open growth plates introduce anatomical constraints, and surgeons typically favour marrow-stimulation techniques for smaller lesions until skeletal maturity is reached rather than committing to an autograft harvest. Body mass index and general activity level also feature in candidate assessment; neither is an absolute bar in isolation, but an elevated BMI places higher mechanical demands on the repair and tends to shift the clinical conversation toward addressing biomechanical load before or alongside surgery.

Donor site, surgical access, and procedural considerations

Deciding where to harvest the osteochondral plug is as important as the repair itself. The knee — specifically the lower-weight-bearing margin of the femoral condyle — remains the most commonly used donor site for talar OATS, and in the 21-patient case series using knee harvest, no patient experienced severe donor-site morbidity. That said, occasional persistent exertional knee pain is a recognised limitation and should be part of any pre-operative consent discussion; one patient in a 32-patient prospective mosaicplasty series recorded a clinically meaningful decline on the Lysholm knee score at follow-up.

For smaller defects — typically in the 8–10 mm diameter range including those with subchondral cysts — the lateral talar articular facet of the same ankle offers a viable alternative that avoids knee harvest entirely. In an 11-patient series followed for a mean of nearly 65 months, this approach improved AOFAS scores from 55 to 92 and VAS from 5.5 to 1.9, with no arthritic change at the donor site and no lateral talar dome collapse.

Where the lesion sits within the talar dome also shapes what surgery involves. Anteromedial lesions are often reachable arthroscopically or through a small incision; posteriorly and centrolaterally placed defects frequently are not. Adequate access to these sites may require a medial malleolar osteotomy — a controlled bone cut to open a surgical corridor — or, less commonly, an anterior chevron tibial osteotomy. Both are established access techniques rather than complications in themselves, but they do extend the overall procedure and recovery. When an osteotomy is performed, non-weight-bearing is typically maintained for around six to ten weeks to allow bone healing before graduated loading begins; without an osteotomy, that protected period is generally shorter. Hardware used to fix malleolar osteotomies is commonly removed at around twelve months in some centres, though practice varies.

Finding a specialist for ankle mosaicplasty in the UK

Mosaicplasty for talar osteochondral lesions sits within foot and ankle surgery as a subspecialty — not every foot surgeon performs it routinely. Relevant markers when identifying a specialist include demonstrated experience with osteochondral lesion characterisation (weight-bearing CT and MRI both contribute), familiarity with lower-limb alignment assessment, and access to a surgical team experienced in malleolar osteotomy when lesion location demands it.

Three questions are worth raising at any consultation: whether the surgeon routinely measures lesion size and area before selecting a technique; how lower-limb alignment is evaluated and whether corrective osteotomy would be discussed alongside the repair; and which donor site is preferred — knee versus ipsilateral talar facet — and the reasoning behind that choice.

Search MSK lists foot, ankle, and cartilage repair specialists across the UK — filter by region and specialty to find a consultant suited to your situation. The goal is a surgeon who addresses the lesion, the joint mechanics, and the patient's activity demands as a single clinical picture rather than in isolation.