The short answer on OATS candidacy

Three variables tend to determine whether OATS or mosaicplasty is the right option: the size and grade of the cartilage defect, the patient's age, and how active they are. When all three align favourably, both procedures offer a single-operation route to restoring a durable, weight-bearing surface in the knee — without the multi-stage laboratory process that other techniques require.

The typical candidate is aged between 16 and 50, physically active, and has a focal, full-thickness cartilage lesion on a weight-bearing surface of the femoral condyle — graded ICRS III or IV, meaning the damage runs through the full depth of the cartilage layer. Defect size matters considerably: the procedures are generally suited to lesions in the 1–4 cm² range, with the specific variant chosen depending on whether one plug or several smaller plugs are needed to cover the area.

Surgery is not the first step. OATS is typically considered only after conservative measures — physiotherapy, activity modification, or injection therapy — have not delivered sufficient relief. The sections below unpack each of these selection criteria in detail.

Defect size: the most decisive factor

Lesion area is what separates a candidate for a single OATS plug from one who needs mosaicplasty — or from one who needs a different procedure altogether.

For defects smaller than approximately 2 cm², a single osteochondral cylinder of 8–10 mm in diameter typically provides complete coverage in one implant. Once a lesion reaches the 2–4 cm² range, a single plug is no longer sufficient; mosaicplasty addresses this by tiling several smaller plugs side by side to extend the repair surface without requiring a second operation or donor tissue from another person.

Above roughly 4 cm², the practical limits of autograft harvest become the constraint. Removing a larger volume of healthy osteochondral tissue from within the same knee risks meaningful donor-site problems, and coverage becomes unreliable. At that threshold, techniques such as ACI, MACI, or osteochondral allograft transplantation (OCA) are generally preferred — the 2021 Cartilage Book notes that for lesions of 3 cm² or larger, MACI demonstrated improved KOOS pain and function scores over microfracture at both two and five years in the SUMMIT trial, supporting a shift away from marrow-stimulation approaches for boundary-size defects.

Size thresholds aside, two characteristics must also be present for OATS or mosaicplasty to be viable: the damage must be focal rather than spread across the joint surface, and it must sit on a weight-bearing area of the femoral condyle. Diffuse cartilage loss — the pattern typical of advancing osteoarthritis — falls outside the scope of either technique regardless of the measured area.

Age and biological healing capacity

Behind the 16–50 year inclusion window endorsed by NICE lies a biological rationale rather than an arbitrary administrative cut-off. Chondrocytes in younger patients are measurably more responsive to growth factors, produce higher levels of sulfated glycosaminoglycans, and deposit collagen more effectively — differences that show up as better repair-tissue fill on post-operative MRI scans. When a graft is placed into a joint where the surrounding cartilage retains those qualities, integration is more reliable and the repair surface is more durable.

Figueroa et al. (2024) confirmed that age functions as an independent predictor of OAT efficacy. Beyond the graft itself, older patients tend to have wider wear patterns in the surrounding cartilage, which undermines the biological environment into which the plug must integrate.

Crucially, age over 50 is a relative contraindication, not an absolute one. The 2021 Cartilage Book frames it that way explicitly, listing it alongside BMI above 40 and a Kellgren–Lawrence osteoarthritis grade above 2 — factors that compound each other rather than individually disqualifying a patient. A 52-year-old with excellent surrounding cartilage, a normal BMI, and a focal traumatic lesion may still be assessed as a reasonable candidate; the threshold signals a need for more individualised evaluation rather than an automatic exclusion.

The lower age boundary is less clearly defined in published guidance. For skeletally immature patients with open growth plates, the surgical approach may need to be modified to avoid physeal damage — though evidence specific to this group remains sparse and clinical decisions are made on a case-by-case basis.

Activity level and why it shapes the decision

Functional demand — what the patient actually wants from their knee after recovery — is the third axis of candidacy, and in many ways the most clinically intuitive.

Both procedures are built around the mechanics of high-impact sport: repeated pivoting, jumping, and loading of the kind that erodes a fibrocartilage patch within roughly two to three years of marrow-stimulation surgery. Microfracture was historically offered as a first-line option for small focal defects, but its role has declined as evidence accumulated showing that the fibrocartilage it produces is structurally inferior to native hyaline cartilage, and that the procedure can damage the subchondral bone plate — compromising future repair options if the fibrocartilage repair fails.

What OATS and mosaicplasty transfer is genuine hyaline cartilage: the same tissue that lines a healthy joint surface. In plain terms, it handles load the way the original cartilage did. Across osteochondral restoration procedures, published series report 66–91% of patients returning to sport at pre-injury levels — a benchmark that carries particular weight for athletes whose primary goal is a return to competitive or high-demand recreational activity.

Lower-demand patients are not automatically excluded, but the calculus changes. Harvesting healthy osteochondral tissue from elsewhere in the knee carries real donor-site morbidity, and if return to impact sport is not the patient's aim, that cost–benefit balance looks different. A specialist assessment should determine whether OATS or mosaicplasty — or a less invasive alternative — is the better match for both the defect and the patient's functional expectations.

When OATS is not the right fit

Not every knee cartilage problem falls within OATS or mosaicplasty's scope, and understanding where the boundaries lie is as useful as knowing the criteria for candidacy.

The most straightforward disqualifier is the nature of the cartilage loss itself. OATS and mosaicplasty are designed exclusively for focal, full-thickness defects — discrete lesions on an otherwise intact joint surface. Patients with diffuse or degenerative cartilage loss across the compartment are outside the procedure's therapeutic logic; filling one patch of a broadly worn surface does not restore joint mechanics and is unlikely to produce meaningful long-term benefit.

Defect area sets a practical upper limit. Once a lesion exceeds approximately 4 cm², the volume of autograft tissue that can be safely harvested from elsewhere in the knee becomes insufficient to fill it adequately. In those cases, the evidence points toward ACI or MACI for biologically healthy joints, or osteochondral allograft (OCA) where donor tissue from a matched cadaveric source can cover the area without the harvest constraint. For intermediate situations, AMIC — a single-stage matrix-augmented approach — may be worth discussing with a specialist.

A Kellgren–Lawrence osteoarthritis grade above 2 signals that the surrounding cartilage is too compromised to support reliable graft integration. Similarly, prior marrow-stimulation procedures such as microfracture can alter the subchondral bone plate in ways that reduce the prospects for any subsequent restorative technique.

BMI above 40 appears in broader relative contraindication frameworks, but it is worth being transparent: there are currently limited OATS-specific outcome data examining BMI as an isolated variable. The threshold is a precautionary one rather than a finding from OATS-focused trials.

None of these boundaries represent a clinical dead end. The cartilage repair pathway encompasses a range of approaches matched to defect size, biology, and patient profile — and a specialist assessment can map out which of those alternatives applies.

Outcomes and finding a specialist

For well-selected patients, the published record is encouraging. Mosaicplasty carries roughly 90% success in reported series (Hangody, ICRS 8th Congress), and Gudas et al. report 86–90% good results with the OATS technique at three-year follow-up. The failure rate sits at approximately 13% at a mean of 3.6 years — a meaningful minority worth holding alongside the headline figures when weighing the decision.

The single-stage design remains a practical advantage: harvest and implantation occur in the same operation, with no cell-culture step or return surgery required.

Evidence gaps are worth naming plainly. Head-to-head randomised trial data comparing OATS and mosaicplasty directly for intermediate-size defects in the 2–4 cm² range remains limited; the comparative picture is drawn largely from cohort studies and registry data rather than controlled trials. The age threshold of 50 is a relative cut-off, not absolute — outcomes near that boundary depend on individual joint biology as much as age alone.

The strongest candidacy profile across these criteria is a young, active patient with a focal ICRS III–IV lesion under 4 cm², healthy surrounding cartilage, and a course of conservative treatment already behind them. A surgeon who routinely assesses the full spectrum of restoration options — not OATS alone — is best placed to confirm whether that profile applies to a given patient. A specialist cartilage directory filtered by region and area of clinical expertise offers a practical route to identifying those practitioners across the UK.