The size-to-technique ladder explained

Choosing a cartilage repair technique begins with a straightforward question: how large is the defect, and where does it sit? As a focal lesion grows in area — measured in square centimetres and graded by depth using the ICRS scale — the appropriate repair escalates in complexity and resource.

The broad ladder runs as follows. Defects below roughly 1–1.5 cm² are typically managed conservatively or with marrow stimulation (microfracture), where fibrocartilaginous fill is sufficient for a small, contained area. Between approximately 1 and 4 cm², osteochondral autograft transfer (OATS or mosaicplasty) becomes the primary structural option, transplanting the patient's own bone-and-cartilage plugs into the defect. From around 2–3 cm² upwards — and especially beyond 4 cm² — cell-based reconstruction with MACI or the single-stage AMIC technique takes over when subchondral bone is intact. Where bone loss is significant, or the defect is large enough that no autograft supply is adequate, fresh osteochondral allograft (OCA) becomes the lead option, with published series reporting femoral condyle survivorship of approximately 95% at 13 years.

Size, however, is not the whole story. Defect location, the condition of the bone beneath the cartilage, limb alignment, and the patient's age and activity level each modify where on this ladder a particular knee sits. The sections that follow unpack the OATS-to-OCA portion of the spectrum — the decisions that matter most for active patients with small-to-large focal defects.

When OATS is the right fit — the 1–4 cm² autograft window

OATS works by relocating the problem. A surgeon harvests one or more cylindrical osteochondral plugs — each containing both the cartilage surface and a column of underlying bone — from a low-demand area of the same knee, typically the periphery of the medial or lateral femoral condyle where loading is minimal. Those plugs are press-fitted into the prepared defect, giving the damaged surface an immediately structural, hyaline-rich repair rather than the fibrocartilage fill that marrow stimulation produces.

The size sweet spot sits between approximately 1 and 3 cm². Within that range, a single plug or a small mosaic can cover the defect without harvesting so much tissue from the donor zone that the harvest site itself becomes a clinical concern. Mills et al. (2024) place the practical ceiling at roughly 2.25–4 cm² — up to a 20 × 20 mm footprint — beyond which the volume of autograft required risks meaningful donor-site morbidity at the harvest area. The upper limit is therefore set by biology and supply, not convention.

At 10-year follow-up, systematic review data show that IKDC and Lysholm clinical scores improved significantly after OAT, and when placed directly against microfracture in the same size range, OATS produces superior long-term functional outcomes. That durability — combined with the fact that the entire procedure is completed in a single operation using the patient's own tissue — makes OATS particularly well-suited to younger, active patients with an isolated focal defect and structurally sound subchondral bone.

Not every knee qualifies. Bipolar (kissing) lesions, where damaged cartilage faces damaged cartilage on the opposing joint surface, are a relative contraindication because viable opposing cartilage is needed for a good result. Relative contraindications also include a BMI above 40, age over 50, and a Kellgren–Lawrence osteoarthritis grade greater than 2.

Large defects and the OCA threshold — when allograft becomes the answer

Two clinical triggers push a knee beyond the autograft window and into allograft territory: the defect is simply too large to fill from the patient's own donor site, or the bone beneath the cartilage has been lost or compromised. Fresh osteochondral allograft (OCA) addresses both problems simultaneously — donor tissue, sized to match the recipient defect exactly, replaces the damaged cartilage and the underlying bone in a single operation, without any harvest from the patient's knee.

The Hinckel et al. (2021) treatment algorithm — widely cited in the field — formally positions OCA and MACI as the primary options once a defect exceeds 4 cm², and published series place the practical start of OCA indication at roughly 3–4 cm², where autograft supply becomes insufficient. Grafts are classified as small (under 5 cm²), medium (5–8 cm²), or large (over 8 cm²); one large femoral condyle series reported a mean treated area of 6.4 cm², reflecting how frequently OCA is used at the heavier end of that spectrum.

OCA versus MACI — bone status as the deciding factor

For defects above 4 cm² where the subchondral bone remains structurally intact, MACI (matrix-induced autologous chondrocyte implantation) and OCA produce comparable clinical outcomes, as Matthews et al. (2022) report. The fork in the decision is bone status: MACI restores the cartilage surface only, so it cannot address subchondral bone loss. When bone is involved — whether through osteochondritis dissecans, avascular necrosis, or a prior failed procedure — OCA is the preferred choice because it reconstructs both layers in one stage.

Long-term survivorship and functional recovery

OCA carries reassuring long-term data. Survivorship for femoral condyle defects reaches approximately 95% at just under 13 years in select series; at a mean six-year follow-up, 75% of OCA recipients returned to sport or recreational activity, and 71% achieved 'very good' to 'excellent' knee function. Patellofemoral OCA shows modestly lower durability — around 87% at five years and 77% at ten — reflecting the mechanical complexity of that compartment.

Suitability is not universal. Advanced osteoarthritis (Kellgren–Lawrence grade above 2), high BMI, and inflammatory arthritis all reduce the likelihood of a successful outcome and are assessed carefully at consultation. One logistical factor specific to OCA is graft timing: chondrocyte viability in fresh allograft tissue falls below acceptable thresholds by approximately 28 days after procurement, making timely surgery a practical necessity once a matched graft is identified.

The contested middle ground — MACI, ACI, and AMIC between 2–4 cm²

Somewhere between 2 and 4 cm², clean lines dissolve. Mosaicplasty, AMIC, MACI, and ACI can all make a credible case in this bracket — and the choice turns on factors beyond defect area alone.

The most useful single number in this zone comes from the SUMMIT trial, which compared MACI against microfracture in patients with lesions of 3 cm² or greater. At both two and five years, MACI produced significantly better KOOS pain and function scores — establishing 3 cm² as the evidence-based inflection point where cell-based repair begins to pull ahead. Below that threshold, mosaicplasty or AMIC remain reasonable first choices; above it, the evidence tips toward cell-based or structural reconstruction.

AMIC — autologous matrix-induced chondrogenesis — occupies an important position here as a single-stage alternative. By combining marrow stimulation with a collagen membrane, it augments the repair environment without demanding a second operation. A 10-year randomised controlled trial found AMIC more durable than standalone microfracture, and published series cluster its practical range at roughly 2–8 cm².

MACI and ACI both deliver cell-based repair but carry the two-stage burden: a cartilage biopsy at the first operation, a period of laboratory cell culture, then re-implantation. One further consideration is surgical history — prior marrow stimulation procedures raise ACI failure risk, which matters where microfracture or drilling has been performed before. An emerging single-stage cell-based variant — sometimes termed STACI or next-generation ACI — aims to compress this into one procedure and is the direction of travel for cell-based repair; evidence is still limited and it sits outside established clinical pathways at present.

Microfracture alone no longer features as a primary option at this scale. Its fibrocartilage fill deteriorates within two to three years and risks structural damage to the subchondral bone plate that can complicate any subsequent repair.

The practical anchor: 3 cm² is where the SUMMIT data tip the balance toward cell-based reconstruction. Whether that means MACI, AMIC, or a move to OCA where subchondral bone is involved depends on bone status, patient profile, and surgeon assessment — but 3 cm² is where the evidence changes the conversation.

Why defect location changes the repair plan

Even two identically sized defects — say, 2 cm² each — may point toward different operations depending on precisely where they sit in the knee. Anatomic location is an independent variable that modifies the size-based ladder at every tier.

Medial femoral condyle — the primary OATS site

The medial femoral condyle accounts for roughly 70% of osteochondritis dissecans (OCD) lesions and is the most common site for focal cartilage surgery overall. Its broadly convex geometry is well matched to the cylindrical plugs used in OATS mosaicplasty, making it the natural primary indication site for autograft transfer in the 1–4 cm² range. The 10-year IKDC and Lysholm improvements cited in the OATS literature come predominantly from medial condyle series.

Patellofemoral joint (the kneecap joint) — where curvature rules out OATS

The patella and trochlea form a joint with a far more complex, asymmetric curvature than the femoral condyle. Cylindrical autograft plugs cannot reliably contour to this surface, leaving gaps at plug margins and risking incongruence under load. For restorative surgery in the patellofemoral compartment, OCA — shaped precisely to the recipient surface — is the preferred structural option regardless of defect size. Survivorship in this compartment reaches approximately 88% at five years and 77% at ten, somewhat lower than condylar figures, reflecting the high contact pressures generated during knee flexion.

Lateral femoral condyle and tibial plateau

The lateral condyle follows the same size-driven selection ladder as the medial side, though published survivorship data are less extensive. The tibial plateau presents a more distinct challenge: its relatively flat, load-sharing surface makes osteochondral plug seating technically more demanding than on a convex condyle, and tibial plateau defects frequently coincide with meniscal damage — a co-pathology that independently affects repair prognosis and must be addressed in the same operation. These factors mean tibial plateau lesions require more individualised surgical planning rather than straightforward application of a size threshold alone.

Location also shapes rehabilitation. Patellofemoral defects place the repaired surface under high load in early deep flexion, so post-operative range-of-motion protocols are typically more restricted after patellofemoral repair than after condylar work.

Patient selection — what else must be right before repair

Cartilage repair is only as durable as the environment surrounding it. Whatever the defect size or chosen technique, five mechanical and biological conditions need to be right before any reconstruction will hold.

Mechanical alignment. Varus or valgus malalignment concentrates load disproportionately on one compartment, meaning even a technically sound repair will be overloaded during walking and sport. Where significant malalignment is present, a high tibial osteotomy (HTO) or distal femoral osteotomy (DFO) is required — either concurrently or staged before cartilage repair — to redistribute force across the joint and protect the reconstruction. Alignment correction in this context is a joint-preservation adjunct, not an alternative to cartilage surgery.

Ligament stability. An incompetent ACL or PCL creates abnormal translational movement with every step. Cartilage repair placed into an unstable knee is subject to shear forces it cannot sustain; concurrent ligament reconstruction is the standard response where instability is confirmed.

Meniscal integrity. The menisci distribute load across the tibial plateau; significant meniscal deficiency raises contact pressure precisely where the repair must bed in. Meniscal repair or transplantation should be addressed at the same time where relevant deficiency is present.

Absence of diffuse osteoarthritis. Cartilage repair targets focal, contained defects. Diffuse joint-space narrowing removes the biological and mechanical foundation for regeneration — the specific grading thresholds that mark this boundary were described in earlier sections.

BMI and age function as patient-selection filters, not absolute barriers, across all techniques; again, the relevant thresholds have been set out in context above.

These five factors form the pre-operative checklist that precedes any technique decision. A structured assessment — combining weight-bearing alignment radiographs, clinical ligament examination, meniscal evaluation, and MRI cartilage analysis — brings all of them into the surgical plan before the operative approach is finalised.