What the choice actually comes down to

When a surgeon mentions cartilage repair and a second operation in the same breath, the natural question is: why do some patients need two procedures when others only need one? The short answer is that the choice is not really a choice — it is clinically governed, and defect size is the primary gate.

Defects below roughly 1.5–2 cm² tend to suit simpler, single-sitting approaches. Those in the 2–4 cm² range open a genuine competition between single-stage and two-stage techniques. Larger lesions — above 4–4.5 cm² — often shift the decision again, toward procedures that replace rather than stimulate tissue.

Size, though, is not the whole picture. Whether the lesion has stable cartilage walls (containment), whether subchondral bone is involved, the patient's age and activity level, and the absence of widespread joint degeneration can all move the recommendation in a different direction.

Both pathways share the same goal: restoring durable, hyaline-like cartilage to delay or avoid joint replacement. What follows maps which defect profiles tend to favour each approach, and what the evidence shows.

Defect size bands and which techniques fit each

Three approximate size bands help organise the clinical options, though surgeons treat them as a guide rather than a rigid rulebook — intraoperative findings frequently refine the plan.

Under approximately 2 cm²

Smaller focal defects are typically addressed in a single operation. Marrow-stimulation techniques such as nano-drilling — and historically microfracture — encourage the body's own cells to migrate into the defect, while OATS (osteochondral autograft transfer, sometimes called mosaicplasty) transplants a small cylinder of the patient's own bone and cartilage from a low-load area. Both are single-stage. However, microfracture's long-term record has tempered enthusiasm: survivorship drops below 60% at three years, with a mean time to failure of roughly four years. For active or younger patients, it is now widely regarded as a temporising step rather than a definitive repair.

2–4 cm²

This is where single-stage and two-stage approaches compete most directly. AMIC — which augments nano-drilling with a bioabsorbable collagen membrane in one operation — carries both RCT evidence (Fossum et al.) and registry data: 57 patients with a mean defect of 3.4 cm² showed sustained improvements in pain and function at two years. Two-stage MACI and ACI also operate confidently in this band and remain the benchmark for evidence volume.

Above approximately 4–4.5 cm²

Larger defects typically outgrow what autograft supply can reliably cover. Fresh osteochondral allograft (OCA) or advanced scaffold constructs are generally required at this size, replacing tissue rather than stimulating the patient's own repair response.

Single-stage repair: what one-operation approaches involve

Three techniques dominate the single-stage surgical landscape, with a fourth emerging at the leading edge of practice.

AMIC

Autologous Matrix-Induced Chondrogenesis builds on nano-drilling by adding a bioabsorbable collagen membrane, which is secured over the drilled defect in the same operation. The membrane concentrates the marrow-derived cells that migrate upward and shields them while they consolidate. AMIC is approved by NICE in the UK and carries the strongest evidence of any single-stage cell-stimulation approach in the 2–4 cm² range. A head-to-head RCT by Fossum et al. found outcomes comparable to ACI at two years in chondral and osteochondral defects of the distal femur and patella. Seven-year follow-up data from Schiavoni Panni et al. supported AMIC's efficacy for defects exceeding 2 cm², though MRI scans in some patients revealed subchondral bone changes worth monitoring over time.

OATS and mosaicplasty

Osteochondral autograft transfer takes a cylindrical plug of the patient's own bone and cartilage — typically from a low-load area of the knee — and presses it into the prepared defect. The transfer is immediate, with no culture step and no foreign material. Mosaic configurations can treat areas up to roughly 4 cm², though the dominant concern is donor-site morbidity: the harvest site in an otherwise healthy part of the knee creates its own small defect and recovery consideration.

Minced cartilage

Minced-cartilage techniques harvest small cartilage fragments, process them on the operating table, and implant them within the same sitting. They carry a strong biologic rationale — the fragments retain viable chondrocytes within their native matrix — and avoid any external culture interval.

STACi

STACi (Single Treatment Autologous Chondrocyte Implantation) takes the biology of staged ACI and compresses it into a single anaesthetic: a specialist laboratory team works alongside the surgical team in theatre, enzymatically freeing chondrocytes from harvested cartilage and combining them with bone marrow MSCs concentrated on-site, before implanting the cell mixture in a collagen scaffold. The approach has been described for defects of around 4 cm². Surgeons may consider it when a patient's defect profile would otherwise indicate staged MACI — particularly where cell-based biology is wanted but a second operation or the associated four-to-six-week wait is a meaningful barrier. It should be understood as an emerging, next-generation pathway, however: large head-to-head RCT data against two-stage MACI are not yet available, and suitability should be discussed carefully with a specialist who offers the technique.

Two-stage repair: what ACI and MACI involve

For patients with medium-to-large focal defects — particularly those at or above 3 cm² — the two-stage pathway centred on ACI and MACI remains the most evidenced cell-based route.

Stage one: biopsy and culture

The first operation is arthroscopic. The surgeon harvests a cartilage specimen of around 200–300 mg from a low-load area — typically the superomedial trochlear margin — and sends it to a specialist laboratory. Over the following four to six weeks, the extracted chondrocytes are cultured and expanded into a population large enough to populate the defect. The patient leaves the first procedure without any cartilage repair yet performed.

Stage two: implantation

At the second, open operation, the cultured cells are implanted into the prepared defect. In first-generation ACI, they are introduced beneath a periosteal flap sutured over the site. MACI — the more widely used current form — seeds the cells onto a Type I/III collagen membrane, which simplifies fixation and reduces some of the technical difficulty of the original technique, though the four-to-six-week culture interval remains.

What the evidence shows

The SUMMIT trial provides the highest-level clinical data for this size range: for cartilage injuries at or above 3 cm², MACI produced superior KOOS pain and function scores compared with microfracture at both two and five years — a meaningful finding given microfracture's historical use as a first-line option.

The 26% figure worth knowing

One multisurgeon series of 46 patients who had a biopsy arthroscopy found that only 12 — just 26% — went on to actual cartilage transplantation. Arthroscopic treatments performed at that first sitting, including debridement and loose body removal, resolved symptoms sufficiently for a substantial proportion. This does not diminish the value of staged ACI where it is genuinely indicated, but it is a realistic part of the conversation: the biopsy arthroscopy is itself a procedure with therapeutic potential.

Weighing the commitment

Two anaesthetics, two separate rehabilitation blocks, and the waiting period between them add up to a patient journey that is considerably longer and more demanding than any single-stage alternative. For the right defect profile — particularly larger, contained lesions in younger, active patients — that commitment is supported by strong evidence. For borderline cases, the practical burden is a legitimate factor in shared decision-making.

Factors beyond size that shape the recommendation

Knowing the size band narrows the field — but rarely settles the question alone. Surgeons work through a short checklist of secondary factors that can shift or restrict what size alone would suggest.

Is the defect contained?

Containment means the lesion is surrounded by a stable rim of healthy cartilage on all sides. Any graft or scaffold needs that rim to hold against and integrate with. An uncontained defect — where damage reaches the cartilage edge — may push the options toward osteochondral allograft, which anchors in bone rather than depending on a cartilage border.

Bone involvement beneath the surface

Pure chondral lesions affect cartilage only. When damage extends into the subchondral bone — as in osteochondritis dissecans or a posttraumatic osteochondral fracture — cartilage-only repair is insufficient. Those cases typically require an approach that also restores the underlying bone architecture, such as fresh osteochondral allograft.

Activity level and life stage

Younger, physically active patients with medium-to-large defects need tissue that tolerates sustained load over years. AMIC and cell-based procedures such as MACI are the stronger clinical fit in this group. The AMIC Registry, in which the mean patient age was 37 years, reflects this cohort; both Runer et al. (2022) and Ow et al. (2023) confirmed that single-stage procedures broadly match staged ACI outcomes when patients are selected appropriately.

Diffuse arthritis changes the conversation

Focal repair is designed for isolated lesions in an otherwise healthy joint. Widespread joint degeneration is a contraindication: cartilage grafting cannot address a global problem. The pathway shifts instead toward joint preservation — including alignment correction via osteotomy (HTO or DFO) to offload the worst-affected compartment — or, where preservation is no longer realistic, joint replacement.

Finding a specialist for cartilage repair in the UK

Defect size sets the initial pathway, but containment and subchondral bone status can shift the answer — and the gap between a planned two-stage procedure and a completed one is wider than many patients expect. The most productive question to raise in a first consultation is not "which technique is better?" but "given the size, containment, and bone depth of this lesion, which approach matches those findings — and why?"

Not every orthopaedic surgeon offers the full range of cartilage repair techniques; case volume and specialist training vary considerably across the UK. A surgeon who regularly performs AMIC, MACI, and single-stage cell-based procedures is better placed to tailor the recommendation to a defect's specific characteristics than one who defaults to a narrower set of options.

Search MSK lists cartilage repair specialists across the UK — filter by region, technique offered, and specialty to find one whose practice fits your situation.