What the two stages actually involve

Planning surgery becomes easier once you understand what 'two-stage' means in practical terms — because both ACI and MACI follow the same two-appointment structure, and the gap between those appointments is something patients need to plan around.

The first stage is a short arthroscopic procedure (keyhole surgery under anaesthetic). The surgeon takes a biopsy of roughly 200–300 mg of healthy cartilage from a low-load area of the knee, typically the superomedial margin of the femoral trochlea — a region that bears little weight during normal movement. That tissue is packaged and sent to a specialist cell laboratory, where technicians isolate the chondrocytes and culture them for several weeks until the cell population has expanded sufficiently.

The second stage is a more involved open operation (an arthrotomy — a direct surgical opening of the joint) to implant those cells into the damaged area. In first-generation ACI, the expanded chondrocytes are injected as a suspension beneath a small patch of periosteal tissue harvested from elsewhere in the knee, which acts as a biological lid to keep the cells in place.

MACi refines this second stage by seeding the cultured cells directly onto a ready-made Type I/III collagen membrane — a scaffold sometimes called ChondroGide — before the operation. The pre-seeded membrane is then press-fitted or sutured into the defect. This removes the need to harvest periosteal tissue and simplifies handling in theatre, but it does not shorten the overall process: patients still attend two separate operations, with the laboratory culture period in between. MACI is a technical refinement of first-generation ACI, not a fundamentally different treatment pathway.

How defect size guides which procedure a surgeon recommends

Defect size is the single most reliable starting point for understanding why a surgeon might recommend one approach over another — though it works as orientation, not a rigid rulebook.

For lesions smaller than roughly 2–4 cm², marrow-stimulation procedures such as microfracture or osteochondral autografting (mosaicplasty) have historically been first-line options. Cell-based repair is generally reserved for larger defects, where the case for it rests on stronger evidence.

At the medium-to-large end — defects of 3 cm² or above — the SUMMIT trial provides the clearest clinical benchmark. It demonstrated that MACI produced significantly better KOOS pain and function scores than microfracture at both 2 and 5 years, and this finding is widely cited as the pivotal evidence supporting two-stage MACI in this size tier.

For very large defects in the 4–12 cm² range, a Phase III hydrogel-ACI trial (100 patients) reported a 92.8% responder rate at 5 years, a mean KOOS improvement of 44.1 points from a pre-operative baseline of 39.8, and a treatment failure rate of just 1%. This suggests that even extensive focal lesions can respond durably to cell-based repair.

Microfracture's role has narrowed considerably. Evidence now associates it with fibrocartilage breakdown at two to three years and damage to the subchondral bone plate — a concern because a compromised bone plate can reduce the likelihood of success if a more advanced procedure such as ACI is attempted later. Patients who have already had a failed microfracture are generally considered at higher risk when being assessed for salvage cell-based repair.

Single-stage alternatives and how their evidence compares

For many patients, the most immediately appealing aspect of the newer approaches is straightforward: one operation instead of two, one recovery period, and no weeks of waiting while cells expand in a laboratory. That practical benefit is real — and three distinct single-stage techniques now exist to deliver it, each through a different mechanism.

STACi (single-treatment ACI) brings the laboratory into the operating theatre. Cartilage is harvested from a non-weight-bearing area of the joint; cells are freed enzymatically, combined with bone marrow mesenchymal stem cells (MSCs) concentrated from the same surgical field, and embedded onto a collagen scaffold — all before the patient leaves the table. Biologically it resembles MACI, but compresses both stages into one session. STACi is best described as next-generation ACI: it carries limited published RCT data at this stage and should be considered an emerging approach.

Minced cartilage implantation (MCI) takes a different route. Harvested cartilage is processed mechanically — cut into small fragments and implanted directly — rather than through enzymatic cell release and laboratory culture. No specialist cell-expansion facility is required, making MCI genuinely single-stage in a broader range of settings.

AMIC (autologous matrix-induced chondrogenesis) works differently again: it augments standard microfracture by laying a collagen membrane over the prepared defect to retain marrow cells more effectively. AMIC sits between marrow stimulation and full cell-based repair — simpler than MCI or STACi, but mechanistically distinct from both.

The 2025 evidence is encouraging. A matched-pair analysis (25 patients per group) found single-stage MCI statistically equivalent to two-stage ACI at 24 months — IKDC 74.3 versus 71.3, with comparable re-operation rates. A separate three-arm RCT (16 patients per group) comparing MACI, AMIC, and MCI found no significant difference across any primary or secondary outcome at two years, including VAS pain and all KOOS subscales.

The critical limitation is that neither study extends beyond 24 months. For young, active patients seeking durable cartilage repair, that gap is consequential: whether single-stage equivalence holds at five or ten years — the horizon at which longer MACI follow-up reveals graft deterioration — is a question the current evidence base cannot yet answer.

Long-term durability: what a 10-year study reveals about MACI

MRI graft fill of 90% at two years sounds encouraging — but a prospective biopsy study that followed 15 patients with a mean defect size of 3.76 cm² out to ten years tells a more complicated story. Fill had declined to 72% by five years; by ten years, it had fallen to 49%. Every participant showed bone oedema and bone cysts at the final follow-up. Biopsy confirmed fibrocartilage — the mechanically inferior tissue type that marrow-stimulation procedures tend to produce — in 73% of cases rather than the hyaline cartilage MACI aims to restore. Around 80% of grafts were measurably softer than the surrounding native cartilage.

These figures carry particular weight for patients being assessed for MACI after a failed microfracture. Two-year clinical scores following salvage cell-based repair can look reasonable, but the tissue-level and imaging data at ten years suggest that assuming long-term mechanical durability would be premature.

One research response is HD-ACI — high-density autologous chondrocyte implantation — which implants five million chondrocytes per cm² compared with the lower density typical of standard MACI. In animal models, the higher-density approach generated hyaline-like tissue where standard techniques produced fibrocartilage. HD-ACI is at present a research-stage approach rather than a widely available clinical service, and it is mentioned here only to illustrate that the field is actively working on the tissue-quality gap the biopsy data expose.

The ten-year picture is also relevant context for interpreting the single-stage equivalence results from 2025: two-year IKDC and KOOS scores look encouraging across all techniques, but the divergence that the MACI biopsy study documents between two-year and ten-year findings is precisely why longer follow-up from single-stage trials still matters before durability claims can be made with confidence.

Patient factors that affect how well cartilage repair works

Two independent predictors of long-term success have emerged from a 2025 PASS (Patient Acceptable Symptomatic State) analysis that followed 112 patients for a mean of 11.3 years after matrix-associated ACI. Male sex carried an odds ratio of 3.1 for achieving an acceptable symptom state at ten years; a BMI in the 20–29 range carried an odds ratio of 3.9 — the stronger of the two. Patients whose BMI fell outside that range showed meaningfully lower odds of reaching the same threshold at long-term follow-up.

A separate UK registry study covering 391 ACI procedures carried out between 1996 and 2020 adds a finding that matters from a planning perspective: socioeconomic deprivation and female sex were both associated with worse outcomes at one year. Age, BMI, and smoking were not significant predictors in that dataset after adjustment — though the longer-term PASS analysis still implicates BMI, underscoring that the effect may take time to become visible in outcome scores.

Neither the sex data nor the BMI finding represent entry criteria. What they offer is a basis for honest pre-operative planning. For patients whose BMI currently sits above the 20–29 range, weight management before surgery is a modifiable factor with a plausible impact on long-term results. The deprivation finding points equally clearly to rehabilitation access — whether transport, time off work, or physiotherapy provision — as something worth raising and, where possible, arranging before committing to a two-stage procedure.

What to ask a cartilage specialist before deciding

The most honest starting point for any specialist consultation is that no published randomised controlled trial has directly compared two-stage MACI against single-stage STACi. The choice between them currently rests on surgeon experience, centre capability, and a patient-by-patient reading of available evidence — not on a definitive head-to-head result. Understanding that gap changes how much weight to place on any single recommendation.

Questions worth raising at a first assessment:

• How large is my defect, and which size bracket does that place me in for treatment planning?
• Does this centre offer single-stage options, and do I meet the criteria for one?
• Given that single-stage equivalence data currently extend only to two years, what does the longer-term evidence suggest for a lesion like mine?
• What does the rehabilitation programme involve, and what support is available if access to physiotherapy is limited?
• Has the proposed technique been used in cases similar to mine in terms of defect size, location, and prior treatment history?

A specialist who works with more than one technique — and can walk through the practical trade-offs between two-stage cell culture and single-stage intraoperative processing — is generally better placed to give an independent recommendation than one whose practice centres on a single approach.

Locating a surgeon whose practice covers the relevant technique may take some research. GP referrals to tertiary cartilage centres, NHS specialist directories, and independent musculoskeletal specialist lists are the usual routes to finding someone with the right range of experience.