Neither procedure is single-stage — and why that matters first

Before comparing ACI and MACI, there is one question worth settling immediately: neither procedure can be done in a single visit to the operating theatre. Both are irreducibly two-stage, and understanding why changes how a patient should plan.

Stage 1 is an arthroscopic biopsy — typically 200–300 mg of cartilage taken from a low-load area of the knee. Those harvested chondrocytes are then sent to a GMP/GLP-certified laboratory, where they are cultured and multiplied over roughly 3–6 weeks. Only once sufficient cells have been produced can Stage 2 — the actual implantation — go ahead. There is no biological shortcut: chondrocytes harvested directly from the joint are far too few in number to fill a clinically meaningful defect, and the expansion step cannot be compressed out of the pathway.

This two-stage commitment is not a quirk of one technique over the other. It is the defining feature shared by both ACI and MACI — and the central practical difference between cell-based repair and single-stage alternatives such as AMIC or minced cartilage procedures. Patients weighing ACI against MACI are therefore choosing between two versions of the same two-operation pathway, not evaluating whether a second operation can be avoided.

How ACI and MACI differ technically

With the biopsy stage shared between them, the two techniques diverge entirely at implantation — and that divergence explains why MACI has become the more common choice.

In first-generation ACI, the expanded chondrocytes are returned to the knee as a liquid suspension. A patch of periosteum — tissue harvested from the shin — is sutured tightly over the defect to create a sealed pocket, and the cells are injected beneath it. The technique works, but it introduces three points of vulnerability: a donor site on the tibia, suture lines that must hold under movement, and the possibility that the liquid cells settle unevenly rather than covering the defect uniformly.

MACI removes all three. Instead of a liquid suspension, cells are seeded directly onto a porcine Type I/III collagen membrane, pre-cut to match the exact shape of the defect. That membrane is fixed with fibrin glue — no periosteal harvest, no sutures, no pooling risk. Because the cells are uniformly distributed across the scaffold before it enters the joint, coverage is more predictable.

The practical result is a smaller incision and, in appropriate anatomy, the option of a fully arthroscopic approach (sometimes called MACI Arthro) — though availability varies by centre.

The 2005 Bone & Joint head-to-head RCT of 91 patients found that graft hypertrophy — overgrowth of repair tissue sometimes requiring revision — occurred in 9% of ACI-C cases versus 6% with MACI. Clinical outcome scores were statistically equivalent at one year (p=0.32). ACI remains a well-evidenced technique with decades of follow-up data; the shift towards MACI reflects technical simplicity rather than a fundamental difference in what the cartilage repair ultimately achieves.

What the outcome evidence actually shows

The comparison that matters most in everyday practice is not ACI versus MACI — that statistical equivalence was established in the 2005 Bone & Joint RCT and is not seriously disputed — but MACI versus microfracture, the procedure it most commonly replaces. The SUMMIT trial, which formed the primary regulatory evidence base for MACI, demonstrated superior KOOS pain and function scores at both two and five years in patients with defects of 3 cm² or larger. That advantage is clinically significant because microfracture generates fibrocartilage rather than hyaline-like repair tissue, and fibrocartilage deteriorates markedly within two to three years of the original procedure.

Across published series, success rates for ACI and MACI are cited at approximately 85–92%, varying with lesion complexity, location, and whether prior marrow-stimulation procedures have already altered the subchondral bone. Patients in these series reported meaningful reductions in pain and functional improvement — gains consistent with return to low-impact daily activity and, in many cases, recreational sport — though individual outcomes depend on defect characteristics and clinical assessment.

What the evidence does not yet provide is a direct long-term RCT comparing ACI with MACI beyond five years. The mid-term case for both is well-supported; the question of decade-scale durability in head-to-head terms remains an open gap in the literature.

Who is a suitable candidate

Defect size is the starting point for any candidacy conversation — not because it is the only factor, but because it shapes which repair strategy is even available.

For lesions below roughly 2 cm², single-stage procedures such as OATS or AMIC are generally preferred: they achieve repair in one operation without the laboratory expansion step that ACI and MACI require. MACI is targeted at focal defects in the range of approximately 2.0–15.0 cm² — the size band for which the SUMMIT trial built its regulatory evidence base. Beyond that range, or where significant bone loss is present beneath the defect, an osteochondral allograft becomes the more likely discussion.

Size alone does not determine candidacy. Bone stock beneath the defect, knee alignment, ligament stability, and the precise location of the lesion all influence whether cell-based repair is appropriate — which is why MRI assessment and a full mechanical workup are necessary before either procedure is planned.

One piece of history worth raising at assessment: prior marrow-stimulation procedures, particularly microfracture, are associated with a higher risk of failure in cell-based repair. Patients who have had microfracture previously should disclose this early in the consultation rather than have it emerge during pre-operative review.

Both techniques are reserved for younger patients with focal, contained defects in an otherwise sound joint. Diffuse or end-stage osteoarthritis sits outside the scope of either approach — that pathway leads towards joint replacement rather than cartilage restoration.

Single-stage alternatives when two operations are not suitable

Patients who cannot manage two anaesthetics, two rehabilitation blocks, and the practical disruption between them have realistic alternatives — though choosing them involves accepting a different set of evidence trade-offs.

AMIC (autologous matrix-induced chondrogenesis) augments standard microfracture by placing a collagen scaffold over the treated defect in the same sitting. No cell culture is needed, no second operation is booked, and the scaffold supports the marrow-derived cells that migrate from the microfracture channels into the repair site.

Minced cartilage implantation (MCI/AutoCart) takes a small cartilage harvest from a low-load area of the knee and reimplants it — as finely cut chips — within the same procedure. Like AMIC, it avoids the laboratory expansion step entirely.

A 2025 retrospective matched-pair study by Schneider et al. (PMC11989345, n=48) compared MACI, AMIC, and minced cartilage at two years and found no statistically significant differences across VAS pain and all five KOOS domains. That finding is worth noting — but the study is small (16 patients per group), follow-up extends only to two years, and its retrospective design limits the conclusions that can be drawn. It does not establish equivalence; it establishes that the question deserves larger, longer investigation.

Single-Treatment ACI (STACI) is a next-generation concept that aims to collapse both stages into one by combining biopsy and implantation in a single visit. It is not yet established in routine NHS or private practice, and published efficacy data at scale remain limited.

The honest summary is that the choice between two-stage and single-stage repair currently rests on patient preference, defect characteristics, and the experience of the treating surgeon — not on evidence that clearly favours either pathway over the long term.

The biopsy-to-implant gap: what patients often do not expect

A piece of real-world data rarely surfaces in pre-operative conversations: a significant proportion of patients who undergo a Stage 1 biopsy do not go on to Stage 2 implantation at all.

In one multisurgeon series of 46 patients who had arthroscopy combined with a planned ACI or MACI biopsy, only 12 — 26.1% — ultimately received a cartilage transplantation procedure. For the majority, arthroscopic treatments carried out at the biopsy sitting itself — debridement, chondroplasty, loose body removal — proved sufficient to resolve their symptoms. That figure comes from a single series and should not be read as a universal conversion rate, but it reflects a clinical reality that is worth understanding before Stage 1 is booked.

The practical implication is that booking a Stage 1 biopsy is not a commitment to implantation. The decision is reassessed after the biopsy, informed by what the surgeon finds arthroscopically and by subsequent MRI review. For many patients, that reassessment never leads to a second operation — not because the plan failed, but because the first procedure resolved what was causing symptoms.

Patients approaching this decision are well placed to ask their surgeon directly what proportion of biopsy cases in their own practice go on to implantation — and what the likely alternative paths are if symptoms settle in the interim. A surgeon experienced across both two-stage and single-stage techniques is best positioned to keep those options genuinely open between procedures.