Which defect size points to MACI or ACI?

The size of the damaged area is the single most important factor when deciding whether MACI or ACI is appropriate — more so than symptom severity alone.

For very small lesions of 1.5 cm² or less, conservative management or focal fixation is typically the first step. The 2–4 cm² range is genuinely a judgement call: microfracture and osteochondral autograft transfer (OATS/mosaicplasty) remain reasonable options, particularly for patients who have not had a previous marrow-stimulation attempt and whose subchondral bone is intact.

Once a defect reaches 3 cm² or more, the evidence shifts towards cell-based repair. The SUMMIT Phase 3 RCT — the largest head-to-head study in this field — found that patients with defects ≥3 cm² treated with MACI reported significantly better KOOS pain and function scores than those treated with microfracture, both at two years and at five years. Real-world data reinforce why this threshold matters: in a database of 2,690 MACI-treated patients (Foldager et al., 2015), the mean defect size was 5.64 cm² and 70% of cases fell in the 3–10 cm² range — considerably larger than the populations enrolled in clinical trials.

Microfracture's declining role is worth noting here: the fibrocartilage it produces tends to degrade within two to three years and the procedure can damage the subchondral bone plate, narrowing future repair options. Both MACI and ACI are indicated broadly for focal, full-thickness defects in the roughly 2–10 cm² range where marrow stimulation has failed or is unsuitable.

What separates MACI from first-generation ACI

Both MACI and ACI follow the same two-stage logic: an arthroscopic biopsy of roughly 200–300 mg of cartilage at stage one, then three to six weeks of laboratory culture before the cells go back in at stage two. The difference lies entirely in how they are returned.

First-generation ACI injects the cultured cells as a liquid suspension and holds them in place with a periosteal patch — a small flap of tissue harvested from a separate knee site and sutured over the defect. The periosteal harvest adds a minor donor wound, and suturing the patch accurately is technically demanding; liquid cell pooling beneath the flap is a recognised risk.

MACI resolves both issues with a single engineering change. The same cultured chondrocytes are seeded uniformly onto a porcine Type I/III collagen membrane (Chondro-Gide), cut to the exact defect dimensions and secured with fibrin glue. No periosteal harvest is needed, no sutures are required, and the membrane is compatible with either a mini-arthrotomy (approximately a two-inch incision, roughly one hour) or fully arthroscopic delivery via MACI Arthro™.

Formal head-to-head RCT data comparing the two approaches are limited, but that has become largely a moot point: first-generation ACI has now been phased out of routine UK practice. MACI is the established two-stage, cell-based standard and is NHS-funded for eligible patients — with the 2024 systematic analysis by Manjunath et al. (PubMed 38739659) further reinforcing its current position.

The two-stage wait and what it risks

Choosing MACI or ACI also means structuring a significant portion of the year around two separate procedures. Real-world data from a 2,690-patient registry (Foldager et al., 2015) put the mean gap between stage one biopsy and stage two implantation at 155 days — more than five months. For someone in employment, training for sport, or managing family commitments, that interval is a concrete planning factor rather than an administrative footnote.

What is less widely appreciated is that the cartilage defect does not simply wait. The same dataset recorded a mean defect expansion of 0.6 cm² during this period — roughly 0.11 cm² of additional damage for each month of delay. More starkly, 16.2% of patients developed a new high-grade cartilage lesion before reimplantation was completed. These figures give clinical weight to minimising the inter-stage gap once biopsy is taken: unnecessary delay is not merely inconvenient, it may measurably worsen the problem being treated.

Practically, patients need to plan two anaesthetics, the lab interval, and at least partial activity restriction across the whole period. For those whose work pattern or sporting commitments make a five-month window particularly difficult to absorb, this staging burden is one of the practical arguments for a single-stage approach.

STACi as a single-stage option

For patients under 40 who are weighing the staging burden described above, STACi — single-treatment autologous chondrocyte implantation — offers a different structure. Rather than harvesting cells at one operation, culturing them for weeks, and returning for a second procedure, STACi processes chondrocytes alongside bone marrow stem cells intraoperatively, seeding them directly onto a three-dimensional scaffold within a single surgical session. The cell-culture phase is removed, as is the second general anaesthetic.

Described in UK literature as 'next-generation ACI' by Taylor and Lee (2019), STACi is typically offered to patients under 40, with selected cases accepted up to approximately 45. MACI is used up to around 55 years. This age difference reflects the biological rationale — younger cartilage environments are generally considered more favourable for single-stage regeneration — rather than a limitation of the technique in the conventional sense.

The key caveat is evidence maturity. MACI has accumulated over a decade of registry data and Phase 3 trial follow-up, including the SUMMIT RCT; STACi lacks comparable randomised trial data, and its long-term picture remains less established as a result.

In the UK, MACI is NHS-funded for eligible patients; STACi is currently private-only, with a guide price of approximately £28,000. For younger patients who are strongly motivated to avoid two operations, STACi is a credible option worth raising at assessment — but it should be understood as evidence-younger rather than a proven equivalent to MACI.

How MACIis delivered and what recovery looks like

Stage two of MACI now comes with a delivery sub-choice: a standard mini-arthrotomy of roughly 2 inches taking approximately one hour, or the fully arthroscopic MACI Arthro™ route. A 2021 systematic review by Migliorini et al. found no statistically significant difference in long-term functional scores — IKDC, Lysholm, or Tegner — between the two approaches. Where they differ is early recovery: arthroscopic delivery is associated with less post-operative pain and a lower risk of post-operative stiffness, which is a meaningful consideration for patients with physically active work or demanding training schedules.

The recovery arc is otherwise identical for MACI and ACI, regardless of delivery route. For the first six weeks, weight-bearing is restricted and a continuous passive motion (CPM) device supports the joint. Full weight-bearing typically resumes between weeks six and twelve. Running and high-impact loading are avoided until around six months. Return to sport is targeted at nine to twelve months. These milestones reflect cartilage biology — remodelling completes over six to nine months and cannot be shortened by surgical technique.

For patients targeting high-demand or semi-professional sport, published MACI cohorts report 85–90% good-to-excellent outcomes overall, with approximately 90% of grafts showing complete defect fill on MRI. These aggregate figures do not, however, disaggregate by sporting level. In published series, the factors most consistently associated with better functional recovery are younger age at treatment, a contained focal defect, and strict rehabilitation adherence — not the choice of delivery route. The long-term, sport-specific return data for both mini-open and arthroscopic MACI remain limited, and anyone with elite sporting goals should raise this directly at assessment rather than rely on overall cohort success rates.

Putting the choice together

The right pathway depends on which of three filters weighs heaviest for a given patient.

• Defect size first. Lesions ≥3 cm² on the femoral condyle or trochlea are where MACI and STACi both come into scope; smaller defects may be better served by OATS or AMIC. Prior procedures — particularly a failed microfracture — and depth of bone involvement can shift this threshold upward and require MRI review before any decision is made.
• Age and staging preference second. Patients under 40 who are strongly motivated to avoid two general anaesthetics can reasonably explore STACi; those up to approximately 55, or for whom NHS funding is a priority, are generally directed to MACI.
• Evidence confidence and cost third. MACI carries the most mature trial base and is NHS-funded for eligible patients; STACi is credible but evidence-younger, and currently private-only.

All three filters depend on accurate defect characterisation. AI-assisted cartilage segmentation and T2 mapping — as used in the clinic's onMRI™ imaging pathway — quantifies lesion dimensions and subchondral depth before the consultation itself, so the planning discussion can focus on options rather than unknowns.

Lincolnshire Knee, part of the MSK Doctors group, accepts patients without referral; an assessment combines imaging review with clinical evaluation to identify the most appropriate pathway. Book at lincolnshireknee.co.uk.