Which operation suits your knee?

By the time ACI or MACI enters the conversation, a consultant has already confirmed what matters most: a focal, full-thickness cartilage defect — typically somewhere between 2 cm² and 10 cm² — in a knee that is otherwise structurally sound. Neither procedure is appropriate for widespread, multi-compartment osteoarthritis; both are reserved for the kind of contained lesion where genuine tissue restoration is realistic.

The choice between them is made by the treating surgeon, not the patient. Four factors shape it: the size and position of the defect, any prior surgery to that knee (particularly previous microfracture, which can complicate cell-based repair), the patient's age and activity targets, and — critically — what the treating centre is equipped to deliver. MACI is technically simpler and removes one of ACI's most persistent complications; however, the branded MACI product is not widely available across the UK, which means ACI or an equivalent scaffold-based technique is the realistic route at many centres.

For a growing number of UK patients, a third question is equally pertinent: whether a single-anaesthetic approach such as STACi — which processes cells in the operating theatre itself — removes the need for two separate procedures altogether.

What ACI and MACI have in common

Beneath the procedural differences lies a shared biological principle: both ACI and MACI use the patient's own cartilage cells — chondrocytes — harvested, multiplied outside the body, then returned to repair the damaged joint surface.

The sequence is the same in both cases. During a first-stage arthroscopy, a small cartilage biopsy is taken from a lower-load area of the knee. That sample travels to a specialist laboratory, where the chondrocytes are expanded over several weeks until sufficient numbers are available. A second procedure then places those grown cells into the defect, where they are expected to integrate and produce new cartilage matrix. It is this two-stage architecture — biopsy, culture, implantation — that separates both procedures from single-stage options and accounts for the weeks-long gap between operations.

Because the cell source and the repair biology are identical, clinical results at medium-term follow-up are broadly comparable between the two techniques. The 2005 Bone & Joint Journal trial — the earliest direct head-to-head comparison — found no significant difference in patient outcomes between ACI-collagen and MACI, consistent with the principle that the chondrocytes themselves are doing the regenerative work. What varies between the two procedures is not the biology but the mechanism used to hold those cells reliably in the defect during and after implantation.

Where the two procedures diverge

The divergence comes down to one surgical step: what holds the cells in place once they reach the defect.

In classic ACI, the laboratory-grown chondrocytes are delivered as a liquid suspension. To stop them washing away, the surgeon first harvests a thin layer of tissue — the periosteal membrane — from the surface of the shin bone, then sutures it over the defect to form a sealed chamber into which the cell suspension is injected. It is a technically demanding step: the flap must be harvested, shaped, and sutured watertight, all within the joint. That secondary shin harvest creates a small but real wound of its own, and graft hypertrophy — overgrowth of the periosteal tissue — has historically been a recognised driver of re-operation in ACI cohorts.

MACI removes this step entirely. The chondrocytes are seeded directly onto a pre-formed porcine collagen scaffold — a thin membrane approximately the dimensions of the defect — during the laboratory phase. At surgery, the surgeon trims the scaffold to fit and fixes it with fibrin glue. No periosteal harvest, no suturing, less operative time, and no donor wound at the shin. The rigid matrix also prevents cells from shifting or leaking unevenly, reducing the risk of the delamination that can complicate liquid-suspension techniques.

There is, however, a recognised limitation. Standard MACI scaffolds are seeded at roughly one million chondrocytes per cm² — a relatively low density that may produce softer, fibrocartilage-like repair tissue rather than true hyaline cartilage. High-density variants such as HD-ACI, which implant around five million cells per cm², have shown more hyaline-like tissue formation in animal models, but these remain experimental and are not part of standard clinical practice. Simpler implantation and better tissue quality are not the same thing, and current evidence does not resolve that tension definitively.

What the evidence actually shows

Longer-term data for MACI are now more substantial than for most cartilage restoration techniques. A 2024 systematic review tracked 168 patients across 188 treated defects (mean age 37) from implantation to a minimum of ten years — and in some cases seventeen. Patient-reported outcomes improved significantly and durably; MRI imaging showed satisfactory defect fill in the majority of cases; the all-cause reoperation rate stood at 9.0%; and only 7.4% progressed to total knee arthroplasty over the study period. For a procedure requiring two surgical episodes, those figures offer meaningful reassurance about durability.

The SUMMIT trial adds a comparative anchor: for defects measuring 3 cm² or more, MACI produced significantly better KOOS pain and function scores than microfracture at both two and five years. This is an important framing point — microfracture is no longer considered a first-line modern choice for lesions of that size, and the SUMMIT data confirm that marrow-stimulation techniques are insufficient at the larger end of the focal-defect spectrum.

Where the evidence is less settled

A 2025 matched-pair study (N=48) complicates the narrative. At two-year follow-up, MACI, AMIC, and arthroscopic minced cartilage implantation produced statistically equivalent improvements in pain (VAS p<0.000) and function (KOOS-Pain p<0.000). MACI is often described as the 'gold standard' for this indication, but the 2025 findings suggest that reputation may partly reflect its longer evidence track rather than inherent superiority over all newer alternatives — particularly single-stage approaches that are technically less demanding.

Two years is also a short interval for cartilage repair; the key question is whether that equivalence holds at five and ten years. Direct head-to-head RCT data comparing classic ACI versus MACI beyond a decade remain limited, with most long-term studies being single-arm cohorts or registry series. Honest counselling at this stage means acknowledging both what the evidence supports and where it has not yet been tested.

The factors that guide the surgeon's recommendation

Several factors shape the recommendation — and the weight given to each shifts depending on the individual.

Defect characteristics

• Size and boundaries. Smaller focal lesions, particularly those below 2 cm², may be well served by single-stage options such as AMIC or OATS. At the other end, very large or osteochondral defects extending into the underlying bone may require fresh osteochondral allograft (OCA) rather than either cell-based technique. ACI and MACI sit in the focal mid-range.
• Location within the joint. Weight-bearing surfaces of the medial or lateral femoral condyle behave differently from the patella or trochlear groove. Scaffold contact, fixation integrity under load, and rehabilitation demands all vary by site — a detail that affects which technique is likely to seat and integrate reliably.

Patient factors

• Prior surgery. A history of microfracture is a meaningful red flag. Previous marrow stimulation increases ACI failure rates and alters the local joint environment in ways that complicate any subsequent cell-based repair.
• Age and activity goals. Cell-based procedures are generally directed at younger, active patients with focal lesions and realistic expectations. The calculus differs considerably between a 28-year-old returning to sport and a 55-year-old whose imaging also shows early degenerative change elsewhere in the joint.

Centre capability and UK availability

The branded MACI product (Vericel) is not routinely commissioned in the UK. Patients assessed for scaffold-based chondrocyte repair here are more often offered analogous approaches — including STACi (Single Treatment Autologous Chondrocyte Implantation), which processes cells within the operating theatre and completes the entire procedure in one anaesthetic rather than two. For many patients, removing the inter-stage wait and the second general anaesthetic is a clinically meaningful difference, not merely a logistical one.

Getting assessed without a referral

The practical conclusion from five sections of evidence is this: ACI and MACI are biologically equivalent routes to the same destination, and the choice between them — or towards STACi — is driven by what a treating centre can offer, what the defect demands, and what a second anaesthetic means for an individual patient. No headline number separates them decisively; context does.

For patients outside London, reaching that conversation does not require a GP referral or an NHS waiting list. MSK Doctors' consultant-led clinics in Sleaford and Grantham (Lincolnshire) assess focal cartilage defects directly, combining clinical consultation, MRI review, and biomechanical assessment to determine whether a cell-based approach — ACI, MACI, STACi, or an alternative pathway — is appropriate. London-based patients can access equivalent assessment through the London Cartilage Clinic. You can book online without a referral at mskdoctors.com.

1. [1] Minimum 10-Year Outcomes of Matrix-Induced Autologous Chondrocyte Implantation in the Knee. (2024). https://doi.org/10.1177/03635465231205309 https://doi.org/10.1177/03635465231205309
2. [2] Comparison of Three Different Techniques for the Treatment of Cartilage Lesions — MACI vs AMIC vs Arthroscopic Minced Cartilage (2-Year Follow-Up). (2025). https://doi.org/10.3390/jcm14072194 https://doi.org/10.3390/jcm14072194
3. [3] Cartilage Defect Treatment Using High-Density Autologous Chondrocyte Implantation (HD-ACI). (2023). https://doi.org/10.3390/bioengineering10091083 https://doi.org/10.3390/bioengineering10091083