Which cartilage defects actually need ACI or MACI

Not every knee cartilage injury sits in the territory where ACI or MACI makes sense. Both procedures are designed for a specific type of damage: a focal, full-thickness lesion — meaning cartilage worn through more than halfway, sometimes all the way to the underlying bone. In clinical grading terms, that translates to Outerbridge Grade III or IV. Diffuse or multi-compartmental osteoarthritis, where cartilage has eroded across broad areas of the joint, is outside the scope of either technique.

Defect size is the other organising principle. Lesions under approximately 2 cm² — roughly the area of a small coin — are typically handled with shorter, single-stage procedures such as microfracture or mosaicplasty (osteochondral autograft), which carry their own solid short-term track record and avoid the two-stage cell-culture pathway that ACI and MACI require. At the other extreme, defects larger than around 10 cm² often exceed what the collagen membrane can practically serve; fresh osteochondral allograft transplantation becomes the more realistic option at that scale.

The 2–10 cm² window is where ACI and MACI sit. Within it, partial-thickness damage is still not an indication for either — MRI assessment of lesion depth is an essential planning step before any surgical route is confirmed. A consultant review that maps defect size, depth, and location is the starting point for determining whether this pathway is appropriate.

How defect size shapes the decision

Within the 2–10 cm² range, size continues to carry clinical weight — the question is where along that span each technique delivers its clearest advantage.

The strongest benchmark is the SUMMIT trial, which examined lesions of 3 cm² or larger. At that threshold, MACI produced meaningfully better KOOS pain and function scores than microfracture at both two and five years — the longest follow-up data currently available for this comparison. One clarification matters: the SUMMIT trial compared MACI against microfracture, not against first-generation ACI, so it establishes MACI's advantage over marrow stimulation rather than over earlier cell-based technique directly.

At the lower end of the indication — roughly 2 to 4 cm² — the picture is less settled. A 2025 matched-pair study of 48 patients found that MACI, AMIC (matrix-augmented microfracture), and arthroscopic minced cartilage implantation produced statistically indistinguishable improvements in VAS pain and KOOS scores at two years. For smaller lesions in this zone, single-stage alternatives may therefore be equally effective in the short term, while avoiding the cell-culture lead-time that a two-stage procedure requires. Longer follow-up data are needed before this can be treated as settled guidance.

Towards the upper end of the range, evidence for ACI and MACI becomes thinner, and the precise point at which osteochondral allograft transplantation offers a more dependable outcome has not been clearly defined — a recognised gap in the current literature.

Defect size, though, is never read in isolation. Geometry, containment within a bony rim, and anatomical location — femoral condyle versus the patellofemoral surface — all modify how the options are weighted. A 3 cm² trochlear lesion and a 3 cm² medial condyle lesion of equivalent depth may lead a consultant to different conclusions.

Why depth and bone involvement change the plan

Cartilage depth and defect size are separate axes — a lesion can be large but partial-thickness, or small but full-thickness, and that distinction determines whether ACI or MACI is even on the table.

Both procedures require full-thickness damage. In Outerbridge terms, that means Grade III (cartilage worn away to more than half its depth, but some tissue remains) or Grade IV (cartilage is entirely absent and the underlying bone is directly exposed). Grade I and II lesions — surface fibrillation and partial erosion — are not indications for either procedure; other management routes apply at those grades, and entering a two-stage cell-culture pathway for partial-thickness damage would not be appropriate.

Subchondral bone involvement adds a further layer of complexity. When a defect has caused cavitary bone loss of more than approximately 8 mm in depth — meaning not just cartilage but a meaningful volume of the underlying bone structure has been lost — cartilage implantation alone is insufficient. Staged bone grafting, or a combined bone-and-cartilage reconstruction, is required either before or alongside MACI in those cases. This is a planning consideration rather than a barrier, but it adds procedural steps that a purely chondral defect does not involve and affects the overall timeline.

MRI is the primary tool for quantifying depth, bone involvement, and whether the defect has a containing bony rim before surgical planning is finalised. Consultant-led interpretation of that imaging — rather than size estimates from clinical examination alone — is the necessary step before any indication for ACI or MACI can be confirmed.

How MACI evolved from ACI — and why it matters

Three generations of the same biological principle sit behind MACI's current form — understanding the progression makes it easier to see what changed and what stayed the same.

First-generation ACI, introduced in the late 1980s and developed through the 1990s, delivered chondrocytes as a cell suspension injected beneath a periosteal patch harvested from the patient's tibia and sutured tightly over the defect. The biology worked: long-term follow-up series confirmed durable cartilage repair in appropriately selected patients. The limiting problem was the periosteal cover itself. Over time it tended to overgrow — periosteal hypertrophy — often requiring a further surgical procedure to trim or remove the excess tissue. That single complication, rather than any failure of the cell-based concept, drove the move to a second generation.

Second-generation ACI (sometimes called CACI) replaced the periosteal patch with a biodegradable collagen membrane, still sutured in place. This step eliminated hypertrophy. The membrane was a genuine improvement, but suturing still created microtrauma at the repair margins and carried a small risk of chondrocyte leakage at stitch sites.

MACI resolved both issues by pre-seeding chondrocytes uniformly onto a resorbable type I/III porcine collagen membrane before it reaches the operating theatre. During implantation, the membrane is custom-cut to the precise shape of the defect and secured with fibrin glue rather than sutures — a faster, less traumatic step that also extends the technique to uncontained defects and patellofemoral sites, including the medial femoral condyle, lateral femoral condyle, trochlea, and patella, which earlier suture-dependent approaches handled less reliably.

One feature has not changed across all three generations: the two-stage structure. An arthroscopic biopsy harvests the cartilage cells first; a laboratory culture period then follows before re-implantation can take place. That lead-time applies to MACI just as it did to original ACI — a practical planning consideration for any patient weighing this route.

What the clinical evidence actually shows

The clearest direct comparison comes from a 2005 randomised controlled trial published in The Bone & Joint Journal, enrolling 91 patients — 44 treated with second-generation ACI using a collagen membrane (ACI-C) and 47 with MACI. At one year, the mean modified Cincinnati knee score had improved by 17.6 points in the ACI-C group and 19.6 in the MACI group — a difference that did not reach statistical significance (p=0.32). Re-operation rates were identical at 9% in both arms; hypertrophy occurred in 9% of ACI-C patients versus 6% with MACI.

The histological picture at one year was counterintuitive. Hyaline or hyaline-mixed cartilage on biopsy appeared in 43.9% of ACI-C grafts compared with 36.4% of MACI grafts; good-to-excellent arthroscopic ICRS scores were recorded in 79.2% and 66.6% respectively. Neither gap was statistically significant, but numerically the earlier-generation technique produced slightly better tissue quality at this timepoint. MACI's advantage over ACI-C is therefore primarily procedural — faster implantation, no suture-related microtrauma, broader anatomical reach — rather than demonstrably superior repair tissue at one year.

Long-term head-to-head data comparing ACI directly with MACI beyond one year remain limited; this is a genuine evidence gap rather than a settled question. MACI's stronger position relative to microfracture at two and five years — established by the SUMMIT trial and discussed in the size section above — does not tell us how the two cell-based approaches compare over a decade.

For patellofemoral lesions specifically, a study of 95 patients found KOOS pain scores significantly better with ACI or MACI than with osteochondral allograft. Within the cell-based group, however, larger defect area independently predicted lower satisfaction and worse outcomes — a reminder that size remains a prognostic factor even within the technique's indicated range.

Age introduces a further boundary: safety and effectiveness data for MACI are not established in patients over 55, and the formal candidate profile covers adults aged 16–55. Older active patients with otherwise suitable focal defects sit in an evidence gap the current literature does not adequately address. Evolving approaches — including High-Density ACI, which implants 5 million chondrocytes per cm² to address MACI's softer regenerated tissue, and single-stage variants such as STACI — aim to extend the concept, but both remain at an early stage of clinical evaluation rather than established alternatives in routine practice.

How the choice is made in practice

Selecting between ACI, MACI, or an alternative pathway is not a protocol to apply but a clinical picture to read. MRI establishes the essentials — defect size, depth, subchondral bone status, and containment — and each variable maps onto the framework covered in earlier sections. Bone loss exceeding 8 mm and an uncontained margin change the surgical plan materially; they are not afterthoughts that emerge only in the operating theatre.

A factor easy to overlook is knee alignment. Varus or valgus malalignment in the same compartment loads any cartilage repair abnormally, compressing its lifespan regardless of which technique is used. Corrective osteotomy — an HTO for varus, DFO for valgus — is therefore often addressed alongside the cartilage procedure rather than deferred to a later date. Objective biomechanical assessment, including load-pattern analysis using MAI Motion® markerless motion capture, helps quantify that risk before surgical planning is finalised. MSK Doctors consultants assess suitability for this pathway without a GP referral; appointments can be booked directly at mskdoctors.com.

For most patients with a full-thickness defect between 2 and 10 cm², MACI is currently the default cell-based option: procedurally simpler than earlier ACI generations, with the SUMMIT trial providing clear support at ≥3 cm². Smaller lesions in the 2–4 cm² zone may be equally well served by single-stage alternatives such as AMIC, and defects exceeding 10 cm² generally point toward osteochondral allograft. Age, prior procedures, and the mechanical environment of the knee modify every one of those defaults — which is why the decision begins with a precise imaging-led assessment, not with a technique name.

1. [1] Comparison of MACI vs AMIC and Arthroscopic Minced Cartilage — 2-Year Follow-Up. (2025). https://doi.org/10.3390/jcm14072194 https://doi.org/10.3390/jcm14072194
2. [2] Cartilage Defect Treatment Using High-Density Autologous Chondrocyte Implantation (HD-ACI). (2023). https://doi.org/10.3390/bioengineering10091083 https://doi.org/10.3390/bioengineering10091083
3. [3] Comparison of OCA vs ACI/MACI for Patellofemoral Articular Cartilage Lesions. (2024). https://doi.org/10.1177/2325967124s00050 https://doi.org/10.1177/2325967124s00050