What 'qualifying for MACI' actually means

The short answer is: qualifying for MACI depends on two things simultaneously — the nature of the cartilage defect itself, and whether the rest of the knee can support a repair that takes months to mature.

MACI is a two-stage procedure. A small biopsy of healthy cartilage is taken first, the cells are cultured in a laboratory, and then the resulting implant is placed into the defect during a second operation. That complexity is deliberate: MACI is reserved for defect profiles where simpler, single-stage procedures are unlikely to hold. It is not a general-purpose treatment for knee pain or cartilage wear.

Critically, the amount of pain someone is experiencing does not determine candidacy. What matters is what MRI and surgical assessment reveal about the defect's depth, size, and location — and what they reveal about the surrounding joint architecture. A graft placed into an unstable, misaligned, or diffusely diseased knee will not survive regardless of technical precision. Selection criteria exist to protect the repair, not to act as arbitrary gatekeeping.

Cartilage defect criteria: grade, size, depth, and location

Four variables from MRI and arthroscopic assessment determine whether a defect is structurally eligible for MACI: grade, size, depth of bone involvement, and location.

How deep is the damage?

Surgeons grade cartilage damage on a scale known as ICRS (International Cartilage Repair Society) grading. Only the two most severe grades qualify. Grade III means the damage extends more than halfway through the cartilage thickness; Grade IV means it has broken through entirely to the underlying bone. Surface softening, early fissuring, or partial-depth loss — all of which can still cause significant pain — do not reach the threshold that makes MACI appropriate.

Does size determine the choice of procedure?

It largely does. For lesions smaller than roughly 2 cm² — about the diameter of a two-pence coin — single-stage procedures such as mosaicplasty are generally considered equally effective and carry far less procedural burden. MACI's two-stage complexity is justified from roughly 2 cm² upwards, and it becomes the preferred option for defects approaching and exceeding 3 cm².

The clearest clinical evidence for this comes from the SUMMIT trial, which compared MACI directly against microfracture and found that patients with lesions of 3 cm² or larger had meaningfully better pain and function scores at both two and five years. It is worth noting that the SUMMIT trial was industry-sponsored; surgeons weigh that context alongside the published data. For defects in the 2–10 cm² range, MACI has become the most evidence-supported restorative option.

Bone involvement and eligible sites

MACI is not limited to pure cartilage loss. Where the underlying bone is also affected — specifically where the bony cavity measures more than 8 mm in depth — the procedure can still be considered, often with additional bone grafting as part of the same plan.

Approved anatomical locations cover the knee's main weight-bearing and articulating surfaces: the medial and lateral femoral condyle, the trochlea, the patella, and the tibial plateau. Qualifying causes include osteochondritis dissecans, osteonecrosis, post-traumatic injury, and localised cartilage loss — but not diffuse, multi-compartment disease.

Why the rest of the knee must be assessed — and what surgeons correct first

Before a surgeon commits to MACI implantation, three structural conditions in the knee require a clear answer: are the ligaments intact, is the leg's alignment straight, and is there enough functional meniscus?

Each of these governs how forces travel through the joint. An ACL or PCL that is deficient allows abnormal movement patterns — ones that shift load unpredictably across the repair site during the many months a graft needs to consolidate. Where ligament insufficiency is identified, reconstruction is planned either at the same time as MACI or in a separate preparatory stage, so the graft matures in stable conditions.

Alignment raises the same concern. The terms varus (bow-legged) and valgus (knock-kneed) describe angular deformities that concentrate load on one compartment of the knee. When the compartment being repaired is also the one being overloaded, that concentration works directly against integration. In those cases, a high tibial osteotomy (HTO) or distal femoral osteotomy (DFO) — bone-angle corrections that redistribute load across the joint — may be planned alongside, or ahead of, the MACI implantation.

Meniscal insufficiency follows the same logic. The meniscus distributes and absorbs load; significant prior meniscectomy or ongoing meniscal loss removes that protection from the repair compartment. Where the deficit is substantial, a meniscal allograft transplant may form part of the overall treatment plan.

None of these findings are automatic disqualifiers. What they mean, in practice, is that the treatment plan becomes considerably more layered: surgeons are constructing a supportive mechanical environment rather than simply addressing a defect in isolation. Most of the genuine complexity and individual variation in MACI candidacy resides here — in these co-procedure decisions — rather than in the implant choice itself.

Formal contraindications: what the approval label rules out

The FDA approval label sets out a defined list of circumstances in which MACI must not be used. These are absolute contraindications — not matters of surgical judgement, but conditions that make the procedure medically inappropriate regardless of defect characteristics.

• Allergy to gentamicin, aminoglycosides, or porcine or bovine materials. The MACI scaffold is manufactured from porcine (pig) collagen and is processed using gentamicin, an antibiotic. Patients with a known sensitivity to any of these — including dietary or religious objections that constitute a clinical concern — are excluded from the outset.
• Severe osteoarthritis. MACI is designed for focal defects within an otherwise functional joint. Diffuse joint-space loss across one or more compartments reflects a fundamentally different condition, and a cartilage graft cannot restore a joint that has deteriorated throughout.
• Inflammatory or metabolic joint disease. Conditions such as rheumatoid arthritis, psoriatic arthritis, and gout create a persistently hostile biological environment — one in which graft integration is unlikely to succeed. The underlying disease process must be distinguished from the localised defect MACI addresses.
• Uncorrected congenital coagulation disorders. Uncontrolled bleeding risk makes the surgical and healing stages unsafe.
• Prior knee surgery within the six months before planned implantation. The exception is the diagnostic biopsy that forms the first stage of the MACI process itself, or a preparatory procedure specifically to ready the joint for implantation.
• Open epiphyseal growth plates. Skeletal immaturity means the joint is still changing; this is the structural basis for the lower age limit.
• Inability to follow a post-surgical rehabilitation programme. This deserves particular attention. MACI recovery typically spans twelve to eighteen months, with a structured and phased return to activity that is integral to graft survival — not optional physiotherapy. Non-compliance is an independent cause of graft failure, and surgeons treat realistic commitment to rehabilitation as a clinical prerequisite, not an afterthought.

Relative disqualifiers surgeons apply beyond the label

Beyond the formal label, experienced surgeons routinely apply three additional filters that reflect real-world outcome data rather than regulatory rules. These are not grounds for automatic refusal, but each requires an honest conversation before candidacy is confirmed.

Damage on both sides of the joint space ('kissing lesions'). When cartilage loss is present on both opposing surfaces within the same compartment — for example, the femoral condyle and the tibial plateau surface facing it — a graft implanted on one side will immediately contact a damaged partner. Graft survival in this configuration is significantly compromised, and surgeons apply it as a practical disqualifier even though the term does not appear by name in the approval label.

BMI of 35 or above. Centres that track surgical outcomes consistently find poorer functional results above this threshold, and a BMI of 35 is widely applied as a cut-off in pre-operative screening. Crucially, this is a modifiable factor: structured weight management before the procedure can shift the picture and keep the pathway open.

Active smoking. Nicotine impairs tissue healing and graft integration. Most surgeons require confirmed cessation — not merely a stated intention to quit — before MACI is offered.

On age, a note of honesty: the upper limit of 55 reflects the population in which the FDA assessed safety and efficacy, not a confirmed biological boundary. Off-label use in fit, active patients beyond this range does occur, but the evidence base for those cases remains limited, and it is not a route any centre should present as routine.

How candidacy is assessed in practice

Candidacy assessment unfolds across several stages rather than a single consultation. A detailed clinical history, weight-bearing X-rays to establish the alignment picture, and MRI to characterise defect depth and boundaries together form the initial picture. Arthroscopy at the biopsy stage then confirms ICRS grade and exact dimensions directly in theatre; the harvested cells are sent for culture only if the findings support proceeding with MACI.

Functional and biomechanical review runs alongside imaging. Where gait or clinical examination raises concern about coronal malalignment, the decision to correct it concurrently or in advance is part of the same consultant-level discussion — not a separate process that happens later.

Suitability is assessed across the full picture — defect grade, size, joint mechanics, general health, and commitment to a structured rehabilitation programme — rather than against any single threshold. Patients who do not meet the MACI criteria are typically assessed for an appropriate alternative: AMIC for smaller defects broadly in the 1–2 cm² range where marrow-stimulation alone is insufficient; a ChondroFiller injection for suitable focal lesions at the lower end of the size spectrum; or an osteochondral allograft for larger post-traumatic defects where autograft tissue alone cannot cover the area adequately. At MSK Doctors, consultants are available without GP referral, so this assessment can begin promptly. What all these pathways share is the same starting point: mapping the defect's characteristics onto the mechanical and biological conditions in which any repair must mature — the two-part framework this article has described throughout.