The right candidate for MACI

Not every knee cartilage problem points to MACI. The procedure is designed for a specific profile, and understanding where that profile begins — and ends — is the most useful starting point for anyone considering it.

The core indication, as set out in the FDA product label, is a symptomatic, full-thickness cartilage defect of the knee in an adult. Clinically, that means ICRS Grade III or Grade IV damage: lesions that extend more than half the depth of the cartilage layer, reaching down to or through the subchondral bone. Superficial damage or partial-thickness wear does not meet the threshold.

Defect size is one of the clearest selection variables. MACI's strongest evidence base sits at defects of 3 cm² or larger — the SUMMIT randomised trial demonstrated superior KOOS pain and function scores against microfracture at both two and five years for defects in this range. For smaller defects, broadly those under 2–4 cm², alternatives such as microfracture, osteochondral autograft transfer (mosaicplasty), or AMIC may be more appropriate first-line choices.

Age is described in published guidelines qualitatively rather than as a hard cut-off: young adults — typically in the 18–55 range — with a single isolated lesion are considered the most suitable candidates. Older patients, or those with multiple lesions across one joint, are less likely to benefit.

Two structural prerequisites must also be in place: stable knee ligaments and correctly aligned mechanics. Untreated ligament instability or uncorrected malalignment places abnormal load on any implant and compromises the outcome; these are addressed before or alongside implantation rather than overlooked.

Generalised osteoarthritis is a clear boundary. Diffuse, multi-compartment cartilage loss moves a patient outside the scope of cartilage repair and towards joint preservation or replacement. Active inflammatory arthritis and a history of joint infection are further contraindications implied across the clinical literature, though published guidelines do not offer a single exhaustive list.

Why knee cartilage cannot heal itself — and what MACI does differently

Cartilage covering the ends of the knee's bones has no blood supply of its own. Without vessels to carry repair cells to the site of damage, a full-thickness defect simply does not close over time — the body has no reliable mechanism to replace lost hyaline cartilage with tissue of equivalent quality. This is why focal lesions that reach ICRS Grade III or IV almost always require active intervention rather than rest alone.

Because hyaline cartilage contains no mineral, it is invisible on a plain X-ray. Accurately grading a defect — its depth, area, and borders — requires MRI or direct arthroscopic inspection, and this imaging is a prerequisite for establishing candidacy.

MACI sits in the third generation of autologous chondrocyte implantation, a biological repair lineage first described clinically by Brittberg et al. in 1994. Earlier ACI relied on securing cultured chondrocytes beneath a periosteal patch sutured over the defect — technically demanding and associated with a risk of patch hypertrophy. MACI replaces the periosteal patch with a porcine Type I/III collagen membrane onto which the patient's expanded chondrocytes are pre-seeded, simplifying fixation and reducing that hypertrophy risk. The product, manufactured by Vericel Corporation and granted FDA approval in December 2016, is the only cell-based cartilage repair product with US regulatory clearance. European and Australian literature sometimes uses the term MACT for comparable matrix-based techniques — both refer to the same membrane-seeded ACI concept.

The two-stage MACI procedure

MACI unfolds across two separate surgical episodes, with a laboratory stage in between.

Stage 1 — the biopsy

The first procedure is day-case and arthroscopic. Under general or regional anaesthesia, the surgeon harvests a small cartilage sample — typically 200–300 mg — from the superomedial margin of the femoral trochlea, a low-load area that tolerates the harvest well. The specimen is preserved at 4°C and dispatched to the laboratory.

The inter-stage period (3–6 weeks)

Laboratory scientists isolate the chondrocytes, expand them in culture, and seed them evenly onto the collagen membrane scaffold. The resulting implant is patient-specific and will be trimmed at Stage 2 to match the exact defect dimensions.

Stage 2 — implantation

The main repair is an open procedure. The surgeon debrides the defect down to stable, perpendicular walls, then uses a sterile template to cut the cell-seeded scaffold to size. The scaffold is fixed with fibrin glue. For lesions on the medial femoral condyle, a subvastus arthrotomy is preferred to minimise disruption to the quadriceps mechanism. Any concomitant work — osteotomy, meniscal repair, or ligament reconstruction — is typically addressed at this same stage, though the optimal sequencing of combined procedures remains an active area of clinical study.

An emerging alternative: STACi

Single Treatment ACI (STACi) is a next-generation development that brings the laboratory team into the operating theatre. Chondrocytes are isolated intraoperatively and combined with bone marrow-derived mesenchymal stem cells, enabling implantation within a single session rather than across two. STACi is distinct from standard MACI — it is not the same procedure — and is currently available only in specialist centres, with its long-term evidence still developing.

Recovery and rehabilitation after MACI

Recovery from MACI is measured in months, not weeks — a fact worth understanding before surgery rather than after.

For the first six to eight weeks, protected weight-bearing with crutches is standard. Keeping load off the repaired surface during this period allows the collagen scaffold to integrate and the implanted cells to begin producing matrix without being disrupted by mechanical stress.

Rehabilitation is structured in overlapping phases. Gentle range-of-motion work begins early — keeping the joint moving reduces stiffness and supports cartilage nutrition. Progressive strengthening of the quadriceps and surrounding musculature follows as weight-bearing increases. Impact activity comes last, and return to sport or strenuous physical work is typically guided by clinical progress rather than a fixed calendar date, but 12–18 months is the commonly cited range in clinical practice. This is considerably longer than the timeline after microfracture, which reflects a meaningful biological difference: the implanted chondrocytes need time to mature, remodel, and integrate into the surrounding native cartilage — a process that cannot be shortened by effort or physiotherapy alone.

MRI at around 12–24 months is used in many centres to assess scaffold fill and graft integration. Early scans will not show normal-appearing cartilage, so imaging findings should be interpreted alongside clinical progress rather than in isolation.

Physiotherapy engagement throughout the entire recovery period is not optional. Published outcome data for MACI consistently reflect the importance of protocol adherence — the biological implant provides the substrate, but rehabilitation drives the functional result. Specific phasing and milestones should be agreed with the treating surgeon and physiotherapist, as protocols vary between centres.

What the evidence shows about MACI outcomes

The strongest clinical evidence for MACI comes from the SUMMIT randomised controlled trial, which compared MACI directly with microfracture in patients with defects of 3 cm² or larger. MACI produced statistically significant improvements in KOOS pain and function scores at both two and five years — the clearest head-to-head evidence available for a cell-based cartilage repair procedure.

Longer-term data from ACI and MACI cohort studies add important context. Published five-year follow-up work by Behrens and Ebert demonstrated sustained clinical improvement alongside MRI evidence of structural scaffold fill. Minas et al. (2014) reported outcomes at a minimum of ten years, supporting the view that repair durability in appropriately selected patients can be meaningful rather than short-lived.

The honest framing of all this evidence is that MACI is a joint-preservation strategy, not a cure. It may delay or avoid total knee replacement, but it does not render a patient immune to future deterioration — particularly if the underlying mechanical environment of the knee is not also addressed.

Two areas carry weaker or less consistent evidence. First, outcomes on the patellofemoral surface are generally positive but less uniform than those for femoral condyle lesions, where the evidence base is strongest. Second, patients who have previously undergone microfracture face a more challenging environment for MACI: marrow-stimulation procedures alter the subchondral bone plate, and published data suggest this prior treatment reduces the likelihood of a successful subsequent ACI or MACI implantation.

What remains less settled across guidelines is the precise upper boundary for defect size and how outcomes vary across different age groups within the adult candidacy range.

Where MACI fits in the wider cartilage repair pathway

Cartilage care for the knee follows a recognisable clinical ladder. Symptom management and physiotherapy sit at the base; biologic support — including injection therapies — occupies the next rung; surgical cartilage restoration, of which MACI is one option, comes third; and joint replacement is the final stage, reserved for advanced or irreversible disease. Most patients do not need to reach the top.

MACI sits in the middle of the restoration rung, suited to the mid-range defect (roughly 3–10 cm²) where its evidence base is strongest. Below that band, single-stage options carry different risk profiles and recovery demands: AMIC (matrix-augmented microfracture) or OATS/mosaicplasty suit smaller focal defects in the 1–4 cm² range; for certain very small, suitable lesions, a ChondroFiller injection — an ultrasound-guided outpatient injectable collagen scaffold — may be considered before committing to theatre. Above the MACI range, defects with significant bone involvement or area beyond roughly 10 cm² may be better served by osteochondral allograft (OCA), which replaces both cartilage and the underlying bone stock using donor tissue.

Alignment is not a separate topic. Uncorrected varus or valgus malalignment concentrates load on the repaired surface, and a corrective osteotomy — HTO for varus, DFO for valgus — is sometimes performed alongside or prior to implantation specifically to protect the graft. Addressing mechanics and addressing cartilage are part of the same clinical plan.

Where disease is diffuse and multi-compartmental, cartilage restoration is no longer realistic. That population belongs in a different clinical conversation — one focused on joint preservation or replacement planning rather than repair.

1. [1] Knee cartilage replacement therapy. https://en.wikipedia.org/?curid=4984243 https://en.wikipedia.org/?curid=4984243
2. [2] Autologous chondrocyte implantation. https://en.wikipedia.org/?curid=19074150 https://en.wikipedia.org/?curid=19074150
3. [3] Articular cartilage repair. https://en.wikipedia.org/?curid=19042351 https://en.wikipedia.org/?curid=19042351