Who these procedures are for

Both ACI and autologous chondrocyte implantation with a matrix carrier (MACI) sit in a specific tier of the cartilage-repair pathway — above marrow-stimulation techniques such as microfracture and single-stage scaffolds, but below large allograft reconstruction or joint replacement. They are designed for a defined problem: a focal, full-thickness chondral defect in a knee that is otherwise structurally sound. Neither is appropriate for diffuse or end-stage osteoarthritis, where the damage is too widespread for a targeted cell-based repair to meaningfully change the joint's trajectory.

Defect size is the main gateway into this tier. For lesions smaller than roughly 2–4 cm², single-stage procedures — microfracture, OATS (osteochondral autograft transfer), or AMIC (matrix-augmented microfracture) — are usually considered first. Once a defect exceeds that threshold, the evidence tilts towards a cell-based approach. The SUMMIT trial, which compared MACI directly against microfracture in patients with cartilage injuries of at least 3 cm², found meaningfully better KOOS pain and function scores in favour of MACI at both two and five years — establishing the cell-based matrix route as the preferred option for larger focal lesions.

Patients who reach this decision point are typically younger adults — often those who have sustained a sporting or traumatic injury — whose lesion size and depth have been confirmed on MRI or during arthroscopy. At this stage, both ACI and MACI share an identical first step: a small cartilage biopsy is taken from a non-load-bearing area of the knee, the harvested chondrocytes are expanded in a laboratory over several weeks, and a second surgery then delivers the cultured cells back into the defect. Where the two procedures differ is in how that delivery is done — and that difference has real consequences for surgical complexity, complication risk, and recovery.

The one stage where ACI and MACI part ways

At Stage 2, everything changes. The cultured cells return to the knee by entirely different routes — and that difference is the reason one technique is considered more technically demanding than the other.

In first- and second-generation ACI, the surgeon injects the cells as a liquid suspension directly into the prepared defect, then seals the cavity with a flap — either periosteum (a thin layer of connective tissue harvested from near the kneecap) or a synthetic membrane — that is secured around the edges of the defect with sutures. The stitching must be meticulous: any gap in the suture line risks the liquid suspension leaking out or pooling unevenly before the cells have had time to integrate with the surrounding cartilage.

MACI takes a different approach before the patient even reaches the operating table. The expanded chondrocytes are seeded onto a porcine Type I/III collagen membrane — a thin, flexible scaffold — and allowed to adhere to it. During Stage 2, the surgeon trims this pre-loaded membrane to the exact shape of the defect and fixes it in place using fibrin glue, a surgical adhesive derived from clotting proteins. No sutures are required. The glue bonds the scaffold to the defect bed within minutes, reducing soft-tissue dissection around the joint and shortening the implantation phase compared with suture-based fixation.

The practical distinction is architectural: MACI's scaffold gives the cells an immediate three-dimensional structure to inhabit from the moment of implantation, whereas the liquid-suspension approach depends entirely on the integrity of the sutured cover to hold cells in position during the critical early weeks of healing.

Complication profiles: where the procedures differ most

Graft hypertrophy is the primary complication of classical, periosteal-flap ACI — and the one most clearly resolved by MACI's design. It occurs when the periosteal patch used to cover the repair overgrows the native cartilage surface, raising a ridge of tissue that can cause the joint to catch, lock, or fill with fluid. In symptomatic cases, a second arthroscopic procedure to trim the excess tissue is needed.

MACI does not carry this risk. Because no periosteal tissue is involved — the repair sits within a porcine collagen scaffold fixed to the defect bed with fibrin glue — there is no biological substrate for overgrowth. Delamination, where the scaffold separates from the underlying bone, is the corresponding concern specific to MACI, but published data indicate its rates are lower than the patch-failure rates associated with periosteal-flap ACI, making its overall complication profile meaningfully better.

This distinction carries a caveat. Modern ACI at high-volume centres now predominantly uses synthetic collagen membrane covers rather than periosteum, which substantially reduces hypertrophy risk and narrows the complication gap between the two approaches. Classical periosteal-flap ACI is today largely a historical reference point rather than routine practice in specialist units.

The practical implication is that complication comparisons depend heavily on which generation of ACI is being described — the periosteal-flap era and the synthetic-flap era produce meaningfully different profiles, and that context matters when interpreting any published data on surgical risk.

What the clinical evidence actually shows

Published evidence offers a clear finding on outcomes, even if it stops short of declaring a winner. A 2024 retrospective comparison by Manjunath found that both ACI and MACI produced good short-term clinical results — with meaningful improvements in pain and activity levels over pre-operative baseline in both groups — and no clear advantage for either technique on patient-reported scores. Histological and arthroscopic data across published studies show broadly similar hyaline-like repair tissue quality between the two generations, suggesting the cellular biology of the repair is roughly comparable once the graft is established.

The strongest randomised trial in this space is the SUMMIT trial, which demonstrated that patients with cartilage injuries of 3 cm² or more had improved KOOS pain and function scores at both two and five years with MACI compared with microfracture. That finding cements the case for cell-based matrix repair over marrow stimulation in larger defects. It does not, however, compare MACI directly with ACI — a distinction worth stating plainly, since the SUMMIT data are sometimes cited in a way that implies it does.

Two evidence gaps are worth acknowledging. Long-term head-to-head RCT data comparing modern synthetic-membrane ACI with MACI are limited; most contemporary comparisons are retrospective and insufficiently powered to detect moderate differences in outcomes. Return-to-sport data specifically contrasting the two approaches are also thin — a meaningful gap for younger and more active patients. These are known limitations of the current literature, not reasons to doubt either procedure's effectiveness, and consultant assessment remains the appropriate route for weighing individual suitability.

Rehabilitation: why MACI recovery is shorter and more predictable

The difference in recovery time between traditional ACI and MACI traces directly back to how each repair is fixed — not to arbitrary protocol policy. With a sutured periosteal flap, early mechanical stress on the suture line carries real risk of disruption, so first-generation ACI required prolonged protected weight-bearing and a structured rehabilitation programme lasting up to 18 months. MACI's fibrin-glue fixation creates a mechanically stable construct from the moment surgery ends, which is what makes a compressed and more standardised timeline achievable.

A US expert panel using a Delphi consensus process, reaching greater than 75% agreement (Flanigan 2020, PMC8808808), established the framework most centres now follow: range of motion to 90° by week 4, and full weight-bearing at 7–9 weeks for tibiofemoral lesions. The most clinically important exception involves patellofemoral lesions — where the repair sits on the underside of the kneecap — for which immediate post-operative weight-bearing in a locked brace is recommended rather than the delayed approach used for femoral condyle defects. Lesion location is the single largest source of variation between individual protocols, and patients with kneecap-surface repairs should expect a distinctly different early weight-bearing schedule.

Broadly, MACI recovery falls into three arcs. The first 12 weeks centre on graft protection: crutch-assisted mobility, isometric exercises, and controlled range-of-motion work. Months three to six shift toward strength rebuilding — stationary cycling and progressive loading form the core, with most patients returning to desk work within two weeks and walking for exercise by around three to four months. From approximately six months onwards, jogging is typically introduced at seven to nine months, while cutting, pivoting, and recreational sport approach the one-year mark. Individual protocols vary according to surgeon preference, lesion characteristics, and any concurrent procedures carried out at the time of implantation.

Getting specialist assessment and next steps

Deciding between ACI and MACI is not something a patient can or should resolve alone — the starting point is confirming defect grade, size, and location through MRI, and sometimes a diagnostic arthroscopy. Without that structural picture, neither procedure can be properly indicated.

At assessment, a knee cartilage specialist will typically examine several factors beyond the defect itself: lower-limb alignment (a varus or valgus knee loads the repair site unevenly and may need correcting first), whether any prior marrow-stimulation procedure has been done (which can affect graft integration), and whether concomitant meniscal or ligament pathology needs addressing at the same time. Arriving at consultation aware of these variables means the conversation can move quickly to what matters for your specific situation.

One practical question worth raising directly is laboratory access. Both procedures require a licensed cell-processing facility, and not every centre that performs cartilage surgery has contracts with a laboratory accredited for both techniques — so the local availability of ACI versus MACI may narrow the choice before biology does.

Search MSK lists knee cartilage specialists across the UK who offer cell-based cartilage repair; filtering by region and specialty is a straightforward way to identify a clinician whose practice covers the procedures discussed here.