How does MACI surgery, the two-operation cell repair, work?

Why does MACI surgery take two operations?

During the first operation, the surgeon looks inside and takes healthy cartilage. The cells grow on a slim sheet in a lab.

Later, the worn spot is cleaned and fitted with that sheet. Its shape is trimmed to match the damaged area.

Who may be offered this operation?

This repair may fit one deep, sore spot rather than wear everywhere. The surgeon checks whether the other knee surfaces still move and bear weight well.

The meniscus is a cushion, while ligaments are bands that steady the knee. Tears, loose ligaments, damaged bone or past operations can change the surgery choice.

What will recovery ask of you?

The new sheet needs protection before the leg takes full weight. Bending, driving, stairs and work may each restart on a different date.

Tell the surgeon about steps at home and the work you do. If months of limits won't fit, say so before the first operation.

Set up rides and help with meals for the restricted days. You'll want those arrangements settled before either operation is booked.

What if soreness doesn't settle without surgery?

For soreness that remains, QC Kinetix offers biologic therapies using your blood without surgery. One choice is concentrated platelet-rich plasma, prepared by separating the drawn blood.

That plasma holds more platelets, the small blood pieces that aid clotting. The provider can discuss goals such as less soreness and easier daily movement.

Sources

1. FDA biologics licence BL 125603 (Vericel) describes MACI verbatim as "an autologous cellularized scaffold product indicated for the repair of symptomatic, single or multiple full-thickness cartilage defects of the knee with or without bone involvement in adults." The licensed indication is a two-stage surgical implantation for full-thickness DEFECTS of the knee in adults. It is not an injection, it is not licensed for osteoarthritis, and it is not licensed for any joint other than the knee.

   U.S. Food and Drug Administration — MACI (autologous cultured chondrocytes on porcine collagen membrane). FDA, Center for Biologics Evaluation and Research, 2024.

2. At two years in the randomised trial of ACI versus microfracture in 80 patients, both groups improved significantly, and the microfracture group's SF-36 physical component score improved significantly MORE than the ACI group's. Biopsies were obtained from 84% of patients and blinded histological evaluation showed no significant difference between the two repair tissues, with no association between histological quality and clinical outcome.

   Knutsen G, et al. — Autologous chondrocyte implantation compared with microfracture in the knee. A randomized trial.. J Bone Joint Surg Am, 2004. DOI: 10.2106/00004623-200403000-00001.

3. At five years the same randomised trial reported nine failures (23%) in each arm and satisfactory results in 77% of patients, with no significant clinical or radiographic difference between ACI and microfracture and again no correlation between histology and outcome. One-third of all patients already had radiographic signs of early osteoarthritis five years after surgery.

   Knutsen G, et al. — A randomized trial comparing autologous chondrocyte implantation with microfracture. Findings at five years.. J Bone Joint Surg Am, 2007. DOI: 10.2106/JBJS.G.00003.

4. In the SUMMIT randomised trial, 144 patients with at least one symptomatic Outerbridge grade III-IV knee defect of 3 cm2 or more (mean lesion 4.8 cm2, mean age 33.8) were randomised to MACI or microfracture. At two years, improvements in KOOS pain (37.0 to 82.5 versus 35.5 to 70.9) and function (14.9 to 60.9 versus 12.6 to 48.7) were significantly greater with MACI.

   Saris D, et al. — Matrix-Applied Characterized Autologous Cultured Chondrocytes Versus Microfracture: Two-Year Follow-up of a Prospective Randomized Trial.. Am J Sports Med, 2014. DOI: 10.1177/0363546514528093.

5. At five years, 128 of the 144 SUMMIT patients remained in follow-up and MACI's advantage over microfracture on the co-primary KOOS pain and function endpoint was maintained and remained statistically significant (P = .022), with activities of daily living also significantly better (P = .007).

   Brittberg M, et al. — Matrix-Applied Characterized Autologous Cultured Chondrocytes Versus Microfracture: Five-Year Follow-up of a Prospective Randomized Trial.. Am J Sports Med, 2018. DOI: 10.1177/0363546518756976.

6. At 14 to 15 years, the Norwegian multicentre randomised trial of 80 patients with a single symptomatic femoral condyle cartilage defect found no significant difference between autologous chondrocyte implantation and microfracture on any clinical scoring system. There were 17 failures in the ACI group versus 13 after microfracture, and more total knee replacements had been needed after ACI (6 versus 3).

   Knutsen G, et al. — A Randomized Multicenter Trial Comparing Autologous Chondrocyte Implantation with Microfracture: Long-Term Follow-up at 14 to 15 Years.. J Bone Joint Surg Am, 2016. DOI: 10.2106/JBJS.15.01208.

7. The ACTIVE randomised trial assigned 390 patients aged 18-55 whose previous cartilage surgery had failed to autologous chondrocyte implantation or alternative surgical management. At five years there was NO evidence of a difference in Lysholm score (2.9 points, 95% CI -1.8 to 7.5; P = .46), about 55% were still experiencing benefit in the ACI arm, time to cessation of benefit was similar, and significantly more participants had a serious adverse event with ACI (P = .02). Previous marrow stimulation had a detrimental effect on ACI outcome.

   Snow M, et al. — A Randomized Trial of Autologous Chondrocyte Implantation Versus Alternative Forms of Surgical Cartilage Management in Patients With a Failed Primary Treatment for Chondral or Osteochondral Defects in the Knee.. Am J Sports Med, 2023. DOI: 10.1177/03635465221141907.

8. At ten years the same 390-patient ACTIVE trial found the ACI arm scored significantly higher on Lysholm (mean difference 7.3 points, 95% CI 2.5 to 12.1), while ten-year treatment failure rates were comparable at 29% versus 25% (HR 1.04). A sensitivity analysis accounting for later operations reduced the advantage to 3.8 points (95% CI -1.8 to 9.5). The five-year and ten-year readouts of the same trial point in different directions, which is why a single follow-up point should never be quoted alone.

   Snow M, et al. — Ten year follow-up of a randomized trial of autologous chondrocyte implantation versus alternative forms of surgical cartilage management in the knee.. Osteoarthritis Cartilage, 2026. DOI: 10.1016/j.joca.2026.03.112.