MACI (matrix-induced autologous chondrocyte implantation)

MACI stands for matrix-induced autologous chondrocyte implantation. "Autologous" means it uses the patient's own cells. In broad terms, cartilage cells taken from the patient are grown in a laboratory, then later placed onto a collagen membrane and implanted into a focal cartilage defect that has been prepared to receive them.

It is one possible option for selected patients with a focal cartilage defect, and it is not suitable for every lesion. Because MACI is a branded procedure, this page keeps to general education; it is not an endorsement, and the details of any individual plan are decided with your surgeon.

MACI is typically carried out in stages rather than a single operation. An initial arthroscopy may be used to evaluate the lesion and to take a small cartilage biopsy. The cells from that biopsy are grown in a laboratory over a period of time. In a later procedure, the cells — now on a collagen membrane — are implanted into the prepared defect.

Not every biopsy leads to an implantation. What is found during evaluation, how the cells grow, and the overall picture of the knee can all change the plan. Being staged is a normal feature of the procedure, not a complication.

After implantation, the repair tissue has to integrate and mature gradually, so rehabilitation protects it early and then progresses in stages under the direction of your surgeon and physical therapist. The pace depends heavily on where the defect is, how large it is, whether other procedures were performed at the same time, and how the tissue matures over time.

Weight-bearing and motion restrictions can differ substantially depending on the situation — for example, a defect on the surface behind the kneecap (a patellofemoral lesion) is protected differently from one on the weight-bearing surfaces where the thigh bone and shin bone meet (a tibiofemoral lesion). When another procedure — such as a realignment osteotomy — is done at the same time, the plan is adjusted and the more protective restrictions generally govern. Your surgical team's specific instructions always take precedence.

A cell-based repair can be less durable if an important contributing problem is left unaddressed. Depending on the individual knee, a surgeon may consider treating alignment, patellar tracking or instability, ligament instability, or meniscus deficiency — at the same operation or in a staged manner — so the repair is protected. Whether any of that applies is individualized and is not part of every MACI plan.

As with any surgery, there are risks, and this procedure has real limits. Possible problems include infection, stiffness, persistent or returning symptoms, the repair tissue not maturing or integrating as hoped, overgrowth of the repair tissue (graft hypertrophy), and the need for additional surgery. This is not a complete list, and which risks matter most depends on your situation. Importantly, MACI is intended to form durable repair tissue rather than recreate the knee's original cartilage exactly; it does not reverse generalized arthritis, and it does not guarantee that arthritis will be prevented or that a person will return to a particular sport. These trade-offs are discussed individually with your surgeon.