MACI is a two-surgery procedure that repairs damaged cartilage in the knee using your own cartilage cells grown on a collagen membrane. It’s FDA-approved for full-thickness cartilage defects, meaning areas where the cartilage has worn all the way through to the bone. Unlike simpler techniques that stimulate the body to fill in damage with scar-like tissue, MACI aims to regenerate cartilage that more closely resembles what was originally there.
How the Two-Stage Process Works
The first stage is a short arthroscopic procedure. Your surgeon removes a small sample of healthy cartilage, roughly 200 to 300 milligrams, from a non-weight-bearing area of your knee, typically from the outer edge of the thighbone where the cartilage isn’t under daily stress. This is a minimally invasive surgery done through small incisions, and recovery from this step alone is relatively quick.
That tissue sample gets sent to a laboratory, where your cartilage cells are isolated and multiplied over several weeks. Once enough cells have grown, they’re seeded onto a small sheet made from porcine (pig-derived) collagen. This membrane acts as a scaffold, giving the cells structure so they can be placed precisely into the damaged area. The result is a living patch custom-populated with your own cells.
The second surgery is the actual implantation. This one requires an open incision into the knee joint. Your surgeon measures the cartilage defect, trims the membrane to match its exact size and shape, then places it cell-side down into the damaged area. For most defects, the membrane is sealed in place with a biological adhesive called fibrin glue. Larger defects, particularly those bigger than 10 square centimeters or those not fully surrounded by stable cartilage, may also need dissolvable stitches for extra security.
Who Qualifies for MACI
MACI is designed for adults with symptomatic cartilage defects that go all the way through the cartilage layer, with or without involvement of the underlying bone. It can treat single or multiple defects. The FDA labeling notes that safety and effectiveness haven’t been established in patients over 55, and the procedure is only approved for the knee.
Several factors can disqualify you. Advanced osteoarthritis (beyond early-stage changes on X-ray), inflammatory arthritis, significant knee malalignment of more than 5 degrees, ligament instability, and major meniscus loss all rule out the procedure. These conditions would compromise the environment the new cartilage needs to survive. Research on long-term outcomes also shows that a BMI between 20 and 29 at the time of surgery is associated with the best results, with patients in that range scoring meaningfully higher on knee function measures at eight years.
Insurance approval adds another layer of requirements. Most major insurers require you to have failed two to three months of conservative treatment (physical therapy, anti-inflammatory medications, activity modification) before they’ll cover MACI. Many also require that a previous surgical attempt, such as microfracture, has already failed. Insurers typically require the defect to be at least 1.5 to 2 square centimeters and classified as a Grade III or IV lesion, meaning substantial or complete cartilage loss. The upper size limit is generally 10 square centimeters.
How MACI Compares to Microfracture
Microfracture is a simpler, single-surgery technique where tiny holes are drilled into the bone beneath a cartilage defect, triggering a healing response that fills the area with fibrous tissue. It’s often the first surgical option tried for smaller defects. In a randomized trial published in the Journal of Bone and Joint Surgery, both MACI and microfracture produced significant improvement over preoperative function, with 77% of patients in each group reporting satisfactory results at five years. The failure rate was also similar: 23% in both groups at five years.
Where MACI tends to show its advantage is in larger defects. Microfracture works best for smaller areas, and the fibrous repair tissue it generates is less durable than the hyaline-like cartilage MACI aims to produce. For defects above 2 to 4 square centimeters, MACI is generally considered the stronger option for lasting repair.
Long-Term Graft Survival
A prospective study tracking patients for up to 18 years found that MACI grafts have a 10-year survival rate of 86%. The breakdown over time: 97% at one year, 96% at two years, 92% at five years, and 87% at ten. Out of 141 patients followed for the full study period, 19 experienced graft failure.
Two factors stood out as risk indicators. Women had 2.8 times the rate of graft failure compared to men, though the reasons aren’t fully understood. Patients who had undergone a previous cartilage repair surgery before MACI also showed a trend toward higher failure rates, though this wasn’t statistically confirmed. These numbers mean the large majority of grafts hold up well over a decade, but the procedure isn’t permanent for everyone.
Recovery and Return to Activity
Recovery from MACI is notably longer than from simpler knee procedures, and the rehabilitation timeline is strict because the new cartilage cells need protected time to mature. For the first two weeks after implantation, you’ll be completely non-weight-bearing on the operated leg, using crutches for all movement. From weeks two through four, you’ll transition to partial weight-bearing, typically limited to about 30 to 40 pounds of pressure on that leg.
Full weight-bearing generally comes between 6 and 12 weeks, depending on the location and size of the defect. Defects on the kneecap or the groove where the kneecap tracks tend to require a slower progression than those on the rounded end of the thighbone. Throughout these early months, physical therapy focuses on restoring range of motion and gradually rebuilding quadriceps strength.
The later phases of rehab, from roughly 9 to 12 months, shift toward sport-specific strengthening and conditioning. Low-impact activities like cycling and swimming come back first. Impact sports, meaning anything involving running, jumping, or cutting movements, are typically cleared around 12 to 16 months post-surgery, and only if you’re pain-free. For defects on the kneecap, the timeline skews toward 16 months. This extended recovery is the trade-off for a procedure designed to produce durable, long-lasting cartilage repair rather than a quick fix.