ACI is a two-stage procedure to treat large cartilage defects in the knee using the patient's own laboratory-cultured cartilage cells. It provides biological regeneration of the joint surface.
Stage 1: A small arthroscopic biopsy of healthy cartilage is taken and sent to a laboratory where the cartilage cells (chondrocytes) are cultured and multiplied over four to six weeks. Stage 2: Through a small incision, the cartilage defect is prepared and the cultured cells are implanted, either on a collagen membrane (MACI — matrix-assisted) or under a periosteal patch. The second operation takes one to one and a half hours.
ACI/MACI is recommended for large, full-thickness cartilage defects (typically greater than 2–3 cm²) in the knee in young, active patients. It is the gold standard for large defects where OATS would require too many plugs.
Physiotherapy, microfracture (for smaller defects), OATS (for defects up to 4 cm²), and osteotomy if alignment is abnormal.
MRI and arthroscopy to assess the defect. The first stage (biopsy) is a short arthroscopic procedure. Four to six weeks between stages for cell culture.
Regeneration of hyaline-like cartilage over the defect. Good to excellent results in approximately 75–85% of patients at five to ten years. Best results in patients under 40 with single defects.
Full return to sport takes twelve to eighteen months.
Overgrowth of the repair tissue, causing catching. May need arthroscopic trimming.
Knee swelling during the healing phase.
The cultured cells may not form adequate cartilage.
The repair tissue may detach.
Joint infection.
Loss of knee flexion.
DVT during prolonged restricted weight-bearing.
Stage 1: local or general anaesthesia. Stage 2: general or spinal anaesthesia.
Non-weight-bearing for six to eight weeks. CPM machine. Progressive physiotherapy for twelve to eighteen months. Running at nine to twelve months. Full sport at twelve to eighteen months. The repair tissue continues to mature for up to two years.
Regular reviews at two weeks, six weeks, three months, six months, and one year. MRI at six months and one year.
The cultured cartilage cells need many months to mature into functional joint surface tissue. Premature loading can damage the immature repair tissue.
MACI (matrix-assisted) is the modern evolution of ACI, using a collagen scaffold instead of a periosteal patch. It is technically simpler and has comparable or better results.