Cartilage Restoration: Microfracture, OATS, ACI and MACI Compared

Articular cartilage, the smooth, glassy tissue that covers the ends of bones inside a joint, has almost no capacity to repair itself. When damaged by trauma or focal injury, the defect does not fill in naturally, leading to pain, swelling and eventual arthritis if left untreated. Cartilage restoration procedures aim to fill these defects with new tissue before the damage spreads.

Who Is a Candidate

Cartilage restoration works best for focal (localized) defects in otherwise healthy joints. The ideal candidate is:

• Under 50 years old (though some procedures work for older patients)
• Has a single cartilage defect of 1 to 6 cm² (not widespread arthritis)
• Has stable knee alignment (malalignment must be corrected with osteotomy)
• Has stable ligaments (ACL deficiency must be addressed first)
• Has intact surrounding cartilage (no widespread "kissing lesions")

The Four Main Procedures

1. Microfracture

The surgeon creates small holes in the bone beneath the cartilage defect using a pointed awl. This allows bone marrow stem cells and growth factors to migrate into the defect and form a clot, which matures into fibrocartilage (a type that is mechanically inferior to native hyaline cartilage). The procedure is simple, low-cost, and performed arthroscopically in about 30 minutes.

Best for: Small defects (under 2 cm²), first-line treatment, lower-demand patients.

Limitations: Fibrocartilage is softer and less durable than hyaline cartilage. Studies show declining outcomes after 2 to 5 years, particularly for defects larger than 2 cm² or in high-demand athletes.

2. OATS (Osteochondral Autograft Transfer System)

Also called mosaicplasty, this procedure transplants one or more small cylindrical plugs of healthy cartilage and bone from a non-weight-bearing area of the knee to fill the defect. The transplanted plugs are true hyaline cartilage, providing the same tissue quality as the original surface.

Best for: Small to medium defects (1 to 4 cm²), particularly on the femoral condyles.

Limitations: Donor-site morbidity (pain or cartilage damage at the harvest location), limited by the number and size of plugs available, and gaps between plugs that fill with fibrocartilage.

3. ACI (Autologous Chondrocyte Implantation)

ACI is a two-stage procedure. First, a small biopsy of healthy cartilage is taken arthroscopically and sent to a laboratory where the cells (chondrocytes) are cultured and multiplied over 3 to 6 weeks. Second, in an open surgical procedure, the cultured cells are implanted into the defect under a membrane (periosteum or collagen patch). The cells then mature in place, producing hyaline-like cartilage over 12 to 18 months.

Best for: Larger defects (2 to 10 cm²), particularly when OATS cannot cover the area.

Limitations: Requires two surgeries, the open second stage has a longer recovery than arthroscopic procedures, potential for graft hypertrophy (overgrowth) requiring reoperation in 5 to 10% of cases.

4. MACI (Matrix-Induced Autologous Chondrocyte Implantation)

MACI is the evolution of ACI. Instead of implanting cells under a patch, the cultured chondrocytes are seeded onto a collagen membrane in the laboratory. The cell-loaded membrane is then trimmed to the exact shape of the defect and glued in place with fibrin adhesive. This eliminates the need for sutures and allows the procedure to be performed through a smaller incision or sometimes arthroscopically.

Best for: Medium to large defects (2 to 6+ cm²), current gold standard for large lesions in young patients.

Limitations: Two-stage procedure, expensive (US list price roughly $35,000 for the implant alone), requires prior authorization from most insurers.

Procedure Comparison

Emerging Approaches

Research continues on several next-generation approaches, though none have yet replaced the four established techniques for clinical use:

• Particulated juvenile cartilage (DeNovo NT): Minced cartilage from juvenile donors, implanted in a single surgery without cell culture. Early results are promising for small and medium defects.
• 3D bioprinting: Experimental scaffolds seeded with a patient's own cells, printed to match defect geometry. Clinical trials are underway but years from routine use.
• Stem cell augmentation: Bone marrow concentrate or adipose-derived stem cells used to enhance microfracture or scaffold-based repairs. Evidence is mixed, with most high-quality trials showing modest improvements over microfracture alone.