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Autologous Chondrocyte Transplantation (ACT) Explained

Autologous chondrocyte transplantation (ACT) is a surgical procedure to repair cartilage damage using the patient's own cartilage cells, which are harvested, multiplied in a laboratory, and then reimplanted into the damaged joint area.

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Things worth knowing about "Autologous Chondrocyte Transplantation"

Autologous chondrocyte transplantation (ACT) is a surgical procedure to repair cartilage damage using the patient's own cartilage cells, which are harvested, multiplied in a laboratory, and then reimplanted into the damaged joint area.

What is Autologous Chondrocyte Transplantation?

Autologous chondrocyte transplantation (ACT), also known as autologous chondrocyte implantation (ACI), is a biological procedure used to restore damaged joint cartilage. The term “autologous” means that the cells used come from the patient's own body. Chondrocytes are the specialized cells responsible for building and maintaining cartilage tissue.

The goal of the procedure is to permanently repair larger, well-defined cartilage defects – particularly in the knee joint – using the patient's own biological material, thereby improving long-term joint function and reducing the risk of developing osteoarthritis.

Indications

ACT is primarily used in the following situations:

Localized, full-thickness cartilage defects of the knee (Grade III–IV according to the ICRS classification)
Traumatic cartilage injuries in younger, physically active patients
Osteochondritis dissecans (a joint condition involving cartilage and underlying bone)
Secondary cartilage defects following prior surgical interventions

The procedure is most suitable for patients between approximately 15 and 55 years of age with a defect size of 2 to 10 cm².

How Does the Procedure Work?

Step 1: Cartilage Biopsy

In a first arthroscopic procedure (keyhole surgery of the knee), the surgeon removes a small sample of healthy cartilage tissue from a low-load-bearing area of the affected joint. This sample contains living chondrocytes.

Step 2: Cell Expansion in the Laboratory

The harvested chondrocytes are sent to a specialized cell culture laboratory, where they are cultivated under sterile conditions for several weeks (typically 3–6 weeks) and expanded to a sufficient number – usually several million cells.

Step 3: Implantation

In a second surgical procedure, the expanded cartilage cells are placed into the defect area of the joint. Several techniques are available:

Classic ACT (1st generation): Cells are injected beneath a periosteal flap (a patch of bone-lining tissue) that covers the defect area.
Matrix-associated ACT (MACT, 2nd/3rd generation): Chondrocytes are seeded onto a three-dimensional scaffold matrix (e.g., made of collagen or hyaluronic acid), which is then placed into the defect. This more modern approach simplifies implantation and improves cell distribution throughout the defect.

Preparation and Aftercare

Prior to surgery, thorough diagnostic imaging (MRI, arthroscopy) is performed to precisely assess the defect and surrounding tissue. Following implantation, an intensive rehabilitation program is crucial for a successful outcome:

Initial weeks: joint offloading, use of crutches
Gradual weight-bearing increase in line with the maturation of the new cartilage
Physiotherapy to restore muscle strength, range of motion, and coordination
Full return to sports typically after 12–18 months

Outcomes and Effectiveness

Clinical studies and long-term follow-up data demonstrate that ACT can lead to lasting pain relief and significant improvement in joint function in suitable patients. Studies report good to excellent outcomes in 70–90 % of treated patients over a follow-up period of 10 years or more. The regenerated tissue resembles hyaline cartilage – the natural articular cartilage – in its structural quality.

Risks and Possible Complications

As with any surgical procedure, ACT carries certain risks:

Infection or wound healing problems
Graft failure
Periosteal hypertrophy (overgrowth of the bone-lining patch) in the classic technique
Joint stiffness or scarring (arthrofibrosis)
Re-injury of the cartilage due to premature physical loading

References

Brittberg M et al. – Treatment of deep cartilage defects in the knee with autologous chondrocyte transplantation. New England Journal of Medicine, 1994; 331(14): 889–895.
Peterson L et al. – Autologous chondrocyte implantation: a long-term follow-up. American Journal of Sports Medicine, 2010; 38(6): 1117–1124.
Moseley JB et al. – Long-term outcomes after knee cartilage repair: a systematic review. Cartilage, 2021; 12(3): 268–281.

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