Autologous Chondrocyte Implantation (ACI):

How It Works

1. How cartilage works in a healthy knee


Cartilage is a form of extracellular matrix secreted by specialized cells called chondrocytes. Articular cartilage covers the end of the femur, the top of the tibia, and the back of the patella. It serves as a shock absorber and is essentially frictionless, providing a smooth surface for the contact and movement of the bones of the knee joint. Meniscal cartilage is on the tibia and serves mostly as a shock absorber. Autologous Chondrocyte Implantation is used to repair defects to the articular cartilage.

Chondrocytes are one of the few cell types that can survive without being close to a blood supply. However, when the cartilage is damaged, the chondrocytes' distance from a blood supply means that they lack the ability to regenerate themselves, a process that requires an increased amount of nutrients and access through the blood stream to other cells and proteins that stimulate the regeneration.


2. Treatment options for damaged cartilage

There are certain procedures that can be done through arthroscopic surgery to relieve the patient's symptoms. The damaged area is usually debrided, cleared of debris and dead cells, and the surface is smoothed. A number of techniques are also used to create scar tissue, including microfracture, drilling and abrasion arthroplasty. Like cartilage, scar tissue covers the joint surface, but unlike cartilage, scar tissue is not as durable or flexible. While these procedures will alleviate symptoms, it is a temporary repair and patients often experience a return of their symptoms. Patients then have the option of living with the pain, undergoing another arthroscopic procedure similar to the one that recently failed, or, in severe cases, receiving a total knee replacement.

Total knee replacement surgery is most common among people between 65 and 75 years old, although younger and older people are also eligible. For patients with knee injuries that would qualify for the ACI procedure, total knee replacement is often not needed. Also, for patients under the age of 50, total knee replacement is considered a poor treatment option because the artificial joint fails after about 10 to 15 years.


3. Who is eligible for ACI

Patients eligible for treatment with ACI suffer "clinically significant, symptomatic defects of the femoral condyle (medial, lateral or trochlear) caused by acute or repetitive trauma".1 These patients usually have joint pain, swelling, catching or grinding. Patients with cartilage damage due to arthritis are not eligible. Arthritis is an autoimmune disease; thus if the immune system attacked and destroyed parts of the patient's cartilage previously, it will probably attack and destroy newly implanted chondrocytes as well.

ACI is generally applied to patients between the ages of 15 and 55, with little or no additional damage to the knee joint. These are patients who do not have enough knee damage to need a total knee replacement, but who are experiencing considerable pain that may be impairing their quality of life. Clinically appropriate patients are identified through traditional diagnostic methods, such as MRI and x-ray evaluation. Final decisions are made at the discretion of the orthopedic physician with the aid of an arthroscopic examination.


4. ACI procedure


Once a patient is determined to be eligible for this procedure, during an arthroscopic examinatino, his orthopedic surgeon takes a biopsy of the patient's articular cartilage. This sample, between 200 mg and 300 mg, is removed from an area of the knee that is not weight-bearing, usually the femoral condyle. The tissue sample is then sent to Genzyme Tissue Repair, where chondrocytes are separated from their surrounding cartilage and cultured for four to five weeks, generating between 5 and 10 million cells.

The procedure to implant the cells is a surgical procedure in which the patient's joint is exposed by the orthopedic surgeon. The defect area is prepared by removing dead cartilage and smoothing the surrounding living cartilage (below).


A piece of periosteum, the membrane which covers bone, is taken from the patient's tibia and sutured over the prepared defect (below). The cultured chondrocytes are injected by the surgeon under the periosteum, where they will grown and mature over time.


Other repairs to the knee joint, such as joint realignment or ligament repair, may also be performed during the implantation of the new chondrocytes. It will be between ten and twelve weeks before the patient can put his full weight on the knee. Cartilage heals slowly and maturation and tempering do not occur before one year.



1. The Center for Orthopaedics and Sports Medicine. "ACI: General Information."





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