ACI - Autologous Chondrocyte Implantation

Transplanted Cells Treat Cartilage Damage

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Autologous chondrocyte implantation, or ACI, is a procedure that was developed in the late 1980s to treat areas of cartilage damage in the knee. ACI has also been used rarely in other joints, such as the ankle, but it is most commonly performed in the knee.

Doctor examining female patient's leg
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The idea of an ACI procedure is to take a few cartilage cells from the knee, grow them in the lab, and once millions of cells have been grown they are implanted into the area of cartilage damage.

ACI is a two-step procedure, requiring two surgeries several weeks apart. First cells are harvested, then they are implanted.

Step One: Arthroscopy

The first step of ACI is to perform an arthroscopic surgery to identify the area of cartilage damage, and determine if it is appropriate for an ACI procedure.

During the arthroscopic procedure, cartilage cells are collected. These cells are sent to a cell expansion laboratory where they are multiplied by growing in a culture. Growing enough cells takes about four to six weeks. Once enough cells have been grown, they are sent to the surgeon, and the second surgery is scheduled.

Step Two: Implantation Surgery

Once sufficient cartilage cells have been grown, a second surgery is scheduled. During this surgery, a larger incision is used to directly view the area of cartilage damage (not an arthroscopy). A second incision is made over the shinbone and an area of tissue called periosteum is harvested. The periosteum is the thick tissue that covers the shin bone. A "periosteal patch," roughly the size of the area of cartilage damage, is harvested.

The periosteal patch is then sewn over the area of damaged cartilage. Once a tight seal is created between the patch and the surrounding cartilage, the cultured cartilage cells are injected underneath the patch. The periosteal patch is used to hold the new cartilage cells in the area of cartilage damage.


ACI is a significant procedure. The recovery is lengthy, and patients must be prepared to participate in intensive physical therapy. ACI is only appropriate for patients with small areas of cartilage damage, not the widespread wear of the cartilage characteristic of knee arthritis. Patients considering ACI should fit the following profile:

  • A focal area of cartilage damage, not widespread arthritis
  • Have pain or swelling that limits their activity
  • A stable knee with no associated ligament damage
  • Weight appropriate for height (not obese)

In addition, patients should have tried other nonsurgical treatments before considering this significant procedure. Furthermore, patients must have a firm understanding of the post-operative rehabilitation from ACI surgery. This step is critical to the success of the ACI procedure. Without proper rehab, the results are usually less than ideal.


The success of ACI is quite variable, with different surgeons reporting varying levels of success. The most common complication is due to scar tissue formation around the edge of the periosteal patch, called periosteal hypertrophy. This problem often requires an additional arthroscopic surgery to remove excess scar tissue.

Other complications include failure of the implanted cells to properly integrate, infection of the knee, and knee stiffness.


The rehabilitation from ACI is very similar to the rehabilitation following microfracture and it depends on the size and location of the area of cartilage damage. The underlying principles of rehabilitation from ACI are:

  • Weight-Bearing: Weight must be limited on the area of the cartilage implant. The reason for limiting weight is that the cells must be allowed to adhere to the area that underwent the ACI procedure. How to limit weight on the area of the ACI depends on the location. When the ACI is on the tibia (top of the shin bone) or the femur (end of the thigh bone), weight is limited by having a patient use crutches. When the ACI is on the kneecap (patella) or within the groove for the patella (the trochlea), motion must be limited because this will cause compression on the implanted cells.
  • Range-of-Motion: Range of motion is usually initiated early on after surgery. However, as stated above, if the area of ACI treatment is on the kneecap or within its groove, then motion will be limited for six to eight weeks. The reason for starting motion as early as possible is that the movement helps to stimulate healthy cartilage growth. However, this motion must be balanced with the pressure caused by motion. You must strictly adhere to your doctor's recommendations as they will be specific to your injury and treatment.

Weight bearing is usually limited for at least six to eight weeks, and then gradually progressed over time. After three to six months, training can increase in load and intensity. Sport-specific activities can begin about 12 months after surgery. Most athletes do not return to full sport until about 16 months after surgery.

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  • Jones DG, Peterson L. "Autologous chondrocyte implantation" Instr Course Lect. 2007;56:429-45.