What Is Autologous Chondrocyte Implantation (ACI)?

Transplanted Cells Treat Cartilage Damage

Autologous chondrocyte implantation, or ACI, is a procedure that treats isolated areas of cartilage damage in the knee. Surgeons remove cartilage cells, grow them in a lab, and then implant them in the area where cartilage is needed.

This procedure requires two surgeries several weeks apart. It generally shows good long-term results in repairing cartilage and allowing patients to return to their usual activities. But is it right for you?

This article explains the two steps involved in ACI surgery, some of the potential side effects and complications, and the rehabilitation process.

Doctor examining female patient's leg
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ACI Step 1: Arthroscopic Harvest

The first step of ACI is an arthroscopic surgery. This helps identify the area of cartilage damage and determine if it's appropriate for an ACI procedure.

During this surgery, a surgeon removes a small piece of cartilage—"typically the size of one or two Tic-Tacs," explains one orthopedics practice.

This cartilage is then treated to isolate the chondrocytes, which are cartilage-producing cells. These are used to culture (grow more) cartilage cells, which takes about six to eight weeks. Then the second surgery is scheduled.

Step 2: Implantation

Unlike step one, the step-two surgery is not arthroscopic. The surgeon uses a larger incision to directly view the area of cartilage damage.

The surgeon also makes a second incision over the shinbone to harvest a section of the periosteum roughly the size of the area of cartilage damage. The periosteum is the thick tissue that covers the shinbone, and it becomes a patch for the knee.

The surgeon sews the periosteal patch over the area of damaged cartilage, creating a tight seal between the patch and the surrounding cartilage. Then they inject the cultured cartilage cells underneath the patch. The patch holds the new cartilage cells in the area of cartilage damage.

About 90% of ACI operations are done on the knee, but the procedure can also be performed on the ankle.

When ACI Is Used

Damage to cartilage and the bone under it (known as osteochondral lesions) can result in joint pain, limited function, and sometimes osteoarthritis. It can happen from repetitive stress on the joint.

Patients over age 60 with osteoarthritis are more likely to need total knee replacement. ACI is better suited to people ages 15 to 50 (and, in some cases, up to age 55).

ACI is appropriate for patients with small areas of cartilage damage and not the widespread wear of the cartilage characteristic of knee arthritis.

Patients considering ACI should have:

  • A focal area of cartilage damage
  • Pain or swelling that limits their activity
  • A stable knee with no associated ligament damage
  • Weight appropriate for height
  • Already tried nonsurgical treatments

ACI is a significant procedure, and the recovery is lengthy. So patients must be prepared to participate in intensive physical therapy. This step is critical, because without proper rehab, the results are likely to be less than ideal.

Overall, patients who experience the best outcomes from ACI are young and have not previously had knee surgery.

ACI can be a good option for athletes, as long as they can avoid intensive training (other than rehab) and competition for nine months to a year.

Side Effects and Complications

Pain, swelling, and bruising are normal after ACI surgery. Applying ice can relieve the first two issues. Elevating the affected leg on pillows should help, too. Place the pillows under your foot or ankle and not under your knee.

Notify your healthcare provider if you develop:

  • A fever above 100 degrees
  • Excessive bleeding on the bandage
  • Significant pain or swelling in your calf

Infection is a general risk of surgery. Tell your provider if your knee or calf feels swollen, tender, or warm or changes color. Smoking increases the risk of infection. And it can also limit the success of cartilage repair.

Serious side effects and complications are less likely when your medical team—from the surgeon to the physical therapist and especially you—pledge to communicate and work together,


The rehabilitation from ACI is extensive. It focuses on restoring range of motion ad weight-bearing ability to the knee.

  • Weight bearing: Patients will need to stay off the knee for the first 8 weeks or so after the surgery so that the new cells can mature. When the graft (new cartilage) is on the tibia (top of the shin bone) or the femur (end of the thigh bone), a patient can use crutches. When it is on the kneecap (patella) or within the groove for the patella (the trochlea), the joint may be immobilized in a brace because motion will cause compression of the implanted cells.
  • Range of motion: Range of motion rehab is usually initiated early on after surgery, often using a continuous passive motion machine. Movement helps to stimulate healthy cartilage growth. But since motion causes pressure, patients must strictly adhere to their healthcare provider's recommendations.

Risk in Doing Too Much, Too Soon

It feels good to feel good after surgery. But there is a risk in feeling so good that you push yourself to the next activity level before you're ready. Remember that regardless of how you may feel, healing from this surgery takes time. Too much activity too soon can damage the graft or cause it to fail.

Weight-bearing restrictions can last for six to eight weeks. 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 their sport until about 16 months after surgery. And the knee joint won't be fully restored until about 18 months after surgery.

Knee Injury and Osteoarthritis Outcome Score (KOOS)

The Knee Injury and Osteoarthritis Outcome Score (KOOS) is a self-reported measure. Patients can use it to share their symptoms and how they are feeling about their knee's health and how it is functioning.

The 42-item questionnaire is divided into five categories: symptoms, pain, ADLs (activities of daily living), sports/recreation, and knee-related quality of life. The minimum score is 0, which indicates severe knee problems. The maximum score is 100, which indicates no knee problems.

Patient Feedback

Researchers have interviewed many patients over the years about their experiences with ACI. The research has revealed mostly positive responses.

  • One study showed that 92% of patients said they would have the procedure again while 84% rated their knee as improved, nearly eight years after the procedure.
  • A study of 73 patients rated their ACI procedure on the KOOS scale at 86.2%. KOOS evaluates both short-term and long-term consequences of knee injury.
  • Nearly four years after their ACI procedure, 26 of 30 patients in another study said they were "highly satisfied" with their surgical result and would undergo the procedure again. They experienced a significant improvement in their pain level, knee function, and quality of life.

You might find the most telling statistic in the professional history of your surgeon. Ask about their track record and request referrals to former patients to help you decide whether to proceed with ACI surgery.


Autologous chondrocyte implantation (ACI) is a technique that replaces cartilage in a damaged or diseased joint by implanting healthy cartilage cells in its place. Pain, swelling, and bruising are typical side effects of the procedure. Serious complications are rare. But the rehab process is intense and can last up to a year.

ACI is a relatively new procedure (introduced in the 1980s), but it has achieved a very good reputation. Patients say the repair holds up well and enables them to return to their usual activities.

A Word From Verywell

If you don't think of yourself as your own best health advocate, preparing for surgery is a fine time to start. While research on other patients can be reassuring, it doesn't reveal how you will respond to ACI surgery.

Ask your surgeon: What outcome do you expect for me? What complications can I expect? What can I do to increase the chance of a successful surgery? How much pain will I feel after surgery, and what will be done about it? How much physical therapy will I need? What will happen if I decide to postpone this surgery?

Take notes so you can refer to them as you weigh the pros and cons. Ultimately, you want to make the best decision for you.

9 Sources
Verywell Health uses only high-quality sources, including peer-reviewed studies, to support the facts within our articles. Read our editorial process to learn more about how we fact-check and keep our content accurate, reliable, and trustworthy.
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  2. Washington University Orthopedics. Autologous chondrocyte implantation (ACI).

  3. International Cartilage Regeneration & Joint Preservation Society. Risks & complications in cartilage repair.

  4. Joint Preservation Center. ACI and joint restoration post-operative instructions.

  5. Joint Preservation Center. After the ACI procedure.

  6. American Physical Therapy Association. Knee injury and osteoarthritis outcome score (KOOS).

  7. Gomoll AH, Gillogly SD, Cole BJ, et al. Autologous chondrocyte implantation in the patella: A multicenter experience. Am J Sports Med. 2014;42(5):1074-81. doi:10.1177/0363546514523927.

  8. Niemeyer P, Laute V, Zinser W, et al. Clinical outcome and success rates of ACI for cartilage defects of the patella: A subgroup analysis from a controlled randomized clinical phase II trial (CODIS study). Arch Orthop Trauma Surg. 2020;140(6):717-725. doi:10.1007/s00402-019-03264-x

  9. Siebold R, Suezer F, Schmitt B, Trattnig S, Essig M. Good clinical and MRI outcome after arthroscopic autologous chondrocyte implantation for cartilage repair in the knee. Knee Surg Sports Traumatol Arthrosc. 2018;26(3):831-839. doi:10.1007/s00167-017-4491-0.

By Jonathan Cluett, MD
Jonathan Cluett, MD, is board-certified in orthopedic surgery. He served as assistant team physician to Chivas USA (Major League Soccer) and the United States men's and women's national soccer teams.