Introduction:
Articular cartilage lines the ends of bones where they come together to form joints. This smooth, gliding surface allows nearly frictionless joint motion. However, cartilage has limited healing potential. Focal cartilage lesions caused by trauma or overuse can progress to widespread joint degeneration and osteoarthritis. For young active patients with isolated cartilage damage, biological joint resurfacing with autologous chondrocyte implantation (ACI) provides a regenerative solution.
What Is Autologous Chondrocyte Implantation?
Autologous chondrocyte implantation (ACI) is a biological method for healing cartilage defects in the knee. It is a two-step technique that uses the patient's cultured chondrocytes to resurface areas of damaged articular cartilage.
ACI was first performed in 1987 by drilling holes into patellar cartilage defects and injecting a suspension of cultured chondrocytes. Modern ACI techniques involve debriding the lesion to a stable vertical wall followed by implantation of chondrocytes under a sealed collagen membrane.
How Is Autologous Chondrocyte Implantation Performed?
Autologous Chondrocyte Implantation involves two stages:
First, a biopsy is done to harvest cartilage cells, which are then grown in a lab for three to six weeks. The biopsy is enzymatically digested (cartilage sample is broken down using enzymes) to isolate the chondrocytes (cells responsible for cartilage production).
In the second open surgery, the chondral lesion is exposed and prepared by debriding all unstable cartilage back to healthy, stable edges. The calculated surface area helps determine the required number of cells to implant. A collagen membrane is shaped to the defect and sutured in place. The expanded chondrocytes are then injected under the membrane to fill the cartilage defect.
The suspended cultured chondrocytes are then injected into the defect through a small cannula inserted underneath the membrane. As the defect fills, the cells adhere and begin synthesizing a new cartilage matrix, eventually filling the lesion with hyaline-like repair tissue integrated with the surrounding cartilage.
The implanted chondrocytes cling to the lesion bed and form a new extracellular matrix, eventually filling the defect with hyaline-like healing tissue. Strict surgical rehabilitation recommendations safeguard the transplant as it matures over 12 to18 months. Parallel treatments, such as realignment osteotomies (surgical procedures that involve cutting and repositioning bones to correct alignment issues), may boost the joint environment.
What Is the Mechanism of Cartilage Repair in Autologous Chondrocyte Implantation?
Articular cartilage lacks blood vessels, lymph drainage, and nerves. Chondrocytes encapsulated in the cartilage matrix maintain the tissue by synthesizing new extracellular components like collagen and proteoglycans. However, cartilage injuries do not trigger the typical wound-healing cascade seen in vascularized tissues.
Focal cartilage defects confined to the avascular cartilage layer do not elicit inflammation or progenitor cell recruitment. For this reason, cartilage has very limited intrinsic healing potential, especially in adult joints. The goal of autologous chondrocyte implantation (ACI) is to stimulate hyaline cartilage regeneration by delivering viable chondrocytes directly into a contained focal defect. The implanted chondrocytes are end-differentiated cells harvested from healthy articular cartilage tissue.
These enlarged chondrocytes fill the cartilage lesion after transplantation and start producing new extracellular matrix molecules. The matrix offers a setting for further cell division and development. Over time, the implanted chondrocytes proliferate within the three-dimensional matrix, eventually filling the original defect with integrated hyaline-like repair tissue. The collagen membrane covering the ACI graft provides a stable environment for this regenerative process to occur. Strict postoperative rehabilitation guidelines then allow the new cartilage tissue to remodel and mature along the lines of biomechanical force as graft healing and integration continue over 12 to 18 months.
What Are the Criteria for Patient Selection for Autologous Chondrocyte Implantation?
Careful patient selection is important for the success of ACI.
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The candidate should have a focal thickness of lesions two to 0.0107639 square foot in size with healthy surrounding cartilage.
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Patients should be under 55 years old, not significantly overweight, and committed to lengthy rehabilitation.
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Those with inflammatory arthritis, extensive degeneration, or multiple prior surgeries tend to have worse outcomes.
Patients with low potential for rehabilitation or those with diffuse osteoarthritis should not utilize ACI. In certain situations, alternative joint repair techniques like osteotomy or arthroplasty can be more appropriate. For older individuals with isolated lesions that do not have extensive degeneration, ACI is still a possibility.
What Is the Rehabilitation Protocol for Autologous Chondrocyte Implantation?
Rehabilitation is essential to promote graft maturation following autologous chondrocyte implantation (ACI). Motion exercises begin immediately after surgery to maintain range of motion and prevent adhesions. Continuous passive motion (CPM) machines are often used. However, weight-bearing is restricted initially to protect the repair site. For the first six to eight weeks, the patient uses crutches and limits weight bearing on the operated leg. Once some graft healing has occurred after around 12 weeks, progressive weight bearing begins along with muscle strengthening exercises. The precise schedule is customized for every patient according to how well they are healing.
Light sports activities like swimming or cycling can be resumed about six months after surgery. However, until at least 12 months have passed, sports involving rotating, impact, or shear pressures are prohibited. This provides time for the newly formed cartilage tissue to completely integrate and reshape along joint pressures. Preserving the graft over this prolonged maturation stage is essential to avoiding overload.
What Are the Outcomes After Autologous Chondrocyte Implantation?
Many studies show ACI provides durable long-term cartilage repair. Success rates over 75 percent are reported at 10 years follow-up. Outcomes are best when alignment and stability are optimized with concomitant procedures like osteotomy. Results are also better without prior surgery, like marrow stimulation. Biopsies show that the repair tissue contains type II collagen and proteoglycans, which are characteristics of hyaline cartilage. However, the new cartilage is less organized and thinner than native cartilage. Clinical outcomes correlate with how well the defect is filled and integrated. ACI also shows good results in younger patients, with over 90 percent returning to sports by 12 months after surgery.
Conclusion:
In suitable patients, autologous chondrocyte implantation (ACI) offers a biologically sound treatment for localized cartilage defects. One can induce hyaline-like cartilage regeneration in an articular defect by introducing cultured autologous chondrocytes. However, strict indications, surgical techniques, rehabilitation guidelines, and addressing associated joint abnormalities are critical to success. While further research aims to optimize repair tissue quality, current evidence shows ACI has good long-term outcomes for isolated cartilage damage without significant osteoarthritis. For such focal defects in younger active patients, ACI is a viable joint preservation approach that can prevent disease progression and restore function.
