Articular cartilage defects in the knee are one of the more problematic injuries seen in sports medicine. Treatment options have evolved over time to attempt to recreate the cartilage surfaces with innovative techniques such as autologous chondrocyte implantation.

 

Articular cartilage is a highly specialized structure composed of hyaline cartilage that covers the bones in our joints. Chondrocytes make up the cellular component of articular cartilage. Although they are metabolically active and are responsible for maintenance of the extracellular matrix, they possess very little ability for tissue repair after injury. The functions of articular cartilage include shock absorption, low friction motion and joint nutrition.

 

Athletic injuries that can damage articular cartilage include harsh pivoting and twisting maneuvers and landing from jumps. These are some of the same injury mechanisms that can cause ACL tears and meniscus tears. Articular cartilage injuries manifest themselves with pain, swelling and difficulties with sports. 

 

Articular cartilage defect treatment methods have included simple debridement and marrow stimulation procedures. The latter techniques attempt to stimulate the ingrowth of fibrocartilage by the pluripotent mesenchymal cells in the bone marrow. Unfortunately, this fibrocartilage does not possess the biomechanical or biologic properties of hyaline cartilage and it lacks durability.

 

Newer methods to treat articular cartilage defects in the knee include replacement of cartilage with plugs of bone and cartilage from other parts of the knee (osteochondral autografts), plugs of bone and cartilage from cadavers (allografts) and implantation of the patient's own cartilage cells (autologous chondrocyte implantation). Autologous chondrocyte implantation offers the ability to use autogenous tissue while not robbing other areas of the knee of significant areas of valuable articular cartilage.

 

Autologous chondrocyte implantation (ACI) is performed as a two stage procedure. The first stage involves an outpatient knee arthroscopy. During this minimally invasive procedure, a very small piece of articular cartilage is harvested from a non-weightbearing portion of the knee. This biopsy is sent to the Genzyme laboratory in Massachusetts for culture. Twelve million of a patient's own chondrocytes are produced in a liquid form. They are available for implantation in four weeks.

 

The second stage for ACI can often be performed as an outpatient procedure. This procedure is performed with a formal open incision. The defect is exposed and prepared by removing any loose cartilage tissue and assuring normal cartilage borders. A precisely shaped patch is meticulously sutured to the edges of the defect with a very fine absorbable suture (6-0 vicryl). The superior edge of the patch is not sutured to allow for cell implantation. This patch can be comprised of a piece of periosteum from the proximal tibia or a resorbable collagen membrane.

 

Once the patch is sutured, the junction of the patch and the surrounding normal articular cartilage is reinforced with fibrin glue to create a water tight seal. This seal is confirmed by injecting saline under the patch from the small opening superiorly. Once the water tight seal is confirmed, the saline is aspirated and the chondrocytes are prepared. With a catheter, the patient's chondrocytes in their liquid form are carefully injected under the patch. The small opening at the superior portion of the patch is then sutured and sealed with the fibrin glue.

 

Postoperatively, the patient's knee is placed into a knee immobilizer in extension for twelve hours. This allows for the chondrocytes to become adherent to the bone at the base of the defect. On postoperative day one, range of motion is encouraged with a continuous passive motion machine. Weight bearing is delayed until four to six weeks in most situations. Physical therapy aims to regain knee motion and rebuild muscle strength. During this time, the chondrocytes proliferate and create an extracellular matrix. This tissue has been shown to resemble hyaline-like cartilage. Running is delayed until one year postoperative since it takes such time for the articular cartilage to harden. Return to pivoting and cutting sports is not recommended until 12-18 months postoperative.

 

The results of ACI in the literature have been favorable. Good results have been reported in the range of 85-93%. Athletes have been able to return to sports. Future directions involve an arthroscopic approach and methods to enhance hyaline cartilage formation.