Four Major Complications After ACI Repair of Knee Cartilage

Cartilage defects in the knee joint that go clear to the bone can be treated these days. The technique developed to repair this problem is called autologous chondrocyte implantation (ACI). The procedure is done by taking normal, healthy cartilage cells from the patient. They use these cells to grow more cells in a laboratory setting and then reimplant the new batch of cells in the damaged area of the knee joint. The new cells usually adapt well to the new environment.

Although this treatment method works well, there can be problems or complications after the procedure. In this study, surgeons from Switzerland and Germany follow a group of over 300 patients who had ACI procedures. Three different ACI techniques were used. They watched to see which patients had the most problems after ACI and what those problems were.

The goal was to find the best way to treat these complications. But in the big picture, researchers hope to find ways to prevent problems from occurring after ACI that require revision (a second) surgery. The three methods used were: 1) periosteum-covered ACI, 2) Chondrogide membrane covered ACI, and 3) a three-dimensional matrix-associated ACI.

Two thirds of the patients had the Chondrogide membrane covered type of ACI. One-fourth had the three-dimensional matrix. The rest (14 per cent) had the periosteum covered ACI procedure. This may be the first large study to report on the problems that can occur with ACI. In the past, ACI failures were only discussed in single reports with a small number of cases. By combining the patients of three surgeons together over a period of five years (2001-2006), they were able to study a much larger group and report specifically on complications.

They found four major problems after ACI: 1) hypertrophy, 2) disturbed or inadequate fusion, 3) delamination, and 4) graft failure. Hypertrophy refers to overgrowth of the transplanted tissue. Insufficient fusion describes patients in whom the transplant just didn’t regenerate like it should. The edges between the healthy, normal tissue and the implanted cells didn’t meld together to form a solid, smooth surface. Delamination is the separation of the cartilage layer from the bone underneath. Shearing forces can cause these two layers to slide apart before fusion takes place.

In a few cases, there was osteonecrosis (death of the bone underneath the cartilage transplant). In all cases, patients were diagnosed on the basis of pain and/or loss of function after the surgery. Symptoms occurred anywhere from the first six months up to three years later. Sometimes MRI scans showed abnormal cartilage or subchondral bone signals. Subchondral refers to the first layer of bone under the cartilage. But surgeons could not rely on the MRIs as the final diagnostic tool. Only after performing arthroscopic revision surgery could they make the diagnosis with certainty.

Of the 349 ACI procedures that were done, 52 required revision surgery due to symptomatic complications as described. Information collected and analyzed included the different types of surgical techniques mentioned, location and size of the defects, and age of the patient.

Most of the defects requiring revision surgery were on the medial (inside edge) of the femoral condyle or on the back of the patella (kneecap). The femoral condyles are two round knobs found on the end of the femur (thigh bone). The condyles rest on the top surface of the tibia (lower leg bone) to form the knee joint.

But the location of the defect did not test out to be a significant factor overall. The type of ACI technique was more important. Patients who had the periosteum-covered ACI had the highest rate of complications. Hypertrophy of the graft was the most common problem in all types of grafts, but especially among patients with the periosteum-covered graft.

Patients with Chondrogide-covered ACI and matrix-associated ACI were more likely to end up with a disturbed fusion site, but the risk of this happening was fairly low. Hypertrophy and malfusion were observed most often in patellar defects.

Age and size of defect did not have any influence on the final outcomes of the surgery. There were some trends noticed by age (hypertrophy in younger patients; insufficient regeneration in older patients). Understanding the significance of this finding may take some time.

Having observed the clinical results of these four types of ACI, the next step is to understand the nature of the repair process and why it works better with some (but not all) graft types. After that, surgeons will look for ways to modify current treatment techniques for improved outcomes.

Right now, the treatment approach in revision surgery is to remove the damaged, dead, or insufficient tissue and regraft the defect. This usually results in a good return of function, but there are no studies yet on the clinical outcomes of revision surgery for failed ACI.

The authors conclude that their study was a good start on clarifying the results of ACI. The observations and summary they made about complications after the primary (first) procedure were the first published. Having a large enough group of patients to be statistically significant was a key benefit of this study.