NEW YORK (Reuters) -- More than one out of every six patients who have an implant of their own tissue to treat a knee cartilage defect will require repeat surgery, according to a new report on 309 patients treated with the procedure.

The surgery, known as autologous chondrocyte implantation (ACI), involves an initial surgery to remove cartilage cells from the knee, which are grown in the lab and then, in a second procedure, are used to fill in the gap in the patient's cartilage.
While the surgery is now a ""well-established therapy,"" Dr. Philipp Niemeyer of Freiburg University Hospital in Freiburg im Breisgau, Germany, and colleagues point out, there is little information on complications that occur after the surgery or how such complications should be treated.
To investigate, the researchers looked at 309 patients who had collectively undergone 349 ACI surgeries at their hospital between 2001 and 2006. Three different ACI techniques were used: the periosteal patch (14.9 percent of surgeries), in which the membrane covering the bone of the adjacent tibia is used to cover the cartilage graft; the Chondro-Gide patch, a membrane derived from pig cells (61.6 percent); or the matrix-associated method, in which the cells are grown within a 3-dimensional scaffold and then implanted into the bone (23.5 percent).
During an average of 4.5 years of follow-up, 52 of the patients, or 16.8 percent, needed repeat surgery due to continuing problems, such as pain or loss of function.
The complications could be classified into four basic categories: overgrowth, or hypertrophy, of the cartilage graft, in 30.8 percent of cases; failure of the graft to fuse adequately with healthy cartilage (23.1 percent); an insufficient amount of cartilage regeneration (17.3 percent); or a tearing away of the intact cartilage near the repair, also in 17.3 percent of patients.
Hypertrophy was most common in patients treated with the periosteal patch technique, and least likely to occur in patients treated with Chondro-Gide.
Hypertrophy is best treated by using devices called bipolar electrocounters, the researchers say, because using a conventional instrument to shave down the tissue can be harmful. Smaller defects can be treated by using drilling or making tiny fractures in the bone to stimulate growth, or with repeat ACI for larger defects.