Group one was treated with autologous chondrocyte implantation (ACI). Normal, healthy cartilage cells were taken from the joint and used to grow more in a lab. The new cells were put back into the joint four weeks later.

The second group had microfracture, a method of increasing bone marrow cells to help repair the damage. Surgeons use a blunt awl (a tool for making small holes) to poke a few tiny holes in the bone under the damaged cartilage. The goal in either treatment method is to restore normal knee function by attempting to replace the damaged cartilage with new tissue.

Patients were followed for two years. Doctors took a small piece of tissue from the repaired site. The amount of actual tissue repair seen during the biopsy was the same between the two groups. Repair tissue after microfracture isn't really normal cartilage. Studies so far report the patient gets a mixture of two kinds of cartilage: fibrocartilage (scar tissue) and hyaline cartilage. Patients with smaller defects had better clinical results with microfracture than those with a bigger defect. The size of the cartilage injury didn't seem to matter in the ACI group.

Overall results showed much more improvement in the microfracture group compared to the ACI group. Microfracture patients had less pain and better physical function. Younger, more active patients had the best overall results in both groups.

The authors report this is the first study to compare ACI and microfracture in repair of knee cartilage. Patient outcome and quality of repair tissue were measured. Researchers think the better physical function after microfracture may be because the operation is much simpler than the ACI procedure. Rehab is easier, too.

A larger, longer study is needed to show what happens years later. It's not clear if one method is better than the other over the long term.