Both the Native American cultures that discovered tobacco and the first Europeans to make it a cash crop believed that tobacco had the ability to cure ailments ranging from toothache to cancer. Not only do we know now that tobacco causes cancer and other diseases, but new research suggests that far from being a cure-all, it actually interferes with the healing process.

Studying mice with knee ligament injuries, researchers from the Washington University School of Medicine in St. Louis discovered that cigarette smoking slows down and impairs healing following ligament-repair surgery.

They focused on the medial collateral ligament (MCL), a ligament that supports the knee joint in both mice and people. MCL injuries are among the most common in competitive and recreational sports.

Previous studies have demonstrated that studying the knees of mice can tell us a lot about what happens in injured human knees.

"This is a good model for knee ligament injury, but it could be a model for ligament injuries anywhere in the body," says co-investigator Linda J. Sandell, Ph.D., professor of orthopaedic surgery. "It's likely the biology is transferable to other knee ligaments, elbow ligaments, shoulder ligaments, you name it."

To test the effects of smoking, Sandel and colleagues used a system in which mice inhale the smoke from two cigarettes daily, the equivalent of a person smoking about four packs per day.

The soft tissue healing that occurs after ligament injuries occurs in stages. First there is a pooling of blood. This is followed by several days of inflammation, in which cells called macrophages go to the injury site and secrete substances called cytokines and chemokines. Those bring in more cells to assist in healing. That process lasts for several days to several weeks. The final stage of healing involves remodeling of tissue and can continue for months or years.

In mice exposed to cigarette smoke, however, the process was significantly slowed.

"Our studies also have shown a decreased macrophage response that may help explain why we see this delayed or decreased healing response," Wright says.

Between 20 and 25 percent of the U.S. population smokes. Wright and Sandell say that although the prevalence of smoking among athletes is slightly lower, a significant percentage of recreational and even professional athletes continues to smoke. Many others use chewing tobacco, which may cause some of the same effects. But that's not yet clear since the mice in this study were exposed to smoke rather than to nicotine only.

"Many patients don't want to hear it, but these results suggest that smoking affects anyone who needs ligament-repair surgery." Wright says. "I counsel surgery patients to at least try to decrease smoking because, if nothing else, that will improve the healing of their surgical incisions. Quitting smoking is good health management regardless, but in patients having this kind of surgery, there are extra advantages."

This research was published in the December 2006 Journal of Orthopaedic Research.