Cigarette Smoking Impairs Ligament Healing

The list of reasons you shouldn't smoke has gotten longer. Researchers at Washington University School of Medicine in St. Louis are reporting that smoking interferes with ligament healing.

Studying mice with knee ligament injuries, the team discovered cigarette smoking impairs the recruitment of cells to the injury site and delays healing following ligament-repair surgery. They reported their findings in the Journal of Orthopaedic Research.

The researchers looked at the mouse medial collateral ligament (MCL), a ligament that supports the knee joint in both mice and people. Each year in the United States there are more than 20 million reported ligament injuries, and MCL injuries are the most common. They also are the most common injuries seen in competitive and recreational sports. It's not clear exactly how many MCL injuries occur annually because many go unreported.

"A lot of MCL injuries never make it to an emergency room because patients will have a sore knee but don't seek treatment," says Rick W. Wright, M.D., associate professor of orthopaedic surgery and a senior investigator on the MCL study.

Previous studies have demonstrated that the mouse provides a good paradigm for 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 look at the effects of smoking, Sandell, Wright and their colleagues used a system developed at the School of Medicine in which mice are placed inside smoking chambers six days per week. The mice don't actually have cigarettes in their mouths, but they get enough passive fumes to "smoke" two cigarettes daily, the equivalent of a person smoking about four packs per day. Mice were placed in the smoking chambers for two months prior to MCL surgery and then again after surgery to mimic the behavior of humans who continue to smoke following an injury.

The researchers say athletes who smoke should keep these findings in mind before driving for a lay-up, sliding into second base or lacing up a pair of ice skates.

The soft tissue healing that occurs following ligament injuries occurs in stages. There is an immediate pooling of blood near the injury, the sort of hemorrhaging that will cause swelling right away. This initial response is followed by several days of inflammation, in which cells called macrophages flock to the injury site and secrete substances called cytokines and chemokines. Those, in turn, recruit more cells to assist in healing. That process of cellular proliferation and synthesis lasts for several days to several weeks. The final stage of healing involves remodeling of the tissue and can continue for months and even years.