If you need another reason to kick the cigarette habit, try this: Smoking interferes with the healing process in injured ligaments—the fibrous tissue that connects bone to bone. Researchers at Washington University School of Medicine exposed mice to cigarette smoke and its toxins for two months before surgery on the medial collateral ligament (MCL), one of the knee joint’s supporting ligaments in mice and people. After surgery, the mice were exposed again, mimicking the behavior of smokers who continue to puff after an injury. Unlike control “nonsmoking” mice, “smoking” mice showed decreased recruitment of healing cells to the injury site, delaying recovery. More than 20 million ligament injuries occur in the United States each year, and MCL injuries are the most common.