Researchers at the Washington University School of Medicine in St. Louis have just given people another reason to kick the butt by revealing that smoking interferes with ligament healing.
The researchers conducted a study on a mouse model and found that cigarette smoking impairs the recruitment of cells to the injury site and delays healing following ligament repair surgery.
The researchers looked at the mouse medial collateral ligament (MCL), a ligament that supports the knee joint in both mice and people.
MCL injuries are the most common, and are also the most common injuries seen in competitive and recreational sports.
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. It's likely the biology is transferable to other knee ligaments, elbow ligaments, shoulder ligaments, you name it," says co-investigator Linda J. Sandell, Ph.D., professor of orthopaedic surgery.
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 were exposed to enough passive fumes to make up for two cigarettes daily, the equivalent of a person smoking about four packs per day.
They were placed in the smoking chambers for two months prior to MCL surgery and then again after surgery to mimic the behaviour of humans who continue to smoke following an injury.
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.
An earlier study found an increase in cell density and in gene activity to produce type I collagen in the first week following MCL injury, so in this study the researchers paid close attention to cell density, biomechanical function and gene expression during the first week after MCL repair. In mice exposed to cigarette smoke, cell density was lower and type I collagen gene expression was reduced.
"Our studies also have shown a decreased macrophage response that may help explain why we see this delayed or decreased healing response," Wright says.
Sandell and Wright say their findings point to yet another reason smokers would do well to quit.
"Many patients don't want to hear it, but these results suggest that smoking affects anyone who needs ligament-repair surgery. 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." Wright says.
The findings of the study are reported in the Journal of Orthopaedic Research.