ST. LOUIS, Missouri— Two new studies funded in part by the National Football League Charities help explain why cigarette smoking impedes the healing of fractured bones and torn ligaments. Researchers in the department of orthopaedic surgery at the Washington University School of Medicine in St. Louis, Missouri, report the two animal studies in the December issue of the Journal of Orthopaedic Research. ^{1, 2}

In the first study from this group, Corey S. Gill, MD, shows that smoking impairs extracellular matrix synthesis after injury to the medial collateral ligament (MCL) injury. In the second study lead author Hossam B. El-Zawawy, MD, shows that smoking delays chondrogenesis, which in turn delays the normal progression of fracture healing through the stages of hematoma and inflammatory response, callus formation, chondrogenesis, ossification, and remodeling.

Smoking impedes ligament healing

The mice were divided into a nonsmoking control group and a group exposed to smoke for 2 months prior to surgical MCL injury. Mice were euthanized at 3 and 7 days after surgery and researchers used propidium iodine staining to quantify cellular density of injured and sham ligaments.  They found that cell density increased significantly from baseline to 7 days after injury in control mice. By contrast, there was a significantly lower cellular density in mice exposed to cigarette smoke at 7 days compared to controls. There was no difference in proliferation between groups at the site of injury. The researchers did not find any evidence of apoptosis in any of the groups at the site of injury. Moreover, type I collagen gene expression was higher in controls compared to smokers at day 7. Almost all of the cells in the substance of the injured MCL at day 7 were spindle-shaped and expressed type I collagen, suggesting that increased cell density from day 3 to day 7 represented an increase in ligament cells rather than an increased inflammatory response.

"Ultimately, a better understanding of the cellular and molecular mechanisms involved in the MCL healing process will allow physicians to improve or speed the healing process, as well as potentially overcome the deleterious effects of smoking on ligament healing," the study authors conclude.

Smoking's slows fracture healing

In this study, mice were divided into two nonsmoking control groups and a group exposed to cigarette smoke for 1 month prior to surgical tibial fracture. Mice were euthanized at 7, 14, and 28 days after surgery. Researchers used immunohistochemical staining for type II collagen protein expression as a marker of cartilage matrix and proliferating cell nuclear antigen staining to measure proliferation at the site of injury. They quantified areas of cartilaginous and noncartilaginous fracture callus with toluidine blue staining and histomorphometry. They analyzed x-rays for evidence of remodeling after injury.

At day 7 after injury, mice exposed to cigarette smoke had a smaller fracture callus with less cartilage matrix compared to controls. Proliferation was present at high levels in both groups at day 7, but proliferating cells had a more immature morphology in the smoking group. At day 14, chondrogenesis was more active in smokers compared to controls, while a higher percentage of bone was present in the control animals. At day 28, x-rays showed a larger fracture callus remaining in the smoking group.

"Together, these findings show that the chondrogenic phase of tibial fracture healing is delayed by smoking," the researchers conclude. "Clinically, if specific events can be identified, smoking cessation in humans, even temporarily, may improve or speed the healing process after injury and decrease the significant morbidity associated with cigarette smoking during fracture healing," they write.

First documentation of smoking effect on soft tissue healing

"This is just another reason to stop smoking, and the mechanisms seem reasonable," said Lewis Maharam, MD, sports medicine expert and medical director of the ING marathon in New York City. "The bottom line is smoking is bad for you, and these studies just point out that the more we investigate, the more we find out that there are more and more problems associated with smoking."

The real contribution of these two reports is that smoking affects soft tissue healing, explained Kurt Spindler, MD, professor and vice-chairman, department of orthopaedics and rehabilitation, at Vanderbilt University Medical School, and director of the Vanderbilt Sports Medicine Center and Orthopaedic Patient Care Center at Vanderbilt Universtiy, in Nashville, Tennessee. Dr. Spindler also serves as head team physician for Vanderbilt's NCAA Division I varsity athletes.

"There is a lot of literature that says people who smoke heal bone less well, but this work says smoking can also affect soft tissue healing and that is its main contribution to the literature," he told CIAOMed. "The research offers a plausible biologic reason that smoking will have some effect and retard soft tissue healing like ligaments," he said.

"Mechanistically, it does make sense and smoking is not going to help your outcome for sure, but how much it delays or hinders your outcome, we don't know yet."

Effects at levels comparable to heavy smokers

"These two studies lend new support to the view that smoking is important in orthopaedics," said CIAOMed editorial board member Stephen Trippel, MD, professor of orthopaedic surgery at Indiana University School of Medicine in Indianapolis. In the first study, smoking was shown to delay chondrogenesis, a critical step in fracture healing, he explained. Using a mouse tibial fracture model, the investigators found that steps in the fracture healing process were adversely affected when mice were exposed to cigarette smoke at levels comparable to those found in humans who are heavy smokers. "This study contributes to the field by demonstrating at the cellular level how smoking exerts a deleterious effect on fracture healing," Dr. Trippel said.

In the second study, a decreased cellular density and decreased type I collagen expression was observed in the injured MCLs of mice exposed to cigarette smoke, he continued, adding that "because other measures of injury repair were not affected, these data begin to provide a cellular and molecular basis for a delayed or deficient early healing imposed by smoking."

Smoking has well established inhibitory effects on bone and soft tissue, and prior studies have demonstrated inhibition of spine fusion in animal models and tibial fractures in humans. Moreoever, smoking has also been shown to inhibit wound healing in animal models and humans. "The present studies expand upon this literature by suggesting that the inhibition also applies to chondrogenesis and ligament healing," Dr. Trippel told CIAOMed. "The effect of smoking on musculoskeletal tissues is important to orthopaedic surgeons and their patients [because] it suggests that the results of orthopaedic surgical procedures may be compromised by smoking," he said. "Already, many orthopaedic surgeons advise their patients to discontinue smoking prior to scheduled surgical procedures and these two studies using mouse models lend theoretical support to this practice. The studies also could serve as the basis for future human studies."


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References

1. Gill CS, Sandell LJ, El-Zawawy HB, et al. Effects of cigarette smoking in early medial collateral ligament healing in a mouse model.  J Orthop Res. 2006;24:1–9.
2. El-Zawawy HB, Gill CS, Wright RW, et al. Smoking delays chondrogeneis in a mouse model of closed tibial fracture healing. J Orthop Res. 2006;24:1–9.