Page 115 - JSOM Spring 2018

Epidemiological Evidence experimentally induced medial collateral ligament injury had
subsequently weaker and less stiff medial collateral ligaments
Studies have shown that smoking is associated with higher with less cell density and less expression of Type I collagen
overall injury risk as well as specific types of injuries. Once an compared with mice not exposed to cigarette smoke. 56,57
injury has occurred, smoking also has detrimental effects on
healing processes that could set the stage for future injuries.
Wound healing in smokers is delayed and less complete, com-
plications are more likely to arise, and cosmetic results are less
Injury Risk satisfying. 43,45,53,58–64 A systematic review and meta-analysis of
It has been a consistent finding that cigarette smoking prior 140 studies examining the effect of smoking on wound heal-
to basic training is associated with increased injury risk in the ing indicated that risk of necrosis, delayed healing, wound
US military 7–17 and in the military forces of other countries. 18,19 complications, and lack of healing were 3.6, 2.1, 2.3, and 2.2
Further, smoking is associated with injuries among in Soldiers times higher, respectively, among smokers compared with non-
in operational units 20–24 and among civilian workers in other smokers. Evidence for longer-term effects of smoking comes
65
occupational groups. 25–32 Military studies that included vari- from studies on skin damage. Tobacco users had more than
ous levels of smoking have shown a dose–response relation- twice the risk of moderate to severe facial wrinkling (indica-
ship; that is, the likelihood of injuries increases with more tive of skin damage) when compared with nonusers, even after
cigarettes smoked or more days of smoking. 7–9,12,16,18 Further, controlling for age, sun exposure, and body mass index. 66–68
cigarette smoking has been shown to be an independent risk
factor for injury when considered in multivariate statistical In summary, there is considerable evidence that smoking im-
models that include a number of other factors. 8,9,12,16,20 Thus, pairs healing processes following fractures, ligament injury,
there is considerable evidence from a variety of sources that and wounding. It appears that these impaired healing pro-
(1) cigarette smoking increases injury risk, (2) the greater the cesses lead to compromised and weakened tissues, possibly
amount of smoking, the higher is the injury risk, and (3) ciga- making smokers more susceptible to subsequent injury.
rette smoking risk is independent injury risk factor.
There is substantial evidence that smoking affects specific Possible Mechanisms of the Smoking Effect
types of injuries, especially fractures and tendon ruptures. Two on Injuries
independent meta-analyses found that smokers had a 25% to One problem in determining the mechanisms through which
26% higher risk for fractures of any type and 39% to 84% smoking may influence injuries is that cigarette smoke is a
higher risk for hip fractures. 33,34 In the US military, smokers heterogeneous aerosol containing at least 3,800 substances
have been shown to have a higher stress fracture risk. 7,10,23,35 in both particulate and gaseous form. More than 90% of to-
Studies in Israeli and Chinese military recruits have found bacco smoke (by weight) is in the gaseous phase. Substances
trends similar to that of the US military, but these trends have in the gaseous phase include carbon dioxide, carbon monox-
not been statistically significant. 36,37 ide, nitrogen oxides, ammonia, hydrogen cyanide, hydrazine,
formaldehyde, acetone, and aerolein. Substances in the par-
Smokers have a higher risk of biceps tendon ruptures and ticulate phase include nicotine, toluene, phenol, and catechol.
38
rotator cuff tears are more likely and larger in smokers. 39,40 Carbon dioxide, nicotine, and carbon monoxide are by far
For rotator cuff tears, there is a dose response such that as the the major components. While the body can readily eliminate
69
number of cigarette packs per day increased so does the risk of carbon dioxide, carbon monoxide is a toxic substance that
40
a tear. Ruptured supraspinatus tendons samples from smok- binds to hemoglobin with at least 200 times the affinity of
ers showed a greater degree of tendon degeneration, lower oxygen, competes with oxygen for active sites, and can result
cell density, and a greater number of apoptotic cells compared in tissue ischemia. Most of the carbon monoxide taken in is
70
with nonsmokers. 41 eliminated by the lungs and the half-life of carbon monoxide
is only about 4 to 5 hours at sea level. Nicotine is a bioac-
71
Healing After Injuries tive substance that increases circulating norepinephrine, epi-
Smoking not only increases the risk of injury, it also appears to nephrine, vasopressin, growth hormone, cortisol, ACTH, and
impair healing processes following fractures, ligament injury, beta endorphins. 72,73 The half-life of nicotine is 30 minutes to
and wounding. Once a fracture occurs, smokers have longer 2 hours. 73,74
healing times and more nonunions and are more likely to de-
velop complications (e.g., infections and osteomyelitis). 42–45 While it is important to consider the number of possible sub-
Experimental fractures in nicotine-exposed rabbits produce stances involved, a number of studies have been conducted
weaker bone tissue, less callus formation, and less vasculariza- that inform on possible mechanisms whereby smoking, with
tion and result in delayed or inhibited bone union. 46–49 all substances included, may affect fractures and wound
healing. Also the effects of smoking on the vascular system,
Several investigations have looked at the effects of smoking psychosocial factors, and effects on muscle tissue should be
on disability and healing after ligament injury and surgical considered in determining the possible mechanisms whereby
repair. Among Soldiers hospitalized with an anterior cruciate smoking may affect injuries.
ligament injury, smokers were 1.36 times more likely to re-
ceive a disability discharge. After anterior cruciate ligament Fractures
50
reconstruction, smokers had lower subjective knee function How smoking might affect fracture risk appears complex and
scores, were less likely to return to sport activities, had greater not fully understood but may involve low BMD, lifestyle,
joint laxity, had lower overall physical activity, and tended to calcium metabolism, and direct effects on osteogenesis and
have higher incidence of infections compared with nonsmok- sex hormones. There is substantial evidence that smoking in-
ers. 51–55 Mice exposed to cigarette smoke 2 months prior to creases the risk of low BMD. A meta-analysis of 82 studies
75
Smoking and Injuries | 109