by Anne Z. Hoch
Stress fractures have two primary causes. They result from excessive bone strain resulting in microdamage to the bone coupled with an inability to keep up with appropriate repair of the bone, or a depressed response to normal strain at the cellular and molecular levels where bone remodeling occurs. The former occurs most often in otherwise healthy female athletes and military recruits, while the latter is likely to occur with other physical problems, such as osteoporosis.
There were 2.4 million high school girls competing in sports in 1997, an 800% increase over 1971. And stress fractures occur more often in female athletes than male athletes. The risk of stress fractures in female recruits in the US military is up to 10 times higher than men undergoing the same training program.
There are many contributing factors to the greater frequency of stress fractures in women. Male athletes may have greater muscle mass, which absorbs shock better. In a study of female athletes, decreased calf girth was a predictor of stress fractures of the tibia. The larger width of male bones may also absorb shock better.
Bone mass and bone mineral density can vary widely in females due to several factors, including hormonal influences and menstrual irregularities. Low calcium intake and eating disorders may contribute to the development of stress fractures. Conversely, oral contraceptive pills appear to help prevent stress fractures in female athletes.
For both men and women, a rigid, high-arched foot absorbs less stress and transmits greater force to the leg bones, which may increase stress fracture risk. And studies of female athletes have shown that having one leg slightly longer than the other can increase the risk of stress fractures.
Other risk factors for stress fractures, in general, include training regimen, footwear and training surface. For example, higher weekly running mileage has been shown to correlate with increased incidence of stress fractures. In another study, ballet dancers who trained more than five hours a day had a significantly higher risk of stress fractures than those who trained less than five hours per day. A sudden change in frequency, duration or intensity of training also affects the risk of stress fractures.
In addition, research has shown that training in athletic shoes older than six months increased the risk for stress fractures. Shoe age, rather than shoe cost, was a better indicator of shock absorbing ability. In theory, training on uneven surfaces, or hard surfaces like cement, could also increase stress fracture risk.
Female Athlete Triad
Stress fractures may be the first sign of a more serious underlying condition, such as the "female athlete triad." This is an inter-related problem consisting of amenorrhea (no menstruation), disordered eating and osteoporosis, a potentially lethal combination. Female athletes, particularly those participating in individual sports, may feel significant pressure to excel where leanness and a low body weight are seen as advantageous.
Abnormal eating patterns include food restriction or fasting, bingeing and purging, or the use of laxatives and diet pills. In combination with decreased body weight and excessive training, this can lead to menstrual disturbance, and in turn, low estrogen levels. Women with disordered eating, estrogen deficiency and menstrual dysfunction are predisposed to osteoporosis. Female athlete triad sufferers are at a significant risk for stress fractures.
Several studies have shown that stress fractures occur more commonly in women who have stopped menstruating or have irregular periods than those who have a regular menstrual cycle. Athletes with menstrual disturbances have lower estrogen levels and this may lead to lower bone mineral densities. Estrogen deprivation may affect the bone's ability to adapt to stress.
There is some evidence that beginning to menstruate at a later age may be a factor in stress fractures. Another issue for young female athletes is abnormally low levels of estrogen and poor nutrition during adolescence. This can lead to lower bone mass, which may be irreversible after a certain age.
Diagnosis and Treatment
A very specific and accurate diagnosis is the key to proper treatment. Pain from a stress fracture of the neck of the femur (thigh bone), for example, may cause pain in the groin, hip, front of the thigh or the knee. Often standard X-rays do not disclose stress fractures. A bone scan, CT (computerized tomography) scan or magnetic resonance imaging may be more effective, depending on the site of the suspected fracture. The pelvis, sacrum (in the lower back), and the femur are areas where females tend to have a higher occurrence of stress fractures. The patella (knee cap), tibia (shin bone), and bones on the outside of the foot are other common areas of stress fractures, the tibia being the most common of all.The type of stress fracture and its location generally determine treatment. In most cases, rest is the cure for stress fractures. Non-weight-bearing exercise, such as swimming, may be prescribed so that the athlete can maintain aerobic fitness. However, some stress fractures require surgery to fix the bone in place so that it can heal properly.
For more information, see "Stress Injury to the Bone Among Women Athletes" in the November 2000 issue of Physical Medicine and Rehabilitation Clinics of North America
Anne Zeni Hoch, DO
Assistant Professor, Physical Medicine & Rehabilitation and Orthopaedic Surgery Medical College of Wisconsin Froedtert and Medical College Sports Medicine Center