Defining stress fracture

Stress fractures are partial bone fractures resulting from repetitive microtrauma. They are classified as fatigue or insufficiency fractures. A fatigue fracture occurs when an abnormal amount of stress is placed on a normal bone. Insufficiency fractures develop when normal stress is applied to an abnormal bone, such as an osteoporotic bone. A third type of fractures occurs when abnormal stress is applied to abnormal bone such as occurs with some female athletes in training who have osteopenia .

Normal bone has ongoing remodeling allowing accommodation to loading. Stress reaction (microfractures) can result if this remodeling system does not keep pace with the applied force. A stress fracture, and most likely a complete fracture, will result if the inciting event continues. The literature debates whether the bone’s failure to adapt has been related to excessive load-bearing forces or inadequate contractile muscular forces . Whichever the cause, early intervention is associated with more rapid healing .

Risk factors

Both intrinsic and extrinsic risk factors have been implicated in the cause of stress fractures ( Table 1 ) . Intrinsic factors include those that are biomechanical (eg, limb malalignment, gait abnormality, muscle imbalance, and small tibia diameter) or biochemical (eg, hormonal imbalance, low bone mass density, bone disease, and nutritional deficits). Extrinsic factors include training errors (eg, overuse, lack of cross training, lack of conditioning, increases in training intensity and duration without an adequate buildup phase, poor technique), environment (eg, nonabsorbent training surface, banked track), and equipment (eg, inappropriate foot wear, prolonged use of foot wear, non–gender specific training equipment).

Although they have been studied individually, the interplay between intrinsic and extrinsic factors likely leads to increased risks for stress fractures. Specifically, the amount, intensity, and timing of training plays a significant role in the bone’s ability to adapt to increasing demands. Overtraining and inadequate training surfaces in combination with poor muscular conditioning may overload the bone, resulting in a stress fracture . Some evidence shows that muscle mass is inversely related to stress fracture incidence. Individuals who have less muscle mass may be predisposed to develop a stress fracture, because studies have shown that one of the major roles of muscles is energy absorption . As muscles become fatigued, the nearby bone is exposed to greater forces. Experts have proposed that bone injury may be a secondary event after a primary failure of muscle function. Further studies are needed to define the role of conditioning and strengthening programs in the incidence and treatment of stress fractures in females.

Risk factors

Both intrinsic and extrinsic risk factors have been implicated in the cause of stress fractures ( Table 1 ) . Intrinsic factors include those that are biomechanical (eg, limb malalignment, gait abnormality, muscle imbalance, and small tibia diameter) or biochemical (eg, hormonal imbalance, low bone mass density, bone disease, and nutritional deficits). Extrinsic factors include training errors (eg, overuse, lack of cross training, lack of conditioning, increases in training intensity and duration without an adequate buildup phase, poor technique), environment (eg, nonabsorbent training surface, banked track), and equipment (eg, inappropriate foot wear, prolonged use of foot wear, non–gender specific training equipment).

Although they have been studied individually, the interplay between intrinsic and extrinsic factors likely leads to increased risks for stress fractures. Specifically, the amount, intensity, and timing of training plays a significant role in the bone’s ability to adapt to increasing demands. Overtraining and inadequate training surfaces in combination with poor muscular conditioning may overload the bone, resulting in a stress fracture . Some evidence shows that muscle mass is inversely related to stress fracture incidence. Individuals who have less muscle mass may be predisposed to develop a stress fracture, because studies have shown that one of the major roles of muscles is energy absorption . As muscles become fatigued, the nearby bone is exposed to greater forces. Experts have proposed that bone injury may be a secondary event after a primary failure of muscle function. Further studies are needed to define the role of conditioning and strengthening programs in the incidence and treatment of stress fractures in females.

Common sites for stress fractures

Stress fractures are seen most commonly in the lower extremities. The most common site for both males and females is the tibia . Sport-specific sites include the tibia, metatarsals and fibula in runners and dancers, the humerus in overhead throwing athletes and the rib and clavicle in rowers . Gender specific stress fracture patterns in women include femoral, pelvic, and metatarsal stress fractures ( Table 2 ) .

Gender-specific issues

The incidence of stress fractures is higher in women than men. A proposed mechanism in young athletic women relates higher stress fracture rates to premenopausal osteopenia or osteoporosis caused by a hypoestrogenic state resulting from hypothalamic amenorrhea. A positive correlation exists between menstrual dysfunction and low bone density . Likewise, menstrual dysfunction and stress fractures are also positively correlated . Cortical bone mass seems to be preserved in amenorrheic women . One study correlated osteopenic dual energy x-ray absorptiometry (DEXA) scan results with cancellous (ie, femoral neck) more strongly than with cortical bone (ie, tibial) stress fractures . This study helps support the use of DEXA scan screening in the active amenorrheic woman who has a stress fracture . Other factors such as inadequate nutrition, inappropriate conditioning, and anatomic differences have been implicated in the increased incidence of stress fractures in female athletes .

Studies have shown a higher risk for stress fractures in female track and field athletes that had more restrictive eating habits . Nutrition’s role in stress fracture development is not fully understood. Matched by age and body mass index (BMI), ballet dancers who have stress fractures tend to show more restrictive eating habits compared with both nondancers and dancers who have no stress fractures .

Common clinical presentation