Definition

Stress fractures are complete or partial bone fractures caused by the accumulation of microtrauma. Normal bone accommodates to stress through ongoing remodeling. If this remodeling system does not keep pace with the force applied, stress reaction (micro fractures) and, finally, stress fracture can result. Stress fracture is the end result of a continuum of biologic responses to stress placed on bone. Adolescent, young adult, and premenopausal women athletes have a higher incidence of stress injuries to bone than men do. Stress fractures in juveniles are rare. Both extrinsic and intrinsic factors have been implicated in this imbalance between bone resorption and bone deposition. Malalignment and poor flexibility of the lower extremities (intrinsic factors) and inadequate footwear, changes in training surface, and increases in training intensity and duration without an adequate ramp-up period (extrinsic factors) can lead to stress fractures.

Stress fractures in athletes vary by sports and are most common in the lower extremities. Stress fractures in the lower extremity account for 80% to 90% of all stress fractures, representing 0.7% to 20% of all sports medicine injuries. Specifically, stress fracture incidence in runners approaches 16% of all injuries. The most common stress fractures occur in the tibia (23.6%), but also develop in the tarsal navicular (17.6%), metatarsals (16.2%), femur (6.6%), and pelvis (1.6%). The fracture site is the area of greatest stress, such as the origin of lower leg muscles along the medial tibia. A narrower mediolateral tibial width was a risk factor for femoral, tibial, and foot stress fractures in a study of military recruits. Studies of female runners demonstrated greater loading rates in those with history of tibial stress fractures compared with those without injury. In contrast, in comparison of runners with and without history of tibial stress fracture, no difference in ground reaction forces, bone density, or tibial bone geometric parameters was found between groups.

Military recruits have been extensively studied in regard to lower extremity stress fractures. In a study of 179 Finnish military recruits aged 18 to 20 years, tall height, poor physical conditioning, low hip bone mineral content and density, and high serum parathyroid hormone level were risk factors for stress fractures. The authors postulated that given the poor vitamin D status, intervention studies of vitamin D supplementation to lower serum parathyroid hormone levels and possibly to reduce the incidence of stress fractures are warranted. A recent systematic review and meta-analysis of nine observational studies examined the association between serum 25 (OH)D levels and stress fractures in the military and concluded that low vitamin D levels may play a role in the development of stress fractures in military personnel and monitoring and ensuring sufficient serum 25 (OH)D levels may be beneficial for reducing the risk of stress fracture. A database of systematic reviews, including 13 randomized prevention trials, concluded that shock-absorbing insert use in footwear probably reduces the incidence of stress fractures in military personnel. There was insufficient evidence to determine the best design of such inserts.

Stress fractures may be related to abnormalities of the bone, such as in female athletes with low bone density due to exercise-induced menstrual abnormalities. Premature osteoporosis leads to an increased risk for stress fractures. One study looked at premenopausal women runners and collegiate athletes and concluded that those with absent or irregular menses were at increased risk for musculoskeletal injuries while engaged in active training. Muscle deficits in the gastrocnemius-soleus complex in jumping athletes have also been implicated in causing tibial stress fractures. Bone injury may be a secondary event after a primary failure of muscle function. Besides nutritional deficiencies in calcium and vitamin D, medications like steroids, anticonvulsants, antidepressants, and antacids are additional risk factors for the development for stress fractures.

Over the last few years, researchers have shown a link between femoral stress fractures and long-term bisphosphonate use. Bisphosphonate medications are indicated for patients with postmenopausal osteoporosis and it has been shown that its use improves bone mineral density, prevents bone loss, and reduces the number of fractures. Bisphosphonates inhibit osteoclastic bone resorption, and therefore bone turnover, by inducing osteoclast apoptosis. The combined and coordinated action of resorbing damaged bone and laying down new bone is fundamental to the process of bone remodeling. If this coupling is impaired, the micro-damage that occurs under physiologic conditions that normally is repaired may accumulate, resulting in a major reduction in the energy required to cause fracture. These fractures are low-energy injuries and have characteristic findings observed on femoral radiographs: a transverse fracture line originating from the lateral tension side of the cortex and lateral cortical thickening adjacent to the fracture. In addition, prodromal thigh pain from the insufficiency changes may be present. The subsequent minimal trauma that often is required to complete the fracture is characteristic, with patients often sustaining a spontaneous non-traumatic fracture during activities of daily living.

Individuals who are non-ambulatory or have limited ambulation due to disability represent another population with abnormal bone and premature osteoporosis. In stroke patients, there is significant bone loss on the paretic side, which is greatest in those patients with the most severe functional deficits. Spinal cord injury may not only cause bone loss, but also alter bone structure and microstructure. Practitioners caring for individuals with limited mobility should consider stress fracture in the differential diagnosis of overuse injuries.

Symptoms

Patients may report an increase in training or activity level or a change in training conditions preceding the onset of symptoms. Because of pain in the affected region of the bone, patients may seek medical attention during the micro fracture or stress reaction phase of injury. Should they forego relative rest (avoidance of the pain-provoking activity), they can progress to stress fracture or even complete fracture; the pain will gradually increase with activity and may occur with less intense exercise, such as walking, or even at rest. In general, however, the pain will improve with rest. The pain can lead to a decline in performance. The individual may also note swelling in the affected region of the bone. Symptoms of paresthesias and numbness should alert the clinician that an alternative diagnosis should be considered.

Physical Examination

On physical examination, the clinician will find an area of exquisite, well-localized tenderness, warmth, and edema over the affected region of the bone. Ecchymosis along the plantar aspect may be present with foot involvement. Percussion of the nearby region can cause pain. Placement of a vibrating tuning fork over the fracture site intensifies the pain. In the tibia, stress fractures primarily occur along the medial border; the frequency, in order, is upper, lower, and midshaft. In the fibula, they usually occur one handbreadth proximal to the lateral malleolus. Tarsal or metatarsal stress fractures present with localized foot tenderness. Weight-bearing activity, such as a one-legged hop test, can provoke the pain by increasing the ground reaction forces. For a presumed femoral stress fracture, the clinician can provoke pain by applying a downward force on the distal femur while the affected individual is seated with the distal femur extending beyond the edge of the seat ( Fig. 79.1 ).

Physical examination technique to evaluate for femoral stress fracture. The examiner applies a downward pressure on the distal femur, using the examination table as a fulcrum, to increase the force across the fracture site. A positive test result reproduces the individual’s pain.

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