Stress fractures are a problem in various populations including runners and military trainees. [1, 2, 3] These fractures can occur with as little as 2-3 weeks of training, be very mild, causing only minimal changes to the bone which eventually heals, or they may progress to a complete fracture that requires surgical fixation. Although rare, poor outcomes may occur in the form of nonunions or avascular necrosis. Certain stress fractures have a higher risk of poor outcome, including anterior tibial and femoral neck stress fractures (FNSFs).
(See also the articles Femoral Head Avascular Necrosis [in the Sports Medicine section], Avascular Necrosis, Femoral Head and Stress Fracture [in the Radiology section], and Stress Fracture [in the Physical Medicine and Rehabilitation section], as well as Risk Factors for Bone tress Injuries: A Follow-up Study of 102,515 Person-Years and Total Hip Arthroplasty in the Older Population on Medscape.)
FNSFs (see image below) are some of the most difficult injuries to diagnose. The pain associated with such an injury may be poorly localized in the hip and may be referred to the thigh or back. Physical examination findings are not very specific for this injury, and diagnostic radiographs in the form of x-ray films, bone scans, and/or magnetic resonance images (MRIs) are often necessary.  Failure to diagnose FSNFs may lead to catastrophic consequences, including avascular necrosis of the femoral head and the need for a hip replacement in otherwise healthy young individuals. [5, 6, 7] A high index of suspicion in the appropriate risk populations is the key to diagnosing and treating FNSFs.
(See also the article Femoral Neck Stress and Insufficiency Fractures.)
Stress fractures may develop in up to 15% of runners and military trainees.  Of those patients who develop stress fractures, about 5-10% of the fractures are in the femoral neck.  Stress fractures on the compression side (the inferior aspect) of the femoral neck are more common than stress fractures on the tension side (the superior aspect).
The femoral neck lies between the femoral head and femoral shaft, demarcated by the greater and lesser trochanters. Weight-bearing forces from the trunk cause a compressive force on the inferior aspect of the femoral neck, whereas the superior aspect is subject to tensile forces. [6, 9] The blood supply to the femoral head runs through the femoral neck; thus, an FNSF may disrupt the blood supply to the femoral head and cause avascular necrosis of the femoral head. 
The load of the runner's body weight is transmitted down the lower extremities through the bones and may exceed 3-5 times the body weight in the femoral neck during running. Muscles help to absorb forces and distribute load, especially the gluteus medius. The weight of the trunk and upper extremities applies compressive forces to the inferior aspect of the femoral neck. Conversely, tensile forces act upon the superior aspect of the femoral neck. These forces become important in the prognosis and management of the stress fracture. A sudden reduction in weight and lower muscle mass combined with daily training was associated with an increased risk of FNSF in US Naval Academy plebes. 
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