Femoral neck stress fractures are a common cause of hip pain in select populations. Chronic, repetitive activity that is common to runners and military recruits predisposes these populations to femoral neck stress fractures. These injuries must be differentiated from insufficiency fractures, which, though similar in appearance and presentation, result from an entirely different pathophysiology and occur in a different population.
The femoral neck area is subjected to large compressive and sheer forces associated with ambulation. Even in the most sedentary individual, the daily cyclic loading of the hip and femoral neck produces high stresses on the bony trabeculae in this anatomic region. In long-distance runners and other high-performance athletes, the forces across the femoral neck are multiplied exponentially because the athletes' training regimens place tremendous physical burdens on this relatively small bridge of bone, which connects the femoral head to the diaphysis.
The greatest physical symptom that stress fractures manifest is pain in the hip, groin, or anterior thigh. Such pain can severely limit an athlete's ability to train, compete, and, ultimately, ambulate if the problem progresses undiagnosed. The ultimate result of an untreated stress fracture can be a complete fracture (possibly displaced) of the femoral neck. Even isolated, this injury could be devastating to a performance athlete. The sequelae of this fracture include avascular necrosis (AVN) of the femoral head and fracture nonunion, both of which can adversely impact athletic careers.
The pain associated with femoral neck stress fractures can be both irritating and disabling to these high-performance individuals. Because the onset of this pathologic entity is insidious and because the results of conventional radiography are frequently equivocal, the diagnosis of femoral neck stress fractures can be missed. Whereas the treatment of stress fractures of the femoral neck is often straightforward, undetected stress fractures can lead to serious complications.
The first description of femoral neck stress fractures in the literature was published in 1936 by Asal in Archiv für Klinische Chirurgie. Since that time, numerous articles have recognized these fractures as difficult entities to treat. In 1963, Ernst recorded what was at the time the largest series of femoral neck fractures and described the resulting disability in military servicemen.  Although the geneses of acutely traumatic fractures are different from those of stress fractures, the treatments are similar
Since the first descriptions of the vascular anatomy of the femoral neck, orthopedists have recognized the importance of prompt reduction of femoral neck fractures in preventing AVN of the femoral head. Closed reduction and impaction of the fracture parts, followed by immobilization in a spica cast application, with the injured extremity in internal rotation, were adopted by numerous orthopedists of the time. However, it was not until 1931 that the first methods of internal fixation were publicly described by Smith-Petersen.  The medical community was hesitant to accept internal fixation, but this method of treatment nonetheless came into wide use and remained so until the 1970s.
Seven years after Smith-Petersen introduced internal fixation, Moore described a multiple-pin fixation technique, which was followed shortly thereafter by the Thompson femoral endoprosthesis. The varied methods of treatment sparked great debate over whether replacing the entire femoral head or simply fixing the neck fracture was more beneficial. The idea that impacting fracture fragments was the key aspect in healing led to the development of the Pugh nail and the Richards screw, in the 1950s and 1960s. Both of these modalities are designed around a sliding fixation process whereby fracture fragments can compress against each other.
Finally, Judet described another method of treatment that has not been practiced widely but still deserves mention: the quadratus femoris muscle pedicle graft. With this technique, a vascularized bone graft is placed across the posterior femoral neck and into the femoral head to prevent the complication of AVN.
The femoral head derives its blood supply from three terminal arterial branches. The lateral epiphyseal artery (an ascending branch off of the medical femoral circumflex artery of the profunda femoris) is the predominant source of blood flow, and its distribution to the head is largely skewed toward the subchondral bone of the femoral articular cartilage. Two accessory arteries supply the remaining 10% of femoral head circulation: the inferior metaphyseal artery (the ascending branch of the lateral femoral circumflex artery) and the medial epiphyseal artery of the ligamentum teres. The latter vessel originates from the obturator artery.
The vascular anatomy of the femoral neck is especially important because fractures of this region can have devastating effects on the already tenuous blood supply to this area. The severity of the vascular disruption generally correlates with the degree of displacement of the fracture.
By midadolescence, the femoral epiphysis is usually closed, providing a reasonable anatomic picture of the femoral head and neck. The neck-shaft angle, which is approximately 130°, is relatively constant between the sexes. Femoral anteversion is estimated at 10.4° and remains unchanged even after skeletal maturity is reached. A fairly large synovial membrane encloses the femoral head and a good portion of the anterior femoral neck. The greater trochanter, a large, posteriorly located, bony prominence, serves as the major attachment for the external rotators; it also provides a definitive surgical landmark for the insertion of numerous femoral internal-fixation devices.
A closer look at the genesis of a stress fracture in the femoral neck reveals that the damage manifested on the physical level derives not from a traumatic event per se but, rather, from a metabolic derangement.
Bone initially responds to increased mechanical loading by increasing resorption. Resorption is normally counterbalanced by an equal but opposite, osteoblast-mediated metabolic repair. Under situations of extraordinarily high levels of training, such as those faced by military personnel and elite athletes, bone resorption begins to exceed the bone's capacity to remodel. Additionally, pharmacologic (glucocorticoids), nutritional (vitamin D and calcium deficiency), and other (postmenopausal, hyperparathyroid) states can adversely affect osteoblasts' ability to keep pace with osteoclastic resorption. If this metabolic imbalance persists, microfractures develop that eventually weaken bone to the point of a complete fracture.
Femoral neck stress fractures in young, otherwise healthy individuals are related to the inability of bony trabeculae weakened by osteoporosis to withstand physical stresses. Unusually high physical demands on normal bone over the long term can lead to mechanical failure of the bone trabeculae.  The phenomenon is seen with exercise beyond the point of muscle fatigue,  alterations of ground reactive forces that yield abnormal stress patterns in bone, and increased muscular contractions.
In contrast, insufficiency fractures of the femoral neck are the result of normal stresses of everyday activity placed on structurally compromised bone. Thus, insufficiency fractures occur in individuals who have concomitant metabolic derangements, such as hyperparathyroidism and renal failure, or menopause. 
At least one example of a crossover group exists: amenorrheic female athletes. Because of their lack of body fat, female distance runners often temporarily halt their menstrual cycle. As a result, they become hypoestrogenic and, therefore, physiologically similar to postmenopausal females.
Estrogen is an essential factor in the development and maintenance of bone strength; in its absence, bones become brittle and osteopenic. This places these female athletes in a sort of double jeopardy, producing characteristics that contribute to stress fractures and causing a background hypoestrogenic state that predisposes these women to insufficiency fractures.
Femoral neck stress fractures occur most commonly in two subsets of the population. The first subset comprises elite distance runners, [6, 7, 8, 9, 10] military recruits, [11, 12, 13] and dancers.  The true prevalence of fractures in this group is difficult to pinpoint because such patients with hip pain and femoral neck stress fractures who never present to a physician and whose fractures go on to heal spontaneously are never identified.
Data from several military hip fracture studies by Stoneham and Morgan, in Britain, and Volpin and colleagues, in Israel, place the prevalence at 0.2-4.7% in patients without a history of a single traumatic episode. [15, 16] The prevalence of stress fractures in the general population may be surmised to be far less than that demonstrated in these two groups.
The second subset comprises hypoestrogenic (postmenopausal) women and individuals with pathologic entities resulting in osteopenia (eg, osteoporosis, Paget disease, hyperparathyroidism). Fractures in this group are termed insufficiency fractures, because bone quality is insufficient to support the diurnal physiologic demands placed on it.
Femoral neck stress fractures in children are exceedingly rare and should be far down on the list of differential diagnoses, behind such conditions as slipped capital femoral epiphysis, Legg-Calve-Perthes disease, infection, and transient synovitis. To date, there have been very few reports of this entity in the English literature. [17, 18] While Blickenstaff and Morris claimed in a 1966 article to have found only compression-sided stress fractures of the femoral neck in children, Lehman and Shah reported on a tension-sided stress fracture in a child of 14 years. [19, 20]
The prognosis for femoral neck stress fractures depends largely on the classification of the fracture.  Patients with compression-type injuries historically fare very well, with the patient recovering full preinjury function after diligent adherence to a physician-prescribed plan of limited weightbearing and walking with an aid.
Patients with transverse-type fractures, if they are identified early and if the only radiographic abnormality is sclerotic changes, tend to recover well after internal fixation. Potential lasting effects of surgical management include hip pain and nonunion or malunion of the fracture. The worst prognosis exists for transverse fractures that are inherently unstable because of mechanical reasons and that can progress to complete displaced fractures.  The rate of nonunion and AVN in these cases is as high as 35%, according to some authors.
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