Osseous structures are subjected to several different types of stress during physical activities, including tensile, compressive, bending, and shear/torsional forces . One or more of these may result in a stress injury to bone if applied in a repetitive manner. A number of mechanisms have been suggested as possible causes for the development of stress fractures.
The compressive forces that result from weight-bearing activities are undoubtedly the source of many stress injuries in the lower extremities . In certain sports, such as gymnastics, the upper extremities are also used for weight bearing; this can lead to stress fractures, most typically in the forearm and wrist .
Even without weight bearing, muscular actions on bone can generate significant forces that may be torsional, tensile, bending, or even compressive in nature . It has also been suggested that, during training, muscle tends to hypertrophy faster than bone, resulting in an imbalance that may lead to stress injuries, especially early in a training regimen [7,8].
Muscle fatigue is also thought to play a role in the production of stress injuries . Normally, muscles act to ''shield'' the bones from excessive forces by dynamically dissipating stresses away from the underlying osseous structures so that the force is not concentrated on a focal portion of the bone. As muscles begin to fatigue, this ''shock absorbing'' effect is lost, and more stress is applied directly to the bone. If a particular muscle group fatigues, this may also result in unopposed pull from an antagonistic muscle group, leading to excessive bending or torsional forces on the bone.
During throwing, tremendous forces are imposed on the shoulder, humeral shaft and elbow, and even the first rib, creating the potential for stress injuries at these sites. The throwing motion may be broken down into four phases: wind-up, cocking, acceleration, and deceleration (follow-through) [10,11]. At the transition point between the cocking and acceleration phases, significant torsional stresses are applied to the humerus, whereas, during the acceleration phase itself, severe valgus forces are produced at the elbow [11-13]. These val-gus forces result in tensile stresses along its medial (ulnar) aspect and compres-sive forces along its lateral (radial) margin . Details of the specific injuries at these sites are discussed in detail later in this article.
Different types of stress injuries are seen in the developing skeleton. Before closure, the growth plate is less resistant to shear and tensile forces and is not very resistant to rotational and compressive forces . Consequently, the physis is quite susceptible to stress injuries in the younger athlete. Epiphysiolysis is a term that denotes a stress reaction of the physis before its closure, in which there is resorption and widening involving an unfused physis in response to chronic stresses. This type of injury most commonly affects the proximal humeral physis, the medial epicondylar apophysis of the elbow, and the distal radial physis [14-16]. These will be discussed more fully later on.
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