A stress fracture is a localized breakdown of a bone, that occurs when the forces directed into the bone tissue exceed the ability of the bone to repair itself. A stress fracture is distinct from the fractures caused by the direct, one-time application of a force to the bone, such as created by a fall or a blow. When bone sustains damage, it mobilizes its reserves of minerals, primarily calcium and phosphates, to repair the bone damage; repetitive stress prevents the repair effort from being effective. Stress fractures occur most often in the tibia and fibula (the bones of the lower leg), the ankle, and the foot. Stress fractures are less common in other parts of the body, but may occur in any sport where repetitive motion isolates forces into a specific skeletal region. Distance running, running-oriented sports, and traditional track and field events such as the high jump and the long jump are the most common activities leading to the development of stress fractures.
Stress fractures recur for a number of reasons, some of which are caused by the structure of the athlete's body, others of which are related to the athlete's preparation, equipment, and attitude toward training. Sports science research confirms that approximately 60% of all athletes who have sustained a single stress fracture will later sustain at least one other. Female athletes are somewhat more likely to suffer a stress fracture than male athletes.
Unlike a direct fracture, stress fractures are more often a progressive condition, identified by the onset of pain while participating in the sport, a sensation that disappears when the athlete is at rest. Stress fractures are observable on an x ray; bone scans and magnetic resonance imaging (MRI) are also commonly employed diagnostic techniques. The recovery from a stress fracture is will primarily require rest, with appropriate stretching or activities that do not direct forces or apply resistance into the fractured bone.
There are two general sets of factors that are typically present to cause a stress fracture. The structure and manner of movement of the body, often referred to as biomechanical factors, will significantly influence how the forces directed into the skeletal structure are distributed and absorbed. The risk of an athlete developing a stress fracture in the lower leg will be increased when different factors are present, such as any sport that requires repetitive leg movement that places stress upon the lower leg. Also, unequal leg length in which the difference in the length of legs of the athlete are greater than 0.25 in (0.5 cm), causing the forces generated by the strike of the foot on the ground to be uneven. The greater force will be repeatedly directed into a region of the foot or the lower leg, causing the formation of the stress fracture.
Another structural factor is a high-arched foot that causes the forces directed into the foot on impact with the running surface to radiate unevenly. High arches are a key contributing factor to stress fractures of the metatarsal bones (the connective bones between the ankle and the toes).
The different manners in which the foot contacts the surface during running are other structural problems. Athletes are generally classed as either "forefoot strikers," where the ball and toes of the foot strike first, or "rearfoot strikers," where the force is first absorbed by the heel, and the foot rotates forward to generate force to push off from the forefoot. Forefoot strikers tend to direct greater amounts of force through the foot into the lower tibia and fibula, in a region located above the ankle, often to the medial (inside) aspect of the lower leg.
Biomechanical deficiencies are unlikely to cause stress fractures alone. A recreational runner who enjoys an easy 3 mi (5 km) run four days per week, will most likely not be affected by any one of these structural factors. It is the combination of greater workout and competitive intensities, training methods, and equipment that elevate the biomechanical factors from the background to prominence.
Stress fractures recur as a result of both a failure on the part of athlete to counter the biomechanical factors, as well as through the imposition of factors personal to the athlete. When an athlete has sustained a stress fracture, aggressive steps must be taken to ensure that the footwear is sufficient protection. An orthotic is often prescribed to correct leg length imbalance of high arch irregularity; a failure to take such steps is often a guarantee of a recurrent
Other training basics must be addressed to prevent a recurring stress fracture. The overall nutrition of the athlete must be examined to ensure that the proper daily intakes of calcium, its companion vitamin D, and related bone-building materials are being ingested. The athlete must ensure that stretching, flexibility, and general warm-up procedures are followed; poor flexibility will create biomechanical problems of its own. In the initial recovery stages from a stress fracture involving running, the athlete must select the training surfaces with care, as uneven or angled ground and roadways creates uneven distribution of forces into the feet and lower legs.
Extreme caution must be exercised in the treatment of a stress fracture that occurs in a young athlete. The long bones, such as the femur, tibia, and fibula, are constructed with a growth plate, located next to the end of the bone, known as the epiphysis. Any damage to the growth plate by way of a stress fracture may compromise the ability of the bone to grow to maturity.