Stress fracture
Specialty	Rheumatology

This article is about stress fractures in bones. For stress fractures in engineering, see fracture and fatigue (material).

A stress fracture is one type of incomplete fracture in bones. It is caused by "unusual or repeated stress"^[1] This is in contrast to other types of fractures, which are usually characterized by a solitary, severe impact.

It could be described as a very small sliver or crack in the bone;^[2] this is why it is sometimes dubbed "hairline fracture". It typically occurs in weight-bearing bones, such as the tibia (bone of the lower leg) and metatarsals (bones of the foot).

It is a common sports injury, and more than half of the cases are associated with athletics.^[3]

Stress fractures usually have only a few symptoms. A stress fracture could present as a generalized area of pain and tenderness associated with weight bearing. Usually when running, a stress fracture in the leg or foot will cause severe pain at the beginning of the run, moderate pain in the middle of the run, and severe pain at the end and after the run.

As with most conditions, a stress fracture is best diagnosed after interview and examination by a physician. Investigations are not necessary to diagnose a stress fracture.

X-rays usually do not show any evidence of stress fractures, so a CT scan, MRI, or 3-phase bone scan may be more effective in unclear cases.

If a stress fracture occurs in a weight-bearing bone, healing will be delayed or prevented by continuing to put weight on that limb.

Rest is the only option for complete healing of a stress fracture. The average time of complete rest from the activity that caused the stress fracture is three weeks. A fracture requires four to eight weeks of recuperation, however, which may include no more than light use of the injured body part, as long as the activity does not cause pain. After the recuperative period, another two weeks of mild activity without any pain may be recommended before the bone may be safely considered healed and activity gradually resumed.

During this time, training errors should be identified (e.g., too much activity too soon)and avoided. One rule of thumb is to not increase the volume of training by more than 10 percent from one week to the next.

Rehabilitation usually includes muscle strength training to help dissipate the forces transmitted to the bones.

Bracing or casting the limb with a hard plastic boot or air cast may also prove beneficial by taking some stress off the stress fracture. An air cast has pre-inflated cells that put light pressure on the bone, which promotes healing by increasing blood flow to the area. This also reduces pain because of the pressure applied to the bone. If the stress fracture of the leg or foot is severe enough, crutches can help by removing stress from the bone.

With severe stress fractures, surgery may be needed for proper healing. The procedure may involve pinning the fracture site, and rehabilitation can take up to a half year.^[4]

Bones are constantly attempting to remodel and repair themselves, especially during a sport where extraordinary stress is applied to the bone. Over time, if enough stress is placed on the bone that it exhausts the capacity of the bone to remodel, a weakened site -- a stress fracture -- on the bone may appear. The fracture does not appear suddenly. It occurs from repeated traumas, none of which is sufficient to cause a sudden break, but which, when added together, overwhelm the osteoblasts that remodel the bone.

Stress fractures commonly occur in sedentary people who suddenly undertake a burst of exercise (whose bones are not used to the task). They may also occur in Olympic-class athletes who do extraordinary quantities of high-impact exercise, in professional and amateur marathon and/or distance runners who run a lot of mileage per-week or in soldiers who march long distances.

Muscle fatigue can also play a role in the occurrence of stress fractures. For every mile a runner runs, more than 110 tons of force must be absorbed by the legs. Bones are not made to stand that much energy on their own and the muscles act as shock absorbers for the excess force. But, as muscles become tired and stop absorbing most of the shock, the bones experience greater amounts of stress. Finally, when muscles (usually in the lower leg) become so fatigued that they stop absorbing any shock, all forces are transferred to the bones.

Previous stress fractures have been identified as a risk factor.^[5]

Dr. Johnathan C Reeser states that, in the US, the annual incidence of stress fractures range from 5% to 30%, depending on the sport and other risk factors.^[6] Stress fractures occur less frequently in those of black African descent than in Caucasians, due to a higher BMD (bone mineral density) in the former. Women and highly active individuals are also at a higher risk. The incidence probably also increases with age due to age-related reductions in BMD. But children may also be at risk because their bones have yet to reach full density and strength. The female athlete triad also can put women at risk, as disordered eating and osteoporosis can cause the bones to be severely weakened.

One method of avoiding stress fractures is to add more stress to the bones. Though this may seem counter-intuitive (because stress fractures are caused by too much stress on the bone), moderate stress applied to the bone in a controlled manner can strengthen the bone and make it less susceptible to a stress fracture. An easy way to do this is to follow the runner's rule of increasing distance by no more than 10 percent per week. This allows the bones to adapt to the added stress so they are able to withstand greater stress in the future.

Strengthening exercises also help build muscle strength in the legs. Strengthening these muscles will prevent them from becoming fatigued quickly, allowing them to absorb the strain of running for longer periods of time. Key muscles that need strengthening with lower leg stress fractures are the calves and the shin muscles. Runners often suffer from overuse injuries or repetitive stress injuries. These include stress fractures, stress reactions, tendonitis, meniscal tears, ITB Friction syndrome, and exacerbation of pre-existing arthritis. Stress fractures, if not diagnosed and treated, can develop into complete fractures.^[7]

Depending on a variety of factors (including weight, running surface and shoe durability), runners should replace their shoes every 300-700 miles to allow adequate mid-sole cushioning. A change in running surfaces can also help prevent stress fractures. However, it is also argued that cushioning in shoes actually causes more stress by reducing the body's natural shock-absorbing action, increasing the frequency of running injuries.^[8]

During exercise that applies more stress to the bones, it may help to increase calcium and vitamin D intake, depending on the individual. Also, it is important to monitor diet, because nutrition plays a vital role in bone development. Some individuals are at risk of osteoporosis, and depending on the country in which medical care is being supplied, there may be an osteoporosis screening program available.

A new study released by Creighton University has shown Calcium and Vitamin D supplementation, even over a short period, can significantly reduce stress fractures in female military recruits. The study results were reported Sunday, Feb. 11, 2007 at the 53rd annual Orthopaedic Research Society meeting at the San Diego Convention Center.^[9]