Q: What is the difference between a stress reaction and a stress fracture in an athlete?

A: This is a controversial question: when does a stress reaction become a stress fracture?

A stress fracture is a microfracture of the bone that is caused by chronic repetitive stresses that exceed the strength of the bone. This is a very common overuse injury -- particularly in runners, basketball players, dancers and gymnasts.

X-rays show a stress fracture only about a third of the time that symptoms develop, which are usually pain, swelling and pain that worsens the more one participates in that sports activity. MRI scans are very accurate and will show ''signal changes'' with bone edema (swelling caused by accumulated fluids) in the area where the overuse injury has occurred.

A stress reaction usually just shows edema in the bone without the beginning of a fracture line. For a true stress fracture to be diagnosed, a fracture line is usually visible. However, it is sometimes hard to interpret from an MRI scan if you're seeing a bone edema or a fracture line.

Either way, the condition is usually treated by a period of rest from the offending activity. Depending on the bone and the location of the injury, the rest period can be anywhere from three weeks to three months. In rare circumstances, some stress fractures of the hip, tibia, fifth metatarsal and spine may require surgery.

The Jock Doc column runs every other week.

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