Bone marrow lesions in runners.

"More than half of the lesions (bone marrow edema) (58%; 26/45) fluctuated during the season, with new lesions occurring (20%; 9/45) and old lesions disappearing (22%; 10/45)."

Stuff happens to your bones during a marathon, or on that long weekend training run. Make sure you give yourself time to recover adequately before you pound out that next run.
The incidental finding of bone marrow edema (BME) on MRI in professional runners is not well understood. Bone takes on load, as it should. In this study, it is suggested that many asymptomatic athletes show BME lesions, many of which will come and go with training. It is most like proper and ample recovery that allows athletes to heal and not let these lesions turn into greater stress responses, or stress fractures. It is when the load comes too often, to long, heavy and hard that things might mount.

Methods:
Sixteen athletes (13 men and 3 women; mean age, 22.9 ± 2.7 years) were recruited from the Dutch National Committee middle-distance and long-distance running selection. All athletes had been injury free for the year before the study. Magnetic resonance imaging scans were obtained before the start of the season and at the end of the season.

14 of the 16 athletes had BME lesions before the start of the season (45 BME lesions in total). Most BME lesions (69%; 31/45) were located in the ankle joint and foot. More than half of the lesions (58%; 26/45) fluctuated during the season, with new lesions occurring (20%; 9/45) and old lesions disappearing (22%; 10/45). The few clinical complaints that occurred throughout the season were not related to the presence of BME lesions.

Am J Sports Med. 2014 May;42(5):1242-6. doi: 10.1177/0363546514521990. Epub 2014 Feb 20.
Bone marrow edema lesions in the professional runner.
Kornaat PR1, Van de Velde SK.