Osteal extravasation is a relatively minor medical condition.
In 2377, the Dinaali Tebbis suffered from osteal extravasation in his leg. However, of more concern was his chromovirus infection. (VOY: "Critical Care")
Physiologically, "extravasation" generally refers to the movement of fluid and/or cells out of transport vessels into the surrounding tissue. In this case, "osteal" implies it is associated with bones.