If the wound has straight, sharp edges (but probably pulled apart), then it's likely to be a dagging cut, if it's grazed-looking, as if the surface has been stripped off, then it could be various other things, including old or current flystrike. When searching for any maggots, look thoroughly right up under the crutch as well as elsewhere.
Wouldn't a maggot icon be brilliant, and unfortunately used a lot.