I agree with Kittychan, this could be arterial insufficiency ulcer and it looks almost exactly like a pressure / trophic ulcer of diabetes i.e punched out look. Pressure or trophic ulcers are classically found on the pressure points, but the best way to differentiate between the two is pain. Arterial ulcer is painful ( unless the nerves are also damaged ) and pressure or trophic ulcers being secondary to lack of sensation are mostly painless. Lack of granulation tissue at the base is common to both.
