(This confidence level - which I entered in two or three cases only in the past - is to express that I am at least as confident as the other 5-confidence answerer on this issue ;-) )

First, distortion is far less common usage for this kind of injury, English medical texts mostly talk of "ankle sprains". (In German, Distorsion is the technical term, thus German native authors as Hackenbruch tend to translate it one-to-one.) Refs on sprain / strain differentiation:

http://www.vsm.vanderbilt.edu/sprains.html
http://www.umm.edu/aniplayer/data.html

Principally, ankle is understood as the joint between Talus and Tibia, the "obere Sprunggelenk"; for the "lower" segment of the joint, "subtalar joint" is the better choice IMO:

http://moon.ouhsc.edu/dthompso/namics/footintr.htm

Looking through websites on this injury, I have got the impression that 'upper' or 'lower' refers to the ligaments/muscles/tendons involved rather than to OSG or USG - maybe one of my English-native doctor colleagues at ProZ can provide his/her opinion?

HTH :-)