The Collaborative International Dictionary of English v.0.48:
Asternal \A*ster"nal\, a. [Pref. a- not + sternal.] (Anat.) Not sternal; -- said of ribs which do not join the sternum. [1913 Webster]WordNet (r) 3.0 (2006):
asternal adj 1: not connected to the sternum or breastbone; "asternal ribs"