Source : Webster's Revised Unabridged Dictionary (1913)

Asternal \A*ster"nal\, a. [Pref. a- not + sternal.] (Anat.)
   Not sternal; -- said of ribs which do not join the sternum.

Source : WordNet®

asternal
     adj : not connected to the sternum or breastbone; "asternal ribs"