Source : Webster's Revised Unabridged Dictionary (1913)

Trepan \Tre*pan"\, v. t. & i. [imp. & p. p. {Trepanned}; p. pr.
   & vb. n. {Trepanning}.] [Cf. F. tr['e]paner. See {Trepan} a
   saw.] (Surg.)
   To perforate (the skull) with a trepan, so as to remove a
   portion of the bone, and thus relieve the brain from pressure
   or irritation; to perform an operation with the trepan.

Source : WordNet®

trepan
     n 1: a surgical instrument used to remove sections of bone from
          the skull [syn: {trephine}]
     2: a drill for cutting circular holes around a center
     v : cut a hole with a trepan, as in surgery
     [also: {trepanning}, {trepanned}]

trepanned
     See {trepan}