The Collaborative International Dictionary of English v.0.48:

Eburnation \E`bur*na"tion\, n. [L. eburnus of ivory, fr. ebur
   ivory: cf. F. ['e]burnation. See Ivory.] (Med.)
   A condition of bone cartilage occurring in certain diseases
   of these tissues, in which they acquire an unnatural density,
   and come to resemble ivory.
   [1913 Webster]

WordNet (r) 3.0 (2006):

eburnation
    n 1: a change that occurs in degenerative joint disease in which
         bone is converted into a dense smooth substance resembling
         ivory