Salvageable or not (2)?

A 37-year-old otherwise healthy gentleman presented to clinic for extraction due to multiple times of crown failure (Fig.1,2).  It appears that crown margin is ~2 mm supragingival.  After RCT retreatment and fabrication of a little longer cast post & core, the tooth seems to be salvageable.  But the patient was adamant in extraction and implant restoration.

Fianlly the tooth was extracted.  In fact, it is indeed nonsalvageable.  Can you guess what is the main reason for nonsalvageability?

Before extraction, photos were taken, including Fig.1 and 3.  One day after extraction, the extracted tooth was examined.  Several crack lines were noted (Fig.4 arrowheads).  Fig.3 was re-examined carefully.  The same pattern of crack lines was confirmed (arrowheads).  Luckily we did not try to save this apparently non-salvageable tooth.  Careful looking for crack lines could save us trouble!

Xin Wei, DDS, PhD, MS 1st edition 08/03/2011, last revision 08/03/2011