1Stone L H,Stroner, W F. Maxillary molars demonstrating more than one palatal root canal[ J]. Oral Surg Oral Med Oral Pathol, 1981,51:649 - 652.
2Libfeld H, Rotstein I. Incidence of four - rooted maxillary second molars: literature review and radiographic survey of 1,200 teeth [ J ]. J Endod, 1989,15 : 129 - 131.
3Peikoff, M D, Christie W H, Fogel H M. The maxillary second molar: variations in the number of roots and canals [J]. Int Endod J ,1996,29:365 -369.
4David J C. Shaping and restoring ovoid canal systems [ J ]. Endodontic Therapy, 2005, (5) : 9 - 13.
