Figure 2: Coronectomy: A, cutting crown below cement-enamel junction (arrow); B, trimming cutting surface to less than 3 to 4 mm below alveolar crest.
From: Coronectomy – oral surgery's answer to modern day conservative dentistry

(Reprinted from Journal of Oral and Maxillofacial Surgery, volume 67, Y. Hatano, K. Kurita, Y.Kuroiwa, H. Yuasa & E. Ariji, Clinical evaluations of coronectomy (intentional partial odontectomy) for mandibular third molars using dental computed tomography: a case-control study, pages 1806-1814, copyright (2009), with permission from Elsevier)