Apical crown technique to model canal roots. A review of the literature.

The goals of endodontic therapy are to shape and clean the root canal system in order to receive a three-dimensional and hermetic filling of the entire endodontic space. The aim of this review is to analyze the development of the different canal shaping techniques from the manual stainless-steel instruments to the recently developed rotary Ni-Ti systems. The need to provide instruments of a suitable size to reach the apex of curved root canals drove the development of preparation techniques with a greater focus on the apical part. The Crown-Down technique involves the widening of the canal orifices with Gates-Glidden drills followed by the incremental removal of organic canal contents proceeding from the canal orifice to the apical portion using manual files. Files are used from larger to smaller with no apical pressure. Nevertheless, the term ''Crown-Down'' does not define the technique in the strictest sense, nor does it provide a specific sequence for the use of instruments, but rather defines a way to use the instruments. The preparation of root canals has been revolutionised by the development of Ni-Ti alloys. The features of this alloy give instruments profound flexibility and allow the manufacture of more tapered instruments compared with traditional stainless-steel instruments. In the second part of this review, numerous examples are provided concerning the comparison between different Ni-Ti systems and their techniques for use. A Crown-Down approach provides certain advantages including early organic debris removal, the creation of a large reservoir for irrigating solutions, a straighter access to the apical region of curved canals, and greater precision with regard to the exact working length and apical size.