Clinical Continued »


instruments were the Gates Glidden drills (sizes ı-6) introduced in ı885. Development of motors in the


ı960’s introduced variation on the norm of 360 degree continuous rota- tion e.g. the Giromatic handpiece rotated 90 degrees clockwise and counterclockwise (reciprocation). Sonic and Ultrasonic handpieces e.g. CaviEndo, were introduced in the ı980s, but fell out of favour.


Discovery of Nickel Titanium William Buehler first developed the shape-retaining alloy of Nickel and Titanium for use in the US Navy in ı959 and called it NiTinoL (Nickel Titanium Naval Ordnance Lab.) after the laboratory where he worked. NiTi has a different structure


from other alloys such as stainless steel, which allows it the status of superelasticity, and shape memory to retain its shape after deforma- tion. This is because of the different atomic structure between the two alloys, particularly in the transfor- mation from austenite to martensite that occurs in NiTi. Steel allows three per cent defor-


mation, NiTi can withstand up to seven per cent deformation without permanent damage. Steel can with- stand up to 20 bending cycles whereas NiTi can undergo ı,000 cycles without fracture. NiTi was introduced into dentistry in ortho- dontic wire but soon its properties were adapted for endo purposes. NiTi files allow for better shaping


of canals with fewer procedural complications. Numerous studies using extracted teeth have concluded that rotary NiTi files maintain the original canal curvature better than stainless steel files. However, NiTi files can fracture without warning and are very difficult to remove or bypass if ‘locked’ into a narrow canal. Two modes of fracture have been


identified for NiTi instruments, (a) Cyclic fatigue where the file is rotating freely but the point of maximum flexure becomes work- hardened, and the file breaks at that point, and (b) Torsional fracture where the tip of the instrument becomes locked into the canal but rotary motion continues. The elastic limit of the metal is exceeded and the file unwinds and then breaks. The goal of manufacturers is to find the best design of file that combines


26 Ireland’s Dental magazine


efficient shaping of the canal with the minimum chance of fracture. Initially, NiTi endodontic files


were copies of hand files in that they had two per cent taper and were used in regular low-speed hand- pieces. Unfortunately, this caused a clinically unacceptable number of instrument fractures so special motors with variable speed and torque controls were introduced. As they evolved, manufacturers dabbled with design features such as cutting angle, tip design, the number of blades, taper size and cross-section. Figure 4 demonstrates the variety of cross-section patterns available in the most common systems avail- able. The variety of designs is an indication that no one pattern has yet proved superior in efficacy. As well as two per cent taper,


files with four, six, eight and up to ı2 per cent taper were intro- duced. ProTaper files introduced the concept of “variable taper” where the degree of taper varied along the length of the cutting section, the purpose being to file different parts of the canal with different files. Having sizes from ı5 to as large


as 60 in these systems involved stocking a lot of expensive files and all the manufacturers have been trying to simplify and reduce the number of files necessary to prepare a canal. Packs are produced with sequences


of files to be used in ‘easy, average or difficult’ cases, or ‘narrow, medium, or large’ canals. More recently, Dent- sply and its European associate firm, VDW, has introduced files to be used with a reciprocating action and claim that just one file will prepare most cases. However, one has to choose between three sizes of file depending on the visibility of the canal on radio- graph and the use of handfiles size ı0, and ı5 is recommended to assess the curvature of the canals. As the tip size is only 25, even Dr


Julian Weber, who was involved in the development, advises that ‘Apical gauging’ be subsequently carried out with hand files to determine the actual diameter of the apical foramen. In a maxillary molar, one can have any combination of wide, medium or narrow canals with degrees of curvature that designates them from easy to difficult – all in the one tooth. So the concept of ‘one size fits all’ is no closer in endodontics than it is


Fig 5


Representations of challenging canal anatomy, (courtesy of Dr Ronald Zapata)


in real life. There are ample anatomical studies which show that canals in normal adult teeth are wider than 350-400 microns in the apical portion, i.e. greater than 0.35mm in diameter and this increases when resorbing apical periodontitis has developed. It is obvious that preparing a canal


to size 25 will not clean the apical part of all canals and larger hand or rotary files are necessary. Many endodontists use handfiles to scout canals initially, NiTi rotary files to carry out the bulk of preparation and fine-tune the final apical millimetres with hand files again. Also, micro computed tomography scans are revealing the diversity of anatomical patterns in canals (Fig 5) and evidence that use of hand or rotary instrumentation often leaves up to 50 per cent of canal surfaces untouched, emphasises the fact that files ‘shape’ but do not ‘clean’ canals. This is carried out by the other aspect of preparation, i.e. irrigation. As preparation time with the newer


files decreases, we must consider using this added time for irrigation, for removing as much bacterial debris from the untreated parts of the canals as possible. Subsequent obturation of the total canal space with a suitable filling material and method will allow healing of the periapical tissues to occur.


® Part two will describe specific NiTi file systems.


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