Hand files can provide tactile sensation when cleaning or shaping root canals. This allows the dentist to feel changes in resistance or angulation, which can help determine curvature, calcification and/or changes in anatomy, in which two dimensional radiographs may not always identify. This information can help determine strategies or avoid complications before moving on to rotary instruments.
The different K files available on the market include K-Flex (Sybron-Endo, Orange, Ca, USA), K-Flexofile (Dentsply-Maillefer, Ballaigues, Switzerland), and EdgeK-File and EdgeK-File NiTi nickel titanium (EdgeEndo, Albuquerque, NM). The cutting edge of K type files is made up of twisted squares of stainless steel alloy. The K-flex file differs for the fact it has a rhomboid shaped cross-section and has an increased flexibility compared to traditional K-files
C-files are stiffer than K-files, and are recommended for calcified canals and ones that are curved and narrow. One example of C files available on the market is the Edge C-File (EdgeEndo, Albuquerque, NM).
Nickel-titanium is a superelastic alloy which allows it to undergo greater stresses compared to stainless steel therefore files have a reduced risk of file fracture. It also has the characteristic of ‘shape memory’ which allows it to return to its initial shape through heating after strain. This reduces the risk of deformation within the root canal as forces of compression and tension are absent.
The superelasticity allows an increase in taper (between 4–8%) compared to stainless steel. This allows an adequate taper of the root canal which takes less time to prepare than with stainless steel and less files needed. The super elasticity also means the risk of zipping and apical transportation is reduced.
Many Nickel-titanium files are available. The files can be used within rotary systems or manually for a higher level of control.
Watch winding and circumferential filing technique
The use of the file in a forwards and backwards motion, as if watch winding, with slight apical pressure. This allows the file to effectively debride the canal dentine by moving slowly down the canal.
For K-type files, once the file has reached the desired working length a push and pulling action is used around the circumference of the canal, while only maintaining contact with the canal wall on the outstroke to minimise a debris blockage apically.
The balanced force technique
This is the most widely used technique and especially good for working with curved canals
Files used for this technique need to be non-cutting edge and flexible. the file is rotated 60 degrees clockwise in the canal when a slight resistance is felt. the file is then rotated 360 degrees anticlockwise to pick up the dentine in the flutes that was made during the first rotation. this should be done no more than three times before the file is removed and cleaned and the canal system irrigated before reinsertion.
The cross-section of a Hedstrom file is made up of a continuous sequence of cones. They are very sharp with a cutting tip. Their use in a push-pull fashion results in a high level of debridement on removal from the root canal. They should not be rotated more than 30 degrees as they are narrow and vulnerable to fracture. They are also used for removal of root canal filling materials e.g. gutta percha during secondary root canal treatment. An example of Hedstrom files available on the market are Edge Hedstroms (EdgeEndo, Albuquerque, NM).
This file is used to remove pulp tissue (extirpation) during root canal treatment. There are sharp barbs on the file to engage the pulp tissue and remove this efficiently. These files are not used to shape the RCS.
The handles of the ISO instruments are colour coded and are available in three different lengths of 21mm, 25mm and 31mm where the extra length is non-cutting shaft. This extra length is particularly useful for posterior teeth where access and visibility is impaired.
ISO files are made of stainless steel. This can be useful in smaller files (<20) but larger files have increased rigidity which can result in procedural errors. At smaller sizes the files can be pre-curved which is a major advantage for the debridement of roots with sharp curvatures. Their rigidity also has an advantage in calcified root canals in the initial stages of debridement.
The ISO stainless steel files on the market today include K-Flex, K-Flexofile and Hedström where the tip size and taper is standardised.
ISO normed hand files have a standardised taper of 2% that equates to 0.02mm increase in diameter per mm of file. This standardised taper allows you to calculate the diameter of any given stainless steel file at any given point. Where the 2% taper means that there is an increase in diameter by 0.02mm every 1mm of file (moved in a coronal direction). The most apical point of any file is deemed D0, so moving coronal on the file by 1mm brings you to D1 and so on, up to D16 as there is a 16mm cutting surface on all files.
For example, an ISO K file size 25 has a D0 value of 0.25mm diameter at its tip. If you were to move 6mm coronally on this file from D0, the cross sectional diameter would be:
0.25mm + (6mmx0.02mm)=0.37mm
The range of files are available as hand and rotary. The first files in the series are termed SX, S1 and S2. These are used to improve access to the canals by first creating a coronal flare in the crown-down technique.
SX files are typically used first as they are shorter in overall length 19mm and so are good in cases of restrictive space. The canal is prepped in the coronal 2/3 with these files as part of the crown-down technique.
After this, files named F1, F2, F3 etc. are used with increasing D0 values. These are used to shape the canal.
Between each of these finishing files, you should recapitulate the canal using the corresponding (with the same D0 value) K file. This prevents procedural errors, confirms the canal remains patent and prevents dentine swarf build up in within the canal. Complete copious irrigation in between each file.
The introduction of Nickel Titanium in dentistry has allowed the use of rotary systems to be used to prepare root canals safely and predictably. Rotary instrumentation is known to have an improved cutting efficiency when compared with hand filing techniques. It is advisable to use a dedicated electric endodontic motor where torque and speed can be easily controlled dependent on the system chosen. Despite the advantages of rotary systems, it is always recommended to create a glide path with hand files in each canal prior to rotary instrumentation. There are numerous rotary files available on the market, including a variety of systems from EdgeEndo, Albuquerque, NM , Micromega , FKG .
Protaper (Dentsply – Maillefer, Ballaigues, Switzerland) is a series of hand and rotary files that include a selection of shaping and finishing files. They create a predictable shape and taper and are widely used by clinicians throughout the world. The files have corresponding K files which allow to diameter of apical tip to match as well as paper points and master cones used in obturation
Reciprocating systems involve rotation of the file in both anti-clockwise and clockwise directions. This is similar to the ‘balanced force’ mechanism used with hand files. When the file is used in an anti-clockwise direction, it engages dentine and is quickly followed by a clockwise turn before re-engaging the root canal wall and shearing the dentine. Benefits of a reciprocating system include:
An example of a reciprocating system is Wave One (Dentsply). Other reciprocating files available on the market are the X1, EdgeOne Fire, and EdgeOne Platinum (EdgeEndo, Albuquerque, NM).
Self-adjusting file systems have been developed to overcome complications that arise due to complex anatomy and canal configurations. These files are used in a rotary hand piece and consist of a flexible, thin NiTi lattice with a hollow centre that adapt three-dimensionally to the shape of a given root canal, including its cross section. The files are operated with vibratory in-and-out motion, with continuous irrigation of disinfectant delivered by a peristaltic pump through the hollow file.A uniform layer of dentin is removed from the whole circumference of the root canal, thus achieving the main goals of root canal treatment while preserving the remaining root dentin. The 3D scrubbing effect of the file, combined with the fresh irrigant, result in clean canals, which in turn facilitate better obturation. More effective disinfection of flat-oval root canals is another goal that is simultaneously attained.
D files are a selection of bespoke rotary files that are commonly used in re-treatment cases for the efficient removal of gutta percha. They are used in sequence to remove the coronal (D1), mid (D2) and apical (D3) ⅓ root filling material more efficiently before the final shaping with conventional instruments. D1 is 16mm in length with a cutting end tip to engage the filling material in the canal. D2 and D3 are 18mm and 22mm in length respectively, both are non end cutting and aim to not remove remaining dentine from canal walls in the process.