Endodontic files and reamers
Endodontic files and reamers are surgical instruments used by dentists when performing root canal treatment.
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.
This allows an adequate taper of the root canal which takes less time to prepare than with stainless steel and less files needed.
At smaller sizes the files can be pre-curved which is a major advantage for the debridement of roots with sharp curvatures.
The ISO stainless steel files on the market today include K-Flex, K-Flexofile and Hedström where the tip size and taper is standardised.
The introduction of Nickel Titanium in dentistry has allowed the use of rotary systems to be used to prepare root canals safely and predictably.
It is advisable to use a dedicated electric endodontic motor where torque and speed can be easily controlled dependent on the system chosen.
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.
[7] The 3D scrubbing effect of the file, combined with the fresh irrigant, result in clean canals, which in turn facilitate better obturation.
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.
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.
[1] In 2007, new legislation documenting the possible risk of prion disease transmission via endodontic files/reamers during root canal treatment was published via the BDJ.
[9] The conclusions made were such that there was no significant risk associated but the implementation of single use instruments was introduced to take all possible precautions.
This was primarily due to the shape and relative surface area of the files making thorough disinfection and sterilisation very difficult.
The more curved the canal, the greater the cyclical fatigue placed on the instrument, as it is undergoing repetitive tensile and compressive stresses upon rotation no matter the flexibility of the alloy.
Pre-curving of the stainless steel files for canal negotiation will work-harden them, rendering them more brittle and therefore are more likely to fracture.
The greater the diameter of the instrument, the more force it can withstand despite needing increased torque however, the less resistant it becomes to cyclic fatigue.
Torsional fatigue can be somewhat limited through creation of a glide path and adopting the Crown-Down technique in a bid to reduce frictional forces.
Beware of surface defects arising from the manufacture of the files, which can propagate under fatigue by creating stress concentrations and ultimately lead to fracture.
Evidence shows that hand instrumentation will result in a lower risk of file fracture compared with rotary and this may be attributed to increased rotational speed, which enhances the effects of cyclic fatigue.
