According to the American Endodontic Association, NiTi is a "super elastic" metallic alloy that, when bent, changes from its initial structure to an austenitic-martensitic state, making it incredibly flexible. When the metal is under stress, as when a root canal is being instrumented, this transformation typically occurs.
NiTi: The Endodontic Rotary File System
When the metal is under stress, as when a root canal is being instrumented, this transformation typically occurs. The NiTi file will, however, break if it is stressed past its elastic limit. NiTi is a special material because of its "shape memory," which enables it to deform while in use and then revert to its original shape if it is not overstressed.
In comparison to stainless steel files, NiTi files, as per the American Endodontic Association, have higher elastic flexibility in bending and torsion. NiTi rotary is known for its flexibility, and it is this quality that enables it to successfully navigate one of the biggest obstacles in root canal instrumentation: the sharp, unanticipated curves in most canals. The instrumentation of the root canal is significantly quicker and more effective when the NiTi rotary is used correctly.
Super Elasticity
When the strain increases while the stress stays constant, a certain range of NiTi alloys exhibit the super elasticity phenomenon. The austenite's transformation into the martensite form gives them their superelastic properties. As per the American Endodontic Association, when the entire NiTi mass has changed to the martensite type, which would imply that the super elasticity stage has ended, the strain stays unchanged throughout this transformation.
Once the capacity of the structure's crystals to absorb strain has been used up, further deformation will result in the stress/strain curve taking the form of a classic relationship. The deformation will still be reversible if the plastic deformation limit is not reached.
Fracture Mechanisms
Instruments made of NiTi carry a risk of separation or fracture due to torsional or cyclic fatigue. Rotating NiTi file distortion is frequently invisible without magnification. In contrast to SS instruments, where distortion can be clearly seen, these instruments can fracture without any prior indications. Torsional fatigue results from the different parts of the file rotating at different speeds, whereas cyclic fatigue is caused by the tool rotating in a curved canal and creating repeated tension and compression cycles that eventually lead to metal fatigue.
In a clinical setting, torsional fatigue happens when an instrument tip becomes stuck to the canal dentin while the shank continues to rotate, producing a torque that leads to a torsional fracture.
The amount of twisting a file can withstand before breaking is known as its torsional fatigue resistance. When used in a small, constrained canal with a high torsional load, this property is desirable. NiTi Endodontic Rotary File Systems are constantly being modified by the manufacturers in an effort to enhance their mechanical characteristics, particularly their resistance to fatigue.
The Bottom Line
Three goals were considered when designing the NiTi Endodontic Rotary File System:
· Cutting effectiveness: The file ought to be able to easily loosen up a tight canal.
· Flexibility: The file should have the ability to follow both anticipated and unforeseen curves in a root canal.
· Separation resistance in curved, calcified canals.
