The success of using instruments while preventing failure depends on how the material, design and technique relate to the forces exerted on the instruments. To fully understand how the file reacts to applied forces, terms have been defined to quantify the actions and reactions to these forces. Common terms related to forces exerted on files have the following definitions:
- Stress — the deforming force measured across a given area.
- Stress concentration point — an abrupt change in the geometric shape of a file, such as a notch, will result in a higher stress at that point than along the surface of the file where the shape is more continuous
- Strain — the amount of deformation a file undergoes
- Elastic limit — a set quantity which represents the maximal strain that, when applied to a file, allows the file to return to its original dimensions; the residual internal forces that remain after strain is removed and return to zero.
- Elastic deformation — the reversible deformation that does not exceed the elastic limit
- Shape memory — the elastic limit is substantially higher than is typical of conventional metals
- Plastic deformation — permanent bond displacement caused by exceeding the elastic limit
- Plastic limit — the point at which the plastic deformed file breaks
These terms are especially useful in understanding the properties of nickel titanium. The significant advantage of a file made of a nickel titanium alloy is its unique ability to negotiate curvatures during continuous rotation without undergoing permanent plastic deformation or failure. Simply restated, nickel titanium alloys were the first, and are currently the only readily available economically feasible materials that have the flexibility and toughness necessary for routine use as effective rotary endodontic files in curved canals.
In 1991, the first commercial nickel titanium manual and rotary files were introduced by Dr. John T. McSpadden. In 1993, Dr. McSpadden also introduced the first series of nickel titanium rotary files having multiple non-conventional tapers (McXIM Series) that had six graduating tapers ranging from the conventional 0.02 taper to a 0.05 taper file. The progressive tapers served to reduce stress by limiting the file’s engagement during the serial enlargement of rotary instrumentation. Nickel titanium is termed an exotic metal because it does not conform to the normal rules of metallurgy. As a super-elastic metal, the application of stress does not result in the usual proportional strain other metals undergo. When stress is initially applied to nickel titanium the result is proportional strain. However, the strain remains essentially the same as the application of additional stress reaches a specific level forming what is termed a loading plateau, during which the strain remains essentially constant as the stress is applied. Eventually, of course, excessive stress causes the file to fail.
This unusual property of changing from an anticipated response to an unanticipated response is the result of undergoing a molecular crystalline phase transformation. NiTi can have three different forms: martensite, stress-induced martensite (superelastic), and austenite. When the material is in its martensite form, it is relatively soft and can be easily deformed. Superelastic NiTi is highly elastic, while austenite NiTi is non-elastic and hard. External stresses transform the austenitic crystalline form of nickel titanium into the stress induced martensitic crystalline structure that can accommodate greater stress without increasing the strain. Due to its unique crystalline structure, a nickel titanium file has shape memory or the ability to return to its original shape after being deformed.