A Brief Introduction of the Generation of Dental Apex Locator

The development of the electronic apex locator (EAL) has helped make the assessment of

working length more accurate and predictable, particularly useful when the apical portion of

the canal system is obscured by certain anatomic structures:Impacted teeth, tori ,zygomatic

arch, excessive bone density, overlapping roots and shallow palatal vault.
The objective of working length determination is to establish the length (distance from the

apex) at which canal preparation and subsequent obturation are to be terminated. Methods

for determining working length are radiographs , electronic apex locators, tactile sense,

mathematics method, apical periodontal sensitivity, paper points, microscopic magnification

and average tooth length.


Root canals are surrounded by dentine and cementum that are insulators to electric current.

At the apical foramen there is a small hole in which conductive materials within the canal are

electrically connected to the periodontal ligament that is a conductor of electric current. The

resistive material of the canal (dentine, tissue, fluid) with a particular resistivity forms a

resistor, the value of which depends on the length, cross-sectional area and the resistivity of

the materials .


The first generation: Resistance between the periodontium and the oral mucous membrane in humans was

constant at 6.5 K Ohm, regardless of the age of the patients or the shape and type of teeth.

Contents of the canal (vital pulp tester vs. necrotic pulp) also had no effect upon the resistance.

First-generation apex location devices measure the opposition to the flow of direct current

or resistance. The resistance was measured between the two electrodes to determine

location within a canal. Pain was often felt with this type of apex locator.
Second-generation apex locatorsmeasure the opposition to the flow of alternating current or

impedance.This generation contains 2 types of apex locator: low frequency and high

frequency apex locator. Low frequency AL is based on the assumption that the impedance

between the oral mucous membrane and the depth of the gingival sulcus closely resembles

the impedance between the canal terminus and the oral mucous membrane.


The 3rd generation apex locator has been called “frequency dependent” apex locators. This

type was supplied by 2 frequencies to measure the impedance in the canal. There are 2

types of the 3rd generation ALs: impedance difference type and impedance ratio type.

Impedance difference AL measures the impedance value at two different frequencies and

calculates the difference between the two values (Yamashita, 1990) while impedance ratio

type measured the position of the file from the ratio between these two impedances.


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Brief Historical Background of Controlled Memory Technology

Optimal cleaning and shaping of root canal systems requires, among many things, the coincident integration and tangible application of numerous anatomical, clinical, and technique driven considerations. For example, the case must be diagnosed correctly; the clinical risk assessed; the technique, clinical supplies, and instruments selected; and all of the above used correctly and simultaneously to achieve the treatment objectives.

The first generation of Ni-Ti was ground from Ni-Ti file blanks and not heat-treated. Such first-generation instruments are superelastic. Superelasticity denotes the ability of the file to deform (strain) from its original shape under a physical load (stress). Clinically, this is manifest as a Ni-Ti file rotating in a curved canal and returning to its original shape upon removal from the root canal treatment equipment.

In essence, the Ni-Ti undergoes a transformation (the instrument is “strained”) from its harder austenite crystalline phase configuration to its softer martensitic crystalline phase configuration while under such “stress.” When the stress is relieved, it returns to its original shape (austenite). Such behavior is termed “shape memory.” First-generation (nonheat-treated) Ni-Ti instruments can generally accommodate approximately an 8% strain before fracture. In contrast, CM instruments do not possess superelasticity and do not undergo the aforementioned transformation.

The second generation of Ni-Ti files is heat-treated, either in the bulk raw material stage before grinding or, alternatively, after grinding. CM instruments are a subset of this second generation of heat-treated instruments. CM technology was introduced in 2010. Heat treatment processes are proprietary.

Interestingly, there is a new file that is heat-treated only in the apical 10 mm of its cutting flutes, providing flexibility at its working end. To the author’s knowledge, for all other current systems, heat treatment encompasses the entire instrument.

CM files are unique among the commercial products available at this time. While made of heat-treated Ni-Ti, they remain curved as they rotate around a curved canal. CM files do not regain their original shape after use. Hence, they have “controlled memory.” The literature suggests this CM feature reduces transportation and conserves tooth structure. The literature also states that CM files are 300% to 900% more resistant to cyclic fatigue and have a statistically significant greater flexibility than their first generation superelastic counterparts. Aside from flexibility, CM files have essentially equivalent torsional strength to nonheat-treated files.

In the existing scientific literature published to date, there are no unfavorable reported findings on CM attributes. The current literature file on CM technology is available by email from the author on request.

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