In endodontic treatments, Nickel-Titanium instruments in continuous rotation (1) optimize root canal shaping. Generally, rectilinear and barely curved root canals with a round or oval section does not cause difficulties and can be prepared by using all standard techniques.
However, particularly thin and moderately or strongly curved canals with a laminar section are more difficult to shape and involve a considerable risk of failure.
Despite its super elastic qualities, Nickel-Titanium alloy has one important inconvenience, namely its low resistance in case of repeated use which results in instrument separation.
Instrument fracture can occur either through material fatigue caused by a significant number of compression-tension cycles or through torsion due to obstruction of the instrument’s tip in the canal (2) (4).
A certain number of factors such as the pressure exercised on the contra-angle head (5), the speed of rotation and the number of clinical applications favour the occurrence of instrument separation.
In addition to these procedural mistakes, instrument diameter, taper, profile (6) (7) and machining as well as canal curvature are crucial for the occurrence (or not) of instrument fracture (8).
Continuous rotation versus reciprocating technique
In recent years, we have seen several alternating movement systems (clockwise – counter-clockwise rotation) come forward, destined to limit instrument separation, for example M4® (Sybron Endo), Endo-Eze AET® (Ultradent), EndoExpress® (Essential Dental System), WaveOne® (Dentsply) and Reciproc® (VDW).
The alternative movement technique varies between 30° and 90°, being thus either symmetric or asymmetric, depending on the manufacturer. The kinetics of reciprocation reproduces the manual movement of the intra-canal file, restricts the risk of instrument fracture and facilitates the penetration into calcified canals (9).
The systems with a 90° alternative and symmetric movement require a large instrumental sequence whereas the systems limited to a 30° movement have a restricted cutting capacity and a tendency to extrude dentine and pulp debris towards the periapex (10).
The latest generation systems with an asymmetric range do not require any pressure being exercised on the contra-angle head.
Although an evolution of the GIROMATIC® technology seemed to be possible, the new One Shape® instrument is used in continuous rotation. The acknowledged benefits of this rotational dynamic are an excellent tactile sensation and a remarkable cutting efficiency.
The difficulty in the instrument’s development lies in its profile which is specifically dedicated to root canal shaping with only one single instrument in continuous rotation.
The instrument’s variable cross-section with a diameter of 25/100 mm and a .06 taper constitutes the innovation of One Shape® (fig.1).
The resistance of a NiTi instrument to separation as a result of torsion and bending depends on its diameter and cross-section (11) (12). 2 cutting edges provide an outstanding resistance to bending whereas a triple helical pitch better withstands torsion (13).
One Shape® presents 3 different cross-section zones along its length to ensure greater flexibility and limit aspiration.
The 16 mm cutting zone consists of:
– A first zone with a length of 2 mm presenting a variable 3-cutting-edge design to ensure a centred progression of the file towards the apex. At the same time the file respects the initial canal path and curvatures, due to the guidance of its non-working tip (fig.2).
– A second transitional zone with a length of 7.5 mm which progressively changes from 3 to 2 cutting edges.
– A third coronal zone with a length of 6.5 mm provided with 2 symmetric and positive cutting edges for an efficient upward debris removal.
The innovative concept of an instrument with variable cross-sections (fig.3) facilitates the downward movement in the root canal, guarantees greater flexibility and respects the original canal path, thanks to a centred progression and the continuous rotation technique.
A single use instrument
One Shape® is a single use instrument. However, it can be used for the endodontic treatment of teeth with one or more roots. Above all, the single instrument concept implies a considerable simplification of the application protocol and thus ensures safe and efficient root canal shaping, whereas the single use concept avoids a systematic control of the tip or the file for unwinding signs.
The single use concept – One Shape® is supplied in sterile blister packaging (14) – which also prevents cross-contamination as a consequence of insufficient instrument decontamination.
Finally, the single use concept evades the weakening of NiTi instruments due to their contact with sodium hypochlorite irrigating solution and autoclaving (15) (16).
The use of One Shape® requires an endodontic contra-angle connected to a “traditional” motor with a rotational speed of 400 rpm. The instrument gradually descends into the root canal by simultaneously brushing the canal walls in a range of 1 to 2 mm without pressure on the contra-angle head. This brushing process eliminates dentine overhangs and constraints (17).One Shape® shapes the root canal and limits obstructions towards the apex. Once the working length is reached, a wide range brushing movement with pressure exercised on the canal walls is recommended in order to verify the free space of the One Shape® instrument in the canal and eliminate the pulp parenchyma.
This mechanical preparation process with a wide taper ensures extensive irrigation and efficient cleaning of the root canal system (19).
Respecting the anatomy and the constriction of the apical zone is essential for the success of each endodontic treatment. An over instrumentation beyond the apical limit with wide tapered NiTi files always results in apical zipping (20), over obturation with apical transgression and a defect in the three-dimensional sealing (21).
This type of complication during the operation often leads to the failure of the endodontic treatment, particularly in case of a preoperative, periapical radiolucency (22).
Expert opinions differ considerably concerning the perfect diameter and taper for the preparation of the last apical third.
A circular preparation of the constriction or an apical limit prepared with a diameter of 40/100 mm and a .06 taper is not “cleaner” than a preparation with a diameter of 20/100 mm and a .08 taper (23).
However, the precise determination of the apical limit and its verification during the operation are vital for a successful endodontic treatment (25).
The working length actually evolves during the root canal preparation due to the instrument’s linear action (24).
The One Shape® method helps to carry out a safe root canal preparation provided that the simple protocol is applied. As for all the root canal preparation methods the pulp chamber opening has to be sufficient for a direct access to the canal system. Dentin overhangs have to be eliminated. The real challenge in endodontics is to locate the canal path, make it permeable and secure it down to the working length (26).
The exploration of the root canal is accomplished by using either a MMC 15 type manual file or mechanized instruments such as G-Files® 12/100 mm or/and 17/100 mm. In the case of a strongly curved canal path, the coronal part of the canal has to be widened and straightened by using EndoFlare®. This procedure also restricts the bending stress on the instrument during the preparation of the canal’s most apical portion (28). After validation of the exploration process, the pulp chamber has to be thoroughly irrigated using sodium hypochlorite (3 % to 5.25 %).
The action of the One Shape® instrument starts with a downward movement of a few millimetres into the canal at a rotational speed of 400 rpm. As soon as a resistance is encountered, a low range up and down movement has to be carried out. This brushing movement on the canal walls facilitates the access to the apical third.
To accurately measure working length and achieve apical patency, a thin diameter file connected to an electronic apex locater will guarantee maximum precision. This determination method of the apical limit after enlargement of the coronal 2/3 yields reliable and reproducible results, particularly in long and curved canals (29). As a matter of fact, the working length varies significantly during root canal shaping.
A MMC 15 file retraces the canal path, frees the foramen from any obstruction and activates the irrigation solution (30). This verification of the apical anatomy is particularly important when using a single instrument method, since over instrumentation leads to significant post-operative symptomatology (31).
The use of an electronic apex locator is highly recommended (32), especially regarding their current precision after elimination of constraints in the coronal third (33).
One Shape® – the single file system for root canal shaping – is a solution destined to practitioners who face the following difficulties:
– reluctance to adopt new techniques
– aseptic chain organization
– insufficient and inadequate root canal preparation
– appearance of overhangs[B1] and constraints
– mechanized instrument separation
– complex instrumental protocol
– long and difficult shaping
The single instrument One Shape® is an innovative concept for root canal shaping. Thanks to its diameter of 25/100 mm and its .06 taper, this instrument with a simple and rapid protocol allows even the shaping of thin and curved root canals. The instrument design combined with a continuous rotation movement guarantees a reliable efficacy all the way down to the apex without stress on the instrument.
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by Dr. Jérome Elias, Dr. Jean-Jacques J-J Bonnin