The Self-Adjusting-File (SAF) Technique
In 1993 the first endodontic files, made from a nickel-titanium (NiTi) alloy, were introduced to the dental market and definitely contributed to a radical change in the operative approach to endodontic treatment . Compared to previous stainless steel files, they were much more flexible, effective and better performing in shaping root canals. Furthermore, in recent years, advances in knowledge and technology have brought about several interesting changes in these alloys, which were made increasingly resistant to torsional load and, therefore, safer to use.This has led to the manufacturing of a “single file” , which enables dental practitioners to shape the full length of the root canal using a single instrument, such as Wave One ® (Maillefer- Dentsply , Ballaugues, Switzerland) or Reciproc ® (VDW, Munich, Germany) .Despite these structural improvements, the file design has remained unchanged : a solid body with more or less sharpened blades, which cut the canal from different angles and collect treatment debris in their flutes. This suggests that, despite the different endodontic variations recurrent in nature, we do nothing but imprint the instrument shape inside the canal, regardless canal anatomy ; this apparently allows us to use similar procedures with either round-shaped and oval- shaped canals , as well as with more or less tightly curved canals.From the studies carried out by Paqué [3,] analyzing MicroCT sections, it is clear that instrumentation with conventional Ni-Ti tools does not allow us to accomplish a proper cleaning of the whole root canal system (fig.1). In fact, these instruments, due to their shape, cannot contact the entire surface of root walls, thus leaving within a certain amount of unremoved tissue, which also reduces the quality of subsequent obturation , as highlighted by De Deus (Fig2) [4.5].
The search for more effective instruments has consequently led to the manufacturing of files with increasingly “active” cutting edges, resulting in greater amounts of treatment debris, as well as greater stress in the dentin, which may cause “micro-cracks” within roots . These are likely to provoke actual root cracks at first , and possible vertical fractures thereafter [6,7,8,9].The introduction of the SAF® (Self Adjusting File, Redent Nova, Israel), in 2010, marked a new definition of the concept of 3D shaping, cleaning and maintenance of endodontium anatomy, thanks to an approach which is completely different from the past.Nowadays, the assertion that endodontic treatment is safer and less invasive than in the past, can be confirmed by over 50 studies published in leading international scientific journals, and by a growing number of clinical applications.
The SAF System
The SAF is the first endodontic file with a hollow tube and not characterized by a solid metal core. As a matter of fact, it consists of a hollow cylinder with two parallel longitudinal beams held together by a Nickel- Titanium lattice; the manufacturing process is not performed using carving techniques, but sand-blasting induced roughening.The working part has no flutes, but only a rough surface that ensures a gentle and effective action of grinding and smoothing the root. (Fig 3).
The hollow core, letting liquid flow within, facilitates debris removal with continuous irrigation flow. This takes place thanks to the small connector, placed on the shaft of the instrument and attached to an endodontic motor (Fig.4), equipped with a peristaltic pump (Endostation ®, Redent Nova, Israel) that provides continuous and pre- calibrated(4ml / min) flow of irrigant solution [10,11,12].
A peculiar hollow design, gives the SAF specific features, such as compressibility along root canals. In fact, the diameter size of a standard instrument is 1,5mm, but if fully compressed, it is reduced to the size of a 0.20 manual K-file. If instead, it is inserted into a oval-shaped canal with mesio-distal diameter of 0.2mm,, the SAF stretches bucco-lingually to an extent of 2.4 mm, thus succeeding in coming in contact with a large portion of the inner surface of the canal. This happens regardless of the operator being aware of the oval canal , since the file itself adjust to it (hence the name of Self-Adjusting -File) .
Its particularly flexible structure, makes the SAF a very safe and reliable instruments with regard to fractures.
In fact, its hollow Ni-Ti lattice enables it to twist , but even if detachment from the grip occurs, the instrument does not get locked in the canal, but can be easily removed used a simple Headstrom file, since it has no blades. This, however, is a rather rare occurrence; it is more likely that, when mechanically damaged while being used, small pieces may come off the net (fig.5) , flow directly into the liquid leaking from the canal, and then into the aspirator.
The file is assembled on a RTD3 headpiece (Fig.6) which performs a dual function: it turns the rotation motion of the motor into an up-and-down motion of the instrument shaft with 0.4 mm amplitude, and also generates a rotation in absence of torque, when the file is free from the canal [10,11,13].
From a clinical perspective, this makes it possible for the SAF to performed in-out movements, changing its circular position each time, thus exerting a dentinal smoothing action, which affects nearly 80% of the canal surface (compared to 50% with normal rotary or reciprocating instrumentation ) [13,14].
2. NO-PRESSURE IRRIGATION
The handpiece is operated by an Endostation® endomotor at a speed of 5000 rpm. The latter is equipped also with a peristaltic pump which injects the irrigant solution into the polyethylene tube connected to the instrument (at a flow rate of 4ml per min), and consequently inside the canal through the hollow file. This way, the irrigant solution, due to the peculiar shape of the instrument is able to reach the LDL, making it possible to disinfect and clean the endodontium continuously throughout the treatment. The irrigant flow has no pressure, which makes the system very safe against possible discharge of hypochlorite beyond the apex.
This allows us to prevent extrusions caused by improper use of syringes ( positive pressure procedures), or from turning to negative pressure procedures (Endovac®, SybronEndo) which, despite being undoubtedly effective, requires substantial enlargement of the canal #40 or #45 to be effective which its used in curved canals where it do not always make it possible for a cannula to get as far as the desired length without risking the integrity of the curved root [15,16].
If all is related to the back-and-forth motion while working , it is well clear how liquid can constantly flow from the canal carrying all debris, and ensure, at the same time, a continuous renewal of fresh fully active sodium hypochlorite solution in apex . This turn-over occurs every 30 seconds,
therefore 8 times during a recommended four-minute treatment , without the least extrusion of liquid fromthe apex of the tooth .
The synergistic action of irrigation and “scrubbing” on the canal walls, ensures a safe cleaning of the endodontium, producing no treatment debris (which is removed as a suspended powder with the liquid coming out from the canal), without needing to use external accessories to increase the efficacy of irrigants themselves. We should not forget that canal anatomy is an important factor conditioning the use of certain irrigation procedures; let us keep in mind that it is difficult to perform a passive ultrasonic irrigation in areas with curved roots, owing to stopping the ultrasonic vibration beyond the point at which the ultrasonic file touches the canal wall at the curvature; the same applies to negative irrigation procedures. In this case, in order to be absolutely effective, 40#.04 or 40#0.6 apical diameter sizes are to be reached , which, in curved canals, might be excessive or even risky [9,18,19].
SEM studies (Fig7) have shown that the cleaning obtainable in the middle and coronal third is in accordance to that obtainable with conventional rotary instruments when used with syringe and needle irrigation. The apical third is the most inaccessible area where even the use of EDTA cannot remove treatment debris and smear layer properly, thus often leaving the area uncleaned; in this case the use of the SAF, with continuos irrigation has shown absence of debris on the walls in all cases and absence of smear layer in 65% of the apical part of the treated samples, which resulted clean [20,21,22]. These characteristics make the SAF the only instrument with chemo-mechanical effect in the apical part of even curved canals.
3. 3-D SHAPING AND CLEANING
We usually consider “satisfactory” a root canal treatment based on observing a 2D image, such as an intraoral Rx , but this allows us to understand very little with regard to real dental anatomy. Studies using Micro-CT, have shown that an oval-shaped root canal anatomy is rarely completely affected by preparation using traditional rotary or reciprocating files, in spite of the nice Rx that may be produced. The same applies to complex roots such as the mesial of the lower molars or upper premolars [23,24,25,39]. Paqué research has shown that in treating the distal root of the lower molar with rotary instruments (Pro-Taper®, Dentsply), approximately 69% of the canal surfaces not changedwith the rotating action of the above mentioned instruments [13,27]
A similar study, carried out after the release of the SAF, shows a 23% residual percentage of untreated surface. . This is certainly not an “idealt”, result but it is much more closer to the objective of our therapy.
Further studies were carried out, in order to prove that rotating files can also reach the above mentioned figures ; they showed that, unless we want to completely decoronate the tooth, or use overly aggressive instruments, which remove much healthy dentin from the canal (Versiani et al ), the SAF is the only file capable of coming in contact with the larger portion of the canal.
Its unique adaptability, makes the SAF particularly suitable also for treating C-Shaped canals , (Fig.8) , as well as curved canals, where a good contact with the perimeter of the canal walls can be achieved, ensuring a minimum loss of dentin and reduced apical foramen transportation (Figure 9) [13,14,23].
Furthermore, the possibility of having a major cleaning action, allows us to reach areas such as those of the isthmus, which we never manage to reach, but where, as we are all aware, removal of the residual tissue is fundamental. Compressibility allows the SAF to pass even through isthmus of 0.2mm diameter size, below which removal action is possible only with the use of irrigants.
Studies by Paqué (Fig10) [30,31] have shown that the rotational motion of conventional Ni-Ti instruments tends to actively pack debris into canal isthmus or recesses.
In these areas, where pulp or bacteria residues may be found, the cleaning out of such packed debris by files and hypochlorite is nearly impossible, and, as highlighted by Paqué himself[31,] even the action of ultrasounds fails to clean out 50% of the packed debris. All this, of course, results in decreased cleaning ability and reduced obturation effectiveness [4,32,33,34].
Having no cutting action, but working with a filing and scrubbing motion, the SAF produces dentin powder ( rather than larger cut particles) which is conveyed by the hypochlorite irritant flow into the file and then outside the canal [10,11,12]. Moreover, studies with micro-CT, have shown that canals treated with the SAF system display radiopaque debris in only 1.7% of volume of the isthmus compared to 10.1% reported in those treated with conventional instruments.
As further below explained, when dealing with obturation, this results in a greater adaptability of gutta-percha to canal walls and, consequently, in better sealing.
As for disinfection effectiveness, there are numerous studies on canals infected with E Faecalis or bacterial biofilms. These have showed the SAF ability to eliminate a higher percentage of bacterial content from canals, especially those with particularly complex anatomies, which are hardly cleanable with the use of instruments that, despite being flexible, cannot enter the deepest canal recesses (Fig11) [32,35]
4. RESPECT THE REMAINING ROOT STRUCTURE
In 2003 Peters’ studies  had already shown that the use of Ni-Ti rotary instruments caused canal transportation, by the Pro-Taper file system (Figure 9). This causes loss of original anatomy and subtraction of what is defined as “sound dentine”.
Straightening the curves of S-shaped canals also tends to distort the natural shape of the canal, and in certain anatomical sites (i.e. mesial roots of lower molars ), there is a greater risk of provoking stripping or thinning of dentine in areas most at risk.
The SAF works by removing evenly a thin layer of dentine from canal perimeter walls, in absolute respect of the original anatomy.
On the other hand, this does not result in a lower effectiveness of the instrument, on the contrary, Metzger studies in 2010 , showed how the SAF accomplishes a closer contact with canal walls, compared to a NiTi rotary instrument. Apart from the risk of modifying the root canal system, it is clear how an excessive removal of dentine may result in a reduction of the ability of the root to endure stress, thus resulting in micro-cracks, that may lead to real vertical root fractures. Therefore, even though an excessive enlargement of the canal was considered favorably in the past ,with regard to cleaning possibility, nowadays it proves counterproductive 
Several studies have been carried out in this direction, either on extracted teeth [6,23,27] and through finite element computerized analysis . From all these studies, it is clear how torques exerted by Ni-Ti instruments on the canal walls may result in dentinal cracks (Fig.12) and that, varying instrument diameters , forces increase considerably. From Kim’s study  it can be inferred that a Pro-Taper® can exert 98 to 386 MPa torsional force, depending on whether we refer to an F1 or F3, respectively compared to 10 MPa exercised by a SAF (fig13).
This fundamental dissimilarity lies in the action of the blades that cut the dentin ( which are completely absent in the SAF) and in rotation force, with often high torque, exerted by conventional engines. The possibility of using engines and instruments with reciprocating motion may potentially ensures lower stress on canal walls (as well as on the instrument), however it is not comparable to an instrument working in absence of torsion.
To date, there are no clinical studies “in vivo” demonstrating this (also considering the difficulty in carrying out such studies), but it is reasonable to assume that we what has been observed in in vitro practice can also be attributed to clinical practice .
5. CLINICAL APPLICATIONS OF THE SAF
Guidelines for use of the SAF can be drawn up, based on observations reported by studies on the instrument mechanics[10,12].
Like with other NiTi instruments, an initial glide path has to be performed using a rotary #20.04, such relatively small diameter and small taper instruments has been recently shown not to produce microcraks even in curved canals (Kfir et al 2015). Afterwards it is suggested to insert the SAF manually into the canal, paying attention to any possible obstacles that might prevent the file from reaching the working length (WL) previously estimated.
At this point, we can assemble the SAF on the handpiece and start canal shaping treatment, which will last for four minutes, as recommended
The choice of a SAF diameter has to be made according to the original canal diameter. For canals with diameter sizes up to 30-35, a 1.5mm SAF can be used, for 35 canal diameter size and over, a 2.0mm SAF is recommended. The procedure itself is very simple but, unfortunately, it often underlies incorrect interpretations of comparative results between SAF and Ni-Ti rotary instruments. Indeed, there are several studies asserting that the SAF procedure turns out to be comparable to others in terms of cleaning, but a more careful reading, shows that 1.5 wide SAF instruments are often used in these studies in wide canals with apex diameter sizes bigger than 35.
Throughout a 4 minute treatment, irrigants (hypochlorite first, then EDTA), perform their action continuously, and eventually enable us to simply calculate the final gauging and then proceed to the final cone test.
Since it is no longer necessary to think in terms of predetermined tapers, we are obliged to test either different tapered or non-standardized cones , in order to find the one that best fits the canal, or, as clearly shown by Metzger, to choose “customized” cones for a single canal, especially when encountering oval-shaped canals with remodelled or extremely tapered apices.
Because of a close coupling between the cone and the walls, which are not carved by a file blades, but are much smoother, the amount of cement required is minimal. All obturation procedures, from cold lateral condensation to hot vertical condensation, either carrier- based or not (Thermafill), have been analyzed in literature, and they have all shown percentages of contact between gutta and canal walls, higher than average (Fig 14) [33,34]
Thanks to its characteristics, the use of the SAF is recommended for any dental practice. It is certainly fundamental in oval-shaped canal treatment; however, considering the possibility of saving root tissue and moderating internal forces exerted in canal roots, we definitely prefer them to conventional NI-Ti instruments
6. CLINICAL CASES
This photo shows how the SAF respects to the best the anatomy of the drop-shaped entrances of the upper premolar, while cleaning the isthmus.
The synergistic use of the SAF and rotary instruments(Pro-Taper®) , has been analyzed in order to demonstrate its effectiveness in re-treatment.
It has been showed  that the action of rotary instruments only is not effective enough to remove all the remaining gutta from canals, whereas the SAF (used afterwards) is able to remove and detach the remaining material more easily from the areas which do not come in contact with the rotary instrument, owing to it structural limitations.
To date, it is not possible to treat a root canal in a predictable way, due to the many variables encountered and ,above all, we are not able to clean and disinfect it completely yet. There are still several issues to solve in order to make endodontics a discipline with definite results, but with the SAF we have certainly taken a great step forward with regard to the objective of root canal therapy; and this change of direction will be able to lay the foundations of the evolution of future endodontic therapy.
This procedure requires a natural learning curve, also due to the fact that we operate using a very flexible and delicate instrument, likely to be damaged, if not properly handled. That is why, even operators with experience in endodontics, will have to be personally instructed and trained in order to re-adjust their tactile sensitivity and their former habits in order to achieve expected result from this procedure.
Video n° 1 and 2 are for courtesy of ReDent Nova®, Israel
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