The role of the operating microscope - in conjunction with ultrasonic in preparation of root canal systems

A review of preparation techniques states that “because of limited efficacy of irrigation in such recesses, debris and smear layer may accumulate and remain on these unprepared root canals walls, decrease the quality of obturation and jeopardize the long-term treatment success” (Hulsmann et al., 2005).

Preparation of root canal system
The cause of failure of endodontic treatment has been attributed to the presence of microorganisms persisting in the apical part of the root canal (Siqueira 2001). Much attention has therefore been focused on preparation and obturation of the apical part of the canal thereby depending on the apical seal to prevent toxins from leaking out into the periradicular tissues. While success rates of endodontically treated teeth without periradicular lesions is very high, there can be a significant reduction in success in both teeth with periradicular periodontitis and in those teeth where endodontic treatment has failed (Ng et al., 2011). This is predominantly due to the failure to remove microbes form the root canal system. The quest is to find more effective irrigants and irrigation techniques, as well as rotary files and preparation techniques to overcome these difficulties.

An ideal preparation shape with a rotary instrument can only be achieved in a canal with a matched cross section. Many canals are variable in shape. They may have irregular and oval cross-sections and while much of the debris is captured within the flutes of the instruments, some is compacted into those spaces between the instrument and the canal wall (Fig. 2). The incidence of isthmuses in both maxillary and mandibular first molars is very high (von Arx 2005). They are particularly liable to have an accumulation of compacted debris after preparation and the inability to clean these areas effectively has been implicated as a major cause of failure of root canal treatment, particularly in both mandibular and maxillary first molars (Fig. 3) (Hsu & Kim, 1997; Tam & Yu, 2002).

The more the debris is compacted, the more difficult it is for chemicals such as sodium hypochlorite and calcium hydroxide to penetrate through the interface. Paque et al. (2010) reported that approximately half of the debris that accumulated during rotary instrumentation of the mesial canals of lower molars remained in the canal system after irrigation.

Failure of endodontic treatment in maxillary molars has been attributed to the failure to locate and treat the mb2 canal (Weine, 1969; Wolcott et al., 2005). Various studies have shown the presence of the mb2 canal in up to 90 % of maxillary first molars. A study by Somma et al. (2009) showed that in 58 % of teeth, the mb1 and mb2 merge apically into one canal. In a proportion of these failed cases where the mb1 canal has been located, cleaned, shaped and obtruded well, the question should be asked, “Was the failure due to inadequate treatment of the apical part of the mb1 canal, or because the mb2 canal and isthmus between the two canals had been missed?” Identification and treatment of the mb2 canal with concomitant re-treatment of the mb1 canal often leads to healing. This suggests that the seals are not always good enough to “entomb” the bacteria. Indeed coronal microleakage has been implicated as a major cause of failure of endodontic treatment (Saunders and Saunders, 1990). Undoubtedly tracts of debris running along side root fillings are conduits for bacteria to cause failure by this method.

In an in vivo study by Nair (2005) the mesial canals of sixteen lower molars with infected root canals were root treated by conventional techniques in a single visit and the apical portions removed by flap surgery and evaluated by corrective light and transmission electron microscopy. In the majority of cases residual microbes were located in inaccessible recesses, uninstrumented areas of the main canals, accessory canals and intercanal isthmuses.

If the lateral extensions feed into the apical part of the canal, then removing bacteria and nutrients from these areas reduces the bacterial load and this has to be beneficial for the outcome of treatment. A variety of techniques have been proposed to overcome the inadequacies of mechanical preparation in non-circular canals including circumferential filing using both hand and rotary files and the use of a rotary self-adjusting file that adjusts to the shape of the canal. The SAF system has been shown to be more effective in cleaning oval canals that conventional rotary nickel titanium instruments, however in the study by De Deus et al. (2011) using mandibular canines, even this technique did not render the canals completely clean. They showed that rotary files were unable to access the recesses of oval canals and that sodium hypochlorite had a “limited ability to compensate for the inadequacy of the file itself”. They further suggested that the common belief that “the file shapes; the irritant cleans” is based more on wishful thinking than on experimental facts. In a review article by Metzger (2014), it was recognised that SAF was unable to prepare the narrow isthmus of less than 0.2 mm. In the case of the narrow isthmus the challenge is to deliver sufficient quantities of irrigant effectively into a very small area in which debris has been compacted during preparation.

Recently new concept files XP 3-D Finisher (Brassler, USA) that change their shape with temperature have been developed with the expectation that they can deal with canal irregularities. While these may be helpful in removing soft tissue in non-circular canals, they may be of limited value in situations where tissue or root filling materials are strongly adherent to the root canal wall.

Among the numerous irrigation techniques that have also been proposed there are those that include the use of ultrasonic energy. Ultrasonic have played a role in endodontics for many years. Initially ultrasonic canal preparation was introduced by Richman (1957) and in subsequent years there was a vogue for using the ultrasonically energised file to cut dentine in root canals. The technique fell out of favour because lack of control produced ledges, apical perforations and instrument separation (Lumley et al., 1992). In the 1980s research showed that passive ultrasonic energy, PUI rendered canals clean more effectively than ultrasonic irrigation with simultaneous instrumentation (UI), where the file is intentionally brought into contract with the canal wall (Weller et al., 1980; Ahmad et al., 1987a). PUI uses an ultrasonically energised file to irrigate the canal and to remove debris utilising a combination of acoustic microstreaming and and cavitational energy (Ahmad et al., 1987a, b; 1988; 1992) (Fig. 4).

PUI was found to be effective in apical part of curved canals and in the isthmus area between two canals. The technique has been shown to remove tissue more effectively than hand irrigation and does not cause damage to the canal wall (Gutarts et al., 2005). Variation in the efficacy of PUI reported in some studies were explained by difficulties in standardizing the position of the instrument in the centre of the canal (van der Sluis, 2007).

Since the introduction of the operating microscope, it has been possible to carry out endodontic treatment at varying magnifications up to approximately 25x with the aid of direct light that can penetrate into the depths of the root canal. This means that visual inspection of the prepared root canal is possible. Once the canal has been shaped by conventional techniques and dried, the canal can be visually inspected both apically and laterally into the extensions of the canal. Straight canals can be inspected to the apical constriction. Since the rotary files straighten the coronal and middle thirds of curved canals, most of these prepared canals can be inspected to within a few millimetres of their full working length.

Inspection through the microscope at about 10x and above can identify those parts of the canal system that have not been touched by the rotary files and contain residual tissue (Fig. 5). These are usually the extensions of oval and flattened canals, isthmuses and fins.

The challenge is to prepare these areas producing a smooth predictable shape, without removing excessive tissue, allowing irrigants to penetrate into the canals more fully and therefore producing cleaner canals. Our expectations are that delivery of irrigants and medicaments using a variety of techniques into these parts of the canal anatomy will digest residual tissue material and entomb remaining bacteria, rendering them ineffective. While they have undeniable advantages in the parts of the canal system that cannot be inspected under the microscope, a significant part of the bacterial load within the canal can be removed by the use of a cutting instrument directed towards a specific part of the root canal such as a fin or isthmus. In the coronal part of the canal this can be done with either a long shank rosehead bur or a dedicated ultrasonic instrument. Long shank burs are very limited in their use, however because of the length of the shank, relatively large diameter of the bur, lack of visual access and they can only be used in the straight part of the canal. In the deeper parts of the canal ultrasonically activated instruments can be used to great effect.

A very effective solution is to use an ultrasonically energised K-file (UEFK), the very instrument that was discarded after the problems identified with ultrasonic instrumentation in the 1980s. The difference between then and now is that in conjunction with the use of the operating microscope the instrument can be used with a great deal of control. Also power settings have been considerably reduced to minimise the possibility of instrument separation. In many situations the UEKF overcomes many of the limitations presented by other ultrasonic instruments. The file can be curved in multiple directions so that the head of the ultrasonic handpiece does not impair visual access and the file can be shaped to follow the curvature of the canal. When used in conjunction with the operating microscope, the file can be directed to the part of the canal that has not been prepared by the rotary files. A size 20 UEKF with a 2 % taper is an optimal size although occasionally a larger file may be used. Because the file is relatively flexible and removes only 0.2 mm of tissue, unnecessary removal of dentine is kept to a minimum (Fig. 6).

The file works in multiple ways; it can be easily pre-curved to follow the canal curvature and can be used as either a cutting instrument by engaging the tip or as a planning instrument by using the flutes along its working length. When used as a planning instrument, it can be used with variable pressure against the walls of the canal such as in an oval canal extension or in an isthmus. The greater the pressure applied, the more effectively the file cuts dentine in the same way as a hand file, at the expense of the ultrasonic effect. As the pressure on the file is reduced, so the ultrasonic effect is increased, achieving the benefits of PUI. The effectiveness of this technique is enhanced by both the flexibility of the k-file so that it can be pre-curved and by its rigidity so that it can cut efficiently into a targeted area. The instrument can be used in both modes interchangeably just by varying the lateral pressure placed on the ultrasonic handpiece.

In endodontic retreatment cases both the UEKF and the dedicated ultrasonic tips can be used to great effect to remove endodontic obturation materials, separated instruments and posts using minimally invasive techniques. While the UEKF has to be used at low power settings to minimise the possibility of fracture, it allows for excellent visual control. The dedicated ultrasonic tips such as the EndoSucess ET 25 tip can be precurved to improve visual access and can be used at higher power settings. It is however only effective at its end. This tip is particularly useful for removing separated instruments. Other ultrasonic tips that cannot be pre-curved can only be used in straight parts of the canal.

The removal of gutta-percha from oval canals often presents a challenge as rotary instruments are not completely effective. A rigid ultrasonic tip is more like to plasticise the gutta-percha, while the UEKF with its increased tip amplitude, fragments the material.

Conclusion
Both ultrasonic and the microscope have become an essential part of the armamentarium in endodontics. When used together they can produce minimally invasive preparations, which produce cleaner canals in both primary and retreatment cases. Conventional irrigation strategies should always be employed, particularly in those areas of the canal system that cannot be visually inspected with the operating microscope such as in the curved apical third. However, the technique described above can aid in the reduction of the bacterial load within the canal system and this can result in more predictable outcomes.

Editorial note: A complete list of references are available from the publisher. This article was published in roots - international magazine of endodontology No. 04/2016.