The Concept of Primary Root Canal Treatment

Primary root canal treatment is a definite treatment with established reports of high clinical and radiographic success rates. Failure in root canal treatment results in an indication for clinical mediation. One of the most common causes of failure is an intricate root anatomy that has not been effectively cleaned or shaped such that microbial flora remains in the apical sections of the root canals. Failure in totally removing the previous obturation can make proper disinfection difficult by restricting the access of antimicrobial agents to certain areas of the root canal system. Debris may cover areas in which residual infection occurs. If bacteria remain in the apical canal, there is an augmented risk of periradicular inflammation.

One of the most important consideration, which influences the outcome of retreatment, is complete removal of root canal obturation material. The efficiency and the precision of retreatment files to remove filling material are important factors for successful treatment. The biggest challenge for these retreatment instruments is to follow the canal anatomy precisely in order to remove all root canal obturation material. Different techniques have been proposed to remove obturation materials, most of the recent ones using nickel-titanium (NiTi) rotary.

Alternatively, some authors have proposed removing the previous obturation material with reciprocating single-instrument systems originally designed for root canal preparation. In this case, filling removal is performed simultaneously with reinstrumentation. Additional enlargement could reduce the amount of residual filling even further. However, extensive enlargement is associated with the risk of canal transportation, and this risk has to be carefully weighed against the added benefit.11 Irrespective of the retreatment technique, numerous studies have shown that complete removal of root canal fillings is not commonly attained,12 particularly in the apical portion of the root canals. Therefore, additional approaches have been suggested to enhance the complete removal of filling material.

Recently, a new NiTi finishing instrument was developed with the purpose of improving root canal cleaning—the XP-endo Finisher R (FKG Dentaire, La Chaux-de-Fonds, Switzerland), which is a size #30 nontapered instrument made with the NiTi MaxWire alloy (Martensite-Austenite Electropolish FleX, FKG Dentaire). Because of this special alloy, this instrument assumes a straight shape in its martensitic phase, which is achieved below 30° C. However, when placed in the canal at the body temperature, it changes to the austenitic phase in which the instrument assumes a spoon shape in the last 10 mm with a depth of approximately 1.5 mm. When rotating, this instrument achieves a natural diameter of 3 mm in the last 10 mm.

According to the manufacturer, when the instrument tip is squeezed, the bulb can be expanded to 6 mm; when the bulb is compressed, the tip will expand to 6 mm. Thus, when the XP-endo instrument is moved up and down for 7 to 8 mm inside the canal, the natural constrictions and expansions in the canal will alternately cause the bulb and tip to expand and contract. This makes the instrument scrape the canal walls and cause turbulence of the irrigant solution. The XP-endo Finisher R instrument has the potential to be applied as an additional procedure in retreatment cases to maximize filling removal. The results, using the XP-endo Finisher R instrument showed better removal of sealer debris compared to not using an adjunct with the ProTaper retreatment files. This is because the instrument expansion at the body temperature added to its helical movement inside the canal, may have allowed it to touch and displace the residual filling material.

In this study, two resin-based sealers were used to evaluate the penetration within the dentinal tubules during treatment and endodontic retreatment. The percentage of AH-Plus sealer remaining in the root canal space after retreatment was statistically greater than percentage of MTA Fillapex sealer after the retreatment procedure (Figure 1(1A)). This could be attributed to the higher bond strength of AH- Plus sealer compared to MTA Fillapex sealer. Gurgel-Filho and Martins concluded that push-out bond strength of AH Plus and gutta-percha core combination were higher than Endofill sealer and MTA Fillapex core combination.13MTA Fillapex showed lesser percentage of sealer remaining in the root canal space. Sarkar et al.14 suggested that the release of calcium and hydroxyl ions from the set sealer will result in the formation of apatite as the material comes in contact with phosphate-containing fluids. The apatite formation promotes controlled mineral nucleation on dentin, seen as the formation of an interface layer with tag-like structures.

The reason for the easy removal of MTA Fillapex in the present study could be the low adhesion capacity of these tag-like structures corroborated by the study made by Sagsen et al.15 MTA Fillapex showed least sealer remaining after the removal procedure and maximum sealer depth penetration after retreatment (Figure 1 (2B)). Silva et al.16 (2013) compared MTA Fillapex and AH-Plus flow in accordance with ISO standards. The authors observed greater flow of Fillapex MTA (31mm) than AH Plus (26 mm).16 MTA Fillapex shows higher solubility and lesser setting time compared to AH-Plus sealer which contributed to easier removal of MTA compared to AH-Plus sealer during removal procedure. In the present study retreatment efficacy was assessed by confocal laser scanning microscope using two contrast fluorescent dyes as it was necessary to distinguish the sealer used in initial treatment from the sealer used on retreatment.

Rhodamine (red) was used during endodontic treatment and Fluorescein (green), in retreatment. Such dyes allowed the formation of high contrast points to show the sealer distribution within the dentinal tubules by CLSM.17.One of the advantages of confocal laser scanning microscopy compared to scanning electron microscopy (SEM) is that the CLSM does not require sample preparation techniques that could cause artifacts.18 Another important advantage of this method is that the same samples were used for the treatment and subsequent retreatment. This aspect eliminates biases such as anatomical issues that may influence the results. Only the apical third section was analysed as the complete seal at the apical third is the most vital part of an endodontic treatment and retreatment.