Keywords
chlorhexidine digluconate - irrigation solutions - sodium hypochlorite - XP-Endo Shaper
Introduction
The development of new nickel-titanium (NiTi) alloy led to a higher success rate of
root canal treatment, reducing clinical time and instrument fracture.[1] The XP-Endo Shaper (XPS; FKG, La Chaux-de-Fonds, Switzerland) plays a leading role
in chemomechanical preparation and root canal disinfection. It is made of MaxWire
alloy[2]; offers high flexibility and fatigue resistance; and can penetrate the canals easily
and quickly, expanding or contracting, increasing canal volume, surface area, percentage
of touched walls, and the amount of dentin removed.[3] This single file system is associated with a lower frequency of postoperative pain
compared to multi-instrument files,[4] is effective in bacterial reduction of oval root canals with necrotic pulps,[4] and is also indicated for use in curved canals as it has the ability to maintain
the original shape with minimal transport.[5]
However, endodontic files are unable to reach all root canal walls,[6] chemical substances assume a fundamental role in acting in these places where the
instrument cannot reach.[7] Of all the substances currently used for root canal irrigation, sodium hypochlorite
(NaOCl) seems ideal because it meets more requirements than any other known irrigating
solution.[8]
[9] The most used formulation of NaOCl is liquid NaOCl (L). However, it is potentially
toxic in its liquid formulation when in contact with periapical tissues.[10] The sodium hypochlorite gel form NaOCl(G) reduces the risk of debris extrusion into
periapical tissues,[11] and it is effective in reducing Enterococcus faecalis biofilm, but this effect is less than that of NaOCl liquid.[12]
Chlorhexidine digluconate (CHX) is an option for endodontic irrigants that could replace
hypochlorite with some limitation.[13] CHX is a cationic compound with excellent antibacterial properties.[14] It has shown antimicrobial activity against both forms of intracanal bacterial growth
(planktonic and biofilm bacteria),[13] and its contact with vital tissues presents low toxicity.[13] For root canal chemomechanical preparation, CHX can be used in a liquid CHX(L) or
a gel CHX(G) presentation. CHX(G) consists of a gel base (natrosol, a hydroxyethyl
cellulose; pH = 6–9) and chlorhexidine gluconate.[15] CHX(G) formulation can perfectly replace the CHX(L), improving the reduction of
smear layer formation, compensating for its incapacity to dissolve organic tissues.
It has a better residual effect due to its substantivity (up to 24 hours).[15] The gel formulation may keep the active principle of CHX in contact with the microorganisms
for a longer time, inhibiting their growth.[16] A brown precipitate is formed by mixing NaOCl and CHX.[17]
Despite technological innovations in endodontics, fractures from mechanized NiTi instruments
still occur in two ways: torsional or cyclic fracture.[18] Therefore, the present study aimed to evaluate the dynamic cyclic fatigue resistance
of the XPS files in association with two solutions in different formulations: NaOCl(L),
NaOCl(G), CHX(L), and CHX(G). The null hypothesis was that the different irrigants
do not influence the XPS files' dynamic cyclic fatigue resistance at body temperature.
Materials and Methods
The sample size and power for statistical testing were calculated by analyzing variance
using the software G*Power 3.1.9.4. For the effect size of 0.571, obtained from a
pilot study (n = 3), the significance level of 5%, and power of 90%, the sample calculation indicated
the need for 60 files (n = 10) in the present study.
Sixty 25-mm long XPS instruments were equally assigned to six groups (n = 10) based on the irrigating solution, CHX(L) chlorhexidine liquid 2% (Arte & Vida,
Bom Jesus de Itabapoana, RJ, Brazil); CHX(G) chlorhexidine gel 2% (Arte & Vida, Bom
Jesus de Itabapoana, RJ, Brazil); NaOCl(L) sodium hypochlorite liquid 5,25% (Fórmula
& Ação, SP, Brazil); NaOCl(G) sodium hypochlorite gel 3% (Fórmula & Ação, SP, Brazil);
NAT natrosol gel (Fórmula & Ação, SP, Brazil) (control group), LO lubricating oil
WD-40 (Ap Winner Ind, Ponta Grossa, PR, Brazil) (control group).
The files were inspected for deformities at high magnification (13.6 × ) (Zeiss Pico;
Carl Zeiss MediTec, Dublin, California, United States), and none of them was discarded.
Noncorrosive stainless-steel blocks against NaOCl or CHX were used to test resistance
to dynamic cyclic fracture. The artificial canal was 1.5 mm wide, 20 mm long, and
3.5 mm depth, with a straight cervical segment of 14.29 mm, a long curved apical segment
of 4.71 mm with a radius of 3 mm and a curvature of 90 degrees, and a long straight
apical segment of 1 mm.[19]
These dimensions allow the file to rotate freely within the artificial canal, both
angularly and in a dynamic motion. The canal was covered with an acrylic plate to
prevent instrument slippage, visualize the instrument during its action, and keep
the irrigating solution within the simulated canal. The interface between the metallic
block and the acrylic plate around the metallic canal was sealed with silicon (Pulvitec
Polystic, São Paulo, SP, Brazil) to prevent leakage and keep the simulated canal full
of irrigating solution. The irrigating solution was inserted into the simulated canal
by using a 3-mL syringe (Ultradent Products Inc – EUA) and needle Navitip 30G 21mm
long (Ultradent Products Inc – EUA).
The stainless-steel block with the artificial canal was positioned vertically on a
heating plate (Fisatom, São Paulo, SP, Brazil) at a constant temperature (37°C ± 1°C),
measured by a laser thermometer pointed at the simulated canal (MT-320 Minipa, Joinville,
SC, Brazil), so the substance temperature into the root canal was accurately measured.
The contra-angle handpiece was fixed to the mechanical system that enables a dynamic
axial movement of the file inside the simulated canal. The mechanical motion system
consists of a linear guide, coupled with a savox sc-12 56t69 engine (Savox, Taichung,
Taiwan) that performs back and forth movements, controlled by an electronic device
that controls the speed amplitude of the axial movement. The files were placed inside
the simulated canal coupled to a 6:1 reduction handpiece (Sirona Dental Systems GmbH,
Bensheim, Germany), driven by a VDW Silver Reciproc motor (VDW, Munich, Germany).
Cyclic fatigue tests were performed by rotating the instruments in continuous rotation
at 800 rpm and torque of 1 Ncm.
The instruments were inserted 20 mm into the canal, fitted with a silicone stop to
register this length. A back-and-forth axial movement at a speed of 3.0 mm/s and amplitude
of 3.0 mm were applied to the instruments to simulate clinical pecking motion. The
continuous rotation of the file's movement occurred until the fracture could be visually
observed. The file movement within the simulated canal was recorded with iPhone 6s
(Apple Inc. Cupertino, California, United States), using 4K recording technology.
The movie was analyzed in Microsoft Movie Maker (Redmond, Washington, United States),
and the time at which the instrument began to rotate until the moment of the fracture
was registered in seconds. The number of cycles to fracture (NCF) was calculated by
using the following formula: NCF = time (seconds) × revolution per minute/60.
The fragment lengths were measured by using a 150-mm digital caliper (accuracy of ± 0.03 mm/0.001).
The maximum point of stress in both artificial canals was also analyzed.
Statistical Analysis
A descriptive analysis of the NCF was performed according to the irrigant type (CHX
and NaOCl) in its different formulations (G or L) and according to the control groups
(LO and NAT). The normality and homoscedasticity were confirmed by the Shapiro–Wilk
and Levene tests, respectively. Tukey's test was used to compare the averages for
the NCF according to the irrigant type (groups = 6). The significance level was set
at 5%.
Results
The CHX(G), CHX(L), and OIL (LO) groups showed no significant difference between them
and presented longer time and NCF. NaOCl(L) shows the NCF without significant differences
between NaOCl(G) and NAT. The NCF of the NaOCl(G) was statistically similar to the
CHX(L) and statistically lower than the CHX(G) and OIL groups. NAT did not present
a statistical difference of the NaOCl(L) and NaOCl(G), and presented a significantly
NCF than CHX(G). The irrigating agents did not significantly influence the length
of the file fragment (p = 0.066; [Table 1]).
Table 1
Means and standard deviations of number of fracture cycles and length of file fragments
according to each type of irrigating substance
|
Irrigating
solution
|
NCF
Mean (SD)
|
Fragments (mm)
Mean (SD)
|
|
CHX gel 2%
|
1,761 (258)a
|
4.36 (0.20)A
|
|
CHX solution 2%
|
1,523 (327)a,b,c
|
4.30 (0.20)A
|
|
NaOCl solution 5.25%
|
1,028 (389)d
|
4.51 (0.29)A
|
|
NaOCl gel 3%
|
1,111 (312)c,d
|
4.25 (0.21)A
|
|
Natrosol gel
|
1,309 (170)b,c,d
|
4.27 (0.16)A
|
|
Lubricating oil
|
1,613 (468)a,b
|
4.42 (0.19)A
|
|
p-Value
|
p < 0.001
|
p = 0.066
|
Note: Different lowercase letters in the columns indicate statistical difference (p < 0.05). Same lowercase letters in the columns indicate that there was no statistical
difference (p > 0.05).
Discussion
The experiment was conducted in a noncorrosive stainless-steel block to ensure the
experiments' standardization without influencing other variables.[20] Because of higher cyclic fatigue resistance than that of the 30/0.04 Ni-Ti rotary
instruments immersed in water at simulated body temperature,[15] XPS instruments were the ideal instrument in this study. The files were subjected
to back-and-forth axial movements at a 3.0 mm/s[21] speed to simulate clinical conditions. In the dynamic model,[22] tensile and compressive stresses are distributed over a wider area throughout the
instrument shaft within the artificial canal curvature by moving the file axially,[23] enhancing fracture resistance, and could reproduce a clinical up-and-down motion.[24]
The radius and the angle of canal curvature are known to have significant roles in
cyclic fatigue failure; lower degrees of curvature will result in longer fracture
time.[25] In the present study, a severe 90-degree curvature of the simulated canal was used
to assess the instrument's behavior in critical conditions.[19]
Synthetic oil is a universal substance used to control cyclic fatigue resistance tests
in a static or dynamic model.[26] The natrosol gel (hydroxyethyl cellulose) was also used as a control substance because
it is the base of the CHX(G) and a nonionic agent, highly inert and soluble in water,[27] similar to the gel used in NaOCl(G). However, the manufacturer does not disclose
the gel substance used in NaOCl(G).
The present study evaluated the instruments at 37°C simulating body temperature (13),
whereas previous studies evaluated at room temperature.[28]
[29]
Despite NaOCl is potentially irritating for periapical tissues, especially in high
concentrations, it is the substance most widely used for root canal irrigation in
endodontics because of its effective antimicrobial activity and ability to dissolve
organic tissues.[30] Furthermore, NaOCl gel has been studied in the efficacy against microorganisms[12] and in the debris extrusion during endodontic treatment[11] with promising results.
On the other hand, CHX (L and G formulation) was used in the present study because
both CHX and NaOCl were equally effective in reducing endodontic infection, despite
their different molecular mechanisms.[8] The main limitation of CHX as an endodontic irrigator is its inability to dissolve
pulp tissue.[24]
In this study, OIL(LO), CHX(L), and CHX(G) did not differ significantly from each
other; however, the NCF were statistically higher compared to NaOCl(L) (p < 0.01). Therefore, the null hypothesis was rejected, corroborating with another
study.[19] This result occurred probably due to deteriorations caused by NaOCl(L) on the surface
of the file[31] and due to a galvanic reaction when the file is exposed to an electrolytic solution
such as NaOCl(L), causing corrosion processes predisposing the instruments to unexpected
fractures.[32] In addition, NaOCl(L) may cause micropitting by removing nickel from the instrument
surface,[33] thereby decreasing the resistance to cyclic fatigue.[34]
On the other side, CHX(G) provided the endodontic file the most significant resistance
to fracture in this experimental model similar to CHX(L). Chlorhexidine gluconate
is a cationic biguanide used as an intracanal irrigant in a liquid[32] and gel[15] formulation. The gel base used in the CHX(G) formulation in the present study was
the natrosol gel (hydroxyethyl cellulose), which is a nonionic and inert and water-soluble
agent[27] widely used to thicken shampoos, gels, and soaps based on cationic substances such
as chlorhexidine gluconate. The cationic chlorhexidine molecule could avoid galvanic
current preventing corrosion of the file metal decreases instrument breakage.[15] In the present study, CHX(G) had no differences regarding NAT, showing that CHX(G)
increases the resistance to file fracture more by avoiding galvanic corrosion than
being a lubricant.
In the NaOCl(G) group, the time to fracture and the NCF was similar to the one verified
with the use of CHX(L) and NAT but did not reach the time until fracture and NCF verified
with the OIL and CHX(G). When exposed to a NaOCl, even in a G formulation, the lower
fracture resistance could be attributed to the induced corrosive zones, which are
likely to reduce the resistance to cyclic fatigue of the instrument.[35] Besides, when exposed to a higher NaOCl concentration, a decrease in cyclic fatigue
resistance is expected due to the increased amount of available chlorine that attacks
the metallic league.[36] NaOCl, when compared with water, negatively affects the fatigue resistance of NiTi
instruments,[37] especially at higher concentrations.[38]
Immersion of instruments in NaOCl before cyclic fatigue testing for 3 to 5 minutes
did not affect the cyclic fatigue of NiTi endodontic instruments.[39] However, they do not reflect the actual clinical status of root canal preparation
in that they are performed in the presence of irrigants in the root canal. The current
study kept the simulated canal full of irrigants throughout the experiment, better
simulating the clinical conditions. Fatigue failure can be caused initially by cracks
on the instrument's surface, which may be due to the concentration of chloride ions
in the corrosion reaction under a titanium gap,[40] influencing instrument fatigue resistance.[41]
The mean lengths of fractured segments were recorded to evaluate the maximum concentration
area of the compression and tensile stresses of the tested files inside the canal
curvature. There was no significant difference between the groups, regardless of the
broken fragments' lengths (4.5 ± 0.5 mm). This matches the file's location inside
the curvature; the point of maximum stress was similar in each circumstance, suggesting
standardization of the experiment.[26]
Based on the current study's findings, it is possible to conclude that CHX (G) and
CHX (L) increased the time to fracture and the NCF of XPS files. In addition, XPS
instruments were significantly more resistant to cyclic fatigue when irrigated with
CHX(G) than with NaOCl(G), NaOCl(L), or CHX(L). However, the circumstances tested
in these dynamic models are very different from those present in clinical practice.
Therefore, further studies are needed to evaluate other irrigating solutions and their
influence on XPS instruments' cyclic fatigue.
