Assessment of Damage of Endodontic Instruments with Naked Eye and Optical Instruments.

OBJECTIVES: To determine the accuracy of visual damage evaluation of rotating and reciprocal endodontic instruments with the naked eye and optical devices.
MATERIAL AND METHODS: Four examiners were involved, divided into two age groups: I. 20-30 years, n = 2, II. 40-50 years, n = 2. They visually assessed the existence of damage to rotating or reciprocal endodontic instruments by their naked eye and with two types of optical aids. A total of 239 instruments from different manufacturers were evaluated by each examiner. Before damage estimation, each examiner calibrated the eye on a new instrument. Sensitivity of the detection method was calculated as by the naked eye. When an instrument was detected as damaged by the naked eye, the damage assessment was stopped and the instrument was considered to be damaged by the other two methods, as well.
RESULTS: Damage was found in 178 instruments. Sensitivity of the naked eye was calculated to be 49.7%, for the loupe 66.2% and for the operating microscope 76.5%. The three methods of damage assessment had significant differences (p < 0.05) in sensitivity.
CONCLUSION: It was concluded that there is a significant difference between age groups in estimating instrument damage. Also, it was noted that the damage assessment with the naked eye and optical aids were different. Finally, in terms of ease of damage recognition, significant differences between the types of engine-driven endodontic instruments were found. However, no type of instrument was more prone to damage than any other type of instrument.

Introduction

Endodontics is a branch of dental medicine that deals with the pathological processes of dental pulp and their consequences on the tooth and surrounding tissues. It is considered to be the only area of dentistry that is performed without a clear field of view (). When performing endodontic procedures, the operator relies heavily on endodontic instruments made of various metal alloys. The advancements in material technology and operating techniques have enabled the use of nickel titanium (Ni-Ti) instruments driven by a machine powered by an external energy source (). Rotary endodontic instruments were described as very flexible and with better cutting ability, when compared to manual instruments () However, Ni-Ti engine-driven instruments, as well as all endodontic instruments irrespective of material, are subject to fatigue, which occurs with repeated loading, thus creating micro cracks (). Specifically, when used in curved root canals, the instruments are subjected to pressure and a tensile load that is localized at the point of curvature in the canal (). On the other hand, in straight canals torsional failure occurs at the very top of the instrument while it is still rotating (). Instrument fracture is a relatively common complication in endodontic procedures. The removal of a broken instrument is extremely demanding, which makes the favorable outcome of endodontic treatment harder to achieve (). The complication of a separated instrument in a root canal often occurs due to damaged instruments; therefore, it is important to visually inspect the instrument during the shaping procedure of multirooted teeth with complicated endodontic space anatomy. Engine-driven endodontic instruments are small and the microcracks due to fatigue are even smaller. Naturally, the question arises whether the clinician is able to see such a small damage and react in time to prevent the unwanted instrument separation.

The working fields where dental procedures are performed are relatively small and at different focal lengths, hence few optical aids have gained widespread acceptance in clinical practice (, ). Apart from better visualization of the working field, the optical aids can be used for better inspection of the surface of the endodontic instrument and possibly detect its damage that cannot be seen with the naked eye. Another factor that has shown to significantly influence the operator’s ability to visualize objects in the working field is their eyesight, which is known to deteriorate over time ().

The aim of this study was to evaluate the accuracy of visual damage assessment of different rotating and reciprocal endodontic instruments with the naked eye and when using optical devices.

The null hypotheses are:

There is no difference between the age groups in estimating the Ni-Ti endodontic instruments damage.

There is no significant difference in damage assessment when using the naked eye or optical aids.

Damage to instruments is equally easy to detect regardless of the type of engine-driven endodontic instrument.

Material and methods

Examined instruments

The study was conducted at the School of Dental Medicine, University of Zagreb and approved by its Ethics Committee (approval number 05-PA-30-XXII-12/20). The study involved four dentists divided into two age groups:

20−30 years, n = 2,

40−50 years, n = 2

Each dentist visually assessed the existence of damage to rotating or reciprocal endodontic instruments used in shaping root canals of multirooted teeth. A total of 239 instruments from different manufacturers were estimated by each examiner. The types of instruments evaluated were: Reciproc (VDW Dental, Munich, Germany n = 126), ProTaper Universal (Dentsply Sirona, York, Pennsylvania, USA, n = 59), ProFile (Dentsply Sirona, York, Pennsylvania, USA, n = 15), One curve (Micro mega, Besancon, France, n = 13), ProTaper Next (Dentsply Sirona, York, Pennsylvania, USA, n = 12), ProTaper gold (Dentsply Sirona, York, Pennsylvania, USA, n = 2), HERO shaper (Micro mega, Besancon, France, n = 6), F360 (Komet Dental, Lemgo, Germany, n = 6). The number of times each file had been used was not recorded. The examined instruments were discarded by different operators because of a perceived decrease in cutting efficiency; fracture; or any defects observed by the naked eye, such as unwinding, curving or bending. All files were cleaned by an ultrasonic cleaner and sterilized in an autoclave before inspection.

Means of examination

Before damage estimation, each examiner calibrated the eye on a new instrument. The instruments were first inspected without optical aids, with freedom of choice of the eye-to-object distance, i.e. focal length illuminated by the light from the work unit (KaVo LUX™ 540 LED, Biberach, Germany). Subsequently, the damaged instruments were selected. Instruments categorized as undamaged by the naked eye were further observed by the Keplerian loupe system (EyeMag Pro S, Zeiss, Oberkochen, Germany) at a magnification of 4.3× and a distance of 400 mm from the object with illumination from the work unit. The damaged instruments noticed by the loupe system were again indicated as damaged. The instruments that appeared undamaged by the loupe system were further observed under an operating microscope (Zeiss Extaro 300, Oberkochen, Germany), at a magnification of 4×, a distance of 250 mm and integrated light (Figure 1). Damaged instruments were singled out in the group where the damage was detected under the operating microscope. All examiners were familiar with the use of loupes and microscopes. They were allowed to use their personal corrective glasses to compensate for visual defects while assessing instrument damage. An instrument was considered damaged if at least one examiner detected it using any method. Within the limitations of this study, instruments categorized as undamaged by all examiners and by all optical methods were not included in the statistical analysis. Damage was found in 178 instruments. Sensitivity analysis of various damage detection methods was performed on these samples.

Figure 1

An instrument under a microscope

Statistical data analysis

The sensitivity of each detection method was then evaluated using detection by each of the methods (naked eye, loupe and the microscope) by each individual evaluator as a binary variable: the differences between the variables were then evaluated using the non-parametric McNemar test for related samples, while the differences between instrument and evaluator groups were tested using the non-parametric Mann-Whitney U test for independent samples. The significance level was set at 0.05 and all analyses were made using SPSS software (Release 20.0.0, by IBM SPSS Statistics).

Results

Of the 239 instruments included in the study, 61 (25.52%) were excluded from sensitivity of damage assessment method analysis since all four examiners agreed they were undamaged. For the instruments included in the analysis of the assessment method, at least one examiner had to record damage by any one of the three optical methods used in the study: the naked eye, loupe system, and operating microscope. The total number of samples included was 178. Descriptive statistics for the two groups of examiners are given in Table 1.

Table 1

Number and percentages of engine-driven endodontic instruments for which damage was not recorded or was recorded by microscope, loupe or the naked eye. Examiners were divided into two age groups: age group I (20−30 years) and age group II (40−50 years).

	Age group I	Age group II	All examiners
Damage not recorded	73 (21%)	94(26%)	167(23%)
Damage recorded under OM	29(8%)	45(13%)	74(10%)
Damage recorded under loupe	47(13%)	70(20%)	117(16%)
Damage recorded by naked eye	207(58%)	147(41%)	354(50%)
Total	356(100%)	356(100%)	712(100%)

When calculating the sensitivity of the damage assessment method, we considered that damage visible under the magnifying glass (the loupe) would also be visible by the naked eye. By the same concept, damage visible under the microscope would also be visible under the magnifying glass and by the naked eye. Sensitivity of the naked eye was calculated to be 49, 7%, for the loupe 66.2% and for operating microscope 76.5%. The three methods of damage assessment differed significantly (p < 0.05) in sensitivity (Table 2).

Table 2

Sensitivity of methods of damage assessment and confidence intervals for all examiners. The difference between the methods of damage assessment was statistically significant (p < 0.05) (McNemar Test).

Method	Sensitivity	CI 95%
Naked eye1	49.7%	46.1–53.4
Loupe2	66.2%	62.5−69.6
Operating microscope3	76.5%	73.3−79.6
Invisible	23.5%	20.4–26.7
Total	100.0%

1 the methods with a different number are significantly different by sensitivity from each other, p<0,001, McNemar test for related samples

In order to compare the sensitivity between the younger and older examiners, it was found that the younger examiners were more sensitive to noticing damage by the naked eye and by the loupe (Table 3).

Table 3

Sensitivity of noticing the damage between age groups. The difference between the methods of damage assessment was statistically significant (p < 0.05).

Method	SensitivityAge group I	SensitivityAge group II	P value*
Naked eye	58.1%	41.3%	< 0.001
Loupe	71.3%	61.0%	0.003
Microscope	79.5%	73.6%	0.063
Invisible	20.5%	26.4%	0.063
Total	100.0%	100.0%

* Mann-Whitney U-test

A further analysis of the instruments was divided into groups according to the type of wire, i.e. heat treatment procedures, to which Ni-Ti alloys had been subjected. The conventional wires (n = 72), M-wire (n = 91) and C-wire (n = 13) were included for further analysis. The gold heat treated group was excluded from further analysis due to a small sample size (n = 2).

It was shown that a significantly greater percentage of damage to conventional instruments was visible by any optical method compared to the M-wire instruments (Table 4). Similar results were obtained when ProTaper Universal and Reciproc instruments were compared, as representatives of the two groups. It was again shown that sensitivity to damage assessment was significantly greater in the case of Pro Taper instruments by all methods (p < 0.05) (Table 5).

Table 4

Sensitivity of all optical methods in noticing the damage when comparing conventional wire to M-wire instruments.

Sensitivity	Conventional	M-wire	C-wire	pConv. vs. M-wire*	pConv. vs. C-wire*	pM-wire vs. C-wire*
Visible by naked eye	54.5%	45.1%	48.1%	0.017	0.392	0.683
Visible under loupe	71.5%	60.4%	71.2%	0.003	0.956	0.137
Visible under microscope	83.0%	70.9%	76.9%	0.003	0.295	0.366
Invisible	17.0%	29.1%	23.1%	0.003	0.29372	0.366
Total	100.0%	100.0%	100.0%

* Mann-Whitney U-test

Table 5

Comparison of visible damage on Protaper Universal and Reciproc, the most numerous representatives of conventional and M-wire instruments.

Sensitivity	ProTaper Universal	Reciproc	P
Visible by naked eye	52.9%	43.2%	0.033*
Visible under loupe	68.6%	58.8%	0.025*
Visible under microscope	82.4%	69.3%	0.001*
Invisible	17.6%	30.7%	0.001*
Total	100.0%	100.0%

* Mann-Whitney U-test

Discussion

The present study determined the sensitivity of the naked eye, a loupe and an operating microscope in assessing the damage to engine-driven endodontic instruments. Generally, there were three principal advantages of magnifying loupes: compensation for presbyopia, ergonomic and optical benefits (). A study by Eichenberger et al. () found that the visual acuity can be improved by 250–961% by the use of magnification devices, independent of age and natural visual acuity. This hypothesis was consistent with our experiments. The sensitivity of the microscope was greater compared with loupes even at comparable magnification factors. This can be explained by a greater angle between the two optical beams and the static position of the microscope, offsetting any disturbances caused by head movements (, ).

We focused on the sensitivity of the eye in evaluating any possible damage to endodontic files that had previously been used in endodontic treatment. We have shown that sensitivity of the eye alone is 49.7%, thus showing that the eye alone detects, on average, about half of damage. Therefore, the other half of it remains unrevealed unless optical aids are used. These findings corroborate the study of Stapan (). It has been observed that fracture of Ni-Ti files can occur with little or no visible evidence of accompanying plastic deformation or, in other words, permanent change of its shape (, ). Such a situation could possibly lead to repeated use of a damaged instrument, thus increasing the risk of instrument separation when endodontic procedures are performed.

Some examiners had better visual acuity by their naked eye. This could be due to the examiner's age (). The first group was made of younger assessors. They were able to notice damage by their naked eye with significantly improved astuteness. On the other hand, the older group of examiners benefited more from the use of optical aids. Consequently, the second null-hypothesis has been rejected.

Instrument observation conditions are another important factor. Our experiments were performed during the day and the light from the work unit was used.

As mentioned, the sensitivity of the magnifying glass measurement was 66.2%, while the sensitivity of the microscope measurement was 76.5%. The difference between these methods was statistically significant (p < 0.05), between the sensitivity of the measurement of the eye, magnifying glass and microscope. It is questionable whether this is clinically relevant since it is subjective: it depends on the values considered to be "good enough", determined by the level of risk associated with a false negative result. Within the limitations of this study, we have obtained no false positive results.

As far as the type of alloy is concerned, the Ni−Ti alloy was introduced for instrumentation in curved canals, mostly because of their shape memory property associated with temperature changes (). In addition, they increase the speed of the endodontic procedures, allowing the dentist to invest more time into the irrigation (). The most visible damages were found on M-wires. The difference was statistically significant (p < 0.05) for conventional materials compared to M-wire, while the differences between the C-wire and conventional materials as well as the C-wire and M-wire were not statistically significant. It should be noted that the number of samples for the C-wire was relatively small (n = 52), which affects the power of testing, but the values for the C-wire are between those of the M-wire and conventional ones. The difference by each method is statistically significant between the two most commonly used instruments in the experiment, Protaper Universal and Reciproc, which does not mean that one material is more prone to damage than the other. In other words, a higher percentage of damage to the Protaper Universal instruments was optically noticeable compared to Reciproc.

Apart from the many benefits of using an operating microscope in endodontics, including visual and ergonomic enhancement (), within the limitations of this study, the results suggest that it is the superior option for therapists to assess an endodontic file for damage. This could reduce the frequency of instrument separation inside the root canal by damaged files. Nonetheless, unexpectedly, there are hardly any studies to address this important aspect of endodontic therapy, possibly due to the relatively recent introduction of optical aids to dentistry (). Further research is therefore needed.

Conclusion

It can be concluded that there is a significant difference between age groups in the estimation of instrument damage with younger examiners being superior to notice damage by the naked eye. Furthermore, there was a noteworthy difference in damage assessment by the naked eye compared to the assessment by optical aids. Finally, significant differences in the ease of identifying damage between the different types of engine-driven endodontic instruments were found. However, this does not necessarily imply that one type of instrument is more prone to damage than the other.