Evaluation of different methods for separated file retrieval from the apical third of curved root canals: In vitro study

Abstract

Background:

This study aimed to evaluate the interaction between different ultrasonic troughing tips and varying retrieval tools in removing separated instruments from the apical third of the mesiobuccal (MB) root canal of maxillary molars, focusing on dentin loss and retrieval time.

Methods:

Sixty MB canals from extracted human maxillary first molars were used. A size 25/04 Nickel–Titanium file was intentionally fractured beyond the curve. Teeth were randomly assigned according to the troughing method (Ultra-X golden tip vs. ET25 L tip) and retrieval tool (BTR Pen vs. modified tube technique). Cone-beam computed tomography was used to measure dentin thickness before and after retrieval. Statistical analysis used two-way analysis of variance, Tukey’s HSD post hoc test, and Kruskal–Wallis tests (P < 0.05).

Results:

ET25 L group paired with the BTR Pen caused the greatest dentin loss (0.3035 ± 0.17 mm), while the ET25 L paired with the tube technique resulted in the least (0.2218 ± 0.13 mm). For retrieval time, the ET25 L tip was significantly faster than the Ultra-X tip (P < .001), regardless of the retrieval tool used.

Conclusions:

The ET25 L tip paired with the modified tube technique provides the most conservative and time-efficient approach for retrieving apically separated instruments.

INTRODUCTION

Instrument separation is an occasional but significant clinical complication during endodontic treatment.[1] The retention of a fractured Nickel–Titanium instrument, particularly in the apical third of curved roots, can impede adequate chemo-mechanical debridement and compromise the long-term prognosis of the tooth.[2]

When a file separates inside the root canal, management options include leaving it in the canal which may affect prognosis, attempting to bypass the separated instrument and continue treatment, or removing it through nonsurgical or surgical methods.[3,4]

Various techniques and devices have been proposed. Often, a combination of ultrasonics with grasping tools, tube techniques, or wire-loop methods is needed for successful retrieval of long fractured files.[4] Ultrasonic tips, such as the ET25 L (Satelec, Acteon, France) with a 3% taper and 0.3 mm tip and the cordless Ultra-X golden tip (Eighteeth, Changzhou, China) with a 2% taper and 0.2 mm tip, are frequently employed to trephine dentin around the separated fragment.[5,6]

Loop systems such as the BTR Pen (Cerkamed, Stalowa Wola, Poland), EndoCowboy, and Yoshi Loop encircle the instrument with a wire after creating space around the coronal part, mainly for fragments over 4.5 mm. The modified tube technique involves exposing the coronal fragment to center it in the canal, then using a tube to extract both fragments as a single unit.[7]

While these instruments have been investigated individually, there is a critical gap in the literature regarding the interaction effects between varying ultrasonic frequencies (cordless vs. corded systems) and specific grasping tools in complex apical curvatures. Most studies focus on incremental modifications of a single technique, not on how the troughing device changes the canal structure or affects grasping efficiency. Unnecessary dentin removal during combined procedures raises the risk of root perforation or vertical root fracture.[8]

This in vitro study aimed to evaluate the effects of two ultrasonic tips (Ultra-X and ET25 L) combined with two retrieval systems (BTR Pen and a modified tube technique) on retrieval time, dentin loss, and success rate in the apical third of mesiobuccal (MB) canals. The null hypothesis was that there would be no significant differences among the combinations.

METHODS

This manuscript was prepared in accordance with the PRILE 2021 guidelines.

The sample size was based on a priori power analysis with α of 0.05, 80% power, and a large effect size from previous studies on dentin loss during file retrieval.[8,9] The calculation needed 15 teeth per subgroup, for a total of 60 teeth.

Sixty extracted human maxillary first molars were selected anonymously from the department of oral and maxillofacial surgery teeth bank. Inclusion criteria required intact, fully formed Vertucci type I MB roots with a curvature of 20°–30° (evaluated via Schneider’s method) and standardized root lengths of approximately 10–12 mm. The study was approved by the research ethics committee (No.: 540/2022).

The selected teeth were cleaned, disinfected, and stored in saline. Access cavities were gained with a round and EndoZ bur (Komet, Rock Hill, USA) in a high-speed handpiece (COXO Medical Instrument Co., Ltd. Foshan, China).

Instrument separation was intentionally performed by notching an MG3 rotary file (PERFECT Medical instruments Co., Shenzhen, China) #25, taper 0.4, using a fine tapered diamond stone 5 mm from the tip of the file to facilitate its separation. The file was inserted into a dry canal and rotated at 500 rpm with a torque of 5 N using an Econnect extreme rotary motor (Eighteenth, Changzhou city, China) until it fractured in the apical 5 mm of the canal; Type III A fractured instrument according to Rao et al.[10]

Teeth were mounted in acrylic resin blocks, and rubber base blocks to ensure consistent positioning. Cone-beam computed tomography in endo mode used a 5 × 5 FOV to measure dentin thickness before canal prep, with 75 kVp, 12.5 mA, 15s exposure, 75 µm voxels. Images were exported as DICOM files and imported into Planmeca Romexis.

The study was double-blinded, specimens were randomly assigned to a 2 × 2 factorial design (n = 15 per subgroup):

• Subgroup A1: Ultra-X tip troughing + BTR Pen retrieval

• Subgroup A2: Ultra-X tip troughing + Modified tube technique

• Subgroup B1: ET25 L tip troughing + BTR Pen retrieval

• Subgroup B2: ET25 L tip troughing + Modified tube technique.

All the procedures of preparation and retrieval were carried out by one specialized Endodontist under a dental operating microscope (Zumax Medical, Suzhou, China) at magnification of × 16. The total procedure time from coronal flaring to instrument retrieval was recorded. A maximum of 50 min was allowed, and any trial exceeding this time was considered a failure.[5]

All groups received coronal flaring on the canal’s outer wall using an MG3 rotary file (size 60, taper 0.2) (PERFECT Medical instruments Co., Shenzhen, China) to achieve straight-line access and enhance visibility of the separated instrument. The burrow platform technique was adopted as it is more conservative for apically fractured files.[11] Troughing was performed in a dry environment for better visibility. Ethylenediaminetetraacetic acid solution (Cerkamed Company, Poland) was used to flush out debris and reduce heat, followed by drying with a Stropko irrigator (Vista apex solutions, Wisconsin, USA).

Group A, Ultra-X:

The Ultra-X golden tip (2% taper) was mounted to the Ultra-X device, activated at 45 kHz, and then introduced into the canal to trough around the fragment on the inner wall 90° to 180°.

Group B, ET25 L:

The ET25 L ultrasonic tip was used with the Satelec Newtron device (Acteon, France) for 60-second troughing cycles at 30% frequency on the inner wall.

For subgroups A1 and B1, a 0.4-mm-tip BTR Pen was inserted into the canal and tightened around the exposed tip. Swaying motion was used to pull the file [Figure 1].

Figure 1.

(A) Retrieved broken file , (B) BTR pen. Retrieved broken file with BTR pen under dental operating microscope at 10X (left) and 16X (right)

For subgroups A2 and B2, a 10-mm hypodermic syringe tip was flattened, curved, and notched below the curve, then inserted into the canal to encircle the separated fragment. A size 25 H-file (MANI Inc., Utsunomiya, Japan) was placed into the notch to secure the fragment, and the assembly was pulled out as one unit [Figure 2].

Figure 2.

(A) Retrieved broken file, (B) Modified tube, (C) H file securing the broken file. Retrieved broken file with modified tube technique under dental operating microscope at 16X

Statistical analysis

Normality of the data was assessed using the Shapiro–Wilk test. Statistical analysis used IBM SPSS v26 (IBM Corp., Armonk, NY, USA). A two-way analysis of variance (ANOVA) evaluated the effects of the troughing method and retrieval technique on dentin loss and retrieval time. Tukey’s HSD post hoc tested pairwise differences. Kruskal–Wallis analyzed success rates, with 95% confidence intervals (CIs) calculated due to limited sample size (n = 15). Significance was P < 0.05.

RESULTS

Descriptive statistics, including mean values, standard deviations, and 95% CIs for retrieval time and dentin loss, are detailed in Table 1.

Table 1.

Descriptive statistics and two-way analysis of variance for retrieval time and dentin loss

Dependent variable	Troughing group	Retrieval group	Mean±SD	95% CI (lower–upper)	Tukey’s HSD	Two-way ANOVA P
Retrieval time (min)	Ultra-X (A)	A1 (BTR)	43.18±3.11	42.7–43.7	a	Troughing: <0.001*
		A2 (tube)	43.28±4.67	42.5–44.1	a	Retrieval: 0.869 (NS)
	ET25L (B)	B1 (BTR)	23.41±4.14	22.7–24.1	b	Interaction: 0.659 (NS)
		B2 (tube)	23.21±4.51	22.5–23.9	b
Dentin loss (mm)	Ultra-X (A)	A1 (BTR)	0.2470±0.20	0.215–0.279	bc	Troughing: 0.929 (NS)
		A2 (tube)	0.2810±0.20	0.201–0.243	ab	Retrieval: 0.108 (NS)
	ET25L (B)	B1 (BTR)	0.3035±0.17	0.275–0.332	a	Interaction: <0.001*
		B2 (tube)	0.2218±0.13	0.246–0.316	c

*P < 0.001 indicates a highly statistically significant difference, Means followed by different superscript letters indicate statistically significant differences between subgroups based on Tukey’s HSD post hoc test (P<0.05). ANOVA: Analysis of variance, CI: Confidence interval, SD: Standard deviation, HSD: Honestly significant difference, NS: Not significant

Retrieval time

The two-way ANOVA revealed a highly significant main effect for the troughing device (P < 0.001). The retrieval technique (BTR vs. Tube) had no significant main effect on retrieval time (P = 0.869), and there was no significant interaction between the troughing and retrieval methods (P = 0.659).

Dentin loss

For linear dentin loss, a highly significant interaction between the troughing device and the retrieval technique was observed (P < 0.001) [Figure 3]. Neither the troughing device (P = 0.929) nor the retrieval technique (P = 0.108) exhibited a significant independent main effect. Tukey’s HSD analysis demonstrated that when the ET25 L tip was paired with the Tube technique (B2), it yielded the significantly lowest dentin loss (0.2218 ± 0.13 mm). Conversely, pairing the ET25 L with the BTR Pen (B1) resulted in the highest overall dentin loss (0.3035 ± 0.17 mm).

Figure 3.

Mean dentin loss (mm) across the experimental subgroups

Success rate of retrieval

No statistically significant difference was found between the groups as revealed by the Kruskal–Wallis test (P = 0.752). The observed success rates were 86.7% for both A1 and B2, 80.0% for B1, and 73.3% for A2. The 95% CIs illustrating the variance within the sample are provided in Table 2.

Table 2.

Success and failure rates among the experimental groups

Troughing group	Retrieval group	Failure rate, n (%)	Success rate, n (%)	95% CI for success rate	(Kruskal–Wallis P)
Ultra-X (A)	A1 (BTR)	2 (13.3)	13 (86.7)	59.5–98.3	0.752 ns
	A2 (tube)	4 (26.7)	11 (73.3)	44.9–92.2
ET25L (B)	B1 (BTR)	3 (20.0)	12 (80.0)	51.9–95.7
	B2 (tube)	2 (13.3)	13 (86.7)	59.5–98.3

No statistically significant difference was observed among the groups (P>0.05). CI: Confidence interval, NS: Not significant

DISCUSSION

Currently, there is no standardized protocol for retrieving separated instruments from root canals, as the process depends on various factors, including the fragment type, its length, location, engagement with dentin walls, and the degree of canal curvature.

Various techniques are available for retrieving separated instruments, including ultrasonic tips, grasping tools, wire loop systems, and tube systems. Sometimes, a single method is enough for short, fractured instruments more coronally,[12] while in other cases, especially longer and apically located ones, more than one technique is required.[13]

Separated fragments longer than 4.6 mm cannot be retrieved using ultrasonics alone and require a grasping tool.[14,15] Moreover, M-wire files are more prone to fracture than deformation.[16]

The null hypothesis was partly rejected; regarding retrieval efficiency, the significant main effect of the ultrasonic troughing device highlights a critical clinical distinction: the ET25 L groups required nearly half the time (approximately: 23 min) of the Ultra-X groups (approximately: 43 min). This difference is attributed to the Ultra-X device’s lower frequency (45 kHz), which may experience power attenuation or micro-troughing stall when encountering severe resistance, compared with the Newtron Booster scaler’s of the ET 25 L broader power range (47–63 kHz), which provides a continuous, high-amplitude acoustic energy stream.[17] Literature also indicates that fragments shorter than 3.1 mm can be retrieved in about 1 min, and each additional 1 mm in length increases troughing time by approximately 79.9 s.[18]

Interestingly, the specific grasping tool (BTR Pen versus the modified tube technique) exerted no significant independent main effect on retrieval time. This implies that the true procedural bottleneck in broken-file removal is the ultrasonic trephination and loosening phase, rather than the subsequent engagement-and-extraction phase.

The most critical finding regarding root structural integrity was the highly significant interaction effect between the ultrasonic troughing device and the retrieval technique on linear dentin loss (P < 0.001), notably in the absence of any independent main effects. This statistical dynamic indicates that neither troughing tips nor grasping instruments can be universally classified as inherently “conservative” or “destructive”; rather, dentin preservation depends entirely on their specific pairing. Specifically, while the ET25 L ultrasonic tip produced the most conservative troughing when followed by the modified tube technique (0.2218 mm), the exact same tip resulted in the highest overall dentin loss when paired with the BTR Pen (0.3035 mm). This discrepancy likely stems from the differing spatial requirements of the retrieval mechanisms. The modified tube technique relies on simple mechanical friction and wedging, requiring minimal lateral clearance to slide over the fractured fragment. Conversely, the BTR Pen utilizes a deployable nitinol loop that necessitates a wider circumferential staging area to adequately expand, pass over the coronal aspect of the fragment, and tighten. This is supported by a previous article stating that loop systems require at least 0.4 mm of space and about 0.7 mm of coronal fragment exposure for effective grasping.[19,20] Moreover, securing the loop in a stable way around the broken instrument is challenging.[21]

In contrast, a previous study reported that BTR Pen does not require excessive dentin removal, as the wire loop’s shape memory allows it to adapt and engage the fragment with minimal space. This contradictory result could be explained by differences in the samples’ curvature, as they selected a lower degree of curvature and a shorter broken tip.[22]

Retrieval success is related to canal curvature and fragment location, achieving 100% success when the fragment is located above the canal curvature. On the other hand, retrieval success decreases to 60% when the fragment is located at the canal curvature and to 31% when it is beyond the curvature.[23]

The failed samples in the current study could have been successfully retrieved if the allowed time had been increased. However, extending the retrieval time will increase the risk of root perforation, secondary fracture of the separated instrument, excessive removal of dentin that predisposes the tooth to root fracture, and heat generation.[24,25]

While this in vitro study offers essential baseline data on the mechanical interactions of modern retrieval tools, future research must translate these findings into in vivo clinical trials. Laboratory models, although standardized, cannot fully replicate the complex clinical challenges of patient movement and restricted oral access.

CONCLUSIONS

The ET25 L tip with the modified tube offers the optimal balance between minimal dentin loss and shorter instrument retrieval time from the apical third of curved root canals.

Conflicts of interest

There are no conflicts of interest.

Funding Statement

Nil.