Retrieval efficiency and root fracture resistance after separated instrument removal using ultrasonic- or trephine bur-assisted BTR-Pen retrieval protocols following thermomechanical aging: an in vitro study

Retrieving separated instruments may require dentin removal and thereby compromise root strength. This study aimed to compare the retrieval efficiency and subsequent root fracture resistance associated with ultrasonic- and trephine bur-assisted BTR-Pen protocols following thermomechanical aging. Fifty-four extracted human mandibular first premolars were allocated to ultrasonic + BTR-Pen and trephine bur + BTR-Pen experimental groups ( n = 22 each), a root canal-treated control group ( n = 5), and an untreated control group ( n = 5). Standardized size 30 K-file fragments were created in the middle third of the experimental root canals. Retrieval success was defined as complete fragment removal within 45 min without perforation, and procedural-time and fracture-resistance analyses in the experimental groups were restricted to successful retrievals. Following retrieval, the experimental specimens and root canal-treated controls underwent standardized root canal preparation, obturation, and restoration. Periodontal ligament simulation was performed for all specimens, after which all groups underwent thermomechanical aging for 120,000 cycles under a 50-N load with simultaneous thermal cycling between 5 °C and 55 °C. Specimens were decoronated and subjected to axial static loading until fracture. Retrieval success was analyzed using a two-sided Fisher’s exact test, procedural time using an independent-samples Student’s t test, and root fracture resistance using one-way ANOVA followed by Tukey–Kramer post hoc comparisons (α = 0.05). Retrieval success was 86.4% for ultrasonic + BTR-Pen and 95.5% for trephine bur + BTR-Pen, with no significant between-group difference ( p = 0.607). Among successful procedures, ultrasonic-assisted retrieval required approximately 35% less time than trephine bur-assisted retrieval, with a mean difference of 6.36 min (95% CI, 2.28–10.45 min; p = 0.003; Cohen’s d = 0.99). Root fracture resistance differed significantly among the four groups ( p < 0.001). No significant difference was detected between the ultrasonic and trephine bur groups ( p = 0.998). Both experimental groups exhibited significantly lower root fracture resistance than the root canal-treated control group (both p = 0.003) and the untreated control group (both p < 0.001). Both retrieval protocols provided high retrieval success, while the ultrasonic-assisted BTR-Pen protocol required less procedural time among successful cases. Nevertheless, both retrieval protocols were associated with reduced root fracture resistance, emphasizing the need to balance retrieval efficiency against the structural cost of fragment exposure and removal.