Comparative evaluation of dentin microcrack formation after root canal preparation with different nickel-titanium instrument systems based on micro-CT: a systematic review and meta-analysis

Dentin microcracks following root canal treatment may progress to vertical root fractures, a severe clinical complication. Engine-driven nickel-titanium (NiTi) instruments are associated with dentin microcrack initiation, but current evidence remains inconsistent. This review systematically synthesizes micro-CT-derived evidence on the effects of different NiTi instrument systems on dentin microcrack formation. A systematic search of PubMed, EMBASE, Scopus, Web of Science, Cochrane Library, and grey literature was performed from inception to October 1, 2025. Eligible studies were ex vivo investigations comparing dentinal microcrack incidence after root canal preparation with various NiTi instruments. Two reviewers independently conducted study selection, data extraction, and risk-of-bias assessment using adapted JBI and CRIS checklists; meta-analysis was performed for sufficient comparable data. Twenty-four ex vivo studies were included from 256 initially retrieved records, covering all tested NiTi instrument subgroups with a pooled total sample of 1082 extracted mandibular and maxillary teeth. Studies testing instruments including RECIPROC, WaveOne, ProTaper Next, the HyFlex series, and TRUShape reported a 0% microcrack incidence under specific experimental conditions. Meta-analysis revealed the following findings: (1) Thermally treated instruments had significantly lower microcrack incidence than conventional ones (pooled incidence: 1% vs. 9%, p = 0.0046); (2) ProTaper Gold had significantly lower microcrack incidence than ProTaper Universal (pooled incidence: 3% vs. 16%, p = 0.0403); (3) ProTaper Universal had higher microcrack incidence in moderately to severely curved canals (20°-40°) (30%) than mildly curved canals (< 20°) (11%), though the difference was not statistically significant ( p = 0.2230); (4) There was no significant difference in microcrack incidence between reciprocating and rotary instruments ( p = 0.2095); (5) No significant difference in microcrack incidence between RECIPROC Blue and WaveOne Gold, both reciprocating systems ( p = 0.3978); (6) WaveOne Gold did not differ significantly from conventional WaveOne ( p = 0.6813), nor did RECIPROC Blue from conventional RECIPROC ( p = 0.5334); (7) XP-endo instruments consistently demonstrated an almost 0% microcrack incidence across studies. They may effectively prevent dentinal microcracks even in moderately to severely curved canals. Funnel plot showed good symmetry, indicating no significant publication bias. Within the limitations of this ex vivo meta-analysis, instrument kinematics did not influence microcrack formation, while thermally treated NiTi instruments significantly reduced risk. Thermally treated systems are preferred, and highly flexible, advanced-design instruments should be prioritized for curved canals to minimize dentin microcracks.