**3. Results**

#### *3.1. Study Selection Results*

PubMed yielded a total of 236 research papers using MeSH keywords, 483 from Cochrane using MeSH keywords, 131 from Scopus using MeSH keywords, and 2179 from ScienceDirect. Following the removal of duplicate articles (98), a total of 2931 studies were recognized for further consideration. After reading the titles of the articles, another 2519 were eliminated. In addition, 352 additional articles were eliminated after reading the abstracts. By reading the full texts of the residual 60 articles, they were evaluated for further selection; nine more articles were eliminated. The data were extracted from the 51 studies that strictly met the eligibility criteria. Figure 3 depicts the selection criteria as it follows the PRISMA guidelines. These 51 articles were examined for the current study based on the quality of the research studies.

#### *3.2. Study Features*

The studies' basic features included in the systematic review are summarized in Table 2. The studies were performed in various countries, lasted varying amounts of time, and were published in various journals. Each included study was published in a good, reputed journal indexed in Web of Science/PubMed/Scopus. The technical characteristics, such as sample size, type of teeth, instrument used, resolution, software, classification system, methods, outcomes, and conclusion, were omitted from the systematic review. Most of the reports comprised were issued in the Journal of Endodontics (*n*-13). In contrast, six were published in the International Endodontic Journal, four in the Clinical Oral Investigations, two in the Scientific reports, two in the Journal of Conservative Dentistry, two in Oral Surgery, Oral Medicine, Oral Pathology, Oral Radiology, and Endodontology, two in the Archives of Oral Biology, two in the Journal of Applied Oral Sciences, two in Clinical Oral Investigations, two in Clinical Anatomy, one in the Australian Endodontic Journal, one in the International Journal of Oral Sciences, one in the Swiss Dental Journal, one in the European Endodontic Journal, one in Acta Odontologica Latinoamericana, one in the Journal of Dental Sciences, one in the Nigerian Journal of Clinical Practice, one in the International Medical Journal of Experimental and Clinical Research, one in the British Journal of Oral and Maxillofacial Surgery, one in The Saudi Dental Journal, one in Imaging Science in Dentistry, one in Medical Principles and Practice, one in The Bulletin of Tokyo Dental College, and one in Annals of Anatomy. All the included studies were published between 2008–2022. The research included was carried out in a variety of nations, including Brazil (*n*-16), China (*n*-7), Egypt (*n*-5), Germany (*n*-5), Poland (*n*-3), the United States (*n*-2), Korea (*n*-2), Turkey (*n*-2), New Zealand (*n*-1), Chile (*n*-1), France (*n*-1), Myanmar (*n*-1), Saudi Arabia (*n*-1), Italy (*n*-1), Japan (*n*-1), and the United Arab Emirates (*n*-1).

**Figure 3.** PRISMA flowchart showing the selection process of articles retrieved from different web sources.

**Table 2.** Studies included in the systematic review.



**Table 2.** *Cont.*

In the studies included, a total of 6696 samples were studied. The studies were conducted on either maxillary (*n*-2222) or mandibular teeth (*n*-3760), permanent anteriors (*n*-625), and third molars (*n*-89). Of the total maxillary and mandibular teeth, 970 were maxillary first molars, 262 were maxillary second molars, 659 were both maxillary first and second molars, 331 were maxillary premolars, 789 were mandibular first molars, 158 were mandibular second molars, 529 were mandibular first premolars, 1254 were mandibular incisors, 281 were mandibular canines, and the remaining 1463 were mixed. The authors used different reagents to store the samples (70% Alcohol, 0.5% sodium azide solution, or 10% formalin). To scan samples, a Scanco Medical machine was used in 10 studies, a Bruker Micro-CT machine in 34 studies, Micro-CT Inveon, Siemens Medical Solutions, Knoxville in two studies, VGStudio Max 2.2 in one study, Nikon Metrology Inc, Brighton in one study, Nanotom S, General Electric in one study, Kodak, Rochester, New York, USA in one study, and HMX 225-ACTIS 4, Tesco, Inc in one study.

Bruker Micro-CT software from Kontich, Belgium (*n*-27), software VG-Studio Max 2.2 from Volume Graphics, Germany Heidelberg (*n*-10), NRecon software (*n*-5), CTAn v.1.12 software Mimics 17.01, Materialize, Leuven, Belgium (*n*-4), MICs 10.01 software Materialise, Leuven, Belgium (*n*-1), Image processing language (*n*-1), On-Demand 3D software from Cybermed, Seoul, Republic of Korea (*n*-2), Cobra software Siemens Medical Solutions, Knoxville (*n*-1), and MeVisLab v3.2 software (MeVis Medical Solutions AG, Bremen, Germany) (*n*-1) was used in the studies to interpret the data about root canal morphologies. The minimum Voxel size (resolution) adopted in included studies was 11.6 μm. However, 60 μm was the maximum resolution adopted in included studies. Most studies classified the root canal morphology using Vertucci's classification system (*n*-16) and the four-digit system (*n*-6). Furthermore, Weine's classification system (*n*-3), Pucci & Reig (1944), a new classification system by Ahmed et al., Pomeranz 's classification and the American Association of Endodontics system for classification were also used. The technical characteristics of the studies are shown in Table 3.


**Table 3.** Characteristics of the included studies.



























**Study Reference Sample Size Sample Type Micro-CT Machine Voxel Size (Resolution) Software Used Classification System Technique Results Conclusion**

**Table 3.** *Cont.*

\* Stored in 70% Alcohol. \*\* Stored in 0.5% sodium azide solution at 4 ◦C. \*\*\* Stored in 10% formalin.
