The first step in treating a patient is to gain a thorough understanding of the patient’s condition and develop an appropriate treatment plan, which first includes a thorough medical examination.5,21 Identifying variables that could compromise RCT will help operators prevent possible medical emergencies throughout the procedure.22 Following the medical evaluation, an objective examination and x-rays should be completed.5,22 Operators can then perform and interpret diagnostic tests to determine a patient’s condition and design high-quality treatment planning that takes into account their needs and preferences. Collecting all the information above avoids misdiagnosis and mistreatment of patients. Proper treatment planning not only helps practitioners avoid procedural pitfalls (e.g., missed root canals, over-removal of dentin, perforation, instrument separation), but also allows dentists to select cases based on their experiences, skills, and comfort levels.5,23

General status of patients

The patient’s own characteristics, including general health status, oral and maxillofacial conditions, and psychosocial status, are closely related to the success of RCT.5,6,22,24 These conditions not only determine whether RCT can be performed safely and successfully, but also how difficult it is for operators, which ultimately affect the efficacy of RCT. Chung et al. discovered whatever patient conditions were included or excluded to assess the RCT difficulty, there was a strong positive association between the difficulty and the operating time on all tooth types.25 The results of this study imply that in clinical practices, the RCT difficulty in relatively healthy patients depends on the tooth itself. Therefore, it is important to take the patient’s situation into account to fully assess the RCT difficulty. Nevertheless, it is commonly known that RCT gets increasingly difficult for elderly patients for a variety of reasons, including pathology, physiologic aging, and the shrinkage of the pulp canal space brought on by the deposition of secondary and tertiary dentine and cementum.25

Systemic diseases

The patient’s systemic condition influences the choice and implementation of the treatment plan. Systemic diseases not only determine whether RCT can be performed safely, but also affect the RCT outcome.5,6,22,24 Before treatment, a comprehensive and detailed medical history should be taken. Multiple criteria for assessing the RCT difficulty use the American Society of Anesthesiologists (ASA) classification to assess the level of risk associated with a patient’s medical history.23,26 Emphasis should be placed on asking patients about systemic diseases and medication, including cardiovascular diseases, bleeding disorders, hypertension, diabetes, mental status (with special attention to dental phobia), and history of anesthesia.27 Patients’ age, gender, and kind of handicap are other demographic variables that may have an impact on the patient’s overall health.28 Nevertheless, several studies have discovered that there is minimal correlation between operating difficulty and the demographic traits of the patients.25,29

Oral and maxillofacial conditions

Patient-derived factors, including mouth opening restrictions, gagging, salivation, tooth arrangement, and occlusal relationship, are associated with the RCT difficulty and prognosis.5,20,22,24,25,27,30 A previous study has reported that more than 40% of the patients experienced limited mouth opening or gaging during RCT.20 Among these patient-derived factors, the pharyngeal reflex is closely related to the occurrence of RCT complications.20 A previous study has reported that gagging patients experienced noticeably greater complications than non-gagging patients.20 Moreover, the ability of mouth opening is closely related to the RCT difficulty.25,27 Limited intraoral spaces make it challenging to insert and maneuver intracanal instruments, even with the use of a mouth prop throughout processes.

Psychosocial status

Patient demographic factors such as fear, type of caregiver, and oral hygiene maintenance may have a strong relationship with cooperation level, thus affecting the RCT difficulty, especially when patients are dental phobic.5,20,31 In addition, some patients may have pain that cannot be resolved using conventional measures, which will make RCT more difficult.30

Tooth conditions

The degree of root canal infection and anatomical diversity of teeth determines the RCT difficulty.5,6,22,24,32,33,34 It is generally accepted that the case difficulty is significantly correlated with the clinical operating time.25 Among the many clinical variables that contribute to the RCT difficulty, the tooth anatomical complexity is the main factor that prolongs the operating time in clinical.25

Infection of dental pulp and root canals

The success of RCT is directly correlated with the state of the dental pulp and the difficulty of debriding the infection in the root canal system.32,33,34 In the early stage of infection, the clearance of the root canal system is easy for viable pulp and non-infected root canals, so the efficacy of RCT is exact.33,35 In the late stage of infection, however, especially in the affected teeth with chronic apical periodontitis or post-endodontic diseases, the root canals are severely infected with microbial biofilms.33,35 This poses a hard challenge to completely eradicate the infection, particularly in complex anatomical structures including root canal isthmus, lateral branch of root canal, and root canal divergence.33,36 Consequently, the effectiveness of RCT is unsatisfactory. Based on the pulp state and the level of root canal infection, root canals are classified into four categories: clean, non-infected, infected, and severely infected root canals.33 The elimination difficulty of the root canal system is based on its infection degree.

Value of tooth preservation: crown defects and periodontal lesions

Extensive loss of dental hard tissue at the crown leads to reduced fracture resistance and reduced bonding surface, resulting in an inability to hold fillings in place and easy dislodgement, as well as loss of coronal seal.37 Periodontal tissue is the supporting tissue for teeth. It is an important part of the chewing function of the tooth. When there is a loss of periodontal tissue due to periodontal disease, it may lead to tooth loss with reduced or even loss of chewing function.38 In addition, a number of chronic systemic disorders, most prominently type 2 diabetes, are independently associated with periodontitis.39

Tooth position in the dentition

The anatomical location of teeth affects the degree of cooperation of the patient, the ease of reaching the affected tooth with instruments, and the difficulty of operators’ maneuvering.22,27 Usually, anterior teeth are fully exposed and instruments are easy to enter, thus RCT is less difficult.27 Posterior teeth, especially molars, are affected by the patient’s mouth opening and operators’ operating field, so instruments and materials are difficult to enter and RCT is more difficult.5,20,22,24,27 Tooth types and positions in the arch were the significant factors affecting operating time and the quality of endodontic treatment outcomes.25,40 During micro-endodontics, the maxillary posterior teeth are easier to be observed and operated under the microscope, but the mandibular posterior teeth are more challenging.

Tooth eruption position in the dentition

Tooth eruption refers to the process by which teeth gradually emerge from the jawbone to the oral cavity, ultimately attaining a functional occlusal position.41 Ectopic eruption may occur as a result of various circumstances.41,42 Ectopic eruption encompasses several forms, such as buccal, rotational, and proximal-distal-medial tilted ectopic eruption, which are determined by the position and orientation of teeth and have a significant influence on the degree of RCT difficulty.22 For example, the buccal inclination of maxillary molars imposes more difficulty on RCT by limiting the operators’ field of view and increasing the difficulties of the instrument to access cavity preparation. Ectopic eruptions increase the difficulty of rubber dam installation and isolation.

Tooth crown morphology and restoration

Abnormalities in tooth morphology result in the variation or loss of important anatomical reference points, thus making it difficult for operators to assess the pulp chamber. Crown morphology is also complicated by development, restoration, or destruction, which influences operators’ judgment of the root canal system.25,30 Common clinical conditions include dens invaginatus, prosthesis of the full crown (especially twisted teeth), dental trauma and so on.23,30 These conditions increase the incidence of RCT complications. Of the cases where complications occurred, 62% had a wide restoration.20 In comparison to patients with normal tooth morphology, patients with abnormal crown morphology also experienced considerably more treatment-related complications.20 If access cavity preparation is performed on the prosthesis of the full crown, there is a greater chance of excessive dentin removal and/or perforation because the crown’s orientation may deviate significantly from the root’s orientation.23 Moreover, fillings at the tooth cervical region may block the pulp space, which raises the possibility of causing a blockage in the root canal during instrumentation.23

Root canal system configuration

Comprehensive and systematic understanding of the pulp cavity, including the pulp chamber and root canal is important. Pulp chamber, number of canals, shapes and negotiability of canals, and apical closure have a strong relationship with root canal system configuration.6 Currently, root canal system configuration is usually obtained clinically by taking preoperative apical radiographs or CBCT.43,44,45

Pulp chamber morphology

A receded pulp chamber is caused by tubular secondary or tertiary dentine deposition as a result of pathologic causes (like caries, wedge-shaped defects, restoration) and age-related changes. This deposition manifests itself as matrix deposition along root canal walls, or dentine bridge formation at the orifice of root canals, or complete pulp canal obliteration. Therefore, access cavity preparation and root canal orifice detection are more challenging.46,47 The pulp calcification index was proposed to categorize the degree of pulp chamber and root canal calcification.48 Grade 1: deep pulp chamber and wide root canal; Grade 2: shallow pulp cavity and thinned root canal with clear root canal imaging; Grade 3: partial disappearance of pulp cavity and root canal; Grade 4: complete disappearance of pulp cavity and root canal. For the purpose of clinical application, pulp chamber calcification was classified into 3 categories. Grade 1: no calcification in pulp chamber; Grade 2: partial calcification in pulp chamber; Grade 3: complete calcification in pulp chamber. In addition, tertiary dentin and dental pulp fillings, such as amalgam core build-up and glass fiber core build-up, can make it more difficult to obtain access to cavity preparation.

Number of root canals

In general, the less the number of root canals in a root, the easier the endodontic operation is; on the contrary, when the number of root canals in a root is more, it may result in a smaller diameter of the root canals and more variations in their configuration, thus the more difficult the endodontic operation is.22,27 The complexity of root canal topologies inside a single root is determined by the manner in which the main root canal divides along its path from the pulp chamber to the root apex.49,50 These configurations include one canal in a single root, two canals in a single root, multiple canals in a single root, three canals in premolars or molars, second mesio-buccal (MB2) canal in maxillary molars, a middle mesial canal in mandibular molars, and atypical root canals.50,51,52 In general, the chance of missing a root canal increases with the number of root canals.53 The high percentage of endodontic treatment failure can be ascribed to an untreated missed root canal with bacteria and necrotic tissue inside the canal.54 It was revealed that 66.0% of RCT failures in the maxillary first molar involved an untreated MB2 canal.55 Moreover, Shah et al. suggested that a tooth with supernumerary roots should be paid extra attention.56

Root canal morphology

To effectively debride bacteria and necrotic pulp tissue in the root canal, a comprehensive understanding of root canal morphology is necessary.50 Based on the root canal morphology, root canals are divided into I-shaped canals, C-shaped canals, J-shaped curve canals, and C/S-shaped curve canals. C/S-shaped curve canal, C-shaped canal system, bifurcating canals in the apical/middle third, and apical delta make root canal negotiability complex, increase the risk of creating a blockage or separating an instrument in canals, and complicate obturation of the canal space.23,57,58,59,60 A C-shaped canal system was likely to present in the fused root tooth, and the prevalence is 39% in Chinese mandible second molars. C-shaped canals appeared to divide into two or more canals towards the canal terminus.61 Isthmuses within the root canal system, may contain necrotic debris, tissue remnants, or organic substrates that suppo