A Brief History of the First Studies on Root Canal Anatomy

Since the Þrst attempts of using contemporary advanced imaging systems, such as X-ray computerized tomography [1Ð5], a lot of research work

M.A. Versiani, DDS, MSc, PhD (*) Department of Restorative Dentistry, Dental School of Ribeirao Preto, University of Sao Paulo,

Avenida do CafŽ, s/n Bairro Monte Alegre, Ribeirao Preto 14049-904, SP, Brazil e-mail: marcoversiani@yahoo.com

J.D. PŽcora, DDS, MSc, PhD M.D. Sousa-Neto, DDS, MSc, PhD

Department of Restorative Dentistry, Dental School of Ribeirao Preto, University of Sao Paulo, Ribeirao Preto, Brazil

has been done in relation to the root canal anatomy and its remarkable inßuence on the endodontic procedures. However, to understand the contemporary approaches regarding this issue, it would be appropriate to take a brief look to the past. Authors that preceded this new image-processing technological era, to whom endodontics is greatly indebted, should be always revisited.

Although the Hungarian dentist and professor Gyšrgy Carabelli, from the University of Vienna, was eternized in the dental literature by his description of an additional cusp on the palatal surface of the mesiopalatal maxillary molar cusp [6], the so-called CarabelliÕs cusp, he was also the Þrst author to provide a comprehensive description of the number and location of root canals. In his textbook, Anatomie des Mundes [6], he reproduced some illustrations of sectioned

teeth detailing the root canal system and the external morphology of all groups of teeth. Thirty years later, MŸhlreiter [7] published the Þrst systematic study on the root canal anatomy in which teeth was sectioned in all planes and the internal anatomy described in details. After a few decades, Greene Vardiman Black published the Þrst edition of his classic book [8] in which he systematized the dental terminology and detailed the internal and external anatomy of the teeth. According to him, Òanatomy is not to be learned from books alone, but also by bringing the parts to be studied into view, and closely examining them in connection with the descriptions given.Ó In 1894, Professor Alfred Gysi, from the University of ZŸrich, published a collection of photomicrographs in which impressive pictures of histological sections of human teeth demonstrated the complexity of the root canal system [9]. Nevertheless, at this point, the methodological approaches for studying the root canal anatomy were predominantly based on sectioning techniques.

At the beginning of the twentieth century, Preiswerk introduced the Òmodeling techniqueÓ for the study of the root canal anatomy [10]. His method consisted in the injection of molten metal (70 ¡C) into the canal space in which, after complete tooth decalciÞcation, it was possible to obtain a metal model of its internal anatomy. The main limitation of this method was that it led to tooth overheating and the replicas were obviously incomplete as the metal could not penetrate the Þner branches of the root canal system. Despite these methodological drawbacks, Preiswerk was one of the Þrst researchers who stated that Òa canal-anastomosis system can be found in some roots and is not rareÓ [10]. In 1908, Fischer [11] obtained better results Þlling approximately 700 teeth with a collodion solution, made up of 1 part small-piece collodion to 8 parts of pure acetone. The collodion solution was able to penetrate all the branches of the root canal system and harden in 2 or 3 weeks, providing a full replica of the root canal system. Fisher deeply studied ramiÞcations and little lateral canal branches, especially those near the apical foramen. However, the hardened collodion solution was fragile, and replicas of the more subtle

ramiÞcations fractured easily. In later years, improved techniques for injecting different materials, such as parafÞn [12], were also used to obtain a model of the root canal space.

In 1914, the German anatomist Werner Spalteholz developed a process in which organs could be made translucent and stained using different colors [13]. This process was based on dehydration of the removed organs and the use of anoptically transparent embedding material that had the same refractive index as the tissue of the organ itself. Some researchers in the endodontic Þeld modiÞed and simpliÞed the SpalteholzÕs method employing this Òclearing techniqueÓ (diaphanization) for the study of the root canal anatomy. Basically, this method renders the surrounding hard tissues transparent through demineralization after injecting ßuid materials, such as molten WoodÕs metal [14], gelatin-containing cinnabar [15], and China ink [16], into the root canal system.

After considering that the available research methods did not Þt for the study of a large number of teeth, Professor Walter Hess developed his own technique and studied the root canal morphology of approximately 3,000 teeth [17, 18]. Basically, he used the demineralizing method, packing and pressing softened natural rubber, which was vulcanized later into teeth. Then, specimens were washed in running water and placed in 50 % hydrochloric acid. After decalciÞcation, the teeth were washed again, organic debris removed, and vulcanite samples mounted on blocks of chalk. Hess corroborated his results performing some histological preparations by carrying out serial sections. He established a correlation between the presence of ramiÞcations and the patientÕs age and published details about the percentage number of root canals in all groups of teeth [17]. A few years later, Okumura speciÞed the percentage values concerning the number and divisions of the main root canal in 1,339 teeth using dye injection and diaphanization technique [19].

In the following decades, the morphology of the root canal system was described by several in vivo and ex vivo methods such as threedimensional wax models [20], conventional radiography [21Ð32], digital radiography [33Ð35],

resin injection [36Ð38], macroscopic evaluation [27, 39, 40], tooth sectioning on different planes [39, 41Ð46], microscopy evaluation [43Ð45, 47, 48], clearing techniques [49Ð59], radiographic methods with radiopaque contrast media [60], and scanning electron microscopy [61].

Without doubt, these techniques have shown potential for endodontic research and have been used successfully over many years [62]; however, some of them may provide questionable data. The accuracy of radiographic methods, longitudinal and transverse cross sectioning, and microscopic approaches in assessing the morphology of the root canal system is reduced because they provide only a two-dimensional image of a threedimensional structure [63]. It may be pointed out that in the process of making the sections, the specimens are also destroyed, and an accurate image of the root canal as a whole cannot be obtained because of the large thickness of the sections [64]. Modeling techniques with the removal of all surrounding tissues from casts of root canals with wood metal, celluloid, resin, or wax, as well as, decalciÞcation and clearing techniques, produce irreversible changes in the specimens [65] and many artifacts [66] which, therefore, cannot accurately reßect the canal morphology [67, 68]. Furthermore, these techniques do not allow for the three-dimensional analysis of the external and internal anatomy of the teeth at the same time [64]. These inherent limitations have repeatedly been discussed, encouraging the search for new methods with improved possibilities [62].

Computational Methods for the Study of Root Canal Anatomy

In 1986, Mayo et al. [69] introduced computerassisted imaging in the Þeld of endodontic research. According to these authors, endodontics needed Òa model for studying canal morphology before, during, or after endodontic therapy on actual teeth.Ó They adapted a technique that allowed three-dimensional imaging of objects [70] for the evaluation of the root canals of single-rooted premolars. Brießy, after

the injection of a contrast medium into the root canal, six radiographs of each tooth were taken from known angles. By combining all six views, a mathematically determined three-dimensional (3D) representation of the canals was obtained. From this data, the volume and diameters of the root canals were estimated using a computerized video image-processing program. Despite a signiÞcant discrepancy in the results, essentially caused by technological computer-processing limitations, authors stated that Òapplications of this technique in the Þelds of research and education are very promising.Ó This radiographic volume interpolation method from two-dimen- sional radiographs taken in different angles was also used in further studies to evaluate the root canal anatomy [71Ð73]. Some years later, a new computerized method for 3D visualization of the root canal before and after instrumentation was introduced [74]. Five cross-sectional images of the mesial root of mandibular Þrst molars before and after canal preparation, at intervals of 1 mm, were obtained. Then, micrographs of these sections were transferred to a graphics computer, which rebuilt, superimposed, and elaborated the sections, providing a 3D model of the root with the image of the canal system. Subsequently, this computer-based method was improved by decreasing the cross-sectional thickness of the root [75Ð79].

These computerized methods allowed the development of 3D models of the root as well as the measurements of parameters such as distance, contour, diameter, perimeter, area, surface, and volume of the canal. Despite the improvements achieved with this newer approach, it was still a destructive technique, and the thickness of sections and material loss were found to inßuence the obtained results [79]. The invention of X-ray computed tomography (CT) brought a signiÞcant step forward in diagnostic medicine [70]. CT produces a two-dimensional map of X-ray absorption into a two-dimensional slice of the subject. This is achieved by taking a series of X-ray projections through the slice at various angles around an axis perpendicular to the slice. From this set of projections, the X-ray absorption map is computed. By taking a number of slices, a three-dimensional map is produced [5]. To maxi-

mize their effectiveness in differentiating tissues while minimizing patient exposure, medical CT systems need to use a limited dose of relatively low-energy X-rays (≤125 keV). Besides, they must also acquire their data rapidly because the patient should not move during scanning. Then, to obtain as much data as possible given these requirements, they use relatively large scale in mm and high-efÞciency detectors [80].

In 1990, Tachibana and Matsumoto [1] were the Þrst authors to suggest and evaluate the feasibility of CT imaging in endodontics. Because of high costs, inadequate software, and a low spatial resolution (0.6 mm), they concluded that CT had only a limited usefulness in endodontics as achieved images were not detailed enough to allow a proper analysis. Further improvements in digital image systems have been used to evaluate the root canal anatomy in either ex vivo or in vivo conditions using nondestructive tools such as conventional medical CT [81Ð86], magnetic resonance microscopy [87Ð93], tuned-aperture computed tomography (TACT) [94, 95], optical coherence tomography [96], and volumetric or cone beam CT (CBCT) [97Ð114]. However, these digital image systems were hampered mainly by insufÞcient spatial resolution and slice thickness for the study of root canal anatomy [3, 4].

A decade after the CT scanner was created, Elliott and Dover [2] developed the Þrst highresolution X-ray microcomputed tomographic device, and using a resolution of 12 μm, the image of the shell of a Biomphalaria glabrata snail was produced. The term ÒmicroÓ in this new device was used to indicate that the pixel sizes of the cross sections were in the micrometer range. This also meant that the machine was smaller in design compared to the human version and was indicated to model smaller objects [115]. X-ray microcomputed tomography (micro-CT) has also been denominated as microcomputed tomography, microcomputer tomography, high-resolution X-ray tomography, X-ray microtomography, and similar terminologies. Nowadays, despite the impossibility of employing micro-CT for in vivo human imaging, it has been considered the most important and accurate research tool for the study of root canal anatomy [63, 67, 68, 116].

The Micro-CT Technology

in Endodontics

Like conventional medical tomography, microCT also uses X-rays to create cross sections of a 3D object that later can be used to recreate a virtual model without destroying the original model [115]. Therefore, whereas a typical digital image is composed of pixels (picture elements), a CT slice image is composed of voxels (volume elements) [80, 115] (Fig. 2.1).

Because micro-CT is mostly used in nonliving objects, the scanners were designed to take advantage of the fact that the items being studied do not move and are not harmed by X-rays. Basically, micro-CT technology employs four optimizations in comparison to conventional CT [80]:

(a)It uses high-energy X-rays which are more effective at penetrating dense materials.

(b)X-ray focal spots are smaller providing increased resolution at a cost in X-ray output.

(c)X-ray detectors are Þner and more densely packed which increases resolution at a cost in detection efÞciency.

(d)It uses longer exposure times increasing the signal-to-noise ratio to compensate for the loss in signal from the diminished output and efÞciency of the source and detectors.

Application of micro-CT technology to endodontic research was recognized only 13 years after its development and described in a paper entitled Microcomputed Tomography: An Advanced System for Detailed Endodontic Research [3]. In this article, Nielsen et al. [3] evaluated the reliability of micro-CT in the reconstruction of the external and internal anatomy of four maxillary Þrst molars, assessing the morphological changes in the root canal after instrumentation and obturation, using an isotropic resolution of 127 μm. Authors concluded that micro-CT had Òpotential as an advanced system for research, but also provides the foundation as an exciting interactive educational tool.Ó In this study, three-dimensional images of the internal

Fig. 2.1 Three-dimensional cross section of the coronal third of a mandibular second molar root (a) illustrating the difference between pixel (b) and voxel (c). The word pixel stands for picture element. Every digital image is made up of pixels. They are the smallest unit of information

arranged in a two-dimensional grid that makes up a picture. Voxel stands for volumetric element, and it is the three-dimensional equivalent of a pixel and the tiniest distinguishable element of a 3D object

and external structures of the teeth were also presented in a format previously unattainable [3].

With further developments of the micro-CT scanners, improvements in the speed of data collection, resolution, and image quality yielded greater accuracy compared with the Þrst studies using computational methods, with voxel sizes decreasing to less than 40 μm [4, 117]. Dowker et al. [4] demonstrated the feasibility of this technology using a resolution of 38.7 μm to evaluate the morphological characteristics of the root canal before and after different steps of root canal treatment. Authors concluded that micro-CT technology would offer the possibility of learning tooth morphology by interactive study of surfacerendered images and slices, contributing to the development of virtual reality techniques for endodontic teaching. Later, the reliability of microCT as a methodological tool was also demonstrated in the quantitative assessment of the root canal preparation [62, 116Ð119], obturation [120], and retreatment [121], using innovative image software that allowed the alignment of preand post-image volumes.

Therefore, micro-CT has gained increasing signiÞcance in the detailed study of canal anatomy in endodontics because it offered a nondestructive reproducible technique that could be applied quantitatively as well as qualitatively for twoand three-dimensional accurate assessment of the root canal system [116]. Conversely, given that scanning and reconstruction procedures take considerable time, the technique is not

suitable for clinical use, the equipment is expensive, and the complexity of the technical procedures requires a high learning curve and an in-depth knowledge of dedicated software. The technical procedures related to the micro-CT methodology with the aim to evaluate aspects related to the morphological analysis of the root canal anatomy are a complicated subject, and a thorough discussion is beyond the scope of this text. However, an understanding of basic principles is desirable to ensure a better comprehension of its potential as a tool for endodontic teaching and researching.

A typical micro-CT scanner consists of a microfocus X-ray source, a motorized highprecision sample rotation stage, a detection array, a system control mechanism, and computing software resources for reconstruction, visualization, and analysis of the root canal anatomy [122]. The source sends X-ray radiation through the tooth attached to the sample stage (Fig. 2.2a), and a detector array Ð coupled to a digital chargecouple device camera Ð records attenuated intensities of the X-ray beam, while the object rotates on its own axis (Fig. 2.2b); i.e., micro-CT involves gathering projection data of the tooth from multiple directions. If many projections are recorded from different viewing angles of the same tooth, each projection image will contain different information about its internal structure. At this stage, the only preparation that is absolutely necessary for scanning is to ensure that the previously cleaned tooth Þts inside the Þeld of

Fig. 2.2 Inside view of the chamber of a SkyScan 1174 v2 (Bruker-microCT, Kontich, Belgium) micro-CT device. Common elements of micro-CT: (a) X-ray source, an object attached to the sample stage to be imaged through which the X-rays pass, and a detector(s) that mea-

sures the extent to which the X-ray signal has been attenuated by the object. The source sends X-ray radiation through the tooth, and a detector array records attenuated intensities of the X-ray beam, while the object rotates on its own axis (b)

view and does not move during the scan [80]. The entire operation of the scanner, including X-ray exposure, type of Þlter, ßat-Þeld correction, resolution, rotation step, rotation angle, number of frames, data collection, etc., is controlled by a software Ð the system control mechanism Ð which allows setting up these parameters in order to improve the further 3D reconstruction of the tooth.

After recording the X-ray images, the projection data of the tooth from multiple directions (Fig. 2.3a) is then used as input for a reconstruction algorithm. This algorithm computes a threedimensional image of the internal anatomy of the tooth, based on the two-dimensional projection images (Fig. 2.3b) [123]. The resulting volumetric images are then subjected to image segmentation using dedicated software. Image segmentation is a manual or automatic procedure that can remove the unwanted structures from the image based on the object density. The goal of segmentation is to simplify the representation of an image into something that is more meaningful and easier to analyze. More precisely, image segmentation is the process of assigning a label to every pixel in an image as such that pixels with the same label share certain visual characteristics [124]. Concerning the tooth, the different radiographic densities of the enamel, dentin, and root canal facilitate the segmentation procedures (Fig. 2.3c).

The result of image segmentation is a set of segments that collectively cover the entire image. When applied to a stack of images, as in the study of the internal anatomy of the teeth, the resulting contours after image segmentation can be used to create 3D models with the help of interpolation algorithms, which can be visualized (Fig. 2.3d) or analyzed using different software.

Evaluation of Root Canal Anatomy

Using Micro-CT

The Þrst attempt to use micro-CT as a quantitative tool for the analysis of the root canal anatomy was done by Bj¿rndal et al. [125]. Authors correlated the shape of the root canals to the corresponding roots of Þve maxillary molars scanned at a resolution of 33 μm. However, the real potential for the analysis of several quantitative parameters using micro-CT was reported in the following year [116]. Peters et al. [116] evaluated the potential and accuracy of micro-CT for detailing the root canal geometry of 12 maxillary molars regarding volume, surface area, diameter, and structured model index. Then, micro-CT was used by different groups to evaluate geometrical changes in root canals after preparation with different instruments and techniques [62, 119, 126Ð129], as well as, for educa-

Fig. 2.3 The projection data of the tooth from multiple directions (a) is used as input for a reconstruction algorithm which computes a 3D image of the internal anatomy of the tooth, based on the 2D projection images (b). The

different radiographic densities of the tooth tissues (c) facilitate its segmentation which can be used to create 3D models (d)

tional purposes [64, 130, 131]. Though, it took over 18 years for the micro-CT scanners gain accessibility [3] and the Þrst in-depth studies evaluating the root canal anatomy started to be published. The main results of the studies published in indexed journals in English language are summarized in Tables 2.1, 2.2, 2.3, and 2.4.

Most of the micro-CT studies on root canal anatomy evaluated anatomical variations present in speciÞc groups of teeth, such as the second canal in the mesiobuccal root of maxillary Þrst

molars [161Ð165, 167Ð170], three-rooted mandibular premolars [135, 143, 144] and molars [154Ð156], four-rooted maxillary second molar [67], two-rooted mandibular canines [68] and premolars [141], C-shaped canals in mandibular premolars [136Ð138] and molars [145, 146, 148Ð 152, 159], radicular grooves [134, 136, 139, 140, 144], and isthmuses [147, 153, 157, 158, 160]. Other authors evaluated the anatomical conÞguration of conventional mandibular incisors [132, 133], mandibular canines [63], mandibular Þrst

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(continued)

Tomography Microcomputed Using Teeth Permanent of Anatomy Canal Root in Update 2

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Tomography Microcomputed Using Teeth Permanent of Anatomy Canal Root in Update 2

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.al et Versiani .A.M

Tomography Microcomputed Using Teeth Permanent of Anatomy Canal Root in Update 2

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Table 2.4 Micro-CT studies on the root and root canal morphology of maxillary molars

Tomography Microcomputed Using Teeth Permanent of Anatomy Canal Root in Update 2

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Table 2.4 (continued)

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3D models

Central incisor

Lateral incisor

Canine

Normal

anatomy(a) Variations

2 canals(b)

1 canal 3 canals(c)

4 canals(d)

2 canals(e)

1 canal 3 canals(f)

4 canals(g)

1 canal 2 canals(h)

Anomalies

Two-rooted(i) Radicular groove(j) Fusion/gemination(k)

Two-rooted(l) Radicular groove(m) Fusion/gemination(n)

Dens invaginatus(o) Dens evaginatus(p)

C-shaped(p) Talon cusp(r) Apical curvature(s)

Dens invaginatus(t)

Clinical remarks(u)

-A total of 79.7 % of all foramina were located approximately 0.5 mm or less from the apex and 94.9 % were approximately 1.0 mm or less away

-56.4 % of the lateral canals has a mean diameter less than an size 10 K-file

-Average length: 22.5 mm

-High frequency of apical root curvature to the disto-buccal direction

-Average length: 22 mm

-Root canal cross-section is usually oval-shaped

-Large midroot canal diameter

-Average length: 26.5 mm

premolars [142], and maxillary molars [166]. Summarized data for canal numbers and its variations, extracted from selected references, are presented in Figs. 2.4, 2.5, 2.6, and 2.7.

The quantitative morphological data of the Þrst studies [41, 61] on root canal anatomy using conventional methods were taken from measuring some parameters such as area, diameter, and perimeter, acquired from a few cross sections of the root. In contrast, these same parameters can be easily measured by means of micro-CT technology using automatic computer tools in hundreds of slices at once. Based on cross sections of the root, the canal shape has been also qualitatively classiÞed as round, ßat, oval, or irregular shaped [242]. Despite its applicability, a qualitative evaluation is always subjective, which may lead to inaccurate results. Algorithms used in micro-CT evaluation allow a mathematical description of these cross-sectional appearances using two morphometric parameters: form factor and roundness. Roundness is deÞned as 4.A/

(p.[dmax]2), where ÒAÓ is the area and ÒdmaxÓ is the major diameter. The value of roundness ranges from 0 to 1, with 1 signifying a circle. The form factor is calculated by the equation (4.p.A)/ P2, where ÒAÓ and ÒPÓ are object area and perimeter, respectively. Elongation of individual objects results in smaller values of form factor. Previous results using these parameters in singlerooted canines have demonstrated different crosssectional forms throughout the root canal [63]. This is an important data as different canal shapes in the same root may have impact on the selected chemomechanical protocol on root canal treatment. Form factor was also used to describe that the shape of the accessory foramen was more round than that of the main foramen in C-shaped canals of mandibular second molars [145] (Fig. 2.8a).

In the earlier studies, 3D analysis was applied qualitatively to evaluate the number and conÞguration of the main canal, as well as, the presence and location of accessory, lateral, and furcation

3D models

First premolar

Second premolar

First molar

Second molar

Clinical remarks(i)

-Most incisors have a single root

-Often a dentinal bridge is present in the pulp chamber that divides the root into 2 canals

-The 2 canals usually join and exit through a single

apical foramen; but, they may persist as 2 separate canals

-Removal of the lingual shoulder is critical, because this tooth often has 2 canals

-Canal cross-section is oval-shaped, wider buccolingually than mesiodistally

-Average length: 20.7 mm

-The root canal is narrow mesiodistally but usually very broad buccolingually

-In two-rooted canines, a lingual shoulder must be removed to gain access to the entrance of a second canal

-The lingual wall is almost slit-like compared with the

larger buccal wall, which makes the canal. - Average length: 25.6 mm

canals, and apical deltas. Nowadays, 3D analysis using micro-CT algorithms allows also for the calculation of volume and surface area [116]. The clinical signiÞcance of such parameters has been emphasized by studies demonstrating that variations in canal geometry before cleaning and

shaping had a greater effect on the changes that occurred during preparation than did the instrumentation techniques [119]. Besides, considering that the main role of laboratory-based studies is to develop well-controlled condition, these morphological data should be taken into account in

3D models

First premolar

Second premolar

First molar

Second molar

Clinical remarks(ak)

-The root canal system is extremely variated.

-The root canal system is wider buccolingually than mesiodistally

-At the cervical third is oval-shaped and tends to become round at the middle and apical thirds

-The lingual canal, when present, tends to diverge from the main canal at a sharp angle

-Average length: 21.6 mm

-The root canal is more often oval than round

-The lingual canal, when present, tends to diverge from the main canal at a sharp angle

-The canal morphology may present many variation

-Average length: 22.3 mm

-It usually has 2 roots, but occasionally it has 3, with 2 or 3 canals in the mesial root and 1,2, or 3 canals in the distal root

-The distal surface of the mesial root and the mesial surface of the distal root have a concavity, which makes the dentin wall very thin

-The presence of root canal isthmuses averages 55% in the mesial root and 20 % in the distal root

-Multiple accessory foramina may be present in the

furcation area.

-Average length: 21 mm

-It may have 1 to 5 canals, although the most prevalent configurations are 3 and 4 canals

-The 2 mesial orifices are located closer together

-A variation in root morphology is the presence of

C-shaped canal

-The apices of this tooth often are close to the mandibular canal

-Average length: 19.8 mm

Fig. 2.8 (a) Two-dimensional micro-CT cross section of the cervical third of a maxillary Þrst molar root showing the 2D parameter measurements of the four root canals. (b) Frontal and (c) lateral views of 3D models of a man-

dibular canine root canal before (green) and after (red) preparation with a conventional multiple-Þle rotary system, demonstrating the qualitative and quantitative changes in the canal geometry

Fig. 2.9 Three-dimensional micro-CT models of the mesial root system of 8 mandibular molars presenting regular (a) and irregular (b) tapered root canals, as well as, canals connected by isthmus (c), after preparation (in

red) with single-Þle reciprocating systems. From left to right, it is possible to observe that with the increase of the complexity of the root canal system, the amount of nonprepared canal surface areas (in green) also increases

the sample selection, as the results of such studies might demonstrate the effect of canal anatomy rather than the variable of interest [63, 68, 119, 243, 244].

Another interesting 3D parameter that can be evaluated using micro-CT is the so-called structure model index (SMI). SMI is derived as 6. ((SÕ.V)/S2), where S is the object surface area before dilation and SÕ is the change in surface area caused by dilation. V is the initial, undilated object volume. An ideal plate, cylinder, and sphere have SMI values of 0, 3, and 4, respectively. SMI is impossible to achieve using conventional techniques such as radiographs or grinding, and describes the plateor cylinder-like geometry of an object. The SMI is determined by an inÞnitesimal enlargement of the surface, while the change in volume is related to changes of surface area, that is, to the convexity of the structure. This parameter has been used to assess root canal geometry three-dimensionally in anatomical studies of different groups of teeth [63, 67, 68, 116] (Fig. 2.8b, c). A recent study has shown a large discrepancy between the minimum and maximum values of SMI in the comparison of the root canal thirds in a same tooth [63]. These dissimilarities should be taken into consideration during the root canal preparation as it might compromise the treatment outcome.

The Influence of Root Canal Anatomy on Irrigation Procedures

Advances with micro-CT analysis brought new perspectives on the overall mechanical preparation quality, conÞrming the inability of shaping

tools in acting within the anatomical complexity of the root canal [81, 118, 126Ð129, 243, 245, 246]. Preparation of oval-, ßattened-, or irregularshaped cross-sectional root canals using different instruments has shown to leave unprepared extensions or recesses which can harbor remnants of necrotic pulp tissue and bioÞlms [242, 243]. The disinfecting effects of instruments and irrigants may be additionally hampered in the presence of complex anatomy such as accessory canals, ramiÞcations, intercanal connections, Þns, isthmuses, and apical deltas, which cannot be properly accessed and cleaned by conventional techniques [147, 153, 158, 168, 243]. These hard-to-reach areas may also be packed with dentin debris generated and pushed therein by endodontic instruments, interfering with disinfection by both preventing the irrigant ßow into them as well as by neutralizing its efÞcacy [247, 248] (Fig. 2.9).

Based on the aforementioned assumptions, spreading and ßushing the irrigant throughout the canal space assumes a pivotal role in treatment because it acts mechanically and chemically on remnants of necrotic pulp tissue and bacterial communities colonizing the main canal [243]. In order to circumvent limitations generated by the unpredictable anatomical conÞgurations of the root canal, making cleaning and disinfection procedures more predictable, several instruments and techniques have been developed and are properly detailed in this book. Ideally, efÞcient irrigation solutions and protocols are required to provide ßuid penetrability to such an extent as to accomplishing a microcirculation ßow throughout the intricate root canal anatomy and to counterbalance the suboptimal debridement quality obtained by currently available

Fig. 2.10 Three-dimensional micro-CT models of a type I root canal conÞguration molar. Original root canal anatomy (in green) prior to treatment (a) and after glide path (b), root canal preparation (c), and ultrasonic passive irrigation technique (d), subsequently to the injection of a contrast solution (in black). Irrigant-free areas are shown

in blue after each preparation step. Below: same cross sections of the root in different levels showing the root canal space (in black) before preparation and the contrast solution (in white) and irrigant-free areas (in black) after glide path, canal preparation, and ultrasonic irrigation

technology in the mechanical enlargement of the root canal space [246].

In laboratory-based studies, several experimental models have been used to understand the intracanal effect of irrigants by different irrigation protocols. It includes artiÞcially created grooves [249], histological cross sections [250], computational ßuid dynamics (CFD) [251Ð253], and in vivo use of radiopaque solutions [254Ð 256]. These methodological approaches provide valuable information about the quality of cleaning and shaping procedures which cannot otherwise be obtained, but they are unable to show some critical factors, such as the volume of the solution or the root canal areas effectively touched by the irrigant [257]. Besides, the destructive approach of these methods stands for its major drawback, since the preoperative condition of the root canal is unknown.

An ideal experimental model should allow a reliable in situ volumetric quantitative evaluation of the root canal space, offering a deeper and

comprehensive understanding on capabilities and limitations of different irrigation protocols. Recently, micro-CT has gained increasing significance in endodontics as it offers a reproducible technique for the three-dimensional assessment of the root canal system [63, 67, 68, 119, 244, 245, 248] in different groups of teeth (Tables 2.1, 2.2, 2.3, and 2.4). Micro-CT technology may also overcome several limitations displayed by the conventional methods on the study of root canal irrigation, as it provides three-dimensional quantitative volumetric and two-dimensional mapping of the irrigant within the root canal space (Fig. 2.10).

Using micro-CT, the volume of irrigant can be correlated to the full root canal volume and with the presence of some anatomical irregularity or the presence of dentin debris that may avoid the spreadability of the irrigant. A comprehensive quantiÞcation of irrigant-free areas can also be calculated and correlated, for example, to the irrigant delivery method, ßuid activation system,