Introduction

Introduction

Of all phases of anatomic study in the human system, one of the most complex is that of pulp cavity morphology as stated by Barett. Variations in external morphological features of crowns of teeth accord with variations in the shape and size of head. As the external morphological features of tooth varies from person to person as does the internal morphology of crown and root. Among these, the anatomy of root apex, its morphological variations and treatment are technical challenges for the endodontist. Much of the knowledge of root apex is based on the extensive work of HESS who studied 3000 permanent teeth and showed in minute detail the extensions, ramifications branching as well as size, shape and no of root canals in different teeth.

Fractures of apical third, resorption, weeping canals immature foramen are some areas which continue to be under constant research.Appreciable knowledge of morphology of this small zone and its variance, ability to interpret it correctly on radiograph and to feel it through tactile sensation during instrumentation are essential for an effective rendering of the treatment of the root canals.

Development of Root On completion of formation of crown, the cervical loop formed at the junction of inner and outer enamel epithelium proliferate downward to form Hertwigs epithelial root sheath. This root sheath determines the shape and size of root of tooth. The tip of epithelial root sheath also proliferates horizontally between the dental papilla and dental follicle to form the epithelial diaphragm. This process partially encloses the dental papilla and delineates the apical foramen/foramina.

The ventral section of the epithelial root sheath continues to grow in the apical direction and frees the fully formed crown towards the oral cavity while maintaining the epithelial diaphragm in a stable position in the jaw. This process marks the beginning of tooth eruption.

According to Orban, root apex remains in its place while the tooth crown and its supporting structures move occlusally. This has been supported by Stallard (1963). The cervical loop is the future cementoenamel junction.

The inner enamel epithelium of Hertwigs epithelial root sheath exerts an inductive effect on the adjacent ectomesenchymal cells of the dental papilla to differentiate into odontoblasts which forms the radicular dentin.

As dentin is formed, the root sheath and epithelial cells disintegrate and loose their continuity. This process allows the ectomesenchymal cells to come in contact with the root dentin.

These ectomesenchymal cells will differentiate into cementoblasts which deposit cementum.

The incremental deposition of cementum continues throughout the life of the tooth leaving rest lines on the tooths surface and makes the layer of cementum on the apical third of the root thicker than cervical third.

The continued deposition of cementum on apical third maintains the length of the tooth, constricts the apical foramen and also deviates the apical foramen from centre of the root apex.

Differential growth of the epithelial diaphragm in multirooted teeth causes the division of the root trunk into two or three roots.

During the general growth of the enamel organ the expansion of its cervical opening occurs in such a way that long tongue like extensions of horizontal diaphragm develop.

Two such extensions are found in the germs of lower molars and three in the germs of upper molars.

Before division of the root trunk occurs, the free ends of these horizontal epithelial flaps grow toward each other and fuse.

The single cervical opening of coronal enamel organ is then divided into two or three openings. On the pulpal surface of the dividing epithelial bridges, dentin formation starts and on the periphery of each opening, root development follows.

Occasionally, in a developing tooth, Hertwigs epithelial root sheath remains attached to underlying dentin especially in cervical third and furcation region of root. The attached epithelium may then deposit enamel resulting in development of enamel pearl. The clusters of cells that are left behind when epithelial root sheath migrates towards the dental sac and remain there as Cell Rests of Malassez.

Apical pulp tissue The apical pulp tissue is found in the apical end of the root canal, continuing into the surrounding periapical region.

The apical tissue differs structurally from the coronal pulp tissue which mainly contains cellular connective tissue and fewer collagen fibers whereas the apical tissue is more fibrous and contains fewer cells.

Histochemical studies by Yamashi et al in 1986, demonstrated large concentration of glycogen in the apical pulp tissue, a condition compatible with presence of anaerobic environment. The fibrous tissue in apical root canal is similar to that of periodontal ligament. This fibrous tissue acts as a barrier against apical progression of pulpal inflammation. In partial or total pulpitis, however complete inhibition of inflammation does not occur.

Inflammatory exudate may be found in the periapical tissues, even though the apical pulp tissue may be free of such exudates.Blood and nerve supply The fibrous structure of apical pulp tissue supports the blood vessels and nerves which enter the pulp. The pulp of the tooth is supplied by number of blood vessels which originate in the medullary space of bone surrounding the root apex. These blood vessels course through the periodontal ligament before entering into the apical foramen as arteries or arterioles. The width of these vessels may be same as the capillaries as they lack muscle elements in the walls. The blood vessels ramify in the apical pulp tissue. On entering the apical foramen the apical artery divides almost into several principal and several arteries. The blood vessels are surrounded by large myelinated nerves which also branch after entering the apical foramen. The intimate relationship between blood and nerve supplies of the pulp and periodontal ligament provides a background for the interrelationship of pulp and periodontal disease.

The clinical significance is especially in giving endodontic therapy for primary teeth with vital pulps. When vital pulp tissue is removed from the root canal especially with barbed broach the severance of pulp tissue from pdl is not under the complete control of operator. The severance can occur anywhere in the root canal or even beyond the apical foramen, somewhere in the pdl. When the latter type occurs, the ensuing hemorrhage causes painful pericementitis.Apical Dentin In the apical region, odontoblasts of the pulp are absent or flattened or cuboidal in shape.

The dentin that is produced is not as tubular as coronal dentin, but instead more amorphous and irregular. This type of dentin is called sclerotic dentin. The amount of sclerotic dentin generally increases with age.

At the orifices of root canals of younger teeth, the dentinal tubules become more oblique. In older teeth, the floor of the pulp chamber is irregular and atubular with the presence of atubular calcospheres. The use of isotopes in a study by Thompson in 1964 has shown that apical dentin is more sclerotic than coronal dentin. The sclerotic apical dentin is considerably less permeable than coronal dentin. This reduced permeability has significance because the sclerosed dentinal tubules are less readily penetrated or are impermeable by microorganisms or other irritants.

Apical Foramen In young incompletely developed teeth, the apical foramen is funnel shaped with wider portion extruding outward, is filled with fibrous tissue which is later replaced by dentin and cementum.

As the root develops, the apical foramen becomes narrower. The inner surface of root apex becomes lined with cementum which extends to a short distance into the root canal. This is the cementodentinal junction and does not end at the extreme end of the root but a few mm with in the main root canal. The apical foramen is not the most constricted portion of the root canal. Constriction occurs before the extremity of root is reached, usually 0.5 1 mm from the root apex. Acc. to Ingle, it is uncommon to find the foramen exiting at the centre of the apex; it may exit on the mesial, distal, buccal, and lingual of the root.

Anatomic studies have shown the apical foramen to coincide with the apex in only 17-34% of cases and it is located on an average of 0.4-0.7 mm away from the anatomic apex. In few cases, the apical foramen was found 2-3 mm away from anatomic apex. A knowledge of the age of calcification of root apex is essential for endodontic procedures especially when dealing with young pulpless immature permanent teeth, in which apexification has to be carried out prior to obturation. Care should be taken to prevent over instrumentation and extrusion of root canal filling materials. Apical foramen can be round, oval, or may have unusual shape such as hourglass, semilunar, c shaped etc. Cementodentinal Junction Cementodentinal Junction is the subject of much controversy. CDJ is the point of meeting of cementum and dentin at the apex of the tooth. The cementum and dentin do not meet at a point but in certain cases cementum overlaps the dentin.

According to Kuttler, the narrowest diameter of the canal is definitely not at the site of exiting of the canal from the tooth but usually occurs within the dentin, just prior to the initial layers of cementum. He referred to this position as the minor diameter of the canal, although others call it the apical constriction

It is at this site that is preferred to terminate canal preparation and build up the apical dentin matrix. The diameter of the canal at the site of exiting from the tooth (major diameter) was found to be approximately twice as wide as the minor diameter.

The average distances between the minor and major diameters were 0.524 mm in teeth examined in an 18-25 year old group and 0.659 mm in a 55-year-old and older age group. This means that the longitudinal view of the canal as a tapering funnel to the tip of the root is incorrect. The funnel tapers to a distance short of the site of exiting and then widens again. Because the adjacent walls of cementum are slightly convex or hyperbolic or funnel shaped when viewed in long section, the configuration of the area between the minor and diameters resembles that of a morning-glory flower.

On viewing post filling radiographs when sealer has been slightly extruded past the apical constriction, the morning-glory configuration is clearly visualized.

The terminal area of canal preparation is not at the major diameter but should be at the minor diameter. For obturation procedures, minor diameter provides a bottleneck area where an apical dentin matrix can be established to prevent over extrusion of the filling materials. However techniques to locate exactly the minor diameter are lacking.

Root Canal Isthmus An isthmus is defined as a narrow strip of land connecting two larger land areas or a narrow anatomic part or passage connecting two larger structures or cavities.

Green in 1973 described the isthmus as a corridor between the two roots. Other authors such as Pineda in 1973 discussed this phenomenon as a lateral connection, while Vertucci in 1984 stated that the isthmus was an anastomosis. Weller and colleagues in 1995 described the canal isthmus as a narrow, ribbon-shaped communication between two root canals that contains pulp tissue.

Isthmus Formation The actual formation of the isthmus from embryonic origin is through the epithelial root sheath.

In teeth with single roots, the inner cells of the root sheath next to the dental pulp differentiate into odontoblasts and start secreting dentin matrix. As this matrix is laid down and begins mineralizing, the epithelial root sheath cells secrete a thin layer of cementum on this newly formed dentin structure. The cells then continue to form dentin and cementum while breaks occur within the root sheath epithelium.

Degeneration of these root sheath epithelial cells allows mesenchymal cells or ectomesenchymal cells to migrate into these areas and differentiate to form cementoid to fill these gaps.

Cementum production continues as the tooth erupts into the oral cavity until root formation is complete. Occasionally, defects in the root sheath can be found. If the epithelial cell is defective, an odontoblast does not differentiate. Thus, dentin formation does not occur.

Without dentin formation, cementum is not deposited in this area. This condition then leads to lateral and accessory canals, which is commonly observed in the apical third of the root.

In multiple-rooted teeth and roots with multiple canals, however, another mechanism takes place.

As these multirooted areas continue to grow, defects can occur during normal root formation similar to a normal single root. Defects occur in multirooted teeth with a high incidence.

In this case, the tongue like projections of the epithelial root sheath do not completely fuse with one another. When this occurs, lateral or accessory canals in the furcation area can form.

An isthmus is formed when an individual root projection is unable to close itself off. Therefore the approximation of the root projections can fuse completely and form one root with one root canal system as in the distobuccal root of maxillary molars.

Alternatively, partial fusion results in the formation of two root canals with an isthmus formed in between, such as the mesial root of the mandibular first molar. No fusion leads to a large ribbon-shaped canal that also forms an isthmus throughout the entire root, which is a common finding in the distal root of the mandibular first molars and maxillary second premolars.

Classification of Isthmus Type I was defined as either two or three canals with no notable communications. Type II was defined as two canals that possessed a definite connection between the two main canals. Type III differs from the latter only with the presence of three canals instead of two. Incomplete C-shaped canals with three canals were also included in this category. Type IV: When canals extended into the isthmus area, Type V was recognized as a true connection or corridor throughout the section.

Clinical Significance Even with the high percentage of isthmuses that were found by researchers, conventional mechanical cleaning and shaping methods cannot physically debride this vitally important area.

According to Senia and colleagues in 1971, the only way to clean such anatomic variations is through the use of chemical irrigants such as full-strength sodium hypochlorite. Even with the use of full-strength sodium hypochlorite and mechanical instrumentation, the amount of tissue dissolution was significantly limited

The significance of an isthmus was taken into consideration conventionally first in 1971; however, it was not until 1983 that Cambruzzi and Marshall first reported this significant finding in molar surgery.

This area, which can lead to failures of conventional orthograde endodontic treatment, must also be considered in failures of traditional endodontic surgery. The tissue that can be left over after traditional surgical procedures with a microhead handpiece and microburs can be a nidus for recurrent infections.

Lateral & Accessory Canals Lateral canal is a canal that is located at approximately right angles to the main root canal.

Accessory canal is one that branches off from the main root canal, usually in the apical region of the root.

Accessory foramina are the openings of the accessory and lateral canals in the root surface.

In case where the root sheath disintegrates before dentin is continuously formed, a lateral canal is formed. In other cases, a failure in induction of dentinogenesis or presence of small blood vessel produces a gap that results in formation of an accessory canal.

Lateral and accessory canals contain fibrous tissue, fibroblasts, collagen fibers, nerves and capillaries and some macrophages. The connective tissue found in these canals is same as that found in pulp and periodontal ligament.

Hess in 1925, by vulcanite corrosion of specimens detected incidence of 16.9% of lateral canals in all teeth.

Kramer in 1960 demonstrated lateral canals in bifurcation and trifurcation region of molars.

Acc. to Hess et al (1938) accessory canal foramina have a mean diameter of 6-60 micron m.

These accessory and lateral canals are avenues for interchange of metabolic and breakdown products between pulp and periodontal tissues.Apical Delta A principal canal may not exit as a single apical foramen but may split near the apex and exit in 2 or more smaller foramina called apical delta.Clinical Implication The presence of multiple accessory and lateral canals is a rule rather than an exception which raises question regarding the fate of pulp tissue in those canals following endodontic therapy.

Following endodontic treatment in teeth with vital pulps, the lateral and accessory canals tend to become obliterated by deposition of cementum with passage of time.

In teeth with totally inflamed or necrotic pulps, granulation tissue is found in accessory canals prior to endo therapy. Following endodontic therapy, the inflamed tissue gets resorbed.

Nichollo (1963) found that lateral canals were associated with lateral areas of rarefactions in 10 of 228 cases (4.4%) observed. The number of accessory canals in the root does not have any role in success or failure of endo Tt. If they were, then most of the endo Tt teeth would fail. Occasionally a communication exists between two canals in one root. Ex: roots of an upper first bicuspid. But this connection is inadvertently ignored during root canal therapy. Yet, despite this repair frequently occurs. When pulp is extirpated from main canals, a clot forms at the site of wound which repairs subsequently.

Another common occurrence is presence of Y shaped branching of the pulp near the apex of the tooth. Following endo Tt, the pulp in uninstrumented canal may become inflamed but usually retain its vitality. With passage of time, continuous deposition of dentin and cementum tends to narrow the lumina of these canals.

Lateral and accessory canals are difficult to clean adequately. Therefore thorough and effective irrigation techniques should be carried out.

Lateral canals found in furcation region can transmit toxins to establish lesion in furca which may give false evidence of periodontal disease.

A tooth with multiple accessory canals in the apical third may harbor micro-organisms and debris which may continue to irritate the periapex causing pain, inspite of proper filling in principal canal. Surgery is indicated in such cases.Resorption Most commonly occurs in the periapical region.

Periapical pathology, orthodontic forces, trauma are few common causes.

Resorption leads to formation of funnel shaped structure.

As inflammation subsides, root apex may repair by deposition of secondary cementum.

If resorption is more, procedures similar to apexification have to be done.Change in anatomy of Root Apex As a result of resorption and repair, change in anatomy of root apex occurs with passage of time. The change is caused by several forces:1. Anterior component of forces which cause the teeth to move mesially.2. Teeth also have a continuous eruptive force. These combined forces cause tension and bone apposition on one side and pressure and resorption on other side. Thus even if the principal apical foramen may be in the centre originally, it may migrate with deposition of cementum.Conclusion Morphologically, the root apex is the most complex region, therapeutically- a challenging zone, prognostically - an important zone and unfortunately radiographically most obscure and unclear zone.

A detailed knowledge of the apical part of root canal is essential as it is a common area for procedural errors during instrumentation.

The morphologic variation and technical challenge involved in the treatment of apical third is infute. The success of endodontics revolves around effective cleaning, shaping and manipulation and obturation of apical third of root canals.

Thus, for an endodontist, it is important to know of the variation in the area for successful negotiation, preparation and filling of the apical part of the root.References Endodontology ; Samuel Seltzer Endodontic therapy; Weine Endodontic practice; Grossman Microscopes in endodontics; DCNA EMBED Photoshop.Image.8 \s

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