Dental Anatomy Chapter 1 PDF

# Chapter 1 - Introduction and Life History of The Teeth ## Introduction Dental anatomy includes: * Surface anatomy of teeth; externally and internally, their arrangement in the dental arches, their relationship to each other and to the skull bone. * Upon looking into the lower facial third an...

# Chapter 1 - Introduction and Life History of The Teeth ## Introduction Dental anatomy includes: * Surface anatomy of teeth; externally and internally, their arrangement in the dental arches, their relationship to each other and to the skull bone. * Upon looking into the lower facial third and the oral cavity you can see: * Lips * Gingiva * Cheek * Tongue * Floor of the mouth * Roof of the mouth ## 1) Lips: (Fig.1) * There are two lips upper and lower. * The corner of the mouth where they meet is called commissure. * Nasolabial groove: run diagonally downward on each side of the nostril toward the corner of the lip. * Labiomental groove: run horizontally under the lower lip and emphasize the chin. ## 2- Gingiva It is that part of the masticatory tissue that surrounds the cervical part of teeth. It is firmly attached to the teeth and their surrounding bone. ## 3- Cheek The lining of the inside of the cheeks is shiny with a white line called linea alba buccalis running posteriorly on each side at the level where the upper and lower teeth come together. The parotid papilla: it is a round elevation of tissue observed on the cheek between first and second molar at just above the occlusal plane. It covers the opening of the parotid duct (stensen's duct). ## 4- Tongue: (Fig. 2) It is broad flat organ composed of muscle fibers and glands. It rests in the floor of the mouth and manipulates food for mastication. It is the principle organ of taste. ## 5- Floor of the mouth It is shiny and some large blood vessels may be seen near the surface. Beneath the tongue there are two bilateral bulges caused by the presence of large salivary glands. ## 6- Roof of the mouth (palate): (Fig. 2) Its anterior part is named as hard palate. The end of hard palate is opposite to third molar. The soft palate is sometimes redder than the hard palate. The junction between hard and soft palate is called vibrating line. Uvula is a small flesh structure hanging from the center of posterior border of soft palate. ## 7- Teeth * Human teeth are arranged in the oral cavity in two arches called dental arches one upper and one lower. * The upper arch is attached to the maxilla. The teeth of upper dental arch are called upper or maxillary teeth. * The lower dental arch is attached to a movable bone called the mandible which joins the skull at the temporo-mandibular joint. The teeth of lower dental arch are called lower or mandibular teeth. * Each dental arch can be divided into nearly equal halves by an imaginary vertical line called the midline. The right and left halves in each arch are called quadrants. There are four quadrants in the entire oral cavity which includes: * Maxillary (upper) right quadrant * Mandibular (lower) right quadrant * Maxillary (upper) left quadrant * Mandibular (lower) left quadrant ## In the oral cavity there are four types of teeth that includes (Fig.3): ### I- Incisors: * They are the two front teeth in each quadrant. * The central incisor is the first tooth next to the midline. The lateral incisor is the second. ### II-Canine (cuspid): * There is one canine in each quadrant. It is the third tooth from the midline in each quadrant. ### III- Premolars (bicuspids): * There are 2 premolars in each quadrant. They are the fourth and fifth teeth from the midline. * The fourth tooth is the first premolar. The fifth tooth is the second premolar. ### IV. Molars: * There are three molars in each quadrant. They are the sixth, seventh and eighth from the midline. The sixth is the first molar. The seventh is the second molar. The eighth is the third molar. * The incisors and canines are considered anterior teeth since they are closer to the midline. * Molars and premolars are considered posterior teeth since they are farther from the midline. ## Overview of The Human Dentitions * The word dentition means all of the upper and lower teeth considered collectively. * Humans have two dentitions; the primary and the permanent. There is a transitional phase when both dentitions are present in the oral cavity is called mixed dentition. ### 1- The primary dentition: (Fig.4) * This is called the deciduous dentition or the one that is shed or exfoliate. * This begins at 6 months and ends at 6 years. * The eruption of primary teeth begins at 6 months (for lower central incisor) and ends at 2 years (for the upper second deciduous molar). These teeth are also called milk teeth, baby teeth, or temporary teeth. * They are 20 in number, 10 maxillary and 10 mandibular, 5 in each quadrant. * Each quadrant contains 3 types of deciduous teeth which are: * Two incisors * One canine * Two molars ### 2- Mixed dentition: * This period begins with the eruption of the first permanent molar at about 6 years. It ends around the age of 12. At this period the deciduous teeth exfoliate or shed and the permanent teeth erupt into their places. * Shedding; it is the physiological loss of the deciduous teeth and their subsequent replacement by permanent teeth ### 3- The permanent dentition: (Fig.5) * It is also called the secondary, adult or succedaneous dentition since many of these permanent teeth succeed deciduous teeth. * This dentition begins at approximately 12 years and persists throughout the life span of the teeth. There are 32 permanent teeth, 16 maxillary and 16 mandibular. In each quadrant 8 teeth are present 2 incisors, 1 canine, 2 premolars and 3 molars. ***It is important to keep in mind that:*** 1. Deciduous molars are succeeded by a group of teeth named as premolars, which are not present in deciduous dentition. 2. All permanent molars are not preceded with deciduous teeth. So they may be termed non-succedaneous teeth. They represent an extra number of permanent teeth to that of the deciduous teeth. 3. Total number of permanent teeth will exceed that of the deciduous teeth with 12 (3 molars in each quadrant). ## Dental Formula For Mammalian Teeth The type and number of all mammalian teeth are expressed by formula that is used to differentiate human dentition from that of other experimental animals. The dental formula includes deciduous and permanent dentition. Also it indicates the dentition of one side of the mouth only (upper and lower). * **Human dental formula for deciduous teeth is:** I 2/2 C 1/1 PM 0/0 M 2/2 = 10 **This Dental formula can be read as:** * Incisors (I): 2 maxillary and 2 mandibular. * Canines (C): 1 maxillary and 1 mandibular. * Premolar (PM): Absence of these teeth. * Molars(M): 2 maxillary and 2 mandibular. *The total number of the human deciduous teeth is 10 teeth for one side upper and lower dental arch.* * **Human dental formula for permanent teeth is:** I 2/2 C 1/1 PM 2/2 M 3/3 = 16 **Dental formula can be read as:** * Incisors (I): 2 maxillary and 2 mandibular. * Canines (C): 1 maxillary and 1 mandibular. * Premolars (PM): 2 maxillary and 2 mandibular. * Molars (M): 3 maxillary and 3 mandibular. *The total number of the human permanent teeth is 16 teeth for one side upper and lower dental arch.* * In biological research the most used model of experimental animals are rats, guinea pigs and dogs. The rat has permanent set only. * **The rat dental formula:** I 1/1 C 0/0 PM 0/0 M 3/3 = 8 The dog dental formula: * **For deciduous is:** I 3/3 C 1/1 PM 3/3 M 0/0 = 14 * **For permanent is:** I 3/3 C 1/1 PM 4/4 M 3/3 = 21 ## Tooth Anatomy ### Macro-anatomy of the tooth: (Fig.6) * Each tooth consists of three anatomical parts; the crown, root(s) and neck. ### 1. Crown The crown terminates either with: * Horizontal cutting end termed the incisal ridge as in incisors. * Tapered sharply pointed end termed the cusp as in canines, premolars and molars. * **Anatomical crown**: It is that part of the tooth covered by enamel. It remains mostly constant throughout the life span of the tooth except for attrition which is the physiological loss of hard dental structure by mastication * **Clinical crown**: It is the part of the anatomical crown which is visible in the oral cavity and not covered by gingiva. It may be shorter, longer or equal to anatomical crown according to the level of gingiva. * **Clinical crown may be:** * Shorter than the anatomical crown: in case of newly erupted tooth * Longer than the anatomical crown: in case of gingival recession. ### 2. Root * **Anatomical root:** It is that part of the tooth which is covered by cementum. * **Clinical root:** It is that part of the tooth which is under the gingiva, not exposed to the oral cavity. It is embedded in the jaw bone. * Teeth may have one or more roots. Teeth may be divided according to the number of roots into: single rooted teeth and multiple rooted teeth. * Multiple roots may be two roots (by bi-furcation) or three roots by tri-furcation. Multiple roots have undivided part called the root trunk. ### 3- Neck It is the junction between the anatomic crown and the anatomic root. It is named cervical line. It is located at the cervical border of the anatomic crown where the enamel and the cementum meet so it is also named as cemento-enamel junction (CEJ). ### Micro-anatomy (structure) of tooth: (Fig. 6) * The main part of tooth is made up of dentin covered by enamel on the crown and cementum on the root. The central part of the tooth is formed of soft tissue called the pulp. * **1. Enamel**: It covers the anatomical crown. It is the hardest and the most mineralized tissue in the body which can resist masticatory force. * **2. Cementum**: It is a dull yellow hard tissue covering the anatomical root. It provides a medium of attachment the periodontal ligament. * **3. Dentin**: It is a hard yellowish tissue which forms the main bulk of the tooth. It is found under enamel coronally and under cementum in the root. It surrounds the pulp. * **4. Pulp**: It is the only soft tissue of the tooth. It supplies the tooth with blood and lymph vessels and nerves. It occupies the central part of the tooth which is called the pulp cavity. ### Anatomically the pulp cavity is formed of two parts: (Fig. 8) * **a- The pulp chamber:** which lies mostly in the center of the anatomical crown of the tooth. It is the enlarged portion of the pulp. Its outline roughly conforms to crown outline. The pulp chamber roof prolongates into the cusps or mamelons of the crown forming pulp horns. Pulp horns are prominent in young age and disappear by age. * **b- Root canal:** It is located in the root. Root canal ends at the apex of the root by an apical foramen. Apical foramen transmits the neuro-vascular and lymphatic bundles to enter the pulp tissue. The shape of root canal usually follows that of the root. Occasionally, root canals send some branches, which open along the lateral surface of the root termed the accessory canals. Some roots may have more than one canal which may end by common foramen or separate foramina. ### The periodontium (Fig. 1, chapter 7) * It is the investing and supporting attachment system of teeth. It consists of: * **a) Two soft tissues:** gingiva and periodontal ligament. * **b) Two hard tissues:** cementum and alveolar bone. * **Terminology:** * **Alveolar process:** it is that portion of the jaw which surrounds and supports the teeth. * **The root is firmly attached to the socket by a ligament known as the periodontal ligament.** * **The alveolar process is covered by pale pink tissue called the gingiva or gum which surrounds the necks of teeth.** ### Junctions of the tooth tissues: 1. Cemento-enamel junction: (see cervical line) 2. Dentino-enamel junction: it is the junction between enamel and dentin in the crown. 3. Dentino-cemental junction: it is the junction between dentin and cementum in the root. ### Functions of teeth: 1. **Mastication:** This is the most important function of teeth. Teeth are designed to perform their function:- * **Incisors:** The form of the incisors is nearly similar to a chisel. This form is suitable for cutting or incising food. * **Canine:** It is wedge shape suitable mainly for piercing or tearing food, also for lesser extent for cutting. * **Premolars:** They have intermediate form and location between canine and molars. So, they have the function of tearing and grinding of food. The first premolar function is mainly tearing, while the second premolar is for grinding mainly. * **Molars:** They have wide surface with multiple elevations (or cusps) and thus they are suitable for grinding food. 2. **Appearance:** well-arranged clean teeth with proper alignment give nice appearance to the face and also support facial expressions. The facial appearance is altered by extraction of teeth especially the anterior one. 3. **Speech:** Teeth are important for clear pronunciation and for production of sounds. 4. **Growth of jaws:** teeth have a role in growth of jaws in some periods. If the child eats on one side the facial growth will be unilateral. ### Surfaces of the teeth: (Fig. 9) * Each tooth have five surfaces: facial, lingual, mesial, distal and masticatory (functioning surface). ### 1. The facial surface: * It is the surface of a tooth facing the lips or cheeks. It can be also known as: * **Labial surface:** the surface of an anterior tooth facing the lips. * **Buccal surface:** the surface of a posterior tooth facing the cheeks. ### 2. The lingual surface * **Lingual surface:** the surface of lower anterior and posterior teeth facing the tongue. * **Palatal surface:** the surface of upper anterior and posterior teeth facing the palate. ### 3. The mesial surface: * It is the surface of tooth close to the midline. ### 4. The distal surface: * It is the surface of tooth away from the midline. * Together mesial and distal surfaces are called proximal surfaces, i.e. surfaces between adjacent teeth. ### 5. The masticatory surface * **Incisal ridge or edge:** it is the biting edge of anterior teeth. * **Occlusal surface:** it is the chewing surface of posterior teeth. ### - Division of tooth surfaces: (Fig.10) * For descriptive purposes, each tooth surface is divided into thirds to facilitate description and location of anatomic landmarks. These thirds are named according to the area they are near to. * The crown can be divided both vertically or horizontally into three thirds: * **a. Horizontally** * Iniso (occluso) - cervically: the crown is divided into incisal (occlusal), middle and cervical thirds. * **b. Vertically** * Labio - (bucco) - lingually: the crown is divided into labial (buccal), middle and lingual thirds. * Mesio-distally: the crown is divided into mesial, middle and distal thirds. * **The root surface is divided horizontally into cervical, middle and apical thirds and vertically into two halves.** ## Line and Point Angles ### -Line angles: (Fig.11 a&b) * It is formed by the junction of two surfaces of the crown and its name is derived from combining the names of the two surfaces. Line angles aid in identification of certain areas of the tooth crown. ### Line angles of anterior teeth: * The anterior teeth have fewer line angles because the meeting of the mesial and distal line surfaces with incisal ridge are rounded, so the mesio-incisal and disto-incisal line angles are practically not exist. * There are 6 line angles: * Linguo-incisal * Labio-incisal * Disto-lingual * Mesio-lingual * Disto-labial * Mesio-labial ### Line angles of posterior teeth: * There are 8 line angles: * Mesio-occlusal * Linguo-occlusal * Mesio-lingual * Mesio-buccal * Disto-occlusal * Bucco-occlusal * Disto-lingual * Disto-buccal ### Point angles : (Fig. 12 a&b) * Point angle is formed by the junction of three surfaces and is named by combining their 3 names. There are 4 point angles for both anterior and posterior teeth. ### Point angles of anterior teeth: 1. Mesio-labio-incisal 2. Disto-labio-incisal. 3. Mesio-lingo-incisal 4. Dist-lingo-incisal. ### Point angles of posterior teeth: 1. Mesio-bucco-occlusal 2. Disto-bucco -occlusal 3. Mesio-lingo-occlusal 4. Disto-lingo-occlusal ### Tooth identification system * To identifying a specific tooth, the following items should be mentioned: * a- Type of dentition (deciduous or permanent). * b- The arch (upper or lower). * c- The quadrant (right or left). * d- The name of the tooth. * There are different systems for numbering or naming teeth. The most popular are: ### 1-Palmer notation system: * This system utilizes simple brackets to represent the four quadrants of the dentition as if you facing the patient. ### The permanent teeth are numbered from 1-8 on each side from the midline as follows: 1. Central incisor 3. Canine 5. Second premolar 7. Second molar 2. Lateral incisor 4. First premolar 6. First molar 8. Third molar ### Example: * 4 Lower left first premolar * 1 Upper right permanent central incisor ### The deciduous teeth are lettered from A-E on each side from the midline as follows: A Central incisor B Lateral incisor C Canine D First molar E Second molar ### Examples: * A Deciduous lower right central incisor. * E Deciduous upper left second molar ### 2- The international numbering system (the two digit system) * Within this system the teeth are designed by two digits: * **a) The first digit** * Located at the left side * It indicates the quadrant * For permanent dentition the quadrants are given the numbers 1 to 4 starting from the upper right quadrant in a clockwise manner. For deciduous dentition, digits from 5 to 8 is used. * **In the permanent dentition.** * U.R. 1 2 U.L. * L.R. 4 3 L. L. * **In the deciduous dentition.** * U.R. 5 6 U.L. * L.R. 8 7 L. L. * **b) The second digit** * Located at the right side of the number. * It indicates the number of the tooth in the quadrant. * For permanent teeth in each quadrant are the numbers from 1 to 8 while for deciduous teeth are from 1 to 5. This is designed from the midline in a distal direction. The two digits should be pronounced separately. * example: 11 (one one) = upper right permanent central incisor. ### Permanent teeth * **UPPER RIGHT UPPER LEFT** * 18 17 16 15 14 13 12 11 21 22 23 24 25 26 27 28 * 48 47 46 45 44 43 42 41 31 32 33 34 35 36 37 38 ### Deciduous teeth * **UPPER RIGHT UPPER LEFT** * 55 54 53 52 51 61 62 63 64 65 * 85 84 83 82 81 71 72 73 74 75 ### 3- The universal numbering system (American numbering system): * The numbers in this system is always preceded by the sign # to designate that the system is used for universal system. * **a) For permanent teeth** * Numbers from 1-32 are used starting from upper right third molar as # 1 to upper left third molar as # 16 descending down to the lower left third molar as # 17 to ends at lower right third molar as # 32 (going around the two arches in a clock wise from upper right to upper left to lower left to lower right at the end). * **Permanent teeth (1 – 32)** * 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 * 32 31 30 29 28 27 26 25 24 23 22 21 20 19 18 17 * **b)For deciduous teeth** * Numbers from 1 to 20 are used with adding a small (d) as a suffix to each number to designate deciduous from permanent e. g. deciduous upper right second molar = # d 1. * Another common system for deciduous teeth using alphabet from A to T instead of numbers along with the same sequence without using the small (d). * **Example: lower right deciduous canine would be # R.** * **Deciduous teeth (1 – 20)** * 1 2 3 4 5 6 7 8 9 10 * 20 19 18 17 16 15 14 13 12 11 * **Deciduous teeth (A - T )** * A B C D E F G H I J * T S B C D Q P 0 N M L K ### Anatomical Landmarks of The Tooth Crown * There are certain terms that are frequently used to describe the surface anatomy of tooth. These include elevations and depressions. * **Crown elevations include:** * A) Lobe * B) Tubercle * C) Ridge * **Crown depressions include:** * A) Developmental groove * B) Supplemental groove * C) Fissure * D) Fossa * E) Pits * F) Sulcus ### Crown Elevations * **A) Lobe : (Fig. 13)** * It is one of the primary centers of calcification and growth formed during the crown development. Each tooth begins to develop from four lobes or more as follows: * Anteriors: have four lobes (3 labial and 1 lingual). * Upper first and second premolars: have four lobes (3 buccal and 1 lingual) * Lower first premolars and second premolars (2 cusp type) : have four lobes (3 buccal and 1 lingual) * Lower second premolars (3 cusp type): have five lobes (3 buccal and 2 lingual) * Upper first molars: have four lobes (2 buccal and 2 lingual) * Lower first molars: have five lobes (3 buccal and 2 lingual) * Upper and lower second and third molars: have four lobes (2 buccal and 2 lingual) * **Important note:** Only some maxillary third molars have as few as 3 lobes. Peg-shaped maxillary lateral incisors and some supernumerary teeth have less than three lobes. * **The mature forms of lobes are:** * **1-Mamelons: (Fig.14)** * These are 3 small rounded projections of enamel present in the incisal third of newly erupted incisors. The pulp chamber of the maxillary incisors has three pulp horns corresponding to these three labial lobes (mamelons). Mamelons wear down due to normal use into flat incisal edge. * **2-Cusp: (Fig.15)** * It is an elevation nearly pyramidal in shape found on the occlusal surface of posterior teeth and the incisal part of canines. It is formed of enamel, dentin and pulp horn. * **3-Cingulum :(Fig.16)** * It is a rounded prominence or bulge on the cervical third of the lingual surface of the crown of anterior teeth. It is also known as the lingual cervical ridge. * **B)Tubercle :(Fig.17)** * It is a small elevation found on some portion of the crown. It is produced by excessive localized formation of enamel. It is commonly found on the palatal surface of upper 6 and upper E (cusp of Carabelli) and sometimes at the lingual surface of incisors over cingulum. * **C) Ridge:** * It is a linear elevation on the different surfaces of the crown. Its name is according to location or shape as follows: * **I) Ridges on the facial surface (Labial and Buccal)** * **1. Labial ridge:** It is found on the labial surface of canine. It represents a highly developed middle labial lobe. * **2. Buccal ridge:** It is found on the buccal surface of premolars. It represents highly developed middle buccal lobe. * **3. Cervical ridge:** It is found in the cervical third of labial or buccal surfaces of all teeth. But it is well developed in the deciduous first molar (D) followed by deciduous second molar (E). * **II) Ridges on the lingual surface :(Fig 19 a&b)** * **1. Incisal ridge:** It is found on the lingual surface of newly erupted incisors at its incisal margin. The ridge is transformed into edge as the attrition removes some of the incisal enamel. * **2. Lingual ridge:** It is found in the lingual surface of canines extending from the cusp tip to the cingulum dividing the lingual surface into two fossae. * **3. Cusp ridge:** They are ridges forming an angle at the tip of the cusp from facial and lingual sides. They also are called cusp slopes or arms. (Usually distal cusp slope is longer than mesial cusp slope in all teeth except upper C & 4 where the mesial cusp slope is longer than the distal one) * **4. Marginal ridge:** It is a linear elevation found on the mesial and distal margins of the lingual surface of anterior teeth. * **III) Ridges on the occlusal surface of posterior teeth** * **1. Marginal ridge:** It is linear elevation found on the mesial and distal margins in occlusal surface of posterior. * **1. Triangular ridge:** It is triangular in shape. It is found in posterior teeth. It descends from the cusp tip to the center of the occlusal surface. All posterior teeth cusps have a triangular ridge except the mesiolingual cusp on maxillary molars which has two triangular ridges. * **2. Transverse ridge:** It is formed by union of two triangular ridges of two cusps facing each other crossing the occlusal surface transversely in a buccolingual direction. It is usually found in lower premolars. * **3. Oblique ridge:** It is formed by union of two triangular ridges of two cusps not facing each other. It crosses the occlusal surface of upper molars obliquely from the mesiolingual cusp to the distobuccal cusp. It is found on occlusal surface of maxillary molars. ### Crown Depressions: * **1- Developmental groove: (Fig. 21a)** * It is shallow linear depression represents the site of union of primary lobes. Grooves are found on the occlusal surface of posterior teeth and may extend to the buccal or lingual surfaces. They are named according to location. Grooves act as spill ways for cusps during lateral and protrusive jaw motions. * **2- Supplemental grooves: (Fig. 21a)** * They are short, irregular linear depressions found usually on the occlusal surface and do not represent union lobes. They are auxillary grooves radiating from the developmental groove. They can be named according to the parts of the tooth on which they are found (mesiobuccal, distolingual supplemental grooves). The third followed by the second permanent molars are characterized by high number of supplemental grooves. * **3- Fissure: (Fig. 21b)** * It is a linear depression found in the bottom of developmental groove as a result of incomplete union of the primary lobes. It is a fault in enamel. Dental caries often begins in a deep fissure. * **4- Fossa:** * It is small depression found in both anterior teeth and posterior teeth. It is named according to location or shape: * **a- Lingual fossa:** found on the lingual surfaces of anterior teeth. * **b- Central fossa:** found on the occlusal surface of molars. * **c- Mesial and distal triangular fossa:** found on the occlusal surfaces of posterior teeth mesial and distal to marginal ridges of posterior teeth. * **5- Sulcus: (Fig. 21b)** * It is a broad depression or valley found between the inclined planes of triangular ridges on occlusal surface of posterior teeth. Its inclines meet in a developmental groove and extend to the cusp tips. * **6- Pit:** * It is a small pinpoint depression present at the junction or at the end of the developmental grooves. They may be found at the bottom of the central fossa (central pit) or at the bottom of mesial and distal triangular fossae (mesial and distal pits). * **A fault pit** usually develops as a result of incomplete formation of enamel. They may be found at the end of buccal developmental grooves of lower molars or palatal developmental grooves of the upper molars. ### Other terms: * **Self cleansing surfaces:** These are the surfaces of teeth which are cleaned by normal functional action of the tongue and cheeks and mouth fluids; facial, lingual and incisal surfaces are considered self cleaning. * **Crypt:** it is bony space in the alveolar bone containing the developing unerupted tooth. * **Socket:** it is bony space in the alveolar bone containing the roots of erupted tooth * **Soft growth stage: (Fig. 23)** * The deciduous teeth start their development inside the jaw before birth. From 6 weeks intra-uterine (wiu) the oral epithelium begins to form the germ of 10 deciduous teeth in each jaw. * Most of the permanent teeth start also their development inside the jaw before birth. The tooth germ is formed of soft tissue and does not appear in the x-ray films. * **Calcification: (Fig. 24)** * Calcification of the hard dental tissues (enamel, dentin and cementum) occurred by specialized cells. The mineralized salts, mostly calcium, from blood will deposited in the soft tissue in form of small spherical bodies, then they will enlarge to form primary lobes which fuse together to form the calcified dental tissues. * **Eruption: (Fig. 25)** * The eruption of the mandibular teeth precedes the maxillary teeth. * The teeth in both sides of the jaw erupt at the same time. * Teeth erupt in girls earlier than boys * The tooth erupts in the oral cavity when its crown is completely formed as well as third of the root. * **Attrition:** * It is the physiological loss of the hard dental structure due to the continuous friction of the teeth with each other during mastication * The attrition is clearly seen in incisal and occlusal surfaces also at the contact point * By attrition, the incisal ridges and cusp tips are lost. * The contact point is lost and becomes contact area * **Shedding of the deciduous teeth: (Fig. 26)** * Shedding: Is the physiological loss of the deciduous teeth and are replaced by their permanent successors. * Before shedding of the deciduous teeth by 3 years, their roots begin resorption apically till reach their whole length then their crowns are exfoliated. * Loss of the permanent or deciduous teeth before their shedding time is considered pathological loss of teeth.