growth and development of mandible both prenatal and postnatal

embryology
growth
development of mandible

1. GROWTH AND DEVELOPMENT OF MANDIBLE PRESENTED BY : GUIDED BY : DR. KAVINCI SIHAG DR. H.S. SHASHIDHARA IST YEAR MDS PROFESSOR DEPARTMENT OF PROSTHODONTICS AND CROWN & BRIDGE COLLEGE OF DENTAL SCIENCES, DAVANGERE

2. CONTENTS TERMINOLOGIES INTRODUCTION REVIEW OF LITERATURE PRENANTAL GROWTH AND DEVELOPMENT ROLE OF MECKEL’S CARTILAGE TYPES OF OSSIFICATION MECHANISMS OF BONE GROWTH THEORIES OF BONE GROWTH POST NATAL GROWTH AND DEVELOPMENT AGE CHANGES IN MANDIBLE ANOMALIES IN MANDIBLE CONCLUSION REFERENCES

3. TERMINOLOGIES EMBRYOLOGY - It is the study of formation and development of embryo or fetus from the moment of its inception up to the time when it is born as an infant. CELL DIVISION – It is the process by which a parent cell divides into two or more daughter cells. In cell division , the cell that is dividing is called the parent cell. The parent cell divides into two daughter cells and the process then repeats in what is called cell cycle. It is essential for growth, repair and maintenance of organisms. There are two main types of cell division : MITOSIS AND MEIOSIS A) MITOSIS – It is the process whereby one cell divides, giving rise to two daughter cells that are genetically identical to the parent cell. Each daughter cell receives the complete complement of 46 chromosomes. B) MEIOSIS – It is the cell division that takes place in the germ cells to generate male and female gametes, sperm and egg cells, respectively. Each gamete contains 23 chromosomes. GAMETE – They are specialised sex cells found in organisms that reproduce sexually. In humans, gametes are the eggs in females and the sperm in males.

4. TOTIPOTENT STEM CELLS - Totipotent stem cells possess the highest potential, as they can generate all cell types in the body, including those found in embryos and supportive tissues such as the placenta. PLURIPOTENT STEM CELLS - Pluripotent stem cells are slightly more specialized and can differentiate into cells from the three germ layers—ectoderm, endoderm, and mesoderm—which give rise to various tissues and organs. These cells are present in embryos during the blastocyst stage and certain adult tissues. MULTIPOTENT STEM CELLS -  Multipotent stem cells have a more limited potential compared to totipotent and pluripotent stem cells. They can differentiate into multiple cell types within a specific lineage or tissue type. Mesenchymal stem cells (MSCs), for example, are multipotent stromal cells capable of differentiating into a range of cell types, including osteoblasts, chondrocytes, myocytes, and adipocytes.  UNIPOTENT STEM CELLS - Unipotent stem cells possess the most restricted potential, being capable of differentiating into a single type of mature cell, specific to a particular tissue or organ.

5. NEURAL CREST CELLS- They are transient, multipotent , migratory cell population. Unique to vertebrates that gives rise to a diverse cell lineage including Melanocytes Craniofacial cartilage and bone, smooth muscle Peripheral and enteric neurons and ganglia

6. INTRODUCTION “FORM TO FUNCTION OR FUNCTION TO FORM” " Growth " is a general term implying simply that something changes in magnitude. It does not, however, presume to account for how it happens. Therefore, the explanatory term “ Development " is added. This connotes a maturational process involving progressive differentiation at the cellular and tissue levels, thereby focusing on the actual biologic mechanism that accounts for growth.

7. Development is a continuous process by which the male gamete, the sperm, and the female gamete, the oocyte, unite to give rise to a zygote. Development involves many changes that transform a single celled zygote, into a multicellular human being. Most developmental changes occur before birth, but this process continues even in the later stages of life.

8. GROWTH MOYER- Growth as the biologic process by which living matter gets larger. PROFFIT - Growth refers to the increase in size or number. MOSS - Growth as any change in morphology which is within measurable parameter.

9. DEVELOPMENT MOYER- Naturally occurring unidirectional changes in the life of an individual from its existence as a single cell to its elaboration as a multifunctional unit terminating in death. MELVIN MOSS – Development can be considered as a continuum of casually related events from fertilization of ovum onwards. Development = Growth + Differentiation + Translocation

10. growth development

11. There are three basic mechanisms by which growth takes place at the cellular/ tissue level : HYPERPLASIA : Growth due to increase in the number of cells. HYPERTROPHY : Growth due to increase in size of cells EXTRACELLULAR MATRIX SECRETION : In this process, there is an increase in size because of the secretions of the cells into the extracellular matrix. The secretions contribute to increase in size.

12. Methods of bone formation Bone formation takes place by two basic methods, namely: Endochondral ossification Membranous ossification

13. ENDOCHONDRAL OSSIFICATION Also known as cartilaginous ossification / Indirect ossification It involves production of bone in areas where there are high levels of compression. Thus, it is seen in cranial base and in movable joints. Mesenchymal cells condensed at the site of bone formation

14. Mesenchymal cells differentiate into chondroblasts and lay down hyaline cartilage. Cartilage is surrounded by a membrane called perichondrium, Which is highly vascular and contains osteogenic cells. Inter-cellular substance surrounding cartilage cells become calcified due to influence of enzyme called alkaline phosphatase which is secreted by cartilage cells. Formation of empty spaces called primary areolae. Blood vessels and osteogenic cells from the perichondrium invade calcified cartilaginous matrix that is now reduced to bars or walls due to eating away of calcified matrix. Formation of secondary areolae.

15. Osteogenic cells from the perichondrium become osteoblasts and arrange along the surface of these bars of calcified matrix. Osteoblasts lay down osteoid The osteoid becomes calcified to form a lamella of bone. Now another layer of osteoid is secreted.

16. Importance of endochondral ossification Cartilage behaves like a soft tissue and growth takes place both by interstitial and appositional growth. In bones, interstitial growth is not possible. Cartilage unlike bone is a pressure-adapted tissue can grow in heavy pressure areas, e.g. cranial base. Direction of growth is not unidirectional like bone. The entire process is continuous and repetitive , one zone transferring into the next.

17. Intramembranous ossification Membranous ossification or bone formation on the outer surface of bone (Periosteum) , Inner surface (Endosteum), Sutures, etc. Intramembranous ossification is the major mode of growth in the skull. Membranous ossification or growth is seen in areas of tension. Growth takes place outward or externally.

18. Review of literature JOHN HUNTER(1771) compared a series of dried mandibles and concluded that to attain space for permanent molar teeth the mandible must grow by posterior apposition of the ramus accompanied by anterior ramus resorption. HUMPHRY (1866 ) studied growth of mandible by inserting metal wires in the mandible of young pigs. BELCHIE (1936) fed pigs the madder plant root which was labelled appositional growth BJORK (1995) conducted implant studies on jaws to determine the growth pattern & rotation, when subject to serial cephalometric methods. DONALD ENLOW proposed the V principle of growth and the counterpart principle.

19. The growth and development of an individual can be divided into : Prenatal growth is characterized by a rapid increase in cell numbers and fast growth rates whereas Postnatal growth comprises declining growth rates and increasing maturation of tissues. PRENATAL GROWTH POSTNATAL GROWTH

20. Stages of Prenatal growth It is a dynamic phase that is divided into : PERIOD OF OVUM - FROM FERTILIZATION TO 14 th DAY PERIOD OF EMBRYO- FROM 14 TH DAY TO 56 TH DAY PERIOD OF FETUS- FROM 56 TH DAY TILL BIRTH PRESOMITE (8 TH -20 TH DAY) SOMITE (21 ST -31 ST DAY) POST-SOMITE (32 ND - 56 TH DAY)

21. NEURAL CREST CELLS

22. NEURAL TUBE FORMATION Ectoderm above notochord thickens at the head end of embryo Neural plate ( acquires raised margins – neural folds) Midline of neural plate deepens Neural groove Neural folds grow towards each other Fuse to form Neural tube- brain and spinal cord Group of cells differentiate at the crest of neural fold – Neural crest cells

23. NEURAL CREST CELLS Special groups of cells arise from the margins of the neural folds that are caused by the infoldings of the neural plate to form the neural tube in response to the inductive activity of the notochord. These neural crest cells are found first in the angle between the developing neural tube and the ectodermal sheet that covers the dorsum of the embryonic disc . They form separate population of cells , each of which is pluripotent and characterized by vigorous migratory capability. They are ectodermal in origin, exhibit properties of mesenchymal tissues. They are thus called ectomesenchyme.

24. Prenatal growth of mandible Mandible is the only movable bone of all the bones of the face. It is horseshoe-shaped bone with the following parts : Body of the mandible Ramus Condylar process consisting of head and neck, head articulates with the glenoid fossa for the formation of the TMJ. Coronoid process Alveolar process Mandible is derived from the first branchial arch, which is called the mandibular arch. The branchial arches start developing at about 4 th week in utero.

25. In the somite period, 4th week IUL, elevations are seen in the ventral foregut resulting in the formation of six pharyngeal arches or branchial arches. They are bilateral mesodermal swellings. Finally only five arches remain.1st arch is known as mandibular arch The arches are separated by : 4 branchial grooves on the ectodermal aspect 5 pharyngeal pouches on the endodermal aspect. Each arch has : Outer covering of ectoderm An inner covering of endoderm Core of mesoderm. BRANCHIAL ARCHES

26. Around 4 th week of IUL, the developing brain and pericardium form two prominent bulges on the ventral aspect of the embryo. These bulges are separated by the primitive oral cavity or stomodeum. The floor of the stomodeum is formed by buccopharyngeal membrane separates the primitive oral cavity from the foregut. The pharyngeal arches develop on the ventral and lateral aspects of the cranial most part of the foregut that lies in close approximation with the stomodeum. There are 6 pharyngeal arches but the fifth one soon disappears. They are separated by 4 branchial grooves. Each of these arches give rise to Muscles, CT, Vasculature, Skeletal components and neural components of the future face. development OF MANDIBLE

27. The first branchial arch is called the mandibular arch and plays an important role in the development of mandible. The mandibular arch forms the lateral wall of stomodeum. It gives off bud from dorsal end which is the maxillary process, it grows ventro -medially, cranial to main part of the arch, which is now called mandibular process. Mandibular processes of both sides grows towards each other and fuse in the midline. Then now form the lower border of stomodeum that is the lower lip and lower jaw.

28. MECKEL’S CARTILAGE The Meckel's cartilage is derived from the first branchial arch around the 41st - 45th day (6 th week) of intra-uterine life as a solid hyaline cartilage surrounded by fibro-cellular capsule It extends from the cartilaginous Otic capsule to the midline or symphysis and provides a template for the development of mandible. A major portion of the Meckel's cartilage disappears during growth and the remaining part develops into: The mental ossicles Incus and Malleus Spine of sphenoid bone's cartilage Anterior ligament of malleus Spheno -mandibular ligament

29. Ossification of mandible Mandible is the second bone in the body to be ossified. There are two types of ossification : INTRAMEMBRANOUS OSSIFICATION Body of mandible except anterior part Ramus of mandible till mandibular foramen ENDOCHONDRAL OSSIFICATION Anterior portion of the mandible (symphysis) Part of ramus above the mandibular foramen Coronoid process Condylar process

30. Intramembranous ossification 1 st structure to develop in primordium of lower jaw is the mandibular division of trigeminal nerve. This is followed by mesenchymal condensation forming the first branchial arch Neurotropic factors produced by nerve induce osteogenesis in ossification centres A single ossification centre for each half mandible arises in 6 th week of I.U.L in the region of bifurcation of inferior alveolar nerve into mental and incisive branches.

31. Ossifying membrane is located lateral to Meckel’s cartilage and its accompanying neurovascular bundle. From this primary centre, ossification spreads below and around the inferior alveolar nerve and its incisive branch and upwards to form a trough for accommodating the developing tooth buds. Spread of intramembranous ossification dorsally and ventrally form the body and ramus of the mandible. As ossification continues the Meckel’s cartilage becomes surrounded and invaded by bone. Ossification stops at site that will later become mandibular lingula from where Meckel’s cartilage continues into middle ear and develops into auditory ossicles (malleus and incus) The sphenomandibular ligament that extends from lingula to sphenoid bone also forms a remnant of Meckel’s cartilage.

32. It is seen in the 3 areas of mandible CONDYLAR PROCESS CORONOID PROCESS MENTAL REGION ENDOCHONDRAL ossification

33. CONDYLAR PROCESS At about 5 th week of I.U.L, an area of mesenchymal condensation can be seen above the ventral part of the developing mandible. This develops into a carrot /cone-shaped cartilage by about 10 th week , till such time malleus and incus function as a temporary joint with the glenoid fossa of temporal bone to permit mandibular movements. The condylar cartilage is a secondary cartilage and its origin is unrelated to Meckel’s cartilage, which is primary cartilage. The process of ossification does not start till 14 th week. The cartilage is replaced by bone except the region of the tip of the head of the condyle superiorly which is maintained till teens for future growth. ( growth and articular cartilage ) Now that condyle is established, the TMJ is shifted anteriorly. The growth of the mandible at 7 th week to permit elevation o palatal shelves is largely contributed by the growth of the Meckel’s cartilage. The ossification of ramus proceeds and the condyle is soon fused to the mandible at about 16 weeks.

34. CORONOID PROCESS Secondary accessory cartilages appear in region of coronoid process by about 10-14 week of I.U.L This secondary cartilage of coronoid process is believed to grow as a response to developing temporalis muscle. The coronoid accessory cartilage (Transient) becomes incorporated into expanding intramembranous bone of ramus and disappear before birth. MENTAL REGION In mental region, on either side of symphysis, one or two small cartilages appear and ossify in 7 th month of I.U.L to form variable numbers of mental ossicles in fibrous tissues of symphysis. The ossification centre is at the site of future Meckel’s cartilage, one on either side. Ossification proceeds anteriorly and posteriorly from here and stops at the site of future lingula. These ossicles become incorporated into intramembranous bone when symphysis ossifies completely during 1 st week of post-natal life.

35. REMODELLING Remodelling can be defined as the process of reshaping and resizing at each level within a growing bone. It takes by selective deposition and resorption of bone. So, bone not only increase in size, but there is constant reshaping of bone also. The mandible remodels differently in directions that are predominantly posterior and superior. The shape of the bone as a whole is maintained. This is a highlight of the remodelling process.

36. MOVEMENTS OF GROWTH

37. Theories of growth Genetic theory Sicher’s sutural dominance theory Nasal septal cartilaginous theory or Scott’s hypothesis Moss Functional matrix theory Neurotrophism Servo system or cybernetic theory

38. ENLOW’S ‘V’ PRINCIPLE Many facial and cranial bone have a V-shaped pattern of growth Bone deposition occurs on the inner side of the V and resorption takes place on the outside surface. V moves from one position to another and also increases in overall dimensions. The direction of growth is towards the wide end of the V. If outer surface of the expanding V is taken, then periosteal surface can be found to be lined with osteoclasts, and the endosteal surface is found to be lined with osteoblasts. The V PATTERN of growth occurs in number of regions such as base of the mandible, end of long bones, mandibular body and palate etc.

39. Post-natal growth and development

40. Mandible undergoes largest amount and longest period of post-natal growth. In accordance with the cephalo-caudal gradient of growth, it exhibits largest variability in morphology. Growth movement of the lower jaw in a downward and forward direction is partially compensated by the ventral displacement of the mandibular fossae on the base of the skull. At the time of birth, mandible is made of 2 halves separated in middle by symphyseal cartilage which is replaced by bone between 4 months to 1 year of post natal life.

41. FUNCTIONAL MATRICES OF MANDIBLE Mandibular growth in the postnatal life shows the integration of the periosteal and capsular matrices of functional matrix theory by Moss. Capsular matrix involves the oropharyngeal functional spaces and the mandible grows according to the functional needs of the particular functional system. The process of surface remodelling usually involves the activity of the periosteal matrix ( Muscle Fibers). The muscular processes of the mandible, like the angle, coronoid and condylar process, are under the influence of the periosteal matrix.

42. Although a single bone, mandible can be functionally and developmentally divided into several subunits on the basis of specific growth stimuli. Condylar process – grows under the influence of TMJ Coronoid process – extends under the traction of the temporalis muscle Alveolar process – which forms with tooth eruption Mandibular Angle – Strengthened under the tensile force of masseter-pterygoid loop Mental protuberance – Arises under the influence of the tension of the basal arch.

43. V principle of growth All these changes take place with the growth of the mandible in the form of an expanding V. It is easier to visualize mandible as the V-shaped bone than the maxilla because of its horse-shoe shape.

44. length The growth of the mandible in length antero-posteriorly is by the deposition of bone at the posterior surface of the ramus and resorption of the leading edge of the anterior surface. This helps in lengthen mandible so that the anterior part of the ramus is occupied by the posterior part of the body in the future and to accommodate the developing permanent molars. POSTERIOR SURFACE ANTERIOR SURFACE

45. As the mandible grows posteriorly, it is displaced anteriorly because the articulation of the condyle to the glenoid fossa is constant and the change in length can take place only by the anterior displacement. As the mandible grows anteriorly, the opening of the mental foramen faces backwards so that the neurovascular bundle leaves the foramen directed backwards. There is corresponding surface remodelling at the anterior border with deposition in the posterior surface of the symphysis and resorption in the superior part of the anterior surface and deposition in the inferior aspect. ANTERIOR BORDER ANTERIOR SURFACE superior part Inferior part

46. width There is deposition on the lateral surface of the ramus and resorption on the lingual surface below mylohyoid ridge. In contrast, the coronoid process, which looks almost like an extension of the ramus in the anterior border, undergoes apposition at the medial surface and resorption at the lateral surface. This expands the mandible like a V. The condyle undergoes reduction of bone on the lateral aspect of neck and deposition corresponding to the V principle, which makes the condyle longer at the neck. Thus, the interramal distance is efficiently increased by the growth of mandible following the V principle. This helps the mandible to keep pace with the growth of the cranial base. Coronoid process Ramus Lateral surface Lingual surface Lateral surface Medial surface

47. Condyle longer at the neck Medial surface Lateral surface

48. height The alveolar process height correlates well with the eruption of teeth. Bone deposition taking place in the lower border of mandible also contributes to increase in the height of the mandible.

49. Unloaded nerve concept The body of the mandible which forms a basal tubular portion in the form of an arc from the foramen ovale through the mandibular to the mental foramen, is the most constant portion of the mandible. This portion of the mandible is in the form of a logarithmic spiral form, the foramen ovale to mental foramen protecting the mandibular nerve.

50. condyle Condylar cartilage Secondary in nature Endochondral growth Hyaline cartilage Regional adaptive growth

51. The main functional role of the condyle is (1) provides a pressure-tolerant articular contact (2) it makes possible a multidimensional growth capacity in response to ever-changing, developmental conditions and variations. The condylar growth mechanism itself is a clear-cut process. Cartilage is a special non-vascular tissue and is involved because variable levels of compression occur at its articular contact with temporal bone An endochondral growth mechanism is required for this part of the mandible because the condyle grows in a direction toward its articulation into the face of direct pressure. An intramembranous type of growth could not operate, because the periosteal mode of osteogenesis is not pressure adapted and has a low threshold for compressive forces.

52. Endochondral growth occurs only at the articular contact part of the condyle, because this is where pressure exists at levels that would be beyond the tolerance of the bone's vascular soft tissue membrane. This regional, endochondral bone-forming mechanism develops as a specific response to this particular local circumstance . The cartilage itself does not contain genetic programming and is a secondary type of cartilage unlike Meckel’s cartilage which is of primary type. In Figure the endochondral bone tissue (b), formed in association with the condylar cartilage (a) is laid down only in the medullary portion. The enclosing bony cortices (c) are produced by the periosteal-endosteal osteogenic activity.

53. NECK OF THE CONDYLE The lingual and buccal sides of the neck characteristically have resorptive surfaces. This is because the condyle is quite broad and the neck is narrow. The neck is progressively relocated into areas previously occupied by the head of the condyle. Therefore, what used to be condyle in turn becomes the neck as one is remodeled from the other.

54. Ramus The ramus moves progressively posterior by a combination of deposition and resorption. Resorption occurs on the anterior part of the ramus while bone deposition occurs on the posterior region. Maintaining antero-posterior dimension of the ramus ; Ultimately lengthening the mandibular body in order to accommodate : Erupting molars Growing pharyngeal spaces Masticatory muscles ANTERIOR SURFACE POSTERIOR SURFACE

55. The lower part of ramus below the coronoid process also has a twisted contour. Its buccal side faces posteriorly toward the direction of backward growth and thus characteristically has a depository type of surface. The opposite lingual side , being away from direction of growth, is resorptive .

56. Ramus uprighting Greater amounts of bone additions on the inferior part of the posterior border than on the superior part. A correspondingly greater amount of matching resorption on the anterior border takes place inferiorly than superiorly. “Remodeling" rotation of ramus alignment thus occurs.

57. Vertical lengthening of the ramus continues to take place after horizontal ramus growth slows or ceases Resorption takes place on the upper part of the posterior border. A forward growth direction can occur on the anterior border in the upper part of the coronoid process. A posterior direction of remodeling takes place in the lower part of the posterior border, this result in more upright alignment and longer vertical dimension of ramus without increase in breadth.

58. Superior part – Periosteal deposition on the lingual surface and Resorption on the buccal surface. Basal part – Deposition on the buccal surface and Resorption on the lingual surface. BUCCAL LINGUAL SUPERIOR BASAL Ramus and coronoid process

59. Growing mandible follows ”Expanding V Principle“ especially in the posterior border thus increasing the inter-ramal distance.

60. CORONOID PROCESS The coronoid process has a propeller like twist, so that its lingual side faces three general directions all at once posteriorly, superiorly, and medially. Increase in height superiorly Widens medially Grows posteriorly When bone is added onto the lingual side of the coronoid process, growth thereby precedes superiorly and this part of the ramus increases in the vertical dimension.

61. Deposition of bone on the lingual side also bring about a posterior direction of growth movement. It produces backward movement of two coronoid process even though deposition on the inside (lingual) surface. Deposition of born on the lingual side also bring about medial direction of growth in order to lengthen corpus Area occupied by anterior part of ramus in mandible 1 becomes relocated and remodeled into posterior part of corpus in mandible 2.

62. Corpus/ body of the mandible The displacement of the ramus posteriorly converts the area occupied earlier by the ramal bone to the posterior part of the mandible so basically it is formed at the expense of ramus re- postioning in posterior direction, to accommodate posterior teeth. Outer surface - Depository Medial surface(inferior aspect) – Resorptive Re-modelling is in the form of ‘L’ Depository area -from the superior half of medial surface of corpus to anterior half of medial surface of ramus(below coronoid) Resorptive area –from inferior half of medial surface of corpus to posterior half of medial surface of ramus (below condyle)

63. Mandibular foramen The mandibular foramen likewise drift backward and upward by deposition on the anterior and resorption on the posterior part of its rim. The foramen maintains a constant position about midway between the anterior and posterior borders of the ramus. Even when the ramus undergoes marked alterations associated with edentulism, the foramen usually sustains midway location

64. Angle of the mandible BUCCAL SIDE Resorption on the antero-superior part Deposition on the postero -inferior part LINGUAL SIDE Deposition on the antero-superior part Resorption on the postero -inferior region. BUCCAL LINGUAL SUPERIOR INFERIOR ANTERIOR POSTERIOR

65. ANTEGONIAL NOTCH A single field of surface resorption is present on the inferior edge of mandible at the ramus corpus junction. This forms the antegonial notch by remodelling from the ramus just behind it as the ramus relocates posteriorly The size of the notch can be increased whenever a downward rotation of corpus relative to the ramus takes place

66. LINGUAL TUBEROSITY Important structure as it is direct anatomic equivalent of the maxillary tuberosity Major site of growth for mandible Effective boundary between basic parts of the mandible ; ramus and corpus. Grows posteriorly by deposits on the posterior facing surface. The prominence of tuberosity is increased by presence of large resorptive fields just below it which produces a sizable depression, the lingual fossa.

67. The combination of resorption in the fossa and deposition on the medial facing surface of tuberosity itself greatly accentuates the contours of both regions Deposition on the lingual surface of the ramus just behind the tuberosity produces a medial direction of drift that shifts this part of the ramus into alignment with the axis of corpus.

68. CHIN During the descent of the maxillary arch and the vertical drift of the mandibular teeth, the anterior mandibular teeth simultaneously drift lingually and superiorly. The re-modelling process involves Periosteal resorption on the labial bony cortex Deposition on the alveolar surface of the labial cortex Resorption on the alveolar surface of the lingual cortex Deposition on the lingual side of the lingual cortex

69. With age the chin becomes more prominent with resorption in the alveolar region below the lower incisors which accentuates the increased deposition at the mental protuberance.

70. Alveolar process Develops in response to the growing dentition Continued growth of the alveolar bone increases the height of the mandibular body. Alveolar process grows upwards and outwards to accommodate the larger permanent teeth. The mental foramina although doesn’t show much change after 6 th year of life.

71. GROWTH TIMING Growth of width of mandible is completed first, then growth in length and finally growth in height Width of mandible Growth in width is completed before adolescent growth spurt Inter-canine width does increase after 12 years Both molar and bicondylar width shows small increase until growth in length ends Growth in length Growth in length continues through puberty Girls—14-15 years boys---18-19 years Growth in height Continues in both the sexes for longer period Growth increase occurs with concomitant eruption of teeth and continues to increase through out life and decreases in adult life

72. ANATOMICAL LANDMARK AT BIRTH ADULT OLD AGE Mental foramen Near the lower border (b/w 2 deciduous molars) Midway between upper and lower border Near upper border Ramus Vertical in direction Oblique in direction Mandibular canal Runs little above mylohyoid line Runs parallel with mylohyoid line Runs close to the upper border Angle Obtuse (175˚) 110-120 Obtuse(140˚) Coronoid process Large and project above the condyle condyle Positioned nearly in the line of body Condylar process larger and lies above coronoid Condylar process is larger and lies at level above coronoid process Extreme old age-bent backwards Symphysis menti Present;2 halves united by fibrous tissue Represented by faint ridge- only in upper part Not recognizable or absent Age changes in mandible

73. MANDIBLE AT BIRTH The mandibular rami are short and wide The condyles are not developed. The alveolar process is not formed. The body of the mandible is shell like with tooth follicles and developing crowns. The angle of the mandible is at 175 degrees with the condyles nearly in line with the body. The initial separation of the two mandibular halves by fibrocartilage and connective tissue is eliminated as ossification occurs between 4 th month to 1 year. The symphyseal suture has not ossified.

74. Mandibular growth during first year The growth of mandible in the first year of life involves growth at the symphyseal suture and lateral expansion in the anterior region to accommodate the erupting anterior teeth. The mental foramen is directed at right angle to the surface of the body / corpus. There is increased deposition in the posterior surface of the ramus of the mandible. The infant mandible is suited for the suckling activity since the condyle and the glenoid fossa is flat, which helps in the anteroposterior movement of the mandible.

75. The body becomes elongated in its whole length, but more evident behind the mental foramen The depth of the body increases owing to increased growth of the alveolar part. The mandibular canal , after the second dentition, is situated just above the level of the mylohyoid line; and the mental foramen occupies the position as in the adult. The angle becomes less obtuse, owing to the separation of the jaws by the teeth; about the fourth year it is 140°. 2) CHILDHOOD

76. The alveolar and sub-dental portions of the body are usually of equal depth. The mental foramen opens midway between the upper and lower borders of the bone, and the mandibular canal runs nearly parallel with the mylohyoid line. The bone is larger on the whole and the condyles is well developed. The ramus is almost vertical in direction, the angle measuring from 110° to 120°. 3) ADULTHOOD

77. Bone reduces in size as teeth are lost and the alveolar region absorbed. Mental foramen and mandibular canals are near the superior border. The ramus is oblique in direction, the angle measures about 140°, and the neck of the condyle is more or less bent backward. 4) OLDAGE

78. Bone mass decreases. Jaw becomes porous Loss of teeth leads to loss of periodontal support – RESORPTION Remodeling Changes take place EFFECTS OF AGING

79. MANDIBLE: Male V/s Female Males generally have squarer, stronger, and larger mandible than females. The mental protuberance is more pronounced in males but can be visualized and palpated in females. The symphysis can be not fully fused which happens more in male, leaving an indentation.

80. MANDIBULAR ASYMMETRIES – ETIOLOGY

81. MANDIBULAR ASYMMETRIES DUE TO EMBRYONIC GROWTH ABNORMALITY

82. MANDIBULAR ASYMMETRIES DUE TO ADVERSE POSTNATAL EVENTS DURING THE GROWTH PERIOD

83. MANDIBULAR ASYMMETRIES DUE TO MIS-REGULATION OF GROWTH AFTER BIRTH

84. MANDIBULAR ASYMMETRIES DEVELOPING AFTER GROWTH HAS CEASED

85. Summary and conclusion Mandibular growth and development is too complex and vast to describe accurately. Present theories and hypothesis cannot stand by themselves in providing or explaining completely the mandibular growth and development. A thorough knowledge of the development of mandible is essential for the diagnosis and treatment of mandibular disorders.

86. references ESSENTIALS OF FACIAL GROWTH- DONALD H. ENLOW, MARK G. HANS LANGMAN’S MEDICAL EMBRYOLOGY-12 TH EDITION. T.W.SADLER BOUCHER’S PROSTHODONTIC TREATMENT FOR EDENTULOUS PATIENTS- 12 TH EDITION MEDICAL EMBRYOLOGY- 10 TH EDITION- IB. SINGH HANDBOOK OF ORTHODONTICS- ROBERT E. MOYERS 4 TH ED TENCATES ORAL HISTOLOGY- 8 TH ED ORBANS ORAL HISTOLOGY AND EMRYOLOGY- 13 TH ED CONTEMPORARY ORTHODONTICS-WILLIAM R.PROFFIT. 4 TH ED CRANIOFACIAL DEVELOPMENT- SPERBER THE DEVELOPING HUMAN: CLINICALLY ORIENTED EMBRYOLOGY, BY DRS. KEITH L. MOORE, T.V.N. PERSAUD FUNDAMENTALS OF CRANIOFACIAL DEVELOPMENT- ANDREW. DIXON