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Development of mandible

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Development of mandible

  2. 2. CONTENTS • Introduction • Pre- natal growth of mandible • Post- natal growth of mandible • Fate of Meckel’s cartilage • Secondary cartilages • Age changes in mandible • Clinical considerations • Conclusion • References
  3. 3. INTRODUCTION Mandible is the largest and strongest bone of the face, serves for the reception of lower teeth. It consists of curved, horizontal portion, the body; and two perpendicular portions the rami, which unite with the ends of the body nearly at right angles.
  4. 4.  About the fourth week of intrauterine life, the pharyngeal arches are laid down.  The first arch is called the mandibular arch. Courtesy:Human Embryology,I.B.Singh
  5. 5.  The first branchial arch, also called the first pharyngeal arch and mandibular arch, is the first of six pharyngeal arches. This arch divides into a maxillary and a mandibular process, giving rise to structures including the bones of the lower two-thirds of the face and the jaw. The maxillary process becomes the maxilla (or upper jaw), and palate while the mandibular process becomes the lower jaw. This arch also gives rise to the muscles of mastication.
  6. 6. The first structure to develop in the primodium of the lower jaw is the mandibular division of trigeminal nerve.
  7. 7. Meckel’s cartilage has a close relationship to the mandibular nerve, beginning two thirds of the way along the length of the cartilage. At this point the mandibular nerve divides into lingual and inferior alveolar branches, which run along the medial and lateral aspects of the cartilage, respectively. The inferior alveolar nerve further divides into incisive and mental branches more anteriorly.
  8. 8. Development of:  Body or Corpus of mandible  Rami of mandible  Condylar process  Coronoid process  Alveolar process DEVELOPMENT OF MANDIBLE
  9. 9.  The cartilage of the first arch, Meckel’s cartilage, has a close relationship to the developing mandible. At 6 weeks of development, this cartilage extends as a solid hyaline cartilagenous rod surrounded by a fibrocellular capsule. Body of mandible
  10. 10. Its proximal or cranial ends are connected with the ear capsules, and their distal extremities are joined to one another at the symphysis by mesodermal tissue. The two cartilages of each side do not meet at the midline but are separated by a thin band of mesenchyme.
  11. 11. During the 6th week of embryonic development, on the lateral aspect of Meckel’s cartilage, a condensation of mesenchyme occurs in the angle formed by the division of inferior alveolar nerve and its incisive and mental branches. At, 7th week intramembranous ossification begins in this condensation, forming the first bone of the mandible.
  12. 12. From this centre of ossification, bone formation spreads rapidly anteriorly to the midline and posteriorly to the point where the mandibular nerve divides into lingual and inferior alveolar branch. Courtesy: Ten Cate
  13. 13.  The new bone forms a trough that consist of medial and lateral plates that unite beneath the nerve. The trough is soon converted into a canal as bone forms over the nerve, joining the lateral and medial plates.
  15. 15. The ramus of the mandible develops by a rapid spread of ossification backwards into the mesenchyme of the first branchial arch diverging away from Meckel’s cartilage. This point of divergence is marked by the lingula in adult mandible, where the inferior alveolar nerve enters mandibular foramen.
  16. 16.   Thus by 10 weeks of development, the rudimentary mandible is formed almost entirely by intramembranous ossification.
  17. 17. Meckel’s cartilage has the following fate: H & E stained section-Meckel’s cartilage Prenatal Development of the Human Mandible. THE ANATOMICAL RECORD 263:314–325(2001) Its posterior end forms the malleus and incus of the inner ear and the sphenomandibular ligament; but its fibrocellular capsule persists to form the sphenomandibular ligament . From the lingula forward to the division of the alveolar nerve into its incisive and mental branches, Meckel’s cartilage degenerates.
  18. 18.   The further growth of mandible is influenced by the appearance of 3 secondary cartilages: 1) Condylar cartilage 2) Coronoid cartilage 3) Symphyseal cartilage
  19. 19. Carrot shaped cartilage appears (at 12 weeks of development) in the region of the condyle and occupies most of the developing ramus. It is rapidly converted to bone by endochondral ossification. It gives rise to: • Condyle head and neck of the mandible. • The posterior half of the ramus to the level of inferior dental foramen. At 20 weeks a thin layer of cartilage remains in the condylar head. Remnants of cartilage persists until the end of 2nd decade of life. The condylar cartilage
  20. 20. The coronoid cartilage: It is relatively transient growth cartilage center ( 4th MIU). It gives riseto: • Coronoid process. • The anterior half of the ramus to the level of inferior dental foramen. Disappears long before birth.
  21. 21.  They are two in number. Appear in the connective tissue between the two ends of Meckel’s cartilage. They are obliterated within the 1st year afterbirth.   Symphyseal cartilage:
  23. 23.  It starts when the deciduous tooth germs reaches the early bell stage.  The bone of the mandible begins to grow on each side of the tooth germ.  By this growth the tooth germs come to be in a trough or groove of bone, which also includes the alveolar nerves and blood vessels.  Later on, septa of bone between the adjacent tooth germs develop, keeping each tooth separate in its bony crept.  The mandibular canal is separated from the bony crypts by a horizontal plate of bone.  The alveolar processes grow at a rapid rate during the periods of tooth eruption.
  25. 25. I. Growth of secondary cartilage II. Development of the alveolar process III. Sub-periosteal bone apposition Growth of Mandible
  26. 26. Growth by secondary cartilage Increase in height of the mandibular ramus Increase in the overall length of the mandible Increase in the inter-condylar distance I. Growth of secondary cartilage
  27. 27. II. Development of the alveolar process:  The alveolar process develops in response to the presence of tooth buds.  As the teeth erupt the alveolar process develops and increases in height by bone deposition at the margins.  The alveolar bone adds to the height and thickness of the body of the mandible.  In case of absence of teeth, the alveolar bone fails to develop and it resorbs in the event of tooth extraction.
  28. 28. III. Sub-periosteal bone apposition: Bone deposition Bone resorption Result in External surface of the mandible Inner surface of the mandible Increase the transeverse dimension Posterior border of the ramus Anterior border of the ramus Adjust the thickness of the ramus Anterior border of the coronoid process Posterior border of the coronoid process Displacement of the coronoid process Chin region ‫ــــــــــــــــــــــــ‬ Modeling of the lower face
  30. 30. At birth The body of the bone is a mere shell, containing the sockets of the two incisor, the canine, and the two deciduous molar teeth, imperfectly partitioned off from one another. The mandibular canal is of large size, and runs near the lower border of the bone. The mental foramen opens beneath the socket of the first deciduous molar tooth. The angle is obtuse (175°), and the condyloid portion is nearly in line with the body. The coronoid process is of comparatively large size, and projects above the level of the condyle.
  31. 31.  Childhood The two segments of the bone become joined at the symphysis, from below upward, in the first year; but a trace of separation may be visible in the beginning of the second year, near the alveolar margin. The body becomes elongated in its whole length, but more especially behind the mental foramen, to provide space for the three additional teeth developed in this part. The depth of the body increases owing to increased growth of the alveolar part, to afford room for the roots of the teeth, and by thickening of the subdental portion which enables the jaw to withstand the powerful action of the masticatory muscles.
  32. 32.   The mandibular canal, after the second dentition, is situated just above the level of the mylohyoid line; and the mental foramen occupies the position usual to it 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°.
  33. 33.  Adulhood The alveolar and subdental 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 ramus is almost vertical in direction, the angle measuring from 110° to 120°.
  34. 34.  Old age The bone becomes greatly reduced in size, for with the loss of the teeth the alveolar process is resorbed. The mandibular canal, with the mental foramen opening from it, is close to the alveolar border. The ramus is oblique in direction, the angle measures about 140°, and the neck of the condyle is more or less bent backward.
  35. 35.  Developmental disturbances: Agnathia Micrognathia Macrognathia Facial hemihypertrophy Facial hemiatrophy Abnormalities of dental arch relation  Developmental cysts: Median mandibular cyst Alveolar cyst of newborn Clinical consideration 
  36. 36.  Agnathia • Hypoplasia or absence of mandible. • The entire mandible or one side may be missing or only the condyle or the entire ramus.
  37. 37. Micrognathia  • Means small jaw. • Maxilla or mandible may be affected. • Can be due to small jaw or to an abnormal positioning or abnormal relation of one jaw to another.
  38. 38.  Macrognathia • Abnormally large jaws. • Increased height of ramus. • Increased mandibular body length. • Increased gonial angle.
  39. 39.  Exhibit an enlargement which is confined to one side of the body, unilateral macroglossia, and premature development, and eruption as well as increased size of dentition. Facial hemihypertrophy
  40. 40.  Progressive wasting of subcutaneous fat accompanied by atrophy of skin, cartilage, bone and muscle. Facial hemiatrophy
  41. 41.   Class II  Class III Abnormalities of dental arch relations
  42. 42.  Median mandibular cyst - Originate from proliferation of epithelial remnants entrapped in the median mandibular fissure during fusion of the bilateral mandibular arches. - Primodial cyst originating from a supernumerary enamel organ in the anterior mandibular segment. Developmental Cysts 
  43. 43.   Alveolar cyst of newborn - Arise from epithelial remnants of deeply budding dental lamina during tooth development.
  44. 44. All the events taking place during development of mandible play an important role in determining the final structure of mandible, any deviation of which can give rise to various abnormalities in the oro-facial region. CONCLUSION 
  45. 45. 1) Ten Cate’s Oral Histology, Development, Structure And Function, 7th Edition. 2) Orban's Oral Histology And Embryology, 12th Edition. 3) Histology and Embryology, James Avery, 3rd Edition. 4) Human Embryology, Inderbeer Singh, 10th Edition. 5) Shafer’s Textbook of Oral pathology, 7th Edition. 6) Orthodontics- The Art and Science, S.I. Bhalajhi, 6th Edition. 7) Textbook of Oral Madicine, Anil Govindrao Ghom, 3rd Edition. 8) Vishwanathan R. Alveolar cyst of the newborn: a case report. Arch Orofac Sci 2012;7: 85-7. REFERENCES 
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