Published on December 19, 2013
Oral Biology Balidhawa Johnson BDS, MAKchs
DEVELOPMENT OF THE SKULL The cartilaginous part – forms bones of the The neurocranium has base of the skull. two portions: The membranous The membranous part portion of the skull is – consists of flat derived from neural bones, which surround crest cells and paraxial mesoderm. the brain as a vault. • Neurocranium
NEW BORN SKULL At birth, separated from each other by narrow seams of connective tissue, the sutures. • Sigittal suture – from neural crest cells • Coronal suture – from paraxial mesoderm The anterior fontanalle, which is the most prominent is found where the two pariental and two frontal bones meet. The bones of the vault continue to grow, mainly because the brain grows.
POSTNATAL GROWTH OF THE SKULL • Fibrous sutures of the newborn calvaria permit the brain to enlarge during infancy and childhood • The increase in size of the calvaria is greatest during the first 2 yrs, the period of most rapid postnatal growth of the brain. • Most paranasal sinuses are rudimentary or absent at birth.
VISCEROCRANIUM Consists of the bones of the face Formed mainly from the first two pharyngeal arches. • The first arch gives rise to a dorsal portion, • The maxillary process, which extends forward beneath the region of the eye and gives rise to the maxilla, the zygomatic bone and part of the temporal bone. • The second pharyngeal gives rise to the incus, malleous and stapes
CRANIOFACIAL DEFECTS • a) Cranioschisis • Cranial vault fails to form and the brain tissue remains exposed (anencephaly). • b) Meningocele • Skull forms but brain protrudes to form a mass in the nape.
CRANIOFACIAL DEFECTS • c) Scaphocephaly Early closure of the sagittal suture • d) Plagiocephaly • Early closure of the coronal and lambdoid sutures on one side of the skull • e) Achondroplasia Characterized by a large skull with a small mid face
CRANIOFACIAL DEFECTS Plagiocephaly Achondroplasia large skull with a small mid face Early closure of the coronal and lambdoid sutures
CRANIOFACIAL DEFECTS • Craniosynostosis Cranial abnormalities caused by premature closure of one or more sutures. • Examples; • - Apert syndrome • - Pfeiffer syndrome • - Crouzon syndrome • Microcephaly -The brain fails to grow and the skull fails to expand
CRANIOFACIAL DEFECTS Apert syndrome Pfeiffer syndrome
DEVELOPMENT OF THE MANDIBLE • The mandible is the largest and strongest bone of the face • i) ii) iii) Represents the primitive vertebrate mandible Meckel’s cartilage attains its full form by the 6th week. The mandible is a membranous bone. • BODY OF THE MANDIBLE • Ossification occurs in the 7th week in the angle formed by the incisive and mental nerves. • It is also the region of the future mental foremen ramus • • body of the mandible • MECKEL’S CARTILAGE 3 Main parts form the mandible • • alveolar bone
DEVELOPMENT OF THE MANDIBLE
THE ALVEOLAR BONE • As the enamel organs of the decidous tooth germs reach the early bell stage, the bone of the mandible begins to come into close relationship to them. • This is brought about by the upward growth on each side of the tooth germs then also the lateral and medial plates of the mandibular bone
THE RAMUS • The backward extension of the mandible to form the ramus is produced by a spread of ossification from the body, behind and above the mandilular foremen, The ramus and its processes are mapped by extension of fibrocechular condensation. The formation of bone in this tissue occurs rapidly so that the coronoid and condylar processes are to a large extent ossified by the 10th week.
THE FATE OF MECHEL’S CARTILAGE • During the later fetal period nodules of cartilage are seen in the fibrous tissue of the sympysis; (remnants of the ventral end of Mechel’s cartilage) The rest of Meckel’s cartilage disappears completely except for apart of its fibrous covering which persists as; • Spheno mandibular and spheno – malleolar ligaments • The most dorsal part of the cartilage ossifies to form the incus and the malleus.
THE MANDIBLE AT BIRTH Though perfectly recognizable, it differs in several respects from the adult bones. The chief differences are; • The wide angle, and small size of the ramus compared with the body. • The chin is usually poorly developed, much of its growth takes place after puberty.
DEVELOPMENT OF THE TEMPORAMANDIBULAR JOINT The constituent features of the temporamandibular joint include; • Articular surface of the mandibular condyle • Articular surface of the temporal bone • Articular disc • Joint capsule • Joint cavities • Articular ligaments
Temporarmandibular joint • • • Formation of the temporarmandibular joint is first indicated by the growth of tissue condensation of the developing mandible. Which everywhere precedes ossification, towards a corresponding condensation in the temporal region (12th week in utero). The mandibular condensation maps out the shape of the condyle. A strip of dense tissue above the upper surface of the condyle appears. It is then connected to the lateral pterygoid muscle and the strip of tissue becomes the articular disc. Formation of joint cavities occurs as the condyle becomes approximated to temporal element of the joint. Joint cavity development is virtually complete between the 17th and 18th week.
CLINICAL ASPECTS Dislocation of the temporomandibular joint • When opening the mouth, the condylar head of the mandible & articular disc move anteriorly up to the articular tubercle of the zygomatic process of the temporal bone. • During excessive opening of the mouth, the condylar head & articular disc of one or both sides goes anteriorly beyond the articular tubercle into the infratemporal fossa. • As a result, there is inability to close the mouth which remains open. • Excessive opening of the mouth can happen during yawning, laughing or even during tooth extraction.
CLINICAL ASPECTS • Operations of the temporomandibular joint • During operations of the TMJ, care should be taken to preserve the branches of the facial nerve which are closely related to it. • If the branches of the facial nerve are cut, facial paralysis will result.
CLINICAL ASPECTS • Failure of growth of the condylar cartilages • Bilateral failure of growth of the condylar cartilages leads to gross underdevelopment of the lower jaw. • Unilateral growth failure produces marked assymmetry of the lower part of the face.
DEVELOPMENT OF THE MAXILLA • Each maxilla consists of; i) A body ii) A zygomatic process iii) A frontal process iv) An alveolar process v) Palatine process
DEVELOPMENT OF THE MAXILLA The maxilla proper is developed in the maxillary process of the mandibular arch. Ossification of the maxilla starts slightly later than in mandible, in the 7th week. • The centre of ossification first appears in a band of fibrocellular tissue which lies outer side of the cartilage of the nasal capsule. • The ossification centre lies above that part of the canine from which develops the enamel organ of the canine tooth germ. • Early in development, the developing maxilla forms a body trough for the infra orbital nerve • About the 8th week, a mass of secondary cartilage appears to form the zygomatic process.
THE FATE OF THE NASAL CAPSULE • In the lower portions of the lateral walls of the capsule, the inferior turbinate bones (conchea) develop. Is the primary skeleton of the upper face & analogous to Meckel’s cartilage in the lower part of the face In the upper portion of the lateral wall of the cartilage, the facial part of the ethmoid bone develops. The two portions of the ethmoid are united after birth by the ossification of the cribriform plate in the roof of the capsule. • The intermediate region of the capsule between the facial ethmoid and inferior turbinate atrophies. It is in this region that the maxillary sinus extends outwards from the nasal cavity to invade the maxillary bone. NB: The front of the nasal septum remains cartilaginous throughout life.
Clinical aspects: • By comparison with the • If teeth do not develop, development of the the alveolar processes mandible which begins are not formed. earlier in the 6th week and that of the maxilla, 7th week, even eruption of teeth begins in the lower jaw.
THE MAXILLARY SINUS First appears about the 4th month of fetal life as a small out-pocketing of the mucosa from the lateral wall of the nasal cavity. Separated from the developing maxilla by the cartilage of the nasal capsule and only comes into direct relationship with the bone after the cartilage has a trophied. Gradual extension, the sinus comes into relation with the maxilla above the level of the palatal process and hollows out the interior of the bone. This leads to separation of its upper orbital surface from its lower dental region. The final height of the maxilla is not reached until near the time of the complete eruption of the permanent teeth.
CLINICAL ASPECTS • An inferior extension of the sinus into the base of the alveolar process is of special practical significance; It establishes a closer relation of the sinus with the root apices of the maxillary premolars and molars. In extreme cases the sinus even extends into the alveolar process between the roots of the teeth so that their sockets protrude into the cavity. The body floor of the sinus may even become defective above the apices of the roots.
CLINICAL ASPECTS • As a result, the following dangers are posed; During and molars can pass into and infect the maxillary sinus. tooth extraction (premolars and molars) the NB: Under development maxillary sinus floor can be or even absence of the damaged thereby creating maxillary air sinuses does an oro-antral not appear to affect the communication. size of the maxilla. Infection originating in the root apices of premolars
DEVELOPMENT OF THE PREMAXILLA Formed in the region of the junction of the maxillary and frontonasal processes. A heavy trabecularized network of bone appears on the labial aspect of the canine alveolus On the facial aspect, the suture between the premaxilla and maxilla can still be seen until after birth extending from the region of the incisive foramen forward to the alveolar process between canine and the lateral incisor. At weeks of age, a separate centre of ossification appear for the premaxilla. 7th •
The End Balidhawa Johnson BDS, MAKchs
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