Normal Craniofacial and Dentition Development and Growth .pdf

Normal Craniofacial and Dentition Development and Growth Basic facts: Facial growth continuous and time-dependent No part of face is developmentally independent The face grows out from underneath the brain Teeth are unique: no change o...

Normal Craniofacial and Dentition Development and Growth Basic facts: Facial growth continuous and time-dependent No part of face is developmentally independent The face grows out from underneath the brain Teeth are unique: no change or growth after eruption * Growth allows orthodontist to correct skeletal discrepancies otherwise treated by orthognathic surgery in adults Growth & Development Terminology Growth : increase in size and number Development : continuous change in a predetermined order Di erentiation: increase in complexity * Cephalocaudal gradient of growth * Neurocranium at birth 25%, 2 yrs 75%, 10 yrs 95% complete * The head will stop growing by the age of 10 years. The maxilla and mandible derive from the rst pharyngeal arch into which have migrated cranial neural crest cells Neural crest cells give rise to a number of pre-speci ed derivatives, the patterning of which is controlled by Hox genes (A) ve prominences around the stomodeum (primitive mouth) (4 weeks) (B) Nasal placodes appear on the frontonasal process (28 days) (C) Nasal placode invaginates to form a nasal pit forming(lateral and medial nasal process (35 days) (D+E) fused medial nasal processes to form the intermaxillary process. The mandibular swellings have now merged to create the primordial lower lip (48 days) 1 of 8 ff fi fi fi Formation of the palate (7-9 weeks) (A)The palatal shelves begin to develop and lie on either side of the tongue (B) The palatine shelves elevate rapidly and tongue drop down (C) The shelves fuse together forming the primary palate and the nasal septum (D) Any disturbance in the process of the shelves elevation is likely to cause clefting. Cleft Lip: 6 weeks, failure of fusion between Medial and Lateral nasal processes and the Maxillary process Cleft Palate Cleft palate: 8 week, failure of fusion between the palatal shelves 2 of 8  ANOMALIES OF DEVELOPMENT A. Micrognathia: Diminutive mandible is characteristic of several syndromes, including Pierre Robin's & Treacher Collins 'syndrome. B. Macrognathia Usually an inherited condition Abnormal growth phenomena such as hyperpituitarism may produce mandibular overgrowth of increasing severity with age. C. Congenital hemifacial hypertrophy Evident at birth Tends to accentuate at puberty. Unilateral enlargement of the mandible, the mandibular fossa, and the teeth. It is of obscure etiology. More common is isolated unilateral condylar hyperplasia. Variability of growth and development All children undergo a spurt of growth at adolescence Growth spurt occurs at di erent times in di erent individuals Chronologic age often is not a good indicator of individual’s growth status. The rst dimension that starts is the transverse, then the anteroposterior then vertical. At Cellular level only three possibilities for growth: 1. Hypertrophy 2. Hyperplasia 3. Extracellular Matrix Interstitial growth: Hyperplasia and hypertrophy of the cells within the tissue (All soft tissue and uncalci ed cartilage) - this kind of grow the only on soft tissues and uncalci ed cartilages - does not occur in bone Appositional growth: Formation of new cells and extracellular material Periosteum (soft tissue that cover bone) Bone Growth Bone does not grow interstitially It grows by activity at the margins of the bone tissue appositional. Overall bone growth is a function of two phenomena: 1. Remodeling 2. Displacement 3 of 8 fi ff fi ff fi  Bone grow by appositional only: 1. Displacement (primary or secondary) 2. Remodeling Ways to study growth: A. Implants (Bjork’s studies) B. Vital dye markers C. Superimposition of head lm tracings Bone formation The process by which new mineralized bone is formed is termed ossi cation Ossi cation occurs in one of two ways: There are two ways of bone formation 1. By membrane activity (Intramembranous) 2. By replacement of a cartilage (Endochondral) Endochondral ossi cation Endochondral ossi cation Seen in long bones and cranial base Ossi cation takes place in a hyaline cartilage framework Clear zones of cell division, hypertrophy, and calci cation Epiphyseal plates of long bones and synchondroses of cranial base Growth causes expansion, despite any opposing compressive forces 4 of 8 fi fi fi fi fi fi fi  Intramembranous ossi cation Direct transformation of mesenchymal cells into osteoblasts Osteoblasts occurs in sheet-like osteogenic membranes Seen in cranial vault, facial bones, mandible and clavicle Craniofacial complex: 1.Cranial vault (bones that cover upper and outer surface of the brain) 2. Cranial base ( bony oor under the brain that divide cranium from face) 3. Nasomaxillary complex (nose, maxilla, and associated small bones) 4. Mandible 1. Cranial Vault Formation : Intramembranous bone formation Growth : Remodeling and growth occur at the periosteum of adjacent skull bones (cranial sutures) At birth the at bones are separated by loose connective tissues (Fontanelles) will be eliminated quickly after birth leaving only sutures that fuses in adult life 2. Cranial base Formation : Endochondral bone formation Growth : Grow at synchondroses looks like two sided epiphyseal plate of long bones Sphenooccipital synchodroses last to fuse late teens 3. Nasomaxillary complex (Maxilla) Formation : Intramembranous bone formation Growth: Grow at the sutures appositional growth Downward and forward 4. The Mandible Formation : Both endochondral & intramembranous Growth : Grow at the condylar cartilage & appositional Condylar cartilage is not like an epiphyseal plate or a synchondroses Downward and forward Forward growth rotation are more common Most of the downward forward is from the condyle Tissue Layers of Mandibular Condyle Division (Hyperplasia) Only of Connective Tissue Cells Only Enlargement (Hypertrophy) of Cartilage Cells Proliferating condylar cartilage cells do not show the ordered columnar The articular surface is covered by a layer of dense brous connective tissue Is the mandible a bony replacement of Meckel’s cartilage????? No, mandible form by intramemranous bone formation lateral to Meckel’s cartilage. Remnants of this cartilage are transformed into a portion of two of the small bones of the middle ear and the Perichondrium persists as sphenomandibular ligament Where does the condylar cartilage come from??????? Developed as an independent secondary cartilage 5 of 8 fl fl fi fi PRENATAL GROWTH A. Embryology and prenatal growth of the mandible 4th week: Meckel’s Cartilage 5th – 6th week: membranous ossi cation sites lateral to Meckel’s Cartilage 10th week: secondary cartilaginous sites (condyle) 14th week: start of endochondral ossi cation. B. Comparison with the size of the maxilla Initially during fetal life: the mandible is bigger than maxilla 8th week : maxilla overlaps the mandible. 11th week: equal size of upper & lower jaws. Between 13th & 20th week : mandible growth lacks behind maxilla. At birth, the mandible tends to be retrognatic relative to the maxilla. PRENATAL GROWTH A. Embryology and prenatal growth of the mandible 4th week: Meckel’s Cartilage 5th – 6th week: membranous ossi cation sites lateral to Meckel’s Cartilage 10th week: secondary cartilaginous sites (condyle) 14th week: start of endochondral ossi cation. B. Comparison with the size of the maxilla Initially during fetal life: the mandible is bigger than maxilla 8th week : maxilla overlaps the mandible. 11th week: equal size of upper & lower jaws. Between 13th & 20th week : mandible growth lacks behind maxilla. At birth, the mandible tends to be retrognatic relative to the maxilla. Growth of the Maxilla I. Vertical growth of the maxilla: A. The downward growth of the maxillary arch is produced by two mechanisms: 1. Downward displacement of the entire nasomaxillary complex due to bone apposition on the sutures sites. Frontozygomatic suture Frontomaxillary sutures 2. The palate remodels downwards by deposition of bone on its inferior surface ( palatal vault) and resorption on its superior surface (the oor of the nose and maxillary sinuses) B. The downward movement of teeth is similarly a two part process 1. Displacement of maxilla as a whole, with alveolar bone not participating. 2. Remodeling growth of alveolar bone 6 of 8 fi fi fi fi fl II. Horizontal (AP) growth of the maxilla: Anterior bone deposition till age 5-6 years. Posterior bone deposition at the tuberosity region which will cause anterior displacement of the maxillary complex. Nasomaxillary & temporozygomatic sutures III. Transversal growth of the maxilla: Remodeling of the vault of the palate will contribute to the widening of the maxilla. Laterally, width is increased by remodeling. Zygomaticomaxillary & Midpalatal sutures Growth of the Mandible Skeletal subunits of the mandible One Basic Arc (the corpus) Three Functional Processes 1. Coronoid Process 2. Alveolar Process 3. Angular Process Mandible: is facial bone that grows for longest period. In the face, areas that are part of one structure sometimes get relocated and become part of another structure. Growth of mandible: A part that was once a part of the ramus is now a part of the corpus. Deposition: bone deposited at posterior surface of ramus. Resorption: bone resorbed at anterior border of ramus Areas of relocation permit eruption of 2nd molars at 12 & 3rd molars at 18- 25. Main areas of postnatal growth: 1. Condylar cartilages 2. posterior border of the ramus 3. alveolar ridges Little change in the anterior part of the mandible (chin) 7 of 8 Mandibular growth: 1. Mandible as a whole In the vertical and sagittal plane: - The whole mandible is displaced away from its articulation in each glenoid fossa - The condyle and ramus grow upward and backward into the “ space “ created by the displacement process. - The ramus remodels as it relocates postero- superiorly. 2. Corpus growth: In length: - As the ramus is relocated posteriorly, the corpus becomes lengthened by a remodeling conversion from what was at one time the ramus during a former growth period. - While the mandible is displaced forward the ramus is repositioned backward and it becomes thicker. - The resorption of anterior border of the ramus is less important than the posterior apposition. - Resorption of 1.5 mm / year of anterior border is observed to manage the space for the 3rd molars in the future. In height: - Growth in height of corpus depends mostly on alveolar growth and results also from remodeling process of basilar border. - This remodeling process keeps the dental canal away of the inferior bony surface. In width: - Synostosis of the symphysal suture occurs at the end of 1st year (postnatal). - Transversal growth then caused by periosteal growth: apposition on lingual surface (basilar border) and resorption on external alveolar surface. 8 of 8