Skeletal Growth and Development
Master the INBDE with Dental Panda: Your go-to resource for expert practice exams and tailored study resources!
FORMATION OF BONE
There are 3 main theories regarding the control of growth:
- Direct genetic control – bony growth is under direct genetic control.
- Epigenetic control – cartilage is the primary controlling factor. Cartilage grows first and is then replaced by bone.
- Environmental control/functional matrix theory – growth is controlled by the transmission of forces through adjacent soft tissues.
In embryonic development, mesenchymal cells differentiate into either fibrous membrane or cartilage. Cartilage grows in 2 ways:
- Appositional growth by the recruitment of chondroblasts from stem cells, adding new matrix to the surface.
- Interstitial growth by the mitotic division and laying down of matrix around the chondrocytes already present in the cartilage (e.g. mandibular condyles, spheno-occipital synchondrosis). Hyaline cartilage can undergo interstitial growth, bone cannot.
Endochondral ossification takes place within a hyaline cartilage frame, which is later transformed into bone. Cartilage cells (chondrocytes) are replaced by bone cells (osteocytes) where calcium and phosphate are deposited into the matrix that has been laid down. This method forms the short and long bones, the bones of the base of the skull (ethmoid, sphenoid) and temporal bones of the skull. Endochondral ossification is likely under more genetic control.
Intramembranous ossification takes place in the membranes of connective tissue. Osteoprogenitor cells in the membrane differentiate into osteoblasts that lay down the collagen matrix, which undergoes ossification. This method forms the flat bones of the face, most of the bones of the skull, and the clavicles. Growth of intramembranous bones are likely more influenced by external forces. There will be no growth within the already calcified bony matrix (interstitial growth). Once formed, bone often grows through apposition growth where new layers are added to the underlying bone. Don’t confuse bone growth with bone formation. You will need to understand and explain how each part of the head and neck grows and develops.
CRANIAL VAULT
The cranial vault forms by intramembranous ossification. Fontanelles are areas in infants where the bone is widely separated by connective tissue. Appositional growth occurs at the edges of these separated bones until they fuse. The soft tissue (brain) growth causes pressure on the sutures, with continued bony apposition. Bone is added along the outer wall of the cranial vault, and removed on the internal surface.
CRANIAL BASE
The ethmoid, sphenoid and occipital bones start as cartilage and are transformed via endochondral ossification. After calcification, areas of cartilage remain, called synchondroses (singular synchondrosis)
- intersphenoid synchondrosis (fusion around age 3)
- sphenoethmoidal synchondrosis (fusion around age 7)
- spheno-occipital synchondrosis (fusion starts in girls at 12-13 years, and in boys at 14-15 years and is complete by 17-18 years)
The three growth centers act like two epiphyseal growth plates growing back to back, with the growth center in the middle and the maturing cartilage heading in both directions. Cartilage then transforms into bone. Recall, these endochondral bones are not largely dependent on soft tissue forces (growth of brain).
THE MANDIBLE
In the mandible we see both endochondral and intramembranous growth. Embryonic development starts with Meckel’s cartilage, but the mandible develops immediately lateral to the cartilage through intramembranous growth (not directly from the cartilage). The cartilage disintegrates to form a few bones of the ear (malleus and incus). Condylar cartilage forms separately and later fuses to the ramus of the mandible, covering the surface of the mandible at the TMJ. Cartilage here turns to bone, the resultant growth “pushing” the jaw downward and forwards.
There is resorption along the anterior surface of the ramus of the mandible, with bone depositing on the posterior surfaces. This causes the mandible to appear to grow in a posterior direction. This coupled with vertical condylar growth causes the upwards and backwards appearance of growth, but because the skull is fixed, the mandible “grows down and forward”. Most of the vertical facial growth is from the growth of the mandible. During infancy the anterior border of the ramus is where the first deciduous molar would be located. Posterior growth creates room for the second deciduous molar and later the adult molar teeth. If the growth stops early there won’t be room for the third molars. At age 6, the greatest increase in size of the mandible occurs distal to the adult first molar.
There is also a rotational component of mandibular growth. In children the chin is more prominent. Closing rotation is where the posterior part (molar area) of the mandible outpaces the anterior (incisor area) in growth, leading to posterior facial height increases that outpace the anterior. This is considered normal closing rotation. However, during development, the mandible can under-rotate (termed opening rotation, long face, tendency towards open bite) or over-rotate (termed severe closing rotation, shorter face, deeper anterior overbite).
The mandibular growth outpaces the maxilla, and stops growing later (cephalocaudal gradient of growth). This is called the late mandibular growth theory, and could be to blame for late mandibular crowding of lower incisors. This crowding is often seen in individuals with congenitally missing lower third molars which suggest it’s not due to the third molars “pushing” the teeth and causing crowding.
THE MAXILLA
The maxilla forms via intramembranous ossification, growing by apposition of bone at the sutures superior and posterior to the maxilla. This causes “movement” of the maxilla in a downwards and forwards direction. The maxilla also experiences surface remodeling, with bone added inferiorly and bone removed anteriorly. Room is made for the teeth by new bone added to the maxillary tuberosity area. The alveolar processes grow divergent, increasing arch width. Horizontal growth of the jaws is usually complete before puberty, but vertical growth continues on after puberty.
ALVEOLAR PROCESS
The alveolar process grows in height and length to accommodate the developing dentition. The alveolar process only exists to support teeth. If the teeth are lost, the alveolar process slowly atrophies. If the tooth never erupts, the process never forms.
