Growth of the mandible

Growth of the mandible

                              Pr Kourad

Plan : 

Introduction : 

1. Anatomical and embryological reminder 

  1.1 Formation of the symphyseal region

  1.2Formation of the condylar region

2. Postnatal mandibular growth

   2.1Growth mechanisms

   2.2Growth of the rising branch

  2.3Growth of the horizontal branch

 2.4 Chin growth

 3. Mandibular growth direction

Conclusion

Introduction : 

• Theoretical knowledge of growth modalities is an essential contribution for any orthodontist because orthodontics applies to subjects in full growth, its action is located at the level of the two maxillae, more particularly at the level of the mandible. 
• In fact, this bone alone constitutes the lower facial mass, mobile in relation to the base of the skull and the maxilla.

Anatomical reminder: 

The mandible is made up of (figure 1):

• A horizontal branch (corpus).
• Two ascending branches (ramus).
• Two cartilaginous condyles.
• Two coronoid processes.
• The teeth.
• On the midline: the mental symphysis

Figure 1: Mandibular anatomy

• This mandible is surrounded by muscles. We note: on the antero-external face of the mandible the insertion of the muscles of the chin tuft, triangular of the lips, and square of the chin
• On the posteroinferior surface: the insertion of the genioglossus, geniohyoid, mylohyoid and superior pharyngeal constrictor muscles.
• On the upper edge, the insertion of the buccinator muscle.
• On the lower edge, the insertion of the digastric and platysma muscles of the neck. (Figure 2) 

Figure 2: The muscles of the mandible

At the level of the rising branch:

• On the external face, the insertion of the masseter muscle. 
• On the internal face, the insertion of the internal or medial pterygoid muscle. 
• On the neck of the condyle, the insertion of the external or lateral pterygoid muscle. 
• On the coronoid process the insertion of the temporal muscle. 

(Figure 3)

Figure 3: The muscles of the mandible

Vascularization of the mandible: 

-The facial artery.

-The submental artery.

-The sublingual artery.

-the arteries: masseteric and pterygoid and the mylohyoid artery. 

-The internal vascular network is provided by the inferior alveolar artery.

Innervation of the mandible: 

-The mandibular nerve (V3):

-the posterior, middle and anterior deep temporal nerve.

• the lingual nerve.
• the inferior alveolar nerve. 

Embryological reminder:

The mandible arises from the mandibular bud located at the lower part of the 1st ^branchial arch.

• Its ossification requires the presence of a support which is represented by Meckel’s cartilage (extension of the chondrocranium). The latter will determine the anteroposterior dimension of the corpus and the mandible will form in its vicinity. (Figure 4)

Figure 4: The chondrocranium and Meckel’s cartilage

• The mandible first forms in two parts which will then fuse at the mental symphysis.                                                                          
• The first ^bony nucleus appears around the 40th day of intrauterine life in the connective tissue lining the external face of Meckel’s cartilage, more precisely in the vicinity of the future mental foramen. (Figure 5)

Figure 5: the 1st ^mandibular bone nucleus

• The main core extends horizontally to form:

    -A bony blade which essentially represents the horizontal branch (corpus)

    -A part of the chin region

• it extends backwards and bends upwards to form a large part of the ascending branch.

However, two regions are formed by a phenomenon of substitution

Formation of the symphyseal region:

• The anterior end of Meckel’s cartilage would be transformed into small bony pieces (ossicula mentalia), the formation of the anterior part of the mandible would therefore be due to a substitution phenomenon.
• Some authors have a different point of view and think that the ossifying role at the symphyseal level would fall to the symphyseal cartilage which appears secondarily.

Formation of the condylar region:

     Around the 3-4th month of intrauterine life, three secondary cartilages independent of Meckel’s cartilage appear. The angular and coronoid cartilages are transient and disappear rapidly before birth.

     The condylar cartilage persists and is considered the most important growth center of the mandible. ((Figure 6)

Figure 6: Secondary growth cartilages 

Postnatal mandibular growth 

-Growth mechanisms: 

Three fundamental mechanisms allow modifications of the mandibular bone: 

• Sutural growth: symphyseal synchondrosis
• Cartilaginous growth: at the level of the condylar cartilage
• Remodeling growth: apposition-resorption
• Mandibular growth occurs in three directions: 

In width, height and length

Width growth:

• In width: the symphyseal synchondrosis closes in the first months of extra-uterine life. (Figure 7)
• The growth in width of the mandible is the result of the vertical and posterior elongation of the mandible.
• In fact, the condyles move upwards and externally.

Figure 7: Growth in width of the mandible 

Growth in length  : 

Backward apposition and forward resorption of the ramus, which gradually frees up space for the development of all the teeth. Since apposition is more important than resorption, the ramus moves back and thickens. This remodeling phenomenon leads to an elongation of the corpus. 

This lengthening is achieved through the remodeling phenomenon (i.e. suprasymphyseal resorption and apposition at the base and top of the chin). (Figure 8)

Figure 8: Length growth of the mandible

Height growth  : 

It is due to growth at the level of:

 -of the condyle

-from the upper edge of the alveolar processes 

 -from the lower edge of the mandible. (Figure 9)

Figure 9: Height growth of the mandible

The growth of the rising branch:

 The ascending branch thickens while undergoing a backward displacement because there is:

     -Resorption of its anterior edge which frees up the space necessary for the placement of the teeth

     -Apposition on its posterior edge thanks to the periosteum which covers it

     -The ascending branch lengthens due to condylar growth which occurs upwards and backwards.  

Averages of condylar growth rates were established

  Up :

      *3 mm per year in juvenile period

      *1.5 mm per year in the prepubertal period (according to Bjork)

      *5.5 mm per year during puberty

  Backwards  : 2.5 mm per year

  Outward  : 1mm per year per condyle. The bicondylar distance increases by 2mm per year. (Figure 10)

The neck of the condyle undergoes a reduction in its diameter by resorption of its external face and apposition of new bone on its internal face. (Figure 10)

 The coronoid process is almost nonexistent at birth.

It is built under the effect of the traction of the temporal muscle with the development of the masticatory function. 

Its growth occurs upwards, backwards and inwards by apposition on its internal face and resorption on the external face. 

Figure 10: Bicondylar distance (black line) and growth of the condylar neck

Noticed : 

• The external pterygoid muscle and the retromeniscal ligament play a crucial role in the growth of the condyle; their resection does not stop condylar growth, but reduces it.
• The tongue also plays a role in condylar and mandibular growth.

The growth of the horizontal branch

The lengthening of the corpus is done following:

-Resorption of the anterior edge of the ramus, this process continues late until the eruption of the wisdom teeth.

-On its internal face we note:

  *A bony apposition except under the region of the retromolar trigone and under the mylohyoid line where we have a resorption 

  * the external face is only the seat of apposition.

Figure 11: Growth of the horizontal branch

Chin growth:

       It begins in the postnatal period. Its prominence would be the result of a periosteal deposit around the base and the top of the chin with a recession of the alveolar processes. (Figure 12)

Figure 12: Chin growth

Mandibular growth direction: 

Bjork differentiates three types of mandibular growth direction, according to the mandibular shapes observable on lateral teleradiography.

 -a type of anterior mandibular rotation: the direction of growth being horizontal

 -a type of average mandibular rotation 

 -a type of posterior rotation: the growth direction is vertical.

The so-called posterior rotation is expressed by the following signs:

• 1-The backward-facing condyle is relatively thin
• 2- The dental canal has a straight path
• 3- The mandibular angle is open
• 4- The pregonial notch is marked
• 5- The symphysis is oriented upwards and backwards
• 6- The symphyseal cortex is thin
• 7- The intermolar angulation is reduced
• 8- There is an increase in the lower level of the face. (Figure 13)

However :

The so-called anterior rotation is expressed with opposite signs.

Figure 13: Growth directions of the mandible

Growth of the mandible

  Deep cavities may require root canal treatment.
Dental veneers correct chipped or discolored teeth.
Misaligned teeth can cause uneven wear.
Dental implants preserve the bone structure of the jaw.
Fluoride mouthwashes help prevent cavities.
Decayed baby teeth can affect the position of permanent teeth.
An electric toothbrush cleans hard-to-reach areas more effectively.
 

Growth of the mandible