THE MANDIBLE AND THE HYOID BONE

THE MANDIBLE AND THE HYOID BONE

I- Introduction

The facial mass is a set of bones attached to the antero-inferior part of the skull in front of the

cervical spine.

The facial skeleton is divided into two parts relative to the oral cavity:

• Lower: mobile lower facial mass made up of a single bone, the mandible, attached to the

upper facial mass through the temporomandibular joint;

• Upper: upper facial mass, composed of thirteen bones of which only one is unpaired, the vomer,

the others are even and symmetrical: the maxilla, malar bone (zygomatic), nasal bone, lacrimal bone or unguis, the palatine and the inferior nasal turbinate.

II- THE MANDIBLE:

• The mandible comes from the Latin word mandibula = jaw

It is an odd, median and symmetrical bone which alone constitutes the skeleton of the lower jaw.

It is related to the masticatory function, carries the lower teeth and provides insertion to the masticatory muscles. It articulates with the two temporal bones by means of menisci. 

                                It is the only moving part of the facial mass.

The mandible is made up of three parts:

• A middle part called: body
• Two side parts: the rising branches

A- THE BODY:

It is horizontal and horseshoe-shaped, with a posterior concavity.

 Its dense lower half forms the base of the body of the bone (pars basalis), its upper half hollowed out by the alveolar cavities constitutes the alveolar portion of the mandible (pars alveolaris). 

It presents:

1. An external face : 

Convex, it presents:

-On the median line a vertical crest: the mental symphysis, this ends towards the bottom with a pyramid-shaped projection, this is the mental eminence.

On either side of the midline, six vertical projections: alveolar jugums, separated by grooves, they correspond to the implantation of the incisors and canines.

On each side of the mental eminence, a crest is the external oblique line, directed upwards and backwards, it continues with the lateral lip of the anterior edge of the ascending branch.

Above the external oblique line, the mental foramen which provides passage for the mental vessels and nerves, it is generally located between the two premolars.

   2- an internal face :

 Concave, it presents:

On either side of the midline, two small superimposed projections are the upper and lower genio processes (two on the right and two on the left), the two upper ones provide insertion to the genioglossus muscle, the two lower ones provide insertion to the geniohyoid muscle.

On either side of the geni processes, a crest corresponding to the internal oblique line or mylohyoid line which runs upwards and backwards, to end on the medial lip of the anterior edge of the ascending ramus. It gives insertion to the mylohyoid muscle.

Below the mylohyoid line is the mylohyoid groove which houses the mylohyoid vessels and nerves.

-The oblique line divides the posterior face into two fields:

• Upper:   has an excavation called: sublingual fossa in relation to the anterior face of the sublingual (sublingual) gland.
• Lower: has a depression called: submandibular fossa, related to the submandibular (submaxillary) gland.

3. An upper or alveolar edge : 

Thick, hollowed out with cavities, intended for the roots of the teeth, called alveoli, the latter are separated by inter-alveolar partitions (inter-alveolar septa).

4.  A lower edge:

 Thick, soft and subcutaneous. It presents a depression outside the midline, it is the digastric fossa where the anterior belly of the digastric muscle is inserted.

B- RISING BRANCHES:

They are rectangular bony blades, flattened transversely, they detach from the posterior ends of the body and present: two faces, four edges, and two apophyses.

1. Side face: 

Presents in its lower part rough ridges which provide insertion to the tendinous blades of the masseter.

2. Medial face : 

Presents:

-At its lower part rough ridges which give insertion to the medial pterygoid muscle;

-In its middle part: the entrance orifice of the lower dental canal, which gives access to the lower dental vessels and nerves. 

This orifice is limited in front by a triangular relief in point called spine of Spix or lingula which gives insertion to the spheno-mandibular ligament. 

Behind the orifice of the dental canal there is often a small projection: the antilingula.

3. The front edge: 

It is limited by two lips, lateral and medial, the latter continuing upwards, to the medial face of the coronoid process, by the temporal crest. These two lips provide insertion to the temporal muscle.

4. The posterior edge: 

Thick, blunt, gives insertion above its lower end to the stylomandibular ligament.

5. Bottom edge: 

It continues forwards along the lower edge of the body of the mandible, and forms with the posterior edge the angle of the jaw or gonion. It is marked forwards by a depression related to the passage of the facial artery.

6. The upper edge : 

Sigmoid notch or notch: thin and sharp, concave at the top, it has two projections:

Anterior called coronoid process,

Posterior called condyle.

The sigmoid notch connects the masseteric and zygomatic regions, and provides passage for the masseteric vessels and nerves.

C-THE LOWER DENTAL CANAL:

The mandibular bone is formed as a whole of two layers of compact bone called: external table and internal table which surround the spongy tissue. Between these two tables, runs from the spine of Spix to the mental orifice a canal called: inferior dental canal, it houses the inferior dental vessels and nerves

The hyoid bone

The hyoid bone from the ancient Greek ὑοειδής / hyoeidếs “u-shaped, u-oid”; referring to the Greek letter “  upsilon  ” ‹  υ  › 

is a bone located above the larynx , in the anterior part of the neck , below the base of the tongue . 

It has the particularity in humans of being the only bone in the skeleton that is not articulated with another bone. 

Description

• The hyoid bone is composed of five parts: a body, two greater horns, and two lesser horns. 
• The body of the hyoid bone is convex anteriorly and inferiorly. 
• Laterally at the rear, the body continues with the two large horns. 
• At the level of the birth of the two large horns, the small horns project upwards. 
• The hyoid bone is an important muscular and ligamentous intersection

Muscle Insertions

The hyoid bone serves as a landmark for categorizing the anterior muscles of the neck  :

• the suprahyoid muscles and the infrahyoid muscles . 
• The middle pharyngeal constrictor muscle inserts on the posterior surface of the hyoid bone . 
• The hyoglossus muscle inserts along the entire length of the superior surface of the greater cornu of the hyoid bone. 
• The genioglossus muscle inserts medially and superiorly on the anterior surface of the body of the hyoid bone. 
• Fibers from the intrinsic muscles of the tongue also insert onto the hyoid bone. 

THE MANDIBLE AND THE HYOID BONE

THE MANDIBLE AND THE HYOID BONE