Parotid gland l human anatomy revision for dental students

 

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Parotid Salivary Gland (The Largest Salivary Gland)

 

 Parts of parotid gland:

A main part:

• Wedge-shaped, lies in the groove between the ramus of the mandible anteriorly and the mastoid process and sternomastoid muscle posteriorly.
• It is divided by the mandible and facial nerve into superficial and deep parts ( lobes ) .

A small accessory part:

• Small pat lies above the parotid duct on the masseter muscle.

Parotid duct ( Stenson’s duct )

• Thick-walled, narrow lumen, 5 cm long.
• It begins at the anterior border of the gland, passes horizontally , with the accessory part of the gland , on masseter muscle, one finger breadth below zygomatic arch.
• At the anterior border of masseter, it pierces 4 structures: buccal pad of fat, bucco-pharyngeal fascia, buccinator muscle and buccal mucous membrane ( 2 F & 2M ).
• It opens in the vestibule of mouth opposite the upper 2^nd molar tooth.
• Surface anatomy of the duct: The middle 1/3 of a line extending from the tragus of auricle to a mid- point between the ala of nose and the angle of the mouth .

Surface Anatomy of the Gland:

Upper end:

• Is represented by a concave line which is drawn from just in front of the tragus of the auricle (head of mandible) to the center of mastoid process, around the external auditory meatus.

Anterior border:

• Is represented by a line drawn from just in front of the tragus , till a point in the center of masseter muscle, then to a point 1 cm below and behind the angle of mandible (lower end).

Posterior border:

• Straight line drawn from the center of mastoid process to a point 1 cm below and behind the angle of mandible (lower end), it runs along the anterior border of sternomastoid muscle.

Relations of Parotid Gland:

• The main part of parotid gland has 3 surfaces, 2 ends and 2 borders.

Relations of the surfaces of main part parotid gland:

• Superficial lateral surface: Related to skin, superficial fascia, superficial parotid lymph nodes, and great auricular nerve.
• Anteromedial surface: It is related to ramus of mandible, masseter muscle (laterally) and medial pterygoid muscle (medially).
• Posteromedial surface related to:

1. Two processes: Mastoid & styloid processes and structures attached to it.
2. Two muscles attached to mastoid process: Sternomastoid muscle (superficially) and posterior belly of digastric muscle (deeply).
3. Two blood vessels: Internal carotid artery and internal jugular vein with the last 4 cranial nerves in between.
4. Two structures enter this surface: Facial nerve and external carotid artery.

Relations of borders and ends of parotid gland:

1. Upper end: It is concave and just below the external auditory meatus. Three structures appear at this end: superficial temporal artery and vein and auriculo-temporal nerve.
2. Lower end: It overlaps the posterior belly of digastric muscle. Three structures leave this end : cervical branch of facial nerve, anterior and posterior divisions of retromandibular vein.
3. Anterior border: The following structures appear at the anterior border of the gland from above downwards: Temporal and zygomatic  branches of facial nerve, transverse facial artery, accessory part of the parotid gland and parotid duct, buccal  and mandibular branches of facial nerve .
4. Posterior border : overlap sternomastoid muscle .

 

Structures within the main part of parotid gland: (from superficial to deep)

1. Facial nerve:  Enters the postero-medial surface of the gland close to stylomastoid foramen , It passes forwards and divides into five branches ( T.Z.B.M.C or To Zanibar By Motor Car ) .
2. Retromandibular vein: It is formed within the gland behind the neck of mandible by the union of superficial temporal and maxillary veins , Leaves the lower end of the gland as two divisions (anterior and posterior).
3. External carotid artery:  It enters its posteromedial surface. It ascends vertically deep to the retromandibular vein , It divides behind the neck of the mandible into 2 terminal branches: maxillary and superficial temporal arteries.
4. Deep parotid lymph nodes: They are embedded in the substance of the gland around the blood vessels.
5. Auriculotemporal nerve: It has a short course within the gland.

Nerve supply of the parotid gland:

Parasympathetic:

• The parotid gland receives parasympathetic secretomotor fibers from the glossopharyngeal nerve. They have the following course:

1. The preganglionic fibers arise from the inferior salivary nucleus in the medulla and pass with the glossopharyngeal nerve. Then, the fibers pass in the tympanic branch of the glossopharyngeal nerve which enters the middle ear cavity to form the tympanic plexus.
2. The tympanic plexus gives the lesser superficial petrosal nerve which passes through the foramen ovale to relay in the otic ganglion.
3. Postganglionic fibers join the auriculotemporal nerve which carries them to the parotid gland.

Sympathetic:

• From the sympathetic plexus around the external carotid and middle meningeal arteries.

Sensory:

• From the auriculotemporal nerve.

Blood vessels:

Arteries:

• from superficial temporal artery.

Veins:

• The retromandibular vein.

Lymphatic drainage: 

• The gland is drained into the superficial and deep parotid lymph nodes which finally drain into the upper deep cervical lymph nodes.

Capsules of the parotid gland :

• Parotid capsule is derived from the general investing deep fascia of the neck which split at the lower end of the parotid gland into 2 layers :

    1- Superficial layer :

• attached upwards to zygomatic arch .

    2-Deep layer:

• attached to the styloid process forming the   stylomandibular ligament which separates lower end of parotid gland from the posterior end of the subamdibular gland .

Applied Anatomy:

• Infection of the parotid gland , usually through the duct, causes acute inflammation (acute parotiditis; acute sialoadenitis).
• Acute sialoadenitis of the parotid gland causes severe pain because the gland is surrounded by dense capsule ® marked increase in the pressure inside the capsule and stretch of the capsule ® pain and rapid necrosis, therefore early decompression is needed.
• In acute sialadenitis of the parotid gland, fluctuation is late, after pus penetrates the parotid capsule with major destruction of the gland. Therefore, nerve wait for fluctuation in acute bacterial parotiditis.
• Parotid pain is worse during chewing because the gland is wrapped around the posterior border of the ramus of the mandible and is compressed against the mastoid process when the mouth is opened.
• In general, pain and swelling originating from salivary glands (parotid or submandibular) increases by eating or sucking a lemon, because it stimulates secretion of saliva ® increase in the swelling and stretch of the capsule ® stimulates the sensory nerve fibres in the capsule.
• Parotid pain usually radiates to the auricle, external auditory meatus, temple and tempro-mandibular joint because the auriculo-temporal nerve, from which the parotid gland recieves sensory fibres, also supplies sensory fibres to the skin of the auricle and over the temporal fossa.
• Inflammation or stone of the parotid duct, producing inflammed, red, raising of its papilla opposite the upper 2^nd molar tooth with pus coming out on compression of the duct.
• Normal papilla of parotid duct can be inspected, in bright light, in the inner aspect of the cheek opposite upper second molar tooth.
• Normal parotid duct or stone inside it can be palpated by rolling the thumb and index fingers about a finger breadth below the zygomatic arch along the middle 1/3 of a line between tragus of the auricle and a point midway between ala of nose and the angle of the mouth.
• Parotid salivary gland swelling is characterized by:

1. Single (this differs from L.Ns swellings which are multiple).
2. In characteristic anatomical site i.e below, extending in front and behind the lobule of ear and raising it. It fills the space between the ramus of the mandible and mastoid process.
3. Superficial to masseter and sternomastoid.

• Surgically, the larger external portion of the gland is called the superficial lobe while the smaller internal portion lying on the internal surface of the mandibular ramus is called the deep lobe. 
• The deep lobe is related to the side of the pharynx ® enlargement of this lobe ®  the tonsile is displaced medially.
• Weak or absent superficial temporal pulsation and facial nerve paralysis, indicate malignancy of parotid gland.
• Conservative parotidectomy: (superficial or total).

1. For benign disease in the parotid gland, explore, isolate and preserve the facial nerve with removal of the affected part of the gland.

• Total radical parotidectomy:

1. For malignant parotid disease, the whole gland is removed with scarification of facial nerve which is replaced by great auricular nerve graft.

• Radiologicaly, parotid duct can be visualized by parotid sialography.
• Drainage of parotid abscess by a vertical preauricular skin incision (to avoid injury of superficial temporal vessels and auriculo-temporal nerve) exposing the parotid fascia which is split by closed forceps and opened in a transverse direction (to avoid injury of facial nerve).