Sunday, 19 October 2014

A Guide to Applied Anatomy in Dentistry

My latest study day this friday was with Dr Apollonius Allen from KCL about anatomy and pain control in dentistry. 
Having studied the anatomy part of the BDS in our first year, I was pretty rusty to say the least! Here is a summary of what I learn from the day. 

Of the 12 cranial nerves, the ones that us as dentists are most concerned about are the Trigeminal (V) nerve and the Facial (VII) nerve. 

There are 4 parasympathetic ganglion in the head:
  1. Pterygopalatine (where the greater palatine nerve originates from)
  2. Otic
  3. Submandibular
  4. Ciliary 

The Trigeminal Nerve (V)

The Trigeminal nerve is the most important nerve in dentistry: this is the nerve that you will anaesthetise when performing dental treatment. 

The nerve originates from the PONS and splits after the trigeminal ganglion into its 3 parts: V1 (opthalmic division), V2 (maxillary division) and V3 (mandibular division).

V1 exits the cranium through the SUPERIOR ORBITAL FISSURE
V2 exits the cranium through the FORAMEN ROTUNDUM
V3 exits the cranium through the FORAMENT OVALE

Other structures that go through the Foramen Ovale:
  • Accessory Meningeal artery
  • Lesser Petrosal nerve
  • Emissary veins

The anterior branch V3 is mainly motor whereas the posterior branch is mainly sensory

Anterior branch: nerve to temporalis, nerve to masseter, nerve to lateral pterygoid and the sensory long buccal nerve.
Posterior branch: lingual nerve, inferior dental nerve, auriculotemporal nerve and the motor nerve to mylohyoid.

Two nerves also come off the main trunk of V3: nerve to medial pterygoid and the tensor nerves (tensor tympani and tensor palatini)

Since V3 is mainly sensory you can give bilateral IDBs, contrary to what most of us have learnt at dental school. 
The reason we were given was because there is a risk of the patient swallowing the tongue, however, since the motor supply to the tongue is by the hypoglossal nerve (XII) which is not affected by the IDB, there is no chance of this actually happening. 

The Facial Nerve (VII)

This nerve emerges from the cranium through the stylomastoid foramen and it has 5 branches:
  1. Temporal
  2. Zygomatic
  3. Buccal
  4. Mandibular
  5. Cervical

This nerve supplies the motor function to the muscles of the face which can become apparent during Bell's palsy or when your IDB infiltrates the parotid gland.
A lot of the time this is due to incorrect operator error i.e. not hitting bone when giving the block, however, if the patient has an accessory lobe of the parotid gland which loops into the pterygomandibular space, this will be anaesthetised when you deposit the LA into the correct space. 

Paranasal Sinuses

There are 4 Paranasal Sinuses:
  1. Maxillary (this is only sinus that is present at birth)
  2. Ethmoidal
  3. Sphenoidal
  4. Frontal
The function of sinuses include:
  • They lighten the skull
  • They are lined with cilia to filter the air we breathe
  • They warm the air we breath
  • They create resonance for your voice
At dental school we were taught that if a root of an upper tooth (usually molars) fractures during an extraction, if it is within the apical third, you should leave it in situ as there is a risk of dislodging the root into the antrum if you try to remove it. 
Dr Allen taught us a novel method to help remove root remnants:
  • Clean the area so that it is free of blood and saliva
  • Locate the canal on the tooth you have removed and determine whether the canal is patent
  • If you can locate the root canal, use a size 20 K file to engage the root (insert and turn a quarter clockwise) and gently pull out.

The Nose

The nasal septum has 3 components:
  1. Septal cartilage
  2. Perpendicular plate of the ethmoid bone
  3. Vomer
Most of the paranasal sinuses drain into the Middle meatus in the nose except from the sphenoidal sinus and the posterior ethmoidal sinus.
  The sphenoid sinus drains into the Suprameatal recess.
  The posterior ethmoidal sinus drains into the Superior meatus.

The Nasolacrimal duct also drains into the nose, into the inferior meatus.

Turbinate bone is the term for the mucous membrane lined conchae.
A TURBINECTOMY can be performed for patients who suffer from chronic sinusitis or have difficulty breathing.

What causes nosebleeds?
Bleeding from the anastomoses of the intra and extracranial blood vessels. Several things can cause these to bleed:
  • Severe hypotension
  • Cold or dry air
  • Vigorous nose picking (yuck!)
  • High blood pressure
  • Blood dyscrasias
So how would you manage a nosebleed? 
Pinch the nose and lean forward to prevent blood from being swallowed (which could cause vomiting)

The Skull

There are 28 bones in the head:
  8 of these are in the cranium
  14 of these are facial bones
  3 are ossicles, the tiny bones in your ear

The Cranial Bones:
  1. Occipital 
  2. Sphenoid
  3. Frontal (this bone ossifies as 2 separate bones)
  4. Ethmoid
  5. Temporal x 2
  6. Parietal x 2
The Facial Bones:
  1. Nasal x 2
  2. Lacrimal x 2
  3. Zygoma x 2
  4. Palatal x 2
  5. Maxilla x 2
  6. Inferior concha x 2
  7. Vomer
  8. Mandible
The Ossicles:
  1. Stapes
  2. Malleus
  3. Incus

When you are born, your cranial bones (and your mandible) are not fused together, they are joined with cartilage: anterior and posterior fontonelles.
The anterior one fuses at 18-24 months and the posterior one fuses at 4-5 months old. If they do not fuse properly a Metopic Suture is formed. 

When you are born you also do not have a Mastoid process - this forms when the muscles for your neck start to develop i.e. when you can independently hold your head up, since the sternocleidomastoid muscle pulls on the bone to form the process. 

I hope you have found this summary useful! Please leave any comments or questions in the section below!

Why not take a look at my other Clinical Guide posts?

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