Anatomical basis of airway management

Question 1

How can you measure adequacy of mouth opening? What is normal?

Answer 1

Interdental distance
* Distance between the free edges of the upper and lower teeth
* Lower 95% confidence limit in young adults is 37mm. Decreases w ith age

Protrusion
* Measured by ability to protrude the lower incisors beyond the upper incisors
* A - in front, B - edge to edge C - not able to touch

Question 2

Temperomandibular joint: anatomy, mechanism of action,

Answer 2

• Formed by head of mandible articulating with mandibular fossa of the temporal bone
• Upper and lower synovial cavity, separated by an articular disc

Affects mouth opening through affecting both interdental distance and mandibular protrusion (ability to move lower jaw forewards)

Mechanism of action: 2 staged
1. Rotation: mandibular head rotates in the **lower articular joint **-> downward movement of mandible
2. Translation: occurs in upper joint, mandibular head moves forwards and downwards -> mandibular protrusion

Question 3

What factors affect IDD?

Answer 3

Mandibular length
* Length of mandible (from angle of mandible to mental protruberance) determines adequacy of mouth opening.
* For a given angle at the TMJ, IDD will be greater as the mandibular length increases

**Craniocervical movement **
* Mouth opening is reduced by craniocervical fixation
* There is a significant difference in IDD with head in extension compared to neutral

**TMJ function **

Question 4

Occipitoatlantoaxial complex

Answer 4

Required for craniocervical extension (required for head-tilt for direct laryngoscopy and contributes to mouth opening)

Comprises:
* Bones: occiput, atlas, axis
* Joints: atlanto-occipital and atlanto-axial
* Ligaments
* ‘Gaps’

Question 5

Joints of occipitoatlantoaxial complex

Answer 5

Atlanto-occipital joint:
* Articular surfaces on the inferior aspect of the occiput articular with the atlas.
* Shape of the atlas is specialised to permit flexion and extension of the occiput on its superior surface.

Atlanto-axial joint
* Between articular surfaces of atlas and axis
* Flexion, extension and rotational movement
* odontoid peg or dens, a cranial protruberance from the axis, occupies the anterior compartment of the atlas. Acts as an axel, allowing rotation of the atlas on the axis

Question 6

Answer 6

Note that the axis is relatively large with substantial ‘lateral masses’ that articulate with the atlas. On lateral radiographs, these are seen as ovoid shadows

Question 7

What are the 5 ligaments of the occipitoatlantoaxial complex

Answer 7

Function: provide strength and stability

1. Tectorial membrane: strong, continuation of posterior longitudinal ligament, runs up to the basilar portion of the occipital bone
   2.** Cruciform ligament**: incorporates the transverse ligament. Attaches to anterior border of foramen magnum, and posterior aspect of body of axis. Transvere ligament holds the dens in the anterior compartment of the atlas
2. Alar ligaments: from the apex of the dens to the occiput close to the condyles. Limit rotation of the head
3. Apical ligament: from apex of dens to anterior margin of foramen magnum
4. **Anteiror and posterior atlanto-occipital ‘membranes’: **complete the attachment of the head to the spine

Question 8

What diseases may cause atlanto-axial instability?

Answer 8

Ligamentous or bony disease of the OAA complex:
* Rheumatoid arthritis
* Down’s syndrome
* Bacterial or TB infection of OAA complex
* Mucopolysaccharidoses

Abnormalities of transverse ligament -> anterior instability (on flexion)
If odontoid peg is deficient the subluxation can be posterior (on extension)

Question 9

Relevance of the atlanto/occipital and atlanto/axial gaps

Answer 9

• Allow for flexion and extension
• If absent (e.g. in osteoarthritis) - range of motion will ilkely be reduced
• If occipito-atlanto and atlanto-axial gaps are absent, high probability of difficult direct laryngoscopy

Question 10

Draw a simple diagram of the OAA complex

Answer 10

Notes
* Spine of C2 is ‘squarer’ than other cervical vertebrae
* Gaps visibel between occiput and posterior elements of the atlas and axis

Question 11

How can you measure craniocervical extension?

Answer 11

Wilson method: pen placed at right angles to the forehead should sweep an arc of ≥90˚ between flexion and extension

Interdental distance: if reduced, poor craniocervical movement may be present

Note craniocervical movement may be poor, but appear normal due to compensatory extension of lower cervical vertebrae

Question 12

Answer 12