The TMJ

Question 1

What are the 4 muscles of mastication?

Answer 1

• Masseter
• Temporalis
• Lateral pterygoid
• Medial pterygoid

Question 2

What does the masseter do?

Answer 2

o Elevates the mandible to close the mouth.
o Fibres run anteriorly so helps to protrude the mandible. It is not good at protrusion, but can assist.
o Sits superficial of the mandible which you can feel on outside of the mouth.

Question 3

What does the temporalis do?

Answer 3

o Can feel this when clenching the jaw. Causes stress headaches in people.

o Has spectrum of fibres. The anterior fibres elevate the mandible when contracting. Posterior fibres point backwards (dorsally) which act to retract the mandible.

Question 4

What does the lateral pterygoid do?

Answer 4

o Has 2 heads: superior and inferior head.

o Superior – acts to control retraction of articular disc. Pulling the capsule and disc forward during mouth opening – to maintain relationship between mandibular condyle and TMJ disc – stabilises condyle during chewing.

o Inferior – acts on mandible to create rotation to allow for opening of the mouth of protrusion. Depresses mandible and protrusion. Lateral/medial movement of the mandible.

o Both have a role in protrusion of the mandible.

Question 5

What does the medial pterygoid do?

Answer 5

o Mainly elevates the mandible.

o Lies parallel to masseter on the deep surface of mandible, felt on the inside of the mouth.

o Has a minor role of protrusion.

Question 6

Where do the muscles of mastication insert on and what are they innervated by?

Answer 6

• All innervated by the mandibular branch of the trigeminal nerve.
• All muscles act on the sphenoid bone (butterfly shape)
• Pterygoid process made of lateral and medial pterygoid plate.
• Pterygoid insert originates on the lateral plate.
• Lateral pterygoid muscles important in TMJ dysfunction.

Question 7

What is the structure of the TMJ?

Answer 7

• The TMJ is a complex synovial joint.
• The condyle of the mandible sits in the mandibular fossa of the temporal bone.
• As the TMJ is a synovial joint, it has a capsule around it. Called joint capsule. Is a fibrous capsule.
• The capsule attaches to the temporal bone superiorly and around the neck of the mandible inferiorly.
• Mandibular fossa also known as glenoid fossa.
• Inside joint capsule is the articular disc which is firmly attached to the condyle.
• Articular tubercle has bony process located on the temporal bone.
• The TMJ is separated into 2 distinct joint cavities (upper and lower) called the superior and inferior cavity. Separated by a fibrocartilaginous articular disc.
• In the inferior cavity is the mandibular head.
• These cavities are very small, but allow for movements of the TMJ.

Question 8

What is the movement of the TMJ?

Answer 8

• Head of the mandible undergoes rotation and antero-posterior translation as the mouth open and closes.
• Change of spaces when we open our mouth.
• First movement is a rotational movement, and we see our coronoid process descends due to rotation of anterior capsule.
• Translation then occurs afterwards, when the mouth is opened equally.
• When we combine this movement, we create a hinge like effect (like in the knees).
• Hinge movements allow depression/elevation of the mandible.
• Gliding movements allow protrusion/retraction of the mandible.
• During protrusion – first element of rotation allows for 20mm space between anterior teeth, further opening needs translation to occur.
• During protrusion of the jaw, translational movement also takes place. The teeth, condyles and rami all move in the same direction and to the same degree.

Question 9

What are the ligaments of the TMJ?

Answer 9

• The joint is most stable when the mouth is closed because we have strong ligaments in place.
• Lateral ligament
• Sphenomandibular ligament
  Stylomandibular ligament

Question 10

What is the joint capsule?

Answer 10

• Joint capsule – a fibrous capsule that is loose above the disc, but is tight below it. Provides a supportive role but is not creating stability in the joint, just holds synovial fluid in place.

Question 11

What is the lateral ligament?

Answer 11

o Strengthens lateral part of the lateral ligament – oop fibres stop the condyle from rotating any further within the mandibular fossa – instead it forces the condyle to jump out the mandibular fossa on to the disc which will have moved anteriorly due to pulling of the lateral pterygoid muscle.

o Provides the most strength.

o Sometimes known as the TMJ ligament.

Question 12

What are the 2 fibres of the lateral ligament?

Answer 12

• Lateral ligament has 2 lots of fibres, inner horizontal fibres on the inside of the joint capsule and the superficial oblique fibres which are on the outside of the joint capsule.

Question 13

What is the inner horizontal part (IHP) of the lateral ligament?

Answer 13

o Prevents excessive posterior displacement (retracting joint).

o In the area posterior to the head of the mandible is some delicate vascular tissue. If crushed, it would compromise the TMJ, so the IHP helps stop this from happening. Don’t want to damage this by retraction.

o Important for stability as well. The condyle head would break before these fibres would.

Question 14

What is the outer oblique part (OOP) of the lateral ligament?

Answer 14

o Can see the OOP is looped around the mandible, so can see why it would move forward when the mouth opens, on to the articular eminence.

o Plays a role in the positioning of the condyle head as the mouth is opened.

o When TMJ put under tension due to the lateral pterygoid, encourage rotation as rotation wants to occur in superior parts.

o Assist in opening mouth, even though they have no active contraction role occurring.

o The limited stretch of the OOP means the axis of rotation moves (from A to B)

o As a result, the head moves forwards on to the articular eminence.

Question 15

What is the sphenomandibular ligament (SML)?

Answer 15

o Spine of sphenoid to lingual of the mandible.

o Related laterally to lateral pterygoid muscle, auriculotemporal nerve and maxillary artery and related medially to chorda tympani nerve and wall of pharynx.

o Near lower end is pierced by mylohyoid nerve and vessels.

o 1st of the accessory ligaments.

o Attaches to lingual of the mandible.

o From the spine of sphenoid to the lingual of the mandible.

o Supports the role of OOP in sliding and prevents excessive inferior displacement.

o Prevents excessively opening the mouth.

Question 16

What is the stylomandibular ligament (STML)?

Answer 16

o Thickened part of deep cervical fascia. It separates the parotid
and submandibular glands. Limits excessive protrusion.

o 2nd of the accessory ligaments.

o From the styloid process to the posterior of the ramus of the mandible

o May limit excessive protrusion of the mandible but the function is unclear.

o Don’t really know what it does. It is tight and tough and strong fibres, but where it originates and inserts hints its limiting protrusion.

Question 17

What is the articular disc?

Answer 17

• Anterior capsular ligament – part of the joint capsule.
• Vascular bundle important for joint integrity.
• Anterior capsule ligament and posterior capsular ligament are part of the joint capsule and allow for actions to take place (rotation and translation) and hold synovial fluid.
• Superior retrodiscal lamina – elastic and important.
• Inferior retrodiscal lamina – inelastic, not as important.
• Retrodiscal tissue – highly vascular. Essential to joint integrity and important for health of the disc.

Question 18

What are the lateral and medial disc ligaments for in the articular disc?

Answer 18

o Looking in (if you slice face off)

o Keep the disc close to the condyle.

o Attach to condyle by lateral and medial discal limits. Keep it close to the mandible rather than the temporal bone as we want it to stick to the head of the moveable part, which is the mandible.

Question 19

What is the superior and inferior articular cavity?

Answer 19

o Superior cavity is much larger than the inferior cavity.

o Translation occurs in the superior cavity

o Translation needs joint to stretch anteriorly during depression from the mouth.

o Rotation occurs in inferior cavity.

Question 20

What are the 9 basic steps of normal opening/closing of TMJ?

Answer 20

1. Rest
2. Inferior head of lateral pterygoid rotates mandible anteriorly
   3, Mandible pushes disc anteriorly during opening by inferior head of lateral pterygoid
3. Superior head of lateral pterygoid relaxed, inferior head contract
4. Mandible head is resting on articular eminence
5. Superior retrodiscal lamina pull disc posteriorly during closing until all elastic tension has been expended
6. Superior head lateral pterygoid contracts to add traction to the articular disc. Inferior lateral pterygoid relaxes
7. Disc snaps in to place on final stage of closing
8. Rest

Question 21

What happens during rest of TMJ?

Answer 21

o Articular disc sat in glenoid fossa

o Condyle in close contact with articular disc.

o Held in place by ligaments

o Everything fits nicely within TMJ capsule, disc in mandibular fossa, heads at rest.

Question 22

What happens when inferior head of lateral pterygoid rotates mandible anteriorly?

Answer 22

o Inferior head inserted into the condyle. Fibres start to contract, creating rotational movement within condyle.

o Lateral ligaments OOP assist due to tension being put on them.

o IHP has rotational movement.

Question 23

What happens when the mandible pushes disc anteriorly during opening by inferior head of lateral pterygoid?

Answer 23

o Tension within inferior head of lateral pterygoid muscle.

o Laxity in superior head (only in closing of the mouth).

o Condyle begins to rotate and push onto articular disc, pushing the disc anteriorly.

o Articular disc moving anteriorly till touching eminence of temporal bone

Question 24

What happens when superior head of lateral pterygoid relaxed and inferior head contract?

Answer 24

o Articular disc move onto the articular eminence of temporal bone.

o TMJ most at risk of dislocation when in this position (open mouth)

o No tension, superior head prevents it shooting back.

Question 25

What happens when the mandible head is resting on articular eminence?

Answer 25

o Condyle going to rest on articular eminence o Sandwich between the articular disc. o Superior lamina has to be elastic to allow for stretching during anterior movement of articular disc. o Full contact with articular eminence.

Question 26

What happens when the superior retrodiscal lamina pulls disc posteriorly during closing until all elastic tension has been expended?

Answer 26

o Starting to close the mouth. o Superior discal lamina super elastic, wants to pull disc back into mandibular fossa due to high degree of tension. o Suring closing, superior head adds tension to create the disc wanting to shoot back, so superior retrodiscal lamina controls the shooting back and keeps it contact with head.

Question 27

What happens whe the superior head lateral pterygoid contracts to add traction to the articular disc and the inferior lateral pterygoid relaxes?

Answer 27

o Contracting to add tension anteriorly to the disc. o Inferior is relaxing because head is beginning to rotate as not under tension anymore. o Control action of disc

Question 28

What happens when TMJ has clicking disorder?

Answer 28

* 1. Mandible head is abnormal position relative to disc (posterior to disc) – Mandible sat posterior to disc on rest. Due to damage to superior lamina disc or tissue. Has lost some degree of elasticity so not pulled into joint on closure. * 2. Normal * 3. Normal * 4. Mandible head snapping back into the correct position on the disc – Articular disc making contact with the eminence, head of mandible jumps onto the articular disc, creating a click on opening. * 5. Normal * 6. Normal * 7. Normal – lateral pterygoid holding the disc anteriorly. * 8. Mandible head about to slip off the disc posteriorly causing a second click. * Clicking can be a precursor to locking if lamina loses it elasticity but doesn’t always happen.

Question 29

What happens when TMJ has locking disorder?

Answer 29

* More problematic than clicking. There is no normal movements in a TMJ that locks. * 1. Mandible head is in an abnormal position relative to disc (posterior to disc). * 2. Articular disc moves anterior to articular tubercle – as mandible starts to open, rotation occurs from later head, disc begins to move anteriorly before we have proportional amount of rotation to push it, pushing it further anteriorly. * 4. Mandible head does not snap back into the correct position on the disc – pushes past articular eminences, compressing the vascular tissue between the head of the mandible (condyle) and the eminence of temporal bone. * 5. Opening of mouth will be restricted. * 8. During closure, there is no normal closure as disc stays anteriorly the whole time to the head of the mandible. * Locking could be due to collagen issue (dysfunction), loose elasticity in disc early on, could be post injury/dislocation, loosing integrity of TMJ, so elasticated superior discal lamina is damaged.

Question 30

What are the 2 jaw reflexes?

Answer 30

- Myotactic (strech, jaw-jerk reflex) - Nociceptive reflex

Question 31

What is the Myotactic reflex?

Answer 31

Stretch, jaw-jerk o Operates continuously to maintain resting position of the mandible o Muscle stretch (for example that caused by gravity pulling the mandible down) – initiates contraction of the muscles of mastication. o Muscle fibres are stretched and closed. o Can illicit this reflex by tapping the chin. It’ll open the mouth and the patient will close their mouth. o Sensory fibres of the tendons of the masseter from the trigeminal nerve travels to pons where they travel up to mesencephalic nucleus, synapse into the motor nucleus and efferent fibres encourages closure of the jaw. o Way of protecting TMJ by keeping it closed.

Question 32

What is the nociceptive reflex?

Answer 32

o Protects teeth. o If we put something into our mouth e.g. a popcorn kernel, it will illicit pain in the teeth, so prevents us biting too hard down. o Pain caused by potentially damaging biting forces. o Synapse to pons, to the nociceptive nucleus (spinal nucleus), sends out to efferent fibres to stop contraction of muscles of mastication and encourages digastric (underneath jaw onto the neck) to contract, to relive the tension of food in the tooth. o Relaxation of the muscles of mastication and simultaneous contraction of digastric (causing opening of the mouth) o Both these reflexes protect the TMJ, but this also protects the teeth.