TMJ Biomechanics

Question 1

Which 5 movements are available at the TMJ?

Answer 1

Depression
Elevation
Protrusion
Retrusion
Lateral excursion

Question 2

What are the osteokinematic movements that occur at the TMJ?

Answer 2

Mandibular condyle ROLLS on inferior surface of disc in rotational movements

• early phase depression
• late phase elevation

Mandibular condyle & disc SLIDE together in translational movements
(called condyle-disc translation)
- protrusion & retrusion
- lateral excursion
- late phase depression
- early phases elevation

Question 3

Describe depression of the TMJ

Answer 3

Two phases:

• early phase (rotation) - 35-50%
• late phase (translation) - 50-65%

Early phase:
rotation of mandibular condyle on inferior surface of disc

Late phase:
translation of mandibular condyle & disc on articular eminence of temporal bone

Main force is gravity
Eccentric control from masseter, temporalis, lateral pterygoids

Question 4

Describe elevation of the mandible

Answer 4

A reversal of the phases of depression:

early phase - primary translation (50-65%)
late phase - primary rotation (35-50%)

Early phase:
condyle & disc glide posteriorly & superiorly against articular eminence

Late phase:
mandible rolls posteriorly within inferior surface of disc

Muscles:

• temporalis
• masseter
• medial pterygoid

Question 5

Describe protrusion and retrusion

Answer 5

Protrusion (translating mandible anteriorly)

• mandible & disc glide (translate) anteriorly & slightly inferiorly in temporal fossa
• pterygoids

Retrusion (translation mandible posteriorly)

• mandible & disc glide (translate) posteriorly & slightly superiorly in temporal fossa
• posterior fibres temporalis

Question 6

Describe lateral excursion

Answer 6

Condyle & disc slide laterally in fossa

Ipsilateral mm:

• masseter
• temporalis

Contralateral mm:
- both pterygoids

Question 7

In which TMJ movements does rolling (rotation) occur, and in which does gliding (translation) occur?

Answer 7

The only time rolling (rotation) occurs is in early phase depression (35-50%), and late phase elevation (35-50%).
The mandibular condyle rolls within the inferior surface of the articular disc.

In all other movements the mandibular condyle and disc glide (translate) together against the articular eminence (late phase depression) or against the temporal fossa.

Question 8

What is the expected ROM in depression and lateral excursion?

Answer 8

Depression - 50mm (2-3 knuckles)

Lateral excursion - 11mm

Question 9

What is the role of the articular disc in the late phase of depression?

Answer 9

The intermediate surface of the disc translates forwards with the mandibular condyle & remains between the condyle & the articular eminence of the temporal bone.

This reduces intra-articular stress and maximises joint congruency.

When this does not happen, there is a dysfunction called condyle-disc derangment. this can be without displacement (clicking occurs as condyle clicks on and off disc) or with displacement (disc remains displaced anterior to articular eminence)