Principles of occlusion

Question 1

Class I

Answer 1

Most people

Question 2

Why bother with occlusion

Answer 2

Unstable occlusion–>

• tooth wear
• fractured cusps/ restorations
• localised periodontitis
• pulpitis
• drifting teeth
• failure of crowns and bridges
• failure of endodontic treatment
• cracked cusp syndrome

Question 3

When restoring dentition your aim should be to achieve

Answer 3

1. contacts on all teeth in ICP
2. anterior guidance will create posterior disclusion
3. use canine guidance where possible to provide lateral guidance (otherwise group function)
4. if occlusion acceptable, restore at existing height (OVD) and position (ICP) = conformative
5. if ICP unacceptable you have to use centric relation = reorganised

Question 4

When restoring dentition your aim should be to achieve

Answer 4

1. contacts on all teeth in ICP
2. anterior guidance will create posterior disclusion
3. use canine guidance where possible to provide lateral guidance (otherwise group function)
4. if occlusion acceptable, restore at existing height (OVD) and position (ICP) = conformative
5. if ICP unacceptable, unavailable, unreproducible you have to use centric relation = reorganised

Question 5

ICP

Answer 5

Intercuspal position

Position of mandible in relation to maxilaa when teeth are maximally intercuspated

Question 6

Centric relation

Answer 6

Relationship between mandible and maxilla, when properly aligned condyle-disc assemblies are in most superior position against eminenta
Allows approx 25mm opening of pure hinge movement.

Question 7

Terminal hinge axis or retruded hinge axis

Answer 7

Imaginary line between mandibular condyles around which mandible rotates when mandible is in CR
Allows an opening of approx 25mm before condyles move down eminentia, when opening

Question 8

Terminal hinge axis or retruded hinge axis

Answer 8

Imaginary line between mandibular condyles around which mandible rotates when mandible is in CR
Allows an opening of approx 25mm before condyles move down eminentia, when opening

Question 9

Retruded contact position

Answer 9

RCP is first tooth contact when mandible is closed in CR

One point within CR arc

Question 10

Significance of RCP

Answer 10

RCP often involves only 2 opposing teeth - these carry all the load of occlusion when px slides from RCP to ICP or from ICP to RCP
If these are un-restored sound teeth, there is uaully no problem, however if heavily restored or root treated, then failure of restorations likely

Question 11

Border movements in a sagittal plane

Answer 11

Posselt described envelope of border movement in sagittal plane
These envelopes of movement are tools which may be used to help diagnose or describe problems of TMJ dysfunction

Question 12

Forward (protrusive) movement

Answer 12

Anterior guidance (mandible)
Pathway taken by lower teeth is guided anteriorly by teeth and condyles
Normally involves anterior teeth but may involve posterior teeth, e.g. anterior open bite or edge to edge class III
Determined by amount of overjet and overbite

Question 13

Condyles in forward movement

Answer 13

Move forwards and downwards on eminentia
Gives rise to condylar angle: angle between occlusal plane and path taken by condyles during protrusion
Average value 30degrees

Question 14

Working and non-working sides

Answer 14

Working side: side to which mandible moves during chewing. As mandible moves to left, left canines function or work, therefore this is working side (and vice versa)
Non-working side: opposite side. As mandible moves to left, no teeth function on right therefore this is non-working side (and vice versa)

Question 15

Lateral excursions

Answer 15

Can be guided by canines or group function

Restoring palatal surface of canine involved in canine guidance can alter guidance and increase load on tooth

Question 16

The ideal occlusion

Answer 16

Forces through axis of teeth-cusp to fossae
Disclusion of posterior teeth in eccentric excursions
Mutual protection
RCP = ICP

Question 17

Forces on teeth

Answer 17

Contact - cusp tip to fossa results in axial forces

Contact on incline results in destructive horizontal forces

Question 18

Posterior disclusion

Answer 18

Any movement away from ICP should ideally result in disclusion of posterior teeth
This results in avoidance of destructive forces on inclines of teeth

Question 19

Why should we have canine guidance in lateral excursions?

Answer 19

Morphology - very strongh tooth with long roots
High degree of proprioception to prevent excessive loading
Distant from hinge of mandible
Distant from powerful muscles of mastication

Question 20

Mutual protection

Answer 20

In ICP all teeth should be in contact, posteriors should contact firmly but anterior teeth should only contact lightly
Most force of occlusion is carried and distributed by posterior teeth in axial direction thus: the posterior teeth protect the anterior teeth
In protrusive excursion, anterior teeth protect posterior teeth

Question 21

Why can anterior teeth take reduced load during protrusive excursion?

Answer 21

Furthest away from hinge of mandible and so force exerted is limited
Distant from masseter and therefore force exerted is limited
Highly innervated resulting in brain limiting force exerted on teeth

Question 22

Why RCP = ICP best?

Answer 22

Optimum position for muscles
If not, we want as many contacts in RCP as possible to share occlusal load
Minimal slide RCP - ICP
Earliest contacts should be balanced or at least be on strong teeth, so occlusal load can be tolerated by teeth involved