SAS 11: PART II

axis location

can be determined using a kinematic face-bow or arbitrary hinge-bow

3 classification of jaw relations

orientation relations

vertical relations

horizontal relations

orientation relations

necessary to do before carrying out other jaw relation

this relation gives the angulation of the maxilla in relation to the base of the skull

jaw relation that orients the mandible to the cranium allowing rotation in the sagittal plane around the transverse axis in its most posterior position

vertical relations

represents the length of the face

jaw relation that establishes the amount of jaw separation allowable for dentures

horizontal relations

jaw relation that establishes the front-to-back and side-to-side relationship of one jaw to the other

three-dimensional movement of mandible

vertical axis → lateral rotation

horizontal / hinge axis → opening/closing

sagittal axis → orbiting condyle movement during lateral excursions

rotation around the horizontal (hinge) axis

produces opening and closing movements of the mandible

can occur in any degree of protrusion as well as in centric relation

rotation around the vertical axis of one condyle

produces lateral rotation of the mandible in the horizontal plane

results in lateral excursion toward the side of the rotating condyle

[ rotating (working) condyle → the condyle around which rotation occurs ]

lateral excursions of the mandible

the orbiting condyle travels downward and forward

produces rotation around a second horizontal axis called the sagittal axis → intersects the rotating condyle

[ orbiting (non-working) condyle → the opposite condyle ]

2 types of vertical jaw relations

vertical relation of occlusion (VRO/VDO)

vertical relation of rest position (VRP/VDR)

vertical relation of occlusion (VRO/VDO)

length of the face when the teeth are in contact

used as a guide to restore lost vertical relation of occlusion

• establishment:

  • natural teeth → by the teeth in occlusion

  • dentures → by the vertical height of the two dentures when teeth are in contact

vertical relation of rest position (VRP/VDR)

longer than the VDO

length of the face when the lower jaw is relaxed

determined by muscles and gravity, not teeth

physiologic rest position

all muscles that open the jaws are in minimal tonic contraction sufficient to maintain posture

difference between VRO and VRP

interocclusal distance or freeway space

interocclusal distance (freeway space)

essential for health of periodontal tissues

distance or gap between upper and lower teeth when mandible is at physiologic rest

absence of freeway space may cause:

clicking during speech

soreness of basal seat tissues

rapid destruction of residual alveolar ridges

normal distance of freeway space (interocclusal)

2-4mm vertically → observed at 1st premolars

centric relation (centric jaw relation)

basic horizontal relationship

most posterior position of the mandible to the maxillae at the established vertical relation

eccentric relations

other horizontal relations

deviations from centric relation in a horizontal plane

• protruded relation

• right, left lateral relations

• intermediate relations

movements of the mandible

complex, varies among individuals and within the same individual

• occurs during:

  • mastication, speech

  • swallowing, respiration

  • facial expression

• parafunctional movements:

  • bruxism, clenching → may cause pain or pathosis

factors that regulate jaw motion

influence of the TMJ

axes of mandibular rotation

influence of opposing tooth contacts

muscular involvement in jaw motion

neuromuscular regulation of mandibular motion

influence of opposing tooth contacts

occlusion of opposing teeth guides movements

condylar movement is not limited solely by TMJ anatomy

teeth should meet evenly on both sides in centric and eccentric positions

• prevents:

  • deflection of mandible from normal path

  • displacement of dentures from residual ridges

muscular involvement in jaw motion

temporal and lateral pterygoid muscles

• are primarily involved

• clinically significant for establishing jaw relations

influence of the TMJ

• each TMJ has two compartments separated by the articular disk:

  • upper compartment → primarily translation

  • lower compartment → primarily rotation

3 axes of mandibular rotation

vertical

sagittal

transverse

[ axes move constantly during normal function ]

neuromuscular regulation of mandibular motion

muscles that move, hold, or stabilize the mandible receive impulses from the central nervous system

jaws slightly opened

hinge → rotation

hinge action predominates

jaws widely opened

gliding → translation

hinge and gliding actions combined

face-bows

it orients casts in the same relationship to the opening axis of the articulator

a caliper-like device that records and transfers the jaw relationship to the TMJs

enables mounting of the maxillary cast on the upper member of the articulator in the same relationship as the maxilla to the glenoid fossa

use of face-bows

helps prevent errors in denture occlusion

records the relationship of the maxilla to the glenoid fossa

orients the casts on the articulator in the same relationship as the patient’s jaws

2 types of face-bows

arbitrary face-bow

kinematic (hinge) face-bow

arbitrary face-bow

fork is attached to the maxillary occlusion rim

condyle rods placed approximately over the condyles

widely used type of facebow and are sufficient for fabrication

uses approximate points on the face as the posterior points and condylar rods are positioned on the points

facia type facebow

this facebow takes it name from the fact that it rests upon the face like the kinematic bow

earpiece type facebow

this type of facebow uses the external auditory meatus as an arbitrary reference point which is aligned with ear pieces

kinematic (hinge) face-bow

fork is attached to the mandibular occlusion rim

opening axis of mandible can be located more accurately

allows for the precise determination of the patient’s hinge axis (terminal hinge axis)