Bài giảng nha khoa Mandibular movements and positions

CLASSIFICATION OF THE MANDIBULAR MOVEMENTSAccording to the form of movementsRotation movement: the act of turning about an axis or a center.. Stationary axis: the axis remains in the sam

§MANDIBULAR MOVEMENTS and POSITIONS

People’s Teacher

Prof Hoàng Tử Hùng, DDS, PhD E: tuhung.hoang@gmail.com

W: hoangtuhung.com

MANDIBULAR MOVEMENTS

1 CLASSIFICATION OF THE MANDIBULAR MOVEMENTS

According to the form of movements

Rotation movement: the act of turning about an axis or a center

Stationary axis: the axis remains in the same position during the movement Momentary axis: the axis shifts its position during the progress of the movement

Translation movement: movement of the mandible in which all parts move in the same direction at the same speed

According to the form of movements

1 CLASSIFICATION OF THE MANDIBULAR MOVEMENTS

According to the form of movements

HoangTuHung.com

According to the direction of movements

Opening and closing: symmetric or asymmetric

Forward and backward

Symmetric or asymmetric With or without contact between the upper and lower teeth

(contact or gliding movement)

Lateral gliding movement

1 CLASSIFICATION OF THE MANDIBULAR MOVEMENTS (cont’d)

According to the kinesiology of occlusion

1 CLASSIFICATION OF THE MANDIBULAR MOVEMENTS (cont’d)

Border movements: the most extreme position to which the jaw is able to move

Contact movements: the movement in which the upper and lower teeth

maintain contact (gliding or sliding movement)

Free movements (intra-border mov.): the mandibular movements in which a given reference point does not reach its border and the teeth do not come into contact

HoangTuHung.com

MECHANICS OF CONDYLAR MOVEMENT

Nature of condyle-disc assembly in mandibular movements

The structure of two synovial compartments facilitates

rotational and translational movements of the condyle disc assemblies

The disc generally moves together with the condyle and

can slide forwards and backwards over the condyle

The loose retrodiscal tissue is seated posteriorly to the

disc, filled the temporal fossa and moved forwards and

backwards toghether with the disc

2 BORDER MOVEMENTS AND POSITIONS OF THE MANDIBLE RECORDED IN THE SAGITTAL PLANE (POSSELT’S DIAGRAM )

If the mandible is held back by either patient or operator

and carries out [guide] the act of opening and closing:

- a hinge movement can be performed

- the incisive point*trace the arc from CR to B (distance

between the incisal edges is about 20 – 25 mm)

This movement is called the terminal hinge movement

The position of condyles is named

‒ Terminal hinge position

‒ Centric relation (CR)

‒ Retruded contact position

The posterior opening border movement

Mandible movement maintains around the horizontal stationary rotation axis (hinge axis) through the two condyles (C)

*The midpoint between the two incisal angles of lower central incisors HoangTuHung.com

mesio-If an attempt is made to open the jaw further below B:

- the condyles move forward and downward on posterior

slope of the articular eminence (translation movement)

- the axis of rotation is moved downward and forward,

about at D

- the incisive point moves down to E (extreme or maximal

opening), the distance between the incisal edges is about

50 – 60 mm)

The diagram recorded in sagittal plane of posterior border movement

is a diphasic curve: upper and lower arcs concave backward

The posterior opening border movement (cont’d)

The protrusive closure movement

The movement of protrusive closure from maximum opening (E) stops when the posterior teeth make contact at F (protruded contact position) The F is located 10 – 12 mm anterior to CR

The diagram is recorded a curve concave backward

This movement has slight practical value only

HoangTuHung.com

The propulsive gliding movement

The propulsive path from retruded contact position (CR) to protruded contact position (F):

− an irregular path due to tooth guidance

− the shift of the incisal point is about 10 – 12mm

passes from CR through:

− (maximal) intercuspal position (IP or MIP),

− edge-to-edge position, and

ends in the protruded position (F)

Note: Tooth-guided protrusive, lateral, and laterotrusive movements are called

”excursive movements” or ”eccentric movements”

INCISAL GUIDANCE AND CONDYLAR GUIDANCE

Voluntary protrusion from MIP is guided:- anteriorly by the incisal guidance, and

- posteriorly by condylar guidance

Incisal guidance (or anterior ̴): the palatal inclines of the

maxillary incisors guide the incisal edges of lower incisors

In natural dentition, contact may occur on centrals, on

centrals and laterals, and on centrals, laterals and

canines

Condylar guidance: Protrusive condylar guidance is

guided by the distal inclines of the temporal eminence as

the condyles-disk assemblies move down their inclines

HoangTuHung.com

Border Movements of the Mandible Recorded in the Horizontal plane:

the Gothic Arch or Gysi’s tracing

If mandible in centric relation (CR), as the mandible moves

in retrusive lateral excursion to the left, the incisive point

records the line CR – L

From L, mandible can be move forward and medially to F

A similar tracing can be record from CR for the right side

(CR – R and R – F)

As mandible move straight anteriorly from CR, the slide

movement tracing the line CR - F that was seen in the

propulsive gliding movement recorded in sagittal plane

CR

L R

F

MIP

BORDER MOVEMENTS IN THE HORIZONTAL PLANE

These border movements give a rhomboid figure

The lateral movement of the mandible starting from retruded contact position:

• The side to which the mandible moves is named

“working side”; the condyle of this side is

“working condyle”

• The opposite side is the “non-working side”; the

condyle of this side is “non-working condyle”

BORDER MOVEMENTS IN THE HORIZONTAL PLANE (cont’d)

− a turn (or swing) to one side

− an asymmetric rotation combined with a slight translation movements

Theoretically, lateral movement could involve a rotation

around static vertical axis at the working condyle

Really, working condyle will shift laterally (Bennette movement)

HoangTuHung.com

Working condyle

Non-working condyle

Working side

Bennette angle: When a mandible move laterally, the

non-working condyle moves down, forward and inward

The angle that non-working condyle makes with the

sagittal plane when projected perpendicularly on

horizontal plane

Bennette movement and Bennette angle

Bennette movement (lateral shift, side shift, Bennette shift):

the lateral shift of mandible toward working side during

lateral excursion

It is measured by the distance that working condyle moves

laterally (may have a straight or retrusive, or protrusive in

direction); the average of bennette movement measured in

the horizontal plane: ≈ 0.5 - 1.5 mm

Working condyle

Non-working condyle

BORDER MOVEMENTS IN THE FRONTAL PLANE

In the frontal view, border movements from MIP generally have shield-shaped pattern which consist of four components

• Left lateral superior border movement

• Left lateral opening border movement

• Right lateral superior border movement

• Right lateral opening border movement

Centric relation (acronym: CR) a maxillomandibular relationship, independent of tooth contact , in which the condyles articulate in the anterior-superior position against the posterior slopes of the articular eminences ; in this position, the mandible is restricted

to a purely rotary movement; from this unstrained, physiologic, maxillomandibular

relationship, the patient can make vertical, lateral or protrusive movements; it is a

clinically useful, repeatable reference position (Glossary of Prosthodontic Terms, 2017)

Centric Relation Definition

Centric relation is the relationship of the mandible to the maxilla when the

properly aligned condyle-disc assemblies are in the most superior position

against the eminences irrespective of vertical dimension or tooth position

(Dawson, 2007)

HoangTuHung.com

Centric relation (CR) has been a controversial subject in dentistry for more than a century

Condyle-disc assembly in centric relation

In centric relation , the disc is positioned on top of the condyle

at the most forward position that posterior ligament allows

“The condyles articulate in the anterior-superior position

against the posterior slopes of the articular eminences” (GPT*

2017)

Centric Relation Definition (cont’d)

Centric relation is the relationship of the mandible to the

maxilla when the condyle-disc complex are properly

aligned against posterior slope of the eminences in

anterior-superior position i.e., condyles are related to

posterior slopes with the intermediate zones*of the discs

interposed

*Intermediate zone is the thinnest, avascular, noninnervated part

Terminal Hinge Movement and Centric Relation

centric relation (CR) position: the position of condyles from which a hinge axis movement can occur

− Centric relation is charactered by the properly aligned

condyle-disc assembly and are positioned in their

superior anterior position in the articular fossae

The condyles rotate against the posterior slope of the

articular eminence in the lower joint compartment

CR is not related to the teeth!

− When the jaw muscles are completely relaxed, the

mandible can be guided to open and closed in a terminal

hinge (hinge axis movement) which is a pure rotation

movement without translation of the condyles (the

terminal arc of rotation [CR – B – CR])HoangTuHung.com

CR is stable position and is the key reference position for analysis, occlusal contact evaluation and rehabilitation or reconstruction of masticatory system

In CR (terminal hinge axis of the mandible),

the opening and closing movements are

- reproducible,

the relation between upper and lower jaws can

- recordable, and

- transferable (to the articulator)

Semantically, the term “centric relation” describes

an arc of rotation of the mandible

Terminal Hinge Movement and Centric Relation: Practical Aspect

CR is not the optimal functional endpoint of occlusal closure with maximal occlusal contact (maximal intercuspal position)

CR has to be determined by the operator without patient muscles participation

Terminal Hinge Movement and Centric Relation: Practical Aspect

CR: the only “centric” that reproducible and stable with or without teeth present

(independent of tooth contact)

Retruded contact position:

- the closing end point of the retruded border movement (terminal hinge mov.)

- initial contact of a tooth or teeth during closure around a transverse horizontal

axis

Teeth commonly make contact in MIP during both chewing and swallowing

CR is a functional border position reached chiefly during swallowing and occasionally during chewing

Centric Occlusion

Centric occlusion (CO): the contact of opposing teeth when the mandible is in

centric relation; this may or may not coincide with the maximal intercuspal position

In normal condition of masticatory system, a stable bilateral distribution of tooth contacts is essential requirement for a physiological occlusion at centric occlusion

In ≈ 10% person with natural, healthy, and harmoniously functioning masticatory

system, the centric occlusion and maximal intercuspal position coincide

Like MIP, CO is not considered to be a stable position: CO is changed or lost with teeth

In ≈ 90% of intact dentition in a normal population,

from the retruded contact position, if the teeth are

squeezed together, the mandible slide 1 – 1.5 mm

forward and upward to reach the MI position (“slide

in centric”

Slide in Centric

Slide in centric occurs between mesial inclines of

the maxillary tooth (teeth) in contact against the

distal inclines of the opposing tooth (teeth)

HoangTuHung.com

Retruded Contact Position

Maximal Intercuspal Position (MIP)

In ≈ 10% of intact dentition in a normal

population, slide in centric is not presence when

the MIP coincides with CO

MIP Coincides with CO (≈ 10%) [condyles in CR]

The characteristic of the maximal intercuspal position

3- The cusps of the mandibular and maxillary teeth to interrelate maximally, i.e., the

most closed position → the smallest occlusal vertical dimension (OVD)*

1- A tooth-to-tooth determined relation of the jaw which is guided by the occlusal surfaces

of the teeth (“tooth-guided” position)

4- A subject to change gradually as a result of alterations in occlusal surfaces by reason

of tooth wear, caries…

2- There is maximal contact between the upper and lower occlusal surfaces

→ create the maximum mechanical stability for mandible (the power position of mandible)

(Physiologic) Rest Position

When the head is positioned erect and upright, the physiologic rest position

(postural of mandible) is located 2 – 4 mm below the MIP

Physiologic rest position is indicated that the mandibular musculature is in a state of

minimal tonic contraction , to maintain posture and to overcome the force of gravity.

Interocclusal rest space or freeway space: the distance between occlusal vertical dimension (ODV) and rest vertical dimension (RVD)

Reference Positions

Reference position is a position with which other positions of the mandible can

be compared

The Important Reference Positions:

- Positions on the terminal hinge path (“horizontal reference positions”)

- Postural position (“vertical reference position”)

These reference positions are required appropriate application in full-mouth rehabilitation cases

3D envelope of movement

The three-dimensional space circumscribed by border movements

(including tooth-to-tooth contacts) of a given point of the mandible is named “the movement space”, or “ envelope of mandibular motion ”

Although each of points on the mandible has its own

envelope movement space, the border movements of

incisive point produce the largest movement space which

can be the best illustrated the capacity for movement and

movement space of the mandible

HoangTuHung.com