Factors of mandibular movement related to occlusal morphology



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Factors of mandibular movement related to occlusal morphology
–the doctor said that all the information in this lecture, we know them from the prosthodontics course so it will be like a revision and she’ll mention the important details that she want us to know from the occlusion course-
Today well discuss the following topics :

*Introduction and definitions

*Static occlusion Angle’s classification of occlusion and malocclusion

Intra- and Inter- arch tooth alignment




*Dynamic occlusion Occlusal contacts during mandibular movements.


*Control of mandibular movements Anterior controlling factors.

Posterior controlling factors.



Introduction and definitions
Supporting and non-supporting cusps..

For each posterior tooth we’ve supporting (functioning) cusps which come in contact with the opposing fossae during occlusion. And the non-supporting cusps (non-functioning) cusp which in normal dentition are free of occlusion


-- Supporting cusps : The palatel cusps of the upper posterior teeth

The buccal cusps of the lower posterior teeth.

-- Non-supporting cusps: The buccal cusps of the upper posterior teeth

The lingual cusps of the lower posterior teeth.


Non-supporting cusps BULL ( Buccal of Upper and Lingual of Lower)

Now, the points of contacts of the supporting cusps with the opposing teeth should be well established and stable. These contact areas are called centric stops, they’re important for :



  1. Occlusal stability which is maintained by axially directed forces which are forces applied against the centric stops.

  2. Mastication.

  3. Maintaining the vertical facial height.


Guiding Cusps..

Which are the non-supporting cusps that we previously mentioned, Or they may be called non centric cusps.

They are relatively sharp with non definite tip that are located 1/6th the distance into the total BL width of the tooth
**remember from the dental anatomy course --- the functioning cusps are located 1/3rd the distance into the total BL width of the tooth.. and the non functioning cusps are located 1/6th the distance into the total BL width of the tooth. **
The importance of the guiding cusps :


  1. Minimize tissue impingement, How?

They retract the cheeks and tongue away from the area of function –mastication-

  1. Maintain the bolus of food on the occlusal table for mastication.

  2. give the mandible stability so that when teeth are in full occlusion there is a tigh definite occlusal relationship.


Guiding inclines..

They’re the planes of occlusal ridges that determine the path of the supporting cusps during normal lateral and protrusive working excursions.

These are the lingual inclines of the buccal cusps of the maxillary posterior teeth, the lingual inclines of the maxillary anterior teeth, and the buccal inclines of the lingual cusps of mandibular posterior teeth.
Incisal guidance..

The influence of the lingual surfaces of maxillary anterior teeth on mandibular movement. It may be expressed in degrees with the horizontal plane.


Condylar guidance angle..

The rate at which the condyle moves away from a horizontal reference plane.


Cusp Angle..

The angle that is formed by the slopes of the cusp with a plane that passes through the tip of the cusp and is perpendicular to a line bisecting the cusp.


The plane of occlusion..

Is an imagionary plane that touches the incisal edges of mandibular central incisors and the tip of the DB cusps of mandibular 2nd molars


Curve of spee..

The curvature of the occlusal surfaces of teeth from the tip of the mandibular canine and follows the buccal cusps of all mandibular posterior teeth.


Curve of Wilson..

When observing the dental arches from the frontal view, the maxillary posterior teeth have a buccal inclination and the mandibular posterior teeth have a lingual inclination.

Bonwill describes the dental arches and noted that an equilateral triangle existed between the centers of the condyles and the mesial contact areas of mandibular central incisors, which has a 4 inch side.
Monson utilized Bonwill’s triangle and proposed a theory that a sphere existed with a radius of 4 inch whose center was an equal distance from the occlusal surfaces of posterior teeth and from the centers of the condyles.

Classification of occlusion

Notall the human beings have the same occlusion; there are a lot of differences in occlusion schemes.

The most widely used is Angle’s classification of occlusion and malocclusion. In this classification we have three classes ( Class I, II and III )

The problem with Angle’s classification is that it doesn’t consider TMJ position into the classification.


Angle’s class I is considered as the normal relationship of the arches of teeth.
Class II and III are considered as deviation from the normal.
Angle’s class II

The mandibular arch is too distal (small) or the maxilla is too large or in combination of both


Angle’s class III

The mandible is forward (large) or the maxilla is too small or in combination of both


The differences in the arches sizes affect the positioning of the teeth in relation to each other (upper and lower).. So you have to memorize the teeth position in each class as we’ll mention next,
Angle’s class I :

* Relation between the arches

Normal relation.

* Relation between the upper and lower 1st molars:

--T e MB cusp of the mandibular 1st molar occludes with the embrasure area between maxillary 2nd premolar and 1st molar

--The MB cusp of the maxillary 1st molar is aligned over the buccal groove of mandibular 1st molar. (( Aligned not occludes remember it’s a non supporting—free of occlusion in normal dentition.))

-- The ML cusp of the maxillary first molar occludes with the central fossa area of mandibular 1st molar.

*In this relationship each mandibular tooth occludes with its counterpart and the adjacent mesial tooth.


Angle’s class II..

* Relation between the arches

The mandibular arch is too distal (small) or the maxilla is too large or in combination of both

* Relation between the upper and lower 1st molars:

--T e MB cusp of the mandibular 1st molar occludes with the Central fossa of the maxillary 1st molar and its aligned with the buccal groove of maxillary 1st molar

--The DL cusp of maxillary 1st molar occludes with the central fossa of mandibular 1st molar.

*In this relationship The mandibular teeth will be positioned more distally than in class I and the maxillary will be more mesially.

Either the central incisors are labially inclined and its called angle’s class II division 1 or they’re lingually inclined so less overjet and its called angle’s class II division 2.


Angle’s class III..

* Relation between the arches

The mandible is forward (large) or the maxilla is too small or in combination of both.

* Relation between the upper and lower 1st molars:

-- The DB cusp of mandibular 1st molar is in the embrasure between the maxillary 2nd premolar and 1st molar.

-- The MB cusp of maxillary first molar is positioned over the embrasure between mandibular 1st and 2nd molars

-- ML cusp of maxillary 1st molar is situated in the mesial pit of the mandibular 2nd molar.

*In this relationship The mandibular teeth will be positioned more mesial than in class I and the maxillary will be more distally.


Notes:


Analysis of occlusion requires careful inspection of maximal inter-cusoation position in relation to both the position and condititon of TMJ.

Class I allows good esthetics, function and stability if its in harmony with the completely seated position of the condyles.

If displacement of one or both of the TMJ is needed to achieve a class I occlusion the result isn’t ideal because the deflective inclines have the potential of hyperactivity in the muscles.

The potential for muscle hyperactivity and pain is highest if the deflective contact is unilateral (if bilateral the muscle activity will be less than if uni.)

Class II & III may achieve optimum stability and equilibrium with the joints and musculature and so be the best occlusion for some patients

Intra-arch tooth alignment

Which is the angulations of the teeth within the same arch.


--the maxillary teeth
The anteriors (MD angulation) (BL angulation)

Distally inclined labially inclined


The posteriors (MD angulation) (BL angulation)

Mesially inclined labially inclined



-- the mandibular teeth
The anteriors (MD angulation) (BL angulation)

Mesially inclined labially inclined


The posteriors (MD angulation) (BL angulation)

Mesially inclined Lingually inclined

The occlusal surface of a posterior teeth is divided into :

1. Occlusal table : area of tooth between the buccal and lingual cusps, to which the forces of mastication is applied and it represents 50-60% of the total BL dimension. Its considered the inner aspect since it lies between the inclines.

2. the occlusal area outside the occlusal table is called the outer aspect.
The inner and outer aspects of the teeth are made from inclines that extend from the cusp tip to either the central fosse or the height of contour on the labial and lingual surfaces of teeth.

The inner and outer inclines are further defined by the cusps of which they are part of. They’re also described with respect to the surfaces toward which they are directed (mesial or distal).



Outer incline


Inner inclines.

Outer incline




Inter-arch tooth alignment

Refers to the relation of the teeth in one arch to those in the other.


Arch length: both arches have approximately the same length with the mandibular arch being slightly smaller. This is due to the narrower MD distance of the mandibular incisors.

Arch width: two lingual surfaces of two teeth opposing each other (right and left) the mandible is slightly narrower.


This relationship protects the surrounding soft tissue… How?

The buccal cusps of maxillary teeth prevent cheek and lip biting during function. The lingual cusps of mandibular teeth protect the tongue.
Sometimes due to eruption patterns, the teeth occlude in a way that maxillary buccal cusps contact in the central fossa of mandibular teeth or the mandibular lingual cusps occlude in the central fossa of the maxillary teeth…. This is called cross bite.


Mesio-Distal occlusal contact relationship

Centric cusps contact opposing teeth in central fossa areas, marginal ridge and embrasure areas.

Cusp tip fossa contacts; when two unlike surfaces meet only certain points come in contact. Which means that there’s only 3 points of contact but its impossible to have the fossa coincide with the cusp tip.

When the cusps contact the marginal ridges this allows better food cutting and penetration (since they’re convex surface to convex)

Each tooth in the dental arch occludes with two opposing teeth so the force of one tooth is distributed into the 2 opposing teeth.

When talking about class I; Mandibular teeth are usually positioned slightly lingual and mesial to their counterparts. (both posteriors and anteriors)


Occlusal relationships of anterior teeth

Maxillary anterior teeth are normally positioned labial to mandibular anterior teeth (overjet)

In angle’s class I the overjet is (2-4 mm)

In angle’s class II its more.

In angle’s class III its Zero

The labial inclination of the anterior teeth guides the mandible through the lateral movements. (anterior guidance)

The anterior teeth also provide the initial act of mastication; they incise food as its introduced into the oral cavity, also they play an important role in speech.

Occlusal contacts during mandibular movement

The masticatory system is extremely dynamic. The TMJ and associated muscle permit the mandible to move in 3 planes; sagittal, horizontal and frontal.

Along with these movements come potential tooth contacts which are called eccentric contacts.

There are three basic eccentric movements are present:



  1. Protrusive …. Forward

  2. Latero-trusive.

  3. Retrusive…. Backward



Protrusive mandibular movement

There is contact anteriorly and disocclusion posteriorly.

Anterior guiding planes for this movement; mandibular incisors move against lingual fossa of maxillary incisors and incisal edge.

Posterior guiding planes; mandibular centric cusps pass anteriorly across the occlusal surfaces of maxillary teeth (without contact they just pass across them)

Posterior protrusive contacts occur on the distal inclines of maxillary lingual cusps and mesial inclines of opposing fossae and marginal ridges, they can also occur between the mesial inclines of mandibular buccal cusps and the distal inclines of the maxillary opposing fossae and marginal ridges.

MESIAL INCLINES MOVES AGAINST DISTAL INCLINE OF MAXILLARY POSTERIOR TEETH.


Latero-trusive mandibular movement

The right and left mandibular posterior teeth move aross, as we said before the only contact is on the canine which is known as (anine guidance) or group function in which the contact occur on the canine with the 1st premolar or 2nd premolar or MB cusp of the 1st molar only if it was more posterior so we have deflective contact.

--now we’ll take an example as if the movement is to the left to make things clear.

**during left lateral movement ( Left Molar )

-contact occur in two contact areas;

one between the inner inclines of the maxillary buccal cusps and outer inclines of mandibular buccal cusps.

The other one is between the outer inclines of maxillary lingual cusps and the inner inclines of mandibular lingual cusps

These contacts are called “Laterotrusive contacts” or “working side contacts.”

**during left lateral movement (Right Molar)

The contacts are between the inner inclines of maxillary lingual cusps and the inner inclines of mandibular buccal cusps. These are called medio-trusive movements or non-working side movement.


Anterior teeth play an important guiding role during left and right mandibular movement. In a normal occlusal relationship maxillary and mandibular canines contact during right and left lateral movement and so have latero-trusive contacts which occur between the labial surfaces and incisal edges of mandibular canines; this is called ‘canine protected occlusion’---- anterior teeth might come in contact during latero-trusive movement or might not; depending on the height of canines.
Remember that;

-the latero-trusive movement is the movement of the working condyle since its moving laterally.. (left side in our example)

-the medio-trusive movement is the movement of the non-working condyle since its moving medially.. (right side in our example)

-contacts on working side either is on canine or group function.



Retrusive movement

It’s the opposite of protrusive movement; the mandible moves more toward the distal.

The mandibular buccal cusps move distal along the occlusal surface of opposing maxillary teeth.

MAXILLARY MESIAL INCLINES CROSS THE MANDIBULAR DISTAL INCLINES.

In the maxillary arch Retrusive contacts occur between the mesial inclines of the lingual cusps and the distal inclines of opposing fossae and marginal ridges.

When closure of the mandible in the musclo-skeletal stable position creates an unstable occlusal condition, the neuromuscular system quickly feeds back appropriate muscle action to locate a mandibular position that will result in a more stable occlusal condition.


**Optimal occlusal condition during mandibular closure is achieved by:

Even simultaneous contact of all possible teeth (posteriors).

Forces applied to teeth should be directed along the long axis of the tooth and this is achieved by :


  1. tooth contacts occur either on the cusp tips or relatively flat surfaces that are perpendicular to the long axis of the tooth. Flat surfaces may be crests of marginal ridges or the bottoms of the fossae.

  2. Tripodization which requires that each cusp opposing fossa in 3 points surrounding the actual cusp tip.

*** 3 points of contact with the opposing tooth each cusp occludes with the opposing fossa in 3 points surrounding the cusp tip the cusp TIP its not in contact; the inclines of the cusp are in contact with the fossa.


TMJ permits lateral and protrusive excursions, which allow teeth to contact during different types of eccentric movement. This allows horizontal forces to be applied to teeth. The teeth that could accept the horizontal forces are the canine and the anterior teeth to a lesser extent. These horizontal forces aren’t well accepted by supporting structures of the tooth.

The lever system of the mandible can be compared to a nut cracker, greater forces can be applied to objects as they are placed closer to the fulcrum (greater forces can be applied to posterior teeth than to anterior teeth; due to the fact that they’re closer to the fulcrum) so the anterior teeth can withstand horizontal forces but not axial.


Canines are best suited to accept the horizontal forces which occur during eccentric movement.. Why??

1. They have the largest and longest roots (crown/root)

2. They’re surrounded by dense compact bone which tolerates forces better than the bone found around posterior teeth.

3. Fewer muscles are active when canines contact during eccentric movements than when posterior teeth contact.


Therefore, in left and right latero-trusive movements of the mandible canines are appropriate teeth to contact and dissipate horizontal forces while disoccluding posterior teeth, this is called canine guidance.

Group function guidance: is the most favorable alternative to canine guidance. Several teeth on the working side contact during latero-trusive movement. The most favorable group consist of canines, premolars and the MB cusp of 1st molar.

Any contact more posterior to that is not desirable because of the increased amount of force that can be placed due to closeness to the fulcrum.

Buccal cusps to buccal cusp is more desirable contact during movement than lingual cusp to lingual cusp.


AS A SUMMARY:

-during protrusive movement only anterior teeth should contact and not posterior teeth.

-both anterior and posterior teeth act differently; posterior teeth can withstand forces applied during closure of the mouth. They are positioned well in the arches to accept forces applied to their long axis.

-anterior teeth are not positioned well in the arches to accept heavy forces during closure of the mandible; but they are well suited for accepting forces of eccentric mandibular movement.



Control of mandibular movements

Structures that control the mandibular movement are divided into:

** Posterior controlling factors; those that influence the movement of the posterior part of the mandible and considered as fixed factors TMJ

**Anterior controlling factors; those that influence the movement of the anterior part of the mandible. This is considered a variable factor and can be affected by dental procedures or pathologic conditions and is determined by the steepness of the lingual surfaces of anterior teeth.


The rate at which the mandible moves inferiorly as it is being protruded depends on the steepness of the articular eminence.

The morphologic characteristics of each posterior tooth must be in harmony with those of its opposing tooth or teeth during all eccentric mandibular movements. The exact morphology of the tooth is influenced by the pathway it travels across its opposing teeth.

The relationship of a posterior tooth to the controlling factors influences the precise movement of each tooth. So the nearer the tooth to the TMJ the more its eccentric movements will be influenced by the anatomy of the joint.

Mandibular movement has both vertical and horizontal components.



The angle of deviation from the H refrence plane is what we study in mandibular movement.


Eman M. Thneibat 


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