tooth form and occlusion2(351)

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Tooth form and occlusion

Part II

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OCCLUSAL ANATOMY OF

RESTORATIONS

Occlusion is defined in dictionary as being

closed or the act of closure,

while in dentistry the term denotes the static

opposing teeth contact relationship.

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On the wider scale, the modern concept of

occlusion describes the term as themultifactorial functional relations between

teeth in contact and the other components of the masticatory system (condyles, glenoids,

ligaments, muscles and jaws).

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The teeth are the elements of the

masticatory system that are directlyinvolved in the act of closure and any

disturbance in the occlusal anatomy orphysiology can result in serious damaging

effects in the other components of the whole

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TYPES AND PHYSIOLOGIC

ASPECTS OF OCCLUSION I- Centric occlusion:

When the jaws are in a closed position

with the maxillary and mandibular teeth

are in maximal interdigitated contact

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Malocclusion.

It is a term describing the abnormal or

malpositioned relationship of maxillary

and mandibular teeth when they are in

centric occlusion .

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The centric relation:

describes the relation between the

mandible and maxilla when the

mandibular condyles are in the most

retruded functional unstrained position in

the glenoid fossae with the articular disc

ro erl inter osed.

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The centric occluding relation

Combines the status of both teeth and

jaws in centric i.e. centric occlusion and

centric relation.

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Eccentric occlusion :

describes the relation between teeth whenthe mandible is moved from centricposition.

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This occurs in cases of :1) lateral or protrusive functional occlusion(contact in the functioning region)2) nonfunctional disclusion (balancing

contacts occurring during jaw movementslike lateral nonfunctional contacts orprotrusive nonfunctional contacts).

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During lateral movement of the mandible,the working side contacts could be either:

1) canine guided lateral functionalocclusion pattern ( as in cases of unworndentition) or

2) grouped lateral functional occlusionwhere there is a dominant guidance by thecanines plus a share of posterior teeth cusp

inclines guidance.(in older age and worn

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OCCLUSAL DIAGNOSIS

The anatomical landmarks present on the

occlusal aspects of posterior teeth and the

incisal and lingual surfaces of posterior

teeth must be restored back during

construction of individual or multiple

restorations.

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The posterior teeth are provided with cusps

with tip and inclined planes, fossae, ridgesto present with the formers the action of

mortars and pestles for grinding food.

Pits, fissures and grooves act as spillways

during mastication.

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Meanwhile, the canines are provided withpointed cusps for tearing of food while the

incisors are provided with horizontal edgesfor incision.The prominence of such landmarks depends

on the age, the type of occlusion and thechewing habits present in the mouth

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Articulation of teeth in centric occlusion on

a carbon paper gives the chance to detectthe sites of centric holding areas prior to

designing of a cavity preparation andselecting the type of restoration.

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The eccentric movement of the mandible in

lateral right, left, and protrusive movementswill give the operator the chance to detect

the range of functional and nonfunctional

contacts.

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Recently, computerized occlusal analysis is

helpful for accurate diagnosis and detectionof occlusion criteria prior to the restorative

procedures.

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The universal key of occlusion known todetect the normality of centric occlusion is

the articulation of the mesio buccal cusp of the upper first molar with the buccal grooveof the lower first molar with the upper

overlapping the loweri.e. the lingual surface of upper cusp

contacts the buccal aspect of the lower

molar buccal groove.

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b) Determinants of Occlusion:

These factors are involved in the control of

articulation of teeth during movement of themandible in response to the action of the

muscles.

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They are:

1. Posterior condylar guidance.(pathway of head of condyle on the articular eminence).

2. Anterior incisal guidance (pathway of incisal edges of lower incisors on thelingual surface of uppers)

3. The arrangement and morphology of teeth (vertical dimension, horizontal overlapor over jet, vertical overlap alignment and

topography of teeth).

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N; B:

The first factor is not under the control of

the operator because it is a fixed anatomicalfactor while the latters can be changed by

the dentist and thus they are termed the

variable factors.

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c) Inter-occlusal records:

They are determined and transferred to

articulators with a variety of typesaccording to the data they can accommodate

in order to carry on the procedure of

analyzing the occlusion or constructing an

inlay restoration.

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A face bow record is needed to mount the

upper model and then the lower model ismounted by determining the relation in

centric and the limits of eccentric

movements.

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Various methods can be utilized to relatethe mandible to the maxilla such as;

1- mounting full mouth casts.2- occlusion blocks.3- wax bites and arch tracing.

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Then, the occlusion is studied on the

mounted models and a removable die isconstructed to enable the accurate re-

orientation of the tooth on which the

restoration will be performed.

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FUNCTIONS OF THE OCCLUSAL

ANATOMY1. Mastication of food.

Each tooth is designed to perform certainfunction, incisors for incision, and canines

for tearing and posterior teeth for grinding.

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2. Intercuspation between upper and lower

teeth preserves the relation between theupper and lower jaws and consequently

between the condyle of the mandible to the

glenoid articulation of the maxilla.

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3- Stabilization of jaw movements by

determination of the limits of protrusive andlateral movements.

4- Prevents drifting , over-eruption andmovement of teeth and preserves theiralignment.

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Food cycling and spillway action (pits,

fissures, fossae and grooves as well as thespillways created by the intercuspation in

cases of quadripoded, tripoded and dipoded

contacts of antagonistic cusps with the

corresponding fossae). This is essential for

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GUIDELINES FOR THE RESTORATIONOF OCCLUSAL ANATOMY

1- During restoration of the anatomicalmarkings, the operator should be aware of

the correct anatomy and the physiologicaspects of the dentition.2- The landmarks (pits, fissures, grooves,

cuspal form and inclined planes, fossae, andridges) must be restored in the same size,form and location.

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3- This is performed by correct carving of

plastic restorations inside the mouth andmore precisely restored with indirect

restorations via the appropriate mounting

procedures and the meticulous carving of

the wax pattern.

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4- The precision of the various steps of

impression making, model and dieconstruction, inter-occlusal records and

mounting models is required to assure the

perfect restoration as an ultimate objective.

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SQULAE OF IMPERFECT RESTORATION OFANATOMICAL LANDMARKS

1. Detrimental stresses on the teeth and

supporting structures due to prematurecontacts resulting from misplacement of

structures or undercarving of restorations as

well as the faulty management of inter-

occlusal records.

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The premature contacts in centric andeccentric movements must be checked and

corrected before dismissing the patient.These stresses may cause fracture of restoration, post restoration pain and pulpal

inflammation and damage to supportingstructures.

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2. Inefficiency of mastication and foodstagnation by incorrect restoration of the

spillways and anatomical landmarks.This will prevent correct cycling and escapeof food and may cause recurrent caries and

periodontal problems.

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3. Drifting, over-eruption and malalignment

of teeth may occur due to failure to securethe inter-arch relations provided by the

occlusion of antagonistic teeth.

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4. Articular and temporo-mandibular

problems may result due to unstable jaw

movements and abnormal chewing habits.