Physiology of tooth form 

function: four main functions 1.mastication : normal tooth form and

proper alignment ensure efficiency of mastication.

2.esthetics :form and alignment of anterior teeth-physical appearance

3.speech:form and alignment of anterior and posterior teeth assist in articulation of certain sounds.

4.protection of supporting tissues: form and alignment of teeth assist in sustaining the teeth in dental arches.

Contours The facial and lingual surfaces posses some

degree of convexity that afford protection and stimulation of supporting tissues during mastication.

Cervical third of crown on the facial surfaces of all teeth and lingual surfaces of incisors and canines.

Lingual surface of posterior tooth-height of contour-middle third of crown.

Too great curvature-tissues receive inadequate stimulation.

Too little contour- trauma to attachment apparatus.

Over contouring-flabby ,red, chronically inflamed gingiva and increased plaque retention.

Proximal height of contours provide 1-contact-prevent food impaction. 2.Adequate embrassure space gingivally of

the contacts for gingival tissue,supporting bone, blood vessels,nerves.

Proximal contact area The area of proximal height of contour of

mesial and distal surface of a tooth that touches its adjacent tooth in the same arch.

They promote normal healthy interdental papillae filling of the interproximal spaces.

Improper contacts-food impaction,carious lesion and possible movement of teeth.

Proximal contacts and interdigitation of teeth through occlusal contacts stabilize and maintains integrity of dental arches.

Central incisors –incisal third. Remaining teeth-junction of incisal and

middle third /middle third.

Embrasures V-shaped spaces that originate at the

proximal contact areas between adjacent teeth and are named for the direction towards which they radiate.

Facial,lingual, incisal/occlusal and gingival. Interdental papilla fills the gingival

embrasure. In faciolingual vertical section papilla

triangular in anterior teeth In posterior teeth papilla shaped like a

mountain range with facial and lingual peaks and the col lying beneath the contact area.

When an embrasure is decreased or absent,additional stress is created in the tooth and supporting structures during mastication.

Lingual embrasures are larger than facial to allow more food to be displaced lingually.

Maxilla and mandible Maxilla-formed by two bones:maxilla proper

and premaxilla-forms the bulk of upper jaw and major portion of hard palate,form the floor of orbit,sides and base of nasal cavity.

Mandible:forms the lower jaw.it is horse shoe shaped and relates to skull on either side via the TMJ.

The mandible is composed of a body of two horizontal portions joined at the midline symphysis mandibulae and the rami,the vertical parts

Oral mucosa Is the mucous membrane that covers all oral

structures except the clinical crowns of the teeth. Composed of two layers-stratified squamous

epithelium and lamina propria. The epithelium may be keratinized,

parakeratinized or nonkeratinized. Lamina propria varies in thickness and supports

the epithelium. It may be attached to periosteum of alveolar bone

or it may be interposed over the submucosa,which may vary in different regions of mouth.

Oral mucosa-3 functional types 1.Masticatory mucosa 2.Lining or reflective mucosa 3.Specialized mucosa 1.Masticatory mucosa-comprises of free and

attached gingiva and the mucosa of hard palate.

Epithelium of these tissues is keratinized and lamina propria is dense,thick,firm connective tissue containing collagenous fibers.

2.Lining or reflective mucosa-covers the inside of cheek,lips,vestibule,lateral surfaces of alveolar process,floor of mouth,soft palate and inferior surface of tongue.

It is a thin ,movable tissue with a relatively thick,non keratinized epithelium and a thin lamina propria.

The submucosa comprises mostly thin,loose connective tissue with muscle and collagenous and elastic fibers.

The junction of lining mucosa with masticatory mucosa is the mucogingival junction.

3.Specialized mucosa-covers the dorsum of tongue and taste buds.

The epithelium is non keratinized except for the covering of the dermal filiform papillae.

Periodontium Consist of oral hard and soft tissues that invest

and support the teeth Divided into:1.gingival unit-consisting of free and

attached gingiva and alveolar mucosa. 2.attachment apparatus-consist of

cementum,periodontal ligament and alveolar process.

Periodontium attaches the teeth to the maxilla and mandible and provides a continually adapting structure for the support of teeth during function.

Gingival unit- 1.free gingiva-is the gingiva from the marginal

crest to the level of base of the gingival sulcus.

Gingival sulcus is the space between the tooth and the free gingiva.

Outer wall of sulcus is lined with a thin,nonkeratinized epithelium.

The outer aspect of free gingiva in each gingival embrasure-gingival or interdental papilla

The free gingival groove is a shallow groove that runs parallel to the marginal crest of the free gingiva and usually indicates the level of the base of the gingival sulcus.

2.attached gingiva-a dense connective tissue with keratinized stratified squamous epithelium, extents from the depth of the gingival sulcus to the mucogingival junction.

A dense network of collagenous fibers connects the attached gingiva firmly to the cementum and periosteum of alveolar process.

3.Alveolar mucosa:is a thin soft tissue that is loosely attached to the underlying alveolar bone.

It is covered by thin ,nonkeratinized epithelial layer.

Underlying submucosa contains loosely arranged collagen fibers,elastic tissue,fat,and muscle tissue.

Attachment apparatus 1.Periodontal ligament-is a complex,soft

connective tissue containing numerous cells,blood vessels,nerves and an extracellular substance consisting of fibers(collagen) and ground substance(protein and polysaccharides)

Function:1.attachment and support 2.sensory 3.nutritive 4.homeostatic.

Bundle of collagen fibers (principle fibers of the ligament)serve to attach the cementum to alveolar bone and act as a cushion to suspend and support the tooth.

The portion of principle fibers embedded in the cementum and alveolar bone are called’ Sharpey’s fibers’.

2.Cementum-is a hard tissue with a calcified intercellular substance covering the anatomic roots of teeth.

3.Alveolar process-a part of maxilla and mandible,forms, supports and lines the sockets into which the roots of the teeth fits.

Has two parts 1.alveolar bone proper-the innerwall of bony socket

consists of the thin lamella of bone that surrounds the root of the tooth.(give attachment to Sharpey’s fibers)

2.supporting alveolar bone-surrounds the alveolar bone proper and supports the socket.

Composed of two parts- a. The cortical plate-consisting of compact

bone and forming the inner(lingual) and outer(facial) plates of alveolar process.

b. The spongy base-that fills the area between the plates and the alveolar bone proper.

 

occlusion Means the contact of teeth in opposing dental

arches when the jaws are closed(static occlusal relationships) and during various jaw movements(dynamic occlusal relationships).

General description Cusps-are drawn as blunt, rounded or pointed

projections of the crowns of the teeth. Cusps are separated by distinct developmental

grooves and have additional supplementary grooves on the cusp inclines.

Facial cusps are separated from the lingual cusps by a deep groove called central groove.

If a toot h have multiple lingual cusps or multiple cusps,the cusps are separated by facial or lingual developmental groove.

Depressions between the cusps-fossa. Grooves having noncoalesced enamel are

fissures. Noncoalesced enamel at the deepest point

of a fossa is a pit.  

Tooth alignment and dental arches Maxillary arch is larger than mandibular

arch,resulting in the maxillary cusps overlapping the mandibular cusps when the arches are in maximal occlusal contact.

Facial occlusal line-an imaginary arc connecting the rows of facial cusps in the mandibular arch

Central fossa occlusal line-an imaginary line connecting the maxillary central fossae.

These two lines coincide exactly when the mandibular arch is fully closed into the maxillary arch

Maximum intercuspation(MI)- is the position of mandible when the teeth are brought into full interdigitation with the maximum number of teeth contacting.

Cusps that contact the opposing teeth along the central fossa occlusal line are termed supporting cusps.(centric,holding or stamp cusps-mandibular facial cusps.

Non-supporting cusps-cusps that overlap the opposing teeth (noncentric/nonholding cusps).-maxillary facial cusps.

In MI the mandibular incisors and canines contact the respective lingual surfaces of their maxillary opponents.

Anterior- posterior interarch relationships. The cusp interdigitation pattern of the first

molar teeth is used to classify the anterior –posterior arch relationships.

Angles classification Class I -most common molar relationship. Mesiofacial cusp of max.1st molar occludes with

the mesiofacial groove of mand.1st molar. Class II -mesiofacial cusp of max.1st molar is

located in the facial embrasure between the mand.1st molar and mand.2nd premolar.

Class III-mesiofacial cusp of max.1st molar fits into the distofacial groove of mand.1st molar.

Inter-arch tooth relationships Incisor relationships-incisal overlap

considered in two dimensions: Overjet -this is the horizontal distance

between the labial surface of mandibular anteriors and lingual surface of max.anterior-horizontal overlap.

Overbite-this the vertical distance between the incisal edges of max. anterior and incisal edges of mand.anteriors.

Usual overbite and overjet varies between 2-3mm.

Variations in incisor relationships. 1.open bite as a result of a.mandibular deficiency b.excessive eruption of posterior teeth c.mandibular growth excess. Tooth –to-tooth relationship:mandibular

facial cusp contacting the maxillary premolar mesial marginal ridge and max.premolar lingual cusp contacting the mandibular distal marginal ridge.(only one antagonist is contacted.

Tooth-to-two –tooth contact: most stable relationship.contact of supporting cusp tips against the two marginal ridges

Cusp-fossa contact:when the mandible is slightly distal to maxilla,each supporting cusp tip occludes in a stable relationship with the opposing mesial or distal fossa.

Variations in molar relationships Facial cross bite-contact of maxillary facial

cusps in the opposing mandibular central fossa and mandibular lingual cusps in the opposing max.central fossa.

Lingual cross bite results in a poor molar relationship that provide little functional contact.

Posterior cusp characteristics Four cusp ridges can be identified as common features

of all cusps. 1.outer incline(facial/lingual ridge)-is indicated by the

line that points to the outer incline of the cusp. 2.Inner incline(triangular ridge)- the inner inclines of

posterior cusps face the central fossa /central groove of the tooth.

The inner incline cusp ridges are widest at the base and become arrow as they approach the cusp tip.

3.mesial and distal cusp ridges- extend from the cusp tip mesially and distally and are named for their direction.

-extent downward from the cusp tips,forming the characteristic facial and lingual profiles of the cusp as viewed from the facial and lingual aspect.

Marginal ridges- are elevated ,rounded ridges located on the mesial and distal edges of tooth’s occlusal surface.

Oblique ridge-the distal cusp ridge of maxillary 1st molar mesiolingual cusp curves facially to fuse with the triangular ridge of distofacial cusp,this juction forms the oblique ridge.(characteristic of max.molars.)

Supporting cusps These cusps contact the opposing teeth in their

corresponding faciolingual centre on a marginal ridge or a fossa.

The supporting cusp central fossa contact has been compared to a mortar and pestle becoz the supporting cusp cuts,crushes and grinds fibrous food against the ridges forming the concavity of the fossa of the opposing tooth.

Supporting cusps also serve to prevent drifting and passive eruption of the teeth and hence the term’ centric holding cusps’

Eg: The palatal cusps of max.posterior teeth and buccal cusps of mand. Teeth.

Non supporting(non holding/non centric cusps) When viewed from the occlusal,these cusps are

farther from the faciolingual centre of tooth than the supporting cusps.

They have sharp cusp ridges that serve to shear the food as they pass close to the supporting cusp ridges during chewing strokes.

The overlap of cusps helps to keep the soft tissue of the tongue and cheek out from the occlusal table preventing soft tissue injury during chewing.

Eg: buccal cusp of max.posteriors and lingual cusp of mand.posteriors.

Mechanics of mandibular motion Types of motion: 1.Rotation-rotation is a simple motion of an

object around an axis. The mandible is capable of rotation about

an axis through the centres located in the condyles.

Rotation of the condyles positioned in the centric relation is termed ‘terminal hinge movement’.

The maximum rotational opening in the terminal hinge is limited to 25mm measured between the incisal edges of anterior teeth.

2.translation:this is the bodily movement of an object from one place to another.

Protrusion refers to the forward thrusting of the mandible due to simultaneous direct anterior movement of both the condyles.

Protrusion is limited to approx.10 mm by the ligamentous attachments of the masticatory muscles and the temporomandibular joint.

3.complex motion:this refers to a combination of rotation and translation in a single movement.

Most mandibular movements during chewing and swallowing consist of both rotation and translation.

4.Lateral movements:it is often described with respect to only one side of the mandible for the purpose of defining the relative motion of the mandibular to the maxillary teeth.

Eg:during left lateral movement of the mandible,the left mandibular teeth move away from the midline and the right mandibular teeth move toward the midline.

The left lateral mandibular motion is due to rotation of the left condyle and translation of the right condyle.

The mandibular pathways that are directed away from the midline are termed ‘working/laterotrusion and mandibular pathways directed toward the midline are termed ‘non working’, ‘balancing’, ‘mediotrusion’.

5.Bennett shift: this is the lateral bodily shift of the mandible towards the working side during function.

In normal TMJ this shift varies from 0-1.5mm and does not have any significant effect on the posterior teeth.

Excessive lateral shift is associated with morphological changes in the TMJ.

Centric relation: described as the most retruded position of the mandible from which lateral movements can be made and the condyles are in the most posterior, unstrained position in the glenoid fossa.

In an ideal occlusion both centric relation and centric occlusion coincide with each other.

But in 10% of natural dentition,centric relation does not coincide with centric occlusion due to premature contact or occlusal interference.

This is compensated by proprioceptive mechanism of periodontal fibers by allowing the mandible to change the path of closure.

This prevents injury to teeth and adjacent periodontium.

Tooth contacts during mandibular movements. Prior to restoration of teeth,it is important to

evaluate the location,direction, and area of tooth contact during various mandibular movements as a preoperative procedure,

Anterior tooth contacts: during protrusion of the mandible,the lower anterior teeth glide along the lingual surfaces of the maxillary anterior teeth in a normal occlusion.

With protrusion ,multiple contacts serve to prevent excessive force on any individual pair of gliding teeth.

Posterior tooth contacts:

In an ideal occlusion ,the posterior teeth should contact only during maximal intercuspation.

Forceful contact or collision of individual posterior tooth cusps during chewing and clenching may lead to discomfort or damage to the teeth.

Group function occlusion: Occurs naturally in a worn dentition. In this ,there is simultaneous gliding contact of teeth in

lateral and protrusive movement. The working guidance in group function occurs on all the

teeth on the working side. The incisal edges of the mandibular anterior teeth glide

down the palatal surfaces of the maxillary anterior teeth. The buccal inclines of the buccal cusps of the

mand.premolars and molars glide against the palatal inclines of the buccal cusps of maxillary premolars and molars.

  

The science of occlusion is complex. Understanding a few basic principles of occlusion

related to natural teeth and knowledge of the methods of examining occlusion are of great value in preventing failures that occur in restorations involving occlusal surfaces of teeth.

Any restoration placed should be in harmony with the supporting tissues ,neuromuscular system and TMJ.

   

Conclusion

The Art & Science of Operative Dentistry, Sturdivant, Mosby U.S.A. 

Principle & Practice of Operative Dentistry, Charbeneu, Varghese Publishing , Mumbai.

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Bibliography