preparation -- principles and technique * * impression material and technique

Preparation -- principles and technique * *Impression material and technique

Prosthodontics in dentistry

Definition Prosthodontics is that discipline of dentistry pertaining the restoration of oral function , comfort , appearance (esthetics) and contiguous oral and maxillofacial tissue with artificial substitute.

There are three main branches of prosthodontics

1 -Fixed prosthodontics

fixed prosthodontics pertains to the restoration or replacement of teeth with artificial substitutes that are supported on natural teeth , roots or implant and they are fixed ether by cement such as crown , post-crown , bridge , inlay and onlay. or by screw such as crown and bridge

supported implant.

Full metal single crown Full ceramic single crown

Metal-ceramic bridge Post- crown

2-Removable prosthodontics

Pertains to the replacement of missing teeth and contiguous oral structures with artificial substitutes (materials). That are readily removable such as metal-based acrylic partials ,acrylic partial denture ,complete denture and over -denture

Metal-based acrylic partials Complete denture

Acrylic partial denture Over-denture

3-Maxillofacial prosthodontics

Pertains to the restoration (replacement) of developmental (conginital) or required defects of the stomatognathic system and associated facial structure with artificial substitutes (materials) such as abturators ,nose – ear – eye

denture!.…

Anatomical Crown

Portion of a natural tooth covered by enamel and that extends from its cementoenamel junction (CEJ) to the occlusal surface or incisal edge

Clinical Crown

Its prepared potion of natural tooth ,that extends from the occlusal surface or incisal edge to the bottom of gingival sulcus (free gingiva) –known as abutment

Artificial Crown

It’s a restoration of major surfaces or the entire coronal part of natural tooth ,that restore (reestablish) anatomy ,function and esthetics . Its made from metal , ceramics ,

acryl and gold.

Fixed Restoration (prosthesis) Fixed restorations divided to two types of

restoration.

Fixed restoration

Extra-coronal Entra-coronal

Its restore (cover) ,external potion of a prepared tooth to tissue-compatible contour and obtain retention an resistance to

displacement.

Extra-coronal restoration

Types of extra-coronal restorations

1 -full veneer crown

Its cover all surfaces od clinical crown , its maybe made from various materials (metal – ceramic – metal-ceramic , metal-acryl combination )

Full metal crown Full (all) ceramic crown

Metal-ceramic crownGold crown

2 -partial veneer crown

its cover ( restore) only a portion of clinical crown such as (three quarter – one half crown).

One half crown

3 -bridge (fixed partial denture(FPD)

Bridge replace one or more missing teeth (named as a pontic) and supported on adjacent remaining teeth (named as abutment) bridge cover all surfaces of abutment to which the bridge is cemented

Missing teeth ( pontic )

Adjacent remaining teeth (abutment)

Missing teeth ( pontic )

Adjacent remaining teeth (abutment)

Fixed bridge

4- resin-bonded bridge (Meryland bridge)

replace one missing tooth in posterior area which cover and bonded to occluso-lingual surfaces , and replace maximum two teeth in anterior area which cover and bonded to lingual surface. The restoration bonds to the prepared surfaces by resin luting cement (etching enamel technique)

Posterior resin bonded bridge Anterior resin bonded bridge

5 -laminates (facing)

Cover only the facial surface of the tooth for esthetic purpose , they are fabricated from porcelain(ceramics) and bonded (micro-retention) to enamel by resin luting cement (etching enamel technique)

Intra-coronal restoration

Its restore the caries damaged areas of tooth crown by MO , MOD , OM surfaces preparations . Its obtain their retention and resistance to dispalcement its restoration within the coronal portion of teeth

Types of intra-coronal restorations

1 -inlay

Its an intra-coronal restoration which used to restore the proximo-occlusal badly decaied surfaces (MO , MOD , OD caries ). Its made from ceramics or gold

And cemented by resin luting cement or by glassionnemer cement

2 -Onlay

Its intra-coronal restoration , which is used to restore a more exstensively damaged posterior tooth and restore the most occlusal

surface.

Indications for fixed restoration

1 -Missing of teeth ( kennedy`s classification)

Class1 , class2 - replace missing teeth with implant supported restoration

Class3 , class4 - replace missing teeth with fixed bridge or with implant supported restoration

2 -tooth with large unsupported filling (weak walls)

3-discoloration of teeth (Tetracycline –florsis)

4 -endodontically treated teeth

5 -post-crown restoration

Factors affecting the success or failure of fixed restoration

There are many factors related to the teeth and to the adjacet oral structures ,that have direct influence on the success or failure of fixed restorations

1 -Crown length

Teeth must have adequate occlusocervical crown to achieve sufficient retention. Teeth with short clinical crown often dosen`t provide satisfactory retention ,maybe additional length achieved through periodontal surgery or by orthodontic

tratment.

2-Crown form

Some teeth have a very tapered crown form (like mesiodense),which interferes with preparation parallelism. Its need to full coverage retainers to improve their

retention and esthetic qualities.

3 -Root length and form

Abutment teeth must retain stability of prosthesis during function , abutment resist the occlusal force that will be transmitted to roots then to bone (as anchorage – stability require anchorage). Roots with invert-taper form better than curved or malformed roots , also multirooted teeth provide greater stability (anchorage) than single-rooted teeth.

Teeth with long roots act as a good abutment

Teeth with invert-taper form of root are better abutment than curved or malformed roots

4 -Crown-root Ratio

A comparison of the tooth length projection out of the alveolar bone and the length embedded in the bone has been traditionally used as guideline In determining the suitability of a tooth as abutment

) higher ratio – less tooth will be able to resist additional occlusal forces.(

{Crown root Ratio can be determined by X-Ray panoramic or pre-apical}

1:1,5 ratio has generally been considered to be satisfactory ,

whereas a 1:1 is considered minimal .

)requires condition of other factors such as number of teeth being replaced , tooth mobility and overall periodontal health(

5 -Ante`s Low

Abutment support may be evaluated by employing a rule referred to as (Ante`s low) says “ that the combined periodontal ligament area of the abutment teeth should be equal or exceed that teeth to be replaced ”

Applying this rule is safe and useful guideline ,when making fixed prosthesis (bridge) with long span ( replace two or more missing teeth )

Ant`s low is useful for determining the prognosis of FPD

Number denotes the root area precentage

6 -Span Length

The distance between two abutment teeth affects the feasibility of placing a fixed prosthesis . Replacement of one or two approximating tooth is routinely accomplished with a fixed prosthesis. however the loss of three adjacent teeth requires carefull evaluation of other factors such as crown-root ratio , root length , occlusion , periodontal health and mobility of tooth or post-crown

Fixed prosthesis,replacing missing four adjacent teeth are sometimes fabricated but only in anterior area of mouth in which

reduced forces encountered.

Note : excessive long span is a common contraindication for FPD

A bridge of shorten length will flex less compared to longer bar with same dimension

Increase in length of the bar to twise its orginal value increase its flexibility by eightaa(8) times

7 -Occlusion

The occlusal forces on the prosthesis are related to:

- the degree of muscular activity

- the patient`s habits such as bruxism

- the number of teeth being replaced

- the leverage on the bridge

- the adequacy of bone support

Excessive occlusal forces can cause loosening of the restorations or fracture of ceramic and acrylic component in restorestion if present , and can also cause tooth mobility

There are three types of force can be directed against a prosthesis during function ( tipping force – twist }rotational}

force – path of insertion force).

1-Tipping force can occur in buccolingual or mesiodistal direction depending on point of force application.

2-Twist force may cause a restoration to move circumferentionally around the prepared teeth

3 -Path of insertion force can be apically or occlusally directed

1 2 3

8 -Arch Form

A curvature in the arches often place pntics facially to a straight line (the fulcrum line) drawn between the teeth immediatley adjacent to the edentulous area, this relationship creates a lever arm which can exert excessive forces on the abutment teeth when occlusion occurs on the pontics

The force can exceed the capacity of these teeth to resist tapping

A-B fulcrum line , straight line drawn between the teeth immediately adjacent to the edentulous area

C-D lever arm

E-F counterbalancing retention

offered by including premolars

9 -Periodontal health

Inadequate periodontal health often results in bone loss , extensive bone loss may require the use of multiple abutment teeth ,or removable prosthesis or even tooth extraction. If periodontal disease is present it must be eliminated before restorative treatment is begun and the patient must receive and adequately respond to instruction in proper oral hygiene procedures. The selection of retainers and pontics must be planed to

promote effective oral hygiene.

10 -pulpal Health

Abutment teeth with poor pulpal health, results to caries , should receive

endodontic treatment

deep reduction of the teeth under metal ceramic or full ceramic restorations when the crwon is shoet its prefered to make endo.treatment to prevent reversable pulpitis after fixation the restoration

Full-coverage retainers are required on most endodontically treated teeth (its not prefered to fabricate three-quarter crown – half-crown – and resin bonded bridge) to

prevent the fracture.

Post and core restoration,its necessary to be covered by full-coverage retainers to increase the retention of proshtesis and to keep the remaining tooth structure.

Classification of partially edentulous arches

The most methods of classifications of partially edentulous arches have been proposed by Kennedy , Bailyn , and Skinner

Kennedy`s classification

Kennedy`s method of classification is probably the most widely accepted classification of partially edentulous arches today . Its simple and more

universal use. Kennedy`s classification was orginally proposed by Dr. Edward Kennedy in 1925 , it attempt to classify the partially edentulous arch in a manner that will suggest certain principles of design for

a given situation.

Kennedy divided all partially edentulous arches into four basic classes.

Edentulous areas other than those determining the basic classes were designed as a modification spaces of kennedy`s classification

1 -Class

bilateral edentulous areas located posterior to the remaining natural teeth

Modifaction of Class

2 -Class

unilateral edentulous area located posterior to the remaining natural teeth

Modification of Class

3 -Class

unilateral edentulous area with natural teeth remaining both anterior and posterior to it

Modification of Class

2

4 -Class

edentulous area where is missing teeth located in anterior or frontal segment . ( edentulous area bilaterally located anterior to the remaining teeth )

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