Prosthodontics

The dental specialty concerned with the making of artificial replacements for missing parts of the mouth and jaw. Also prosthetic dentistry or prosthodontia.

Fixed prosthodontics (Crown and Bridge prosthodontics)

It’s a branch of dental science that deals with restoration of damaged teeth with artificial crown and replacing the missing natural teeth by a dental prosthesis permanently cemented in place [Fixed partial denture].

Fixed Prosthodontics involve:

 Inlays-gold and all ceramic.  Gold crowns-full and partial veneer.  Porcelain & Zirconium-all porcelain and PFM.  Fixed partial dentures.

Crown: It's fixed extra coronal artificial restoration of the coronal Portion of a natural tooth .It must restore the morphology, contour and The function of the tooth and should protect the remaining tooth Structures from farther damage.

Types of crowns: (classification)

A) According to coverage area

1. Complete crown: It is the crown that covers all the coronal portion of the tooth, such as full metal crown, all Ceramic crown which is a complete crown made of ceramic material. 2. Partial Crown: It is a crown that covers part of the coronal portion of the tooth such as 3/4 Crown, 7/8 Crown.

3. Complete replacement: it involve those which replace the natural crown entirely. This type of crown retains itself by means of a metal extended inside the root canal space of the tooth such as a post crown.

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B) According to Materials: According to Materials used in the construction of crown and Bridge crown restorations could be made of:

1. Metal Crowns {Gold alloy and its alternatives} as in Full metal Crown and 3/4 Crown.

2. Non-metal crowns: Such as Acrylic resin, Zirconium or Porcelain as in jacket crown.

3. A combination: of metal and plastic materials as in PFM Crown restorations.

Bridge: It is a fixed dental prosthesis (appliance) which replaces and restores the function and esthetic of one or more missing natural teeth; it cannot be removed from the mouth by the patient. It is primarily supported by natural teeth or root. Tooth that give support to the bridge or part of the bridge to which retainer is cemented an abutment tooth.

Components of the bridge:

1. Retainer: It's the part that seat over (on or in) the abutment tooth which could be major or minor, connecting the pontic to the abutment.

2. Pontic: It is the suspended member of fixed partial denture that replaces the missing tooth or teeth, usually it occupies the position of the missing natural tooth.

3. Connector: It Part of F.P.D that join the individual components of the bridge together (retainer& pontics), which could be fixed (rigid) or movable (flexible) connector. When the retainer is attached to a fixed connector it's called a major retainer, but when it is attached to a flexible [movable] Connector it is called a minor retainer.

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Connector

Retainer

Pontic

Span

Abutment

Purposes Of crown Construction

1. To restore the grossly damaged tooth, fractured tooth or a tooth with a heavy filling [amalgam or composite]. 2. To restore the masticatory function and speech. 3. To restore the esthetic [hypoplastic condition whether heredity defect or Acquired defect]. 4. To maintain the periodontal health by re-contouring the occlusion and prevents food impaction. 5. To alter the occlusion (occlusal relationship) as a part of occlusal reconstruction to solve occ. Problem or to improve function. 6. As a retainer for the bridge.

Steps in crown construction

1. Diagnosis: The first step should be diagnosis of the tooth and Surrounding Structures.

a) Periodontal Examination: Proper oral hygiene should be available to ensure that no plaque accumulation is formed on the crown margins, which might lead if left to caries. b) Dental examination

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Retainer i. Visual: The occlusion, Crowding, Spacing, Rotation of teeth are examined .The condition of remaining tooth Structure and future treatment is also analyzed. ii. Radiographic: The radiographic film reveals the condition and shape of the roots and surrounding structures. A lesion in the bone, Root canal treatment, Fracture in the tooth, bone Loss, un-erupted teeth, Size and number of teeth etc…These Information Affects the prognosis of the treatment.

2. Tooth Preparation: It is the cutting or instrumentation procedure that carry on the tooth during crown construction procedure. Prepared tooth: It's the final form or shape of a tooth after cutting (Preparation) procedure. The tooth is prepared so the crown restoration can slide into place and be able to withstand the forces of occlusion. Rotary instruments are used to reduce the height and contour of the tooth. Hand cutting and rotary instruments prepare the gingival margins.

Objectives of tooth Preparation

1. To eliminate undercuts from the axial surface of the tooth. 2. To provide enough space for the crown restoration to withstand the force of mastication, this space depends on the material used, so the metal material needs little space while the plastic material needs more space.

Steps in crown construction (continued):

3. Final impression.

4. Temporary restoration (Crown).

5. Construction of working model.

6. Waxing.

7. Investing.

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8. Wax Elimination.

9. Casting.

10. Finishing and polishing.

11. Try-in &Cementation Of the restoration.

Disadvantages Of crowns

1. Heat generation during cutting procedure of the teeth, might affect the health of the pulp. Therefore water coolant must be used during preparation procedure. 2. Over preparation can cause pulp irritation or even pulp exposure which might lead to death of the pulp. Excessive tooth preparation can also weaken tooth structure. 3. Periodontal problems, food Impaction, and secondary caries might develop.

Finishing line of the preparation (F.L.): It’s a line that separates between the prepared and the unprepared tooth portion. It represents the end margin of our preparation, it should be smoothly continuous from one surface to the other otherwise it will interfere with seating of the crown if it’s poorly done.

Requirement of F.L.:-

1. It must be clear, smooth and well defined.

2. It must be continuous from one surface to the other. It must lie on sound tooth structure.

FINISH LINE TYPES (CONFIGURATION)

1. Knife edge.

2. Shoulder.

3. Shoulder with bevel.

4. Radial shoulder.

5. Chamfer.

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6. Heavy chamfer.

Bridge classification:

1. Fixed-fixed bridge: The most common type used anteriorly and posteriorly, the pontics are connected rigidly to the retainers at both ends of bridge, so we have only one path of insertion.

2. Fixed-movable bridge: the Pontic is attached to fixed retainer on one side while the other side is movable joint that connected with the other retainer.

3. Cantilever bridge: two types:

a. Simple: consist of one or two retainers with the Pontic that replace the missing tooth.

b. Spring: the Pontic is connected to a bar that also connected to retainer, which is away from the pontic. This type is usually used in case of missing maxillary central or lateral on one side of the maxillary arch with the presence of spaces between the anterior teeth.

It is not advised to use it on mandibular arch because:

• Instability of spring.

• Calculus formation.

• Tongue discomfort.

4. Resin –bonded bridge. (Maryland, Rochett)

There is minimal preparation within enamel, used for short span (3-units), less retentive than other types and use composite or resin for cementation to the abutment tooth.

Contraindication:

• Heavy occlusion.

• Decreased over jet.

• Attrition.

• Large surface.

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• Edge to edge.

Advantages of fixed bridge

1. Replacing missing teeth to provide function and aesthetic.

2. As a space maintainer following extraction to prevent over eruption of opposing tooth.

3. To restore fracture and badly carious tooth by retainer part of bridge.

4. Maintain periodontal health of abutment by the retainer.

5. Restoring the psychological demand of patient (aesthetic, function, phonetic).

Disadvantages:

1. Secondary caries attacking the abutment due to open margin.

2. Loss or fracture of the facing material due to technical errors or poor occlusal diagnosis by the dentist.

3. Periodontal problem due to poor Pontic design or under preparation for facing area

4. Poor retention of the retainer due to excessive cutting of abutment.

5. Over preparation and heat generation during cutting can cause pulp irritation or even pulp exposure.

Contra indication of bridge:

1. Long span edentulous area.

2. Free end extension area.

3. Abutment teeth with unstable bone support.

4. Limited inter jaw space.

6. Patient with dry mouth (xerostomia) because of great risk of recurrent caries.

Clinical consideration for crown and bridge construction:

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1. Oral hygiene of patient: high caries index and bad oral hygiene are contraindicated.

2. Age of patient: under the age of 16 due to large pulp and continuous eruption of teeth.

3. Condition of abutment: periodontal weak abutment is very problematic.

4. No. of missing teeth: useful to replace single or 3-5 teeth rather than large no. of missing teeth because, the more missing the more abutments to be included with bridge.

Advantages of FPD over RPD:

1. Oral hygiene: What do you think?

2. More stable and comfortable to the patient because it covers less tissue surface (no acrylic flange, no base, no clasp).

3. Provide more stable occlusion.

4. Better aesthetic.

5. Has splinting action, while RPD cause mobility of the tooth.

6. More suitable for handicapped & psychological patients.

7. Inert and apply no pressure on the ridge tissues.

8. Less phonetics problems than RPD.

9. More suitable for badly tilted abutment teeth.

10. Anatomical limitation of RPD

11. Abnormal large tongue

12. Muscular disorder

Evaluation of Abutment Teeth:

Every restoration must be able to withstand the constant occlusal forces to which it is subjected, this is of great significance when designing and fabricating a fixed partial denture,

8 since the forces that normally be absorbed by missing tooth are transmitted through pontic, connectors, and retainers, to the abutment teeth.

The root of abutments and their supporting tissues should be evaluated for three factors:

1. Crown- root ratio.

2. Root configuration.

3. Periodontal ligaments area (Root Surface Area).

Crown- root ratio: the optimum crown-root ratio for tooth to be utilized as abutment is

2:3. A ratio of 1:1 is the minimum acceptable ratio for abutment. The ratio is a measure of the length of tooth occlusal to alveolar crest of bone compared with length of root embedded in the bone.

Root configuration:

 Roots that are broader labio-lingually than mesio-distally are preferable to roots that are round in cross section.  Multi-rooted posterior teeth with separated roots will offer better periodontal support than converge, fuse and conical roots.  Single-rooted tooth with irregular configuration or some curvature is preferable to tooth that has perfect taper.

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Periodontal ligaments area (Root Surface Area):

Ante’s law stated: the root surface area of abutment teeth had to equal or surpass that of missing teeth, that being replaced with pontics.

Span Length:

Excessive flexing under occlusal loads may cause failure of a long-span fixed partial denture. It can lead to fracture of a porcelain veneer, breakage of a connector, loosening of a retainer, or an unfavorable soft tissue response and thus, render a prosthesis useless. All FPDs flex slightly when subjected to a load where the longer the span, the greater the flexing. The relationship between deflection and length of span is not simply linear but varies with the cube of the length of the span. Thus, other factors being equal, if a span of a single pontic is deflected a certain amount; a span of two similar pontics will move 8 times as much.

When a long bridge is fabricated, the pontic and connector should be made as bulky as possible to get optimum rigidity

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