retention in fpd /certified fixed orthodontic courses by indian dental academy

Retention in fixed partial denture

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Biomechanical principles of tooth preparation

MECHANICAL ESTHETIC

BIOLOGICAL


Biomechanical principles of tooth preparation

Preservation of tooth structure.Retention and resistance form.Structural durability.Marginal integrity.Preservation of periodontium.


Introduction

Restoration should remain firmly attached to tooth structure to meet functional biological and esthetic requirements.


Definition (GPT)

That quality inherent in the prosthesis acting to resist the forces of dislodgement along the path of placement


Factors influencing retention

Magnitude of dislodging forces.Geometry of tooth preparation .Roughness of the fitting surface of the

restoration.Materials being cemented.Thickness of luting agent.


Magnitude of dislodging forces.

Depends on the stickiness of the food and the surface area and texture of the restoration.


Geometry of tooth preparation

TaperSurface areaStress concentration.Type of preparation.


Geometry of tooth preparation

Most fixed prosthesis depends on the geometric form of the preparation rather than on adhesion.

As most cements are non adhesive( act by increasing the frictional resistance between tooth and restoration.

Cylindrical tooth preparation.


Fig 7-27 and 7-28®

Preparation is cylindrical if two horizontal sections of prepared tooth surface is coincident


A crown depends on external retention to resist removal

An inlay depends on internal retention to hold it within preparation


Taper Axial walls of the preparation

must taper slightly to permit the restoration to seat.

Inclination of the wall. Angle of convergence Angle of divergence. Two opposing surfaces with 3°

taper would give a taper of 6 °. Maximum retention is obtained if

a tooth preparation has parallel walls .


Parallel walls are impossible to create in the mouth.

Preparation walls are tapered• To visualize preparation walls• Prevent undercuts.• Compensate for the inaccuracies in the

fabrication process• More nearly complete seating of the restoration

during cementation.

Ward was one of the first to recommend taper of 3 to 12 °respectively.


Relationship between the degree of axial wall taper and magnitude of retention was first demonstrated by Jorgensen in 1955


To minimize stress in the cement interface between the preparation and restoration a taper of 2.5 ° to 6.5 ° is optimum.

Slight increase in the stress as the taper increased from 0-15 °

At 20 ° stress concentration was found to increase sharply.


Some of the tapers suggested by authors

Ohm and silness on vital teeth on non vital teeth 19.2° MD and 23 FL 12.8 ° MD and 22.5 ° FL

Mack 16.5 °

Weed worked on dental students preparation 12.7 ° on typhodonts and 22.5 ° in clinical preparations.

Kent and his associates mean of 15.8 ° between mesial and distal walls 13.4 ° facial and lingual walls. Overall mean of 14.3 °


Recommended convergence angle between the opposing walls is 6 degree which has been shown to optimize the retention for zinc phosphate cement.

Guyer describes optimal taper to be in range of 2-6.5 °.


OPTIMUM DEGREE OF TOOTH PREPARATION TAPER

ARCH M/D F/L OverallMaxillary Anterior tooth 10 10 10Premolar 14 14 14Molar 17 21 19Isthmus 7Box 7Mandibular Anterior tooth 10 10 10Premolar 16 12 14Molar 24 20 22Isthmus 12Box 12www.indiandentalacademy.com

Too small a taper will lead to unwanted undercuts.

Too large a taper will not be retentive.


Length and surface areaOcclusogingival length is an important

factor in retention and resistance. Longer preparation will have more surface area

Length must be great enough to interfere with the arc of casting pivoting about a point on the opposite side of restoration.


Walls of shorter preparation should have as shorter taper as possible.


Greater the area of contact of the cement film bound to the preparation and to the internal detail of the casting the greater the retention of the casting.

Greater the surface area greater the retention. Total surface area is influenced by the Size of the tooth Extent of coverage of the restoration Features such as grooves, boxes pin holes that

are placed in the restoration


Doubling the height of the preparation would nearly double the area of its axial walls

Doubling the diameter of the preparation would doubles the area of its axial walls and quadraples the area of occlusal surface. www.indiandentalacademy.com

Basic unit of retention for a cemented restoration is two opposing walls with a minimal taper.

It may not always be possible to use opposing walls for retention.

It may be desirable to leave a surface uncovered for a partial veneer.

Features like boxes grooves and pinholes can be incorporated to enhance the retention


Kent etal reported difference between degree of taper of full crown preparation 18.4 – 22.2 degree and boxes and grooves in axial surface 7.3degreee

Taper of these features nearly same as the taper of the instruments used to cut them.


Freedom of displacement

Retention is improved by geometrically limiting the number of paths along which a restoration can be removed from the tooth preparation.

Maximum retention can be achieved by only one path ..


Full veneer crowns with long parallel axial walls and grooves will produce such retention.

Short over tapered preparation would be without retention because the restoration can be removed along an infinite number of paths


Stress concentration

Stresses not uniform throughout the cement.Concentrated around the junction of

occlusal and axial walls.Changes in the geometry of the tooth

preparation (rounding the internal line angles) may reduce stress concentrations and thus increase the retention.


Type of preparation

Different types of preparation have different retentive values.

Different types of preparation include Type of coverage of the restoration.

Inclusion of internal features.


Retention of different preparation designswww.indiandentalacademy.com

Roughness of the surfaces being cemented.

If the internal surface of the restorations smooth failure occurs through the cement restoration interface

Worley and Smith in their study conclude that the retention of the restoration can be increased by roughening or grooving the restoration.


Casting must be prepared by the air abrading the fitting surface by 50 micro m alumina.

It has shown to increase the retention by 64 % Deliberately roughening of the preparation hardly

influences the retention and it is not recommended as this adds to difficulty of impression making and waxing.

Oilo and Jogerson found retention of castings on cemented with zinc phosphate cement on test dies with 10 degree taper to be twice as great on preparations with 40 micro m scratches as on those with 10 micro m scratches.

Smith found no significant difference in castings cemented on 14 degree taper preparations whose roughness varied by a factor of 24 from smoothest to roughest.


Materials being cemented.

More reactive the alloy more adhesion is with the luting agent.

Base metal alloys are better retained than less reactive high gold content metals.

Effect of adhesion on different core materials. Chan kc etal in his study found that cement adhered better

to amalgam than to composite resin or cast gold De wals JP etal found that higher value of retention was

found with composite rather than amalgam cores. It may be due to dimensional changes of the core materials although the clinical implications are not clear.


Type of luting agentDecision regarding which agent to use

depends on other factors.


Film thickness of luting agent

Conflicting evidence about the effect of the increased thickness of the cement film on retention of the restoration.

It may be important if a slightly oversized casting is made.


Loss of retention

Loss of retention should be detected early, otherwise extensive caries develops.

Detection of loss of retention Patients awareness to looseness or sensitivity to

temperature or sweets. Periodical examination which includes attempt to

seat prosthesis by lifting retainers up and down while they are held between the hands and a curved explorer placed under the connector.


If the casting is loose the occlusal motion draws the fluids to be drawn under the casting and when the casting is reseated with the cervical force the fluid is expressed generally producing bubbles as the air and liquid are simultaneously displaced.

When the retainer becomes loose the retainer should be removed so that the abutment teeth can be evaluated.

If the restoration can be dislodged from the other prepared teeth without damage and no caries present it is possible to recement the restoration.


If the prosthesis removal reveals lack of adequate retention as evidenced by the preparation form the teeth should be modified to improve their retention and resistance.

Some fixed partial dentures may become loose even when maximal retentive preparations have been developed . It may be caused by excessive span length or heavy occlusal forces.


Summary Greater retention Lesser retention

Taper Parallel 6 degree ExcessiveSurface area Large Small Type of preparation

Molar complete crown

Premolar complete crown Partial crown

Intracoronal restoration

Surface texture

Rough Smooth

Film thickness

Effect uncertain

Luting agent Adhesive resin Glass ionomerPolycarboxylate Zinc phosphate

Zinc oxide eugenol


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