Mechanical Means of Retention Retention Form:It is the form given to the cavity to resist the displacement or removal of the restoration from its place..

Types of retention: According to direction

- Axial - Lateral

According to location - Coronal (b, L, G, …)- Radicular

According to principle- Primary - Secondary

Primary Retention Form:During initial tooth preparation, the form & shape of the preparation need to provide resistance against fracture & retention of the restorative materials in the tooth for non-bounded restorations..Often, features that enhance the retention form of a preparation also enhance the resistance form..

Definition of primary retention form:It is that shape or form of the conventional preparation that resists displacement or removal of the restoration from tipping or lifting forces..The retention form developed during initial tooth preparation may be adequate to retain the restorative material in the tooth..Sometimes, however, additional retention features must be incorporated in the final stage of tooth preparation..

Principles of primary retention form:Vary depending on the restorative material used..

Amalgam - Convergence of the cavity walls- Undercuts- Amalgam bonding

Composite- Micromechanical bond to enamel & dentin - Walls direction

Cast metal- Frictional retention (close parallelism)- Cement interloacking , adhesive cemen

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☺ In most class I & all class II conventional preparation

Convergence of the cavity walls occlusally

Advantages:1. Producing retention 2. Allows slight facial &/or

lingual extension of the proximal portion of the preparation in the gingival area while conserving the marginal ridge reducing the forces of mastication on critical areas of the restoration..

- Can't be dislodged without some type of fracture occurring

- This convergence should not be overdone for fear of leaving unsupported enamel rods on the cavosurface margin of the occlusal surface

- The cavosurface angle where the proximal facial & lingual walls meet the marginal ridge is a desirable 90º because of the occlusal convergence of the preparation..

☺ Classes III & IV The external walls diverge out-wardly to provide strong enamel margins & therefore retention coves or grooves are prepared in the dentinal walls to provide the retention form

(Secondary retention form )

☺ Adhesive systems Provide some retention by micromechanically bonding amalgam to tooth structure

Also reducing or eliminating microleakage However, until longetivety, studies demonstrate that bonding systems

provide complete retention form, traditional retention features should be provided for amalgam restoration especially for root surface restorations..

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♣ Retention by micromechanical bond that develops between the material & etched and primed prepared tooth structure..

♣ Sometimes the tooth preparation for a composite restoration requires the use of mechanical retention form (Wall directions) , which is considered part of the final stage of preparation..

♣ Example in class V campsite tooth preparation on the root surface, groove retention may be recommended in addition to the use of a bonding system..

♣ Because of the strong & rapid bond that developed between etched enamel & composite , the initial tooth preparation of many composite restorations should result in a beveled or flared (>90º Enamel marginal configuration) that's ready to be etched modified composite tooth preparation

♣ N.B: Conventional tooth preparation + Beveling in the final stage beveled conventional preparation..

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Frictional retention (close parallelism)- Intracoronal restorations rely primarily on almost parallel vertical (longitudinal) walls to provide

retention of the casting in the tooth..- During initial tooth preparation, the preparation walls must be designed to provide:

1. for draw or draft (in order for the casting to be placed into the tooth)2. For an appropriate small angle of divergence (2-5º/wall) from the line of draw that will enhance

retention form..- The degree of divergence needed primarily depends on the length of the prepared walls: (The

greater the vertical height of the walls, the more divergence is permitted & recommended but within the range (2-5º/wall)..

- In inlay & only preparations for cast metal restorations, the opposing vertical walls diverge out-wardly by only a few degrees to each other & to a draw path that is usually perpendicular to the floor of the preparation

Cement interlocking, adhesive cement

- Having sufficient length of these most parallel walls allows enough frictional resistance & mechanical locking of the luting agent into minute irregularities of both the casting & the preparation walls to counteract the pull of sticky foods.

Conclusion:Close parallelism of prepared vertical walls is a principle retention form for casting metal restorations, anther being the use of a luting agent that bonds to tooth structure..

2ry retention features:Although features of 1ry retention form are very important, sometimes 2ry retentive features are necessary in the final stage of tooth preparation, usually for non-bonded restorations..

Secondary Retention Form:The 2ry retention forms are two types:

1. Mechanical preparation features2. Treatment of the preparation walls with etching, priming & adhesive materials

(1st step for the insertion of the restorative material)

I) Mechanical features:Retention locks, grooves & coves

Vertical oriented retention locks & retention grooves are used to provide additional retention for proximal portions of some tooth preparations

- The locks are for amalgam - The grooves are for cast metal restorations

Horizontally oriented retention grooves are prepared in:1. Most classes III & V preparations for amalgam

2. Some root-surface tooth preparation for composite

Retention coves are appropriately placed undercuts for:1. The incisal retention of class III amalgam

2. Occlusal portion of some amalgam restoration

3. Some class V amalgams

4. Occasionally for facilitating the start of insertion of certain gold foil restorations.

Retention locks in class II preparation for amalgam restorations:1. ↑ retention of the proximal portion against movement proximally due to creep 2. They are beveled to ↑ the resiatance form of the restoration against fracture @ the

junction of the proximal & occlusal portions3. Locks are recommended for extensive tooth preparation for amalgam involving

for example , wide facio-lingual proximal boxes &/or cusp capping Groove extension

By arbitrary extending the preparation for molars onto the facial or lingual surface to include a facial or lingual groove..When performed for cast metal restorations results in additional vertical (longitudinal) almost // walls for retention This features also enhances resistance for the remaining tooth due to the development..

Skirts, undercuts

It's a preparation features used in cast gold restorations that extend the preparation around some or all of the line angles of the tooth..Provide additional, opposed vertical walls for added retention when properly prepared..Significantly ↑ resistance form by enveloping the tooth resisting fracture of the remaining tooth from occlusal forces..

Mesiolingual & distolingual skirts

Beveled enamel margins

Cast gold/metal :1. Slightly ↑ retention form

2. Primarily afforded better junctional relationship between metal & the tooth.. Composite:1. ↑ surface area of etchable enamel 2. Maximize the effectiveness of the bond by etching more enamel rod ends

Pins, slots, steps & amalgam pins

Pins & slots ↑ retention & resistance Amalgampins & properly positioned steps ↑ retention but not as pins & slots

II) Treatment of the preparation walls:Enamel wall etching , dentin treatment

For porcelain , composite or amalgam restorative materialsDone by an appropriate acid roughened surface mechanical bonding

Adhesive luting cement

For porcelain, composite & amalgam restorative materialsThe actual ttt varies with the restorative material used:

- Composite dentin bonding agent is recommended- Amalgam glass ionmer material is used as a base before the restoration of the

tooth - Adv & dis

Retention from indirect restorations (Fabricated extraorally) enhanced by the luting agent used

Causes of poor composite retention (IMPORTANT):Inadequate preparation formContamination of operating areaPoor bonding techniqueIntermingling of bonding materials from different systems

Potential solutions include:Prepare the tooth with appropriate bevels or flares & 2ry retention feature, when necessary Keep the area isolated while bondingFollow the manufacture's directions explicitly Don't intermingle bonding materials from different systems

NOTE:Amalgam bonding is adjunct to mechanical retention form & not a substitute.Adequate mechanical retention features still must be incorporated into the preparation.Reviewing the retention form for different classes, different restorative materials(direct & indirect)

The new concept in restorative dentistry is adhesive restorations which should modify out traditional cavity preparations..

Regards,