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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

Amalgam
☺	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.. Composite	♣	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.. Cast metal (Usually a gold alloy)	 	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 features 2.	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 portions 3.	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..

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 materials Done by an appropriate acid roughened surface  mechanical bonding Adhesive luting cement 	 	For porcelain, composite & amalgam restorative materials The 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 form Contamination of operating area Poor bonding technique Intermingling 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 bonding Follow 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,

 * Ahmad Ghassan Al Wazani
 * Misr university for science&technology
 * college of Dental surgery
 * Department of operative dentistry
 * 3th year