The Different Forms of Secondary Retention (Mechanical Features, Placement of Etchant, Primer or on Prepared Walls)
Secondary retention form is of two types:
1. Mechanical features.
2. Placement of etchant, primer or adhesive on prepared walls.
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Mechanical features
Retention locks, grooves and coves
(I) Various type of retention grooves and retention locks provide retention for proximal portion of preparation.
(II) Horizontal retention grooves are useful in class III and IV preparation.
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(III) Retention coves are appropriately placed undercuts for the incisal retention of class III amalgams.
Groove extensions
(I) Extension of the preparation onto facial/lingual fissures provides additional retention.
(II) Skirts, beveled enamel margins.
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(III) Pins, slots, steps.
When there is great need of increased retention form, pins and slots are incorporated into the preparation.
Placement of etchant, primer or adhesive on prepared walls
In addition to mechanical features, certain alterations in the preparation walls increase retention.
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Enamel walls are etched for bonded restorations. Etching of enamel by appropriate acid increases the total surface area and improves the bonding of material to the tooth structure.
Dentinal surfaces may require etching and priming when using bonded amalgam restorations. Sometimes, a glass ionomer is used as a base before the restoration of tooth with amalgam.