Amalgam is a material that is commonly used for restorations; however, there are key features of the cavity that must be considered when preparing it for amalgam. This post will look at these features and why they are relevant (generally for Class 1 cavities).

Why is it important?

We restore cavities for a few main reasons: to remove any caries (learn more about how caries forms here), to remove weakened tooth structure and to place margins in areas that are easy to access and clean.

There are properties of amalgam that are crucial when we consider the cavity design, some of these are listed below:

  • Amalgam is non-adhesive – it relies on being mechanically retained in the cavity
  • Amalgam is brittle in thin sections – therefore there is a minimum 2mm depth for an adequate cavity, as well as many other key features in the cavity.

Steps of design

1) Outline Form

The outline form determines the extent of the final cavity, except for finishing margins, and generally determines the external boundaries of the preparation. The preparation usually remains at less than a 1mm depth.

The aim is to remove all weakened tooth enamel and extend the margins into suitable areas. Factors that will impact this include: aesthetics, occlusal contacts, the extent of the carious lesion.

Whilst preparing the cavity at this stage, and later in the preparation, any enamel that is no longer supported by underlying dentine must be removed; otherwise, it is liable to fracture. This would then cause marginal deficiencies in the restoration and increase the likelihood of failure.

Unsupported enamel (red) liable to fracture in the future.
Unsupported enamel (red) liable to fracture in the future.

In addition to this, we need to consider the cavosurface angle. This is the angle between the surface of the tooth and the cavity walls. In an amalgam cavity, we aim for an angle of 90°. This is once again to protect the margins of the restoration.

The diagrams below demonstrate this. On the left the cavosurface angle is obtuse and if this was left then a very thin layer of amalgam would be packed here. As we know, amalgam is brittle in thin sections and therefore this is likely to fracture off and leave poor margins.

If the angle was too acute then it would leave unsupported enamel as shown in the earlier diagram. Therefore, the ideal is 90° as shown in the right-hand image.

Bad cavosurface angle, leaving thin amalgam (green).
Bad cavosurface angle, leaving thin amalgam (green).
Good cavosurface angle
Good cavosurface angle

Along with this, we need to ensure the depth of the cavity is a minimum of 2 mm to allow for a sufficient depth of amalgam.

When considering proximal cavities, e.g. Class II, then a few more considerations need to be made:

  • It is important to extend the cavity buccally and lingually beyond the contact point, into the embrasure, such that margins are in easily accessible areas for cleaning and inspection
  • The floor should also extend gingivally beyond the contact point to a cleansable area

The primary aim at this stage is to clear the amelo-dentinal junction (ADJ) of caries before progressing further into the cavity.

2) Resistance Form

Resistance form involves designing the cavity to allow the tooth and restorative material to withstand masticatory forces, down the long axis of the tooth.

Basic aspects of this include:

  • A cavity with a flat, even floor
  • A sufficient depth of restorative material – 2mm in this case
  • Ensure enamel margins are supported by dentine underneath and is strong areas, with preservation of marginal ridges and cusps wherever possible
  • Rounding off of any internal line angles to reduce stress concentration and subsequent fracture

3) Retention Form

Retention form involves designing the cavity to resist displacement of the restorative material from lifting forces. Generally, design principles are the same as resistance form. With amalgam, retention form is crucial due to its non-adhesive properties.

Undercut cavity (red)
Undercut cavity (red)

The most basic way to provide retention is through undercut creation in the cavity. Often this can occur through the bur used e.g. a pear shape or inverted cone diamond bur. If necessary, small occlusal locks/dovetails can be placed to enhance retention. In more substantial cavities, an intermediate bonding agent may be used e.g. Panavia, or amalgapins, slots or grooves may be cut, providing further retention (these are done during the final cavity preparation).

4) Convenience Form

Convenience form is ensuring the cavity has been extended and prepared sufficiently to allow accessibility during preparation and restoration. It is important to be able to visualise the entire cavity to ensure it is caries free.

The cavity may have to be extended to a particular direction to achieve this. Additionally, tidying up of the cavity can be carried out ensuring the cavosurface angle is corrected and unsupported enamel is removed.

5) Final Preparation

Finally, you must ensure all the cavity, especially the dentine, is caries free. The pulp must be protected wherever possible and care must be taken when preparing close to the pulp. Liners or bases may be considered if the pulp is thought to be in close proximity.

You must ensure the margins are clear and in appropriate areas, as well as the walls being smooth and relatively flat.

Amalgam Cavity Design Summary

  • Amalgam requires specific cavity design to allow for an appropriate restoration
  • There must be a minimum 2mm depth
  • There must be some form of retention, primarily an undercut cavity
  • Unsupported enamel should be removed, whilst maintaining a 90° cavosurface angle
  • The cavity still must be caries free, with margins in cleansable areas


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References and Recommended Reading

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BDS (Hons.) MFDS RCPS (Glasg.) Cert Med Ed FHEA - Currently working as a Speciality Doctor in OMFS and as an Associate Dentist


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