Preclinics of Conservative Dental Medicine · MU Sofia

3. Main principles for cavity preparation – resistance and retention forms creation.

Explains resistance and retention forms as the second and third of Black's cavity preparation steps, covering the three forces a restoration must withstand and the flat floors, box shape, isthmus width, marginal ridge thickness and rounded angles that resist them. Includes a table of retention by material, showing how amalgam, composite, glass ionomer and inlay walls differ, and walks through primary and secondary retention, including proximal grooves in Class II. Ends with a step-by-step Class II amalgam example, common mistakes, and key points.

Inside: 7 sections · 3 labelled pictures · 1 table · 17 flashcards · 10 min

Topic 3 of 53 in the MU Sofia syllabus for Preclinics of Conservative Dental Medicine (Dental Medicine, Year 3). The free AceMed note covers it in 7 sections, about 1,464 words (10 min of reading), with 17 flashcards.

In this note

  1. Introduction
  2. Definitions and the forces involved
  3. Resistance form
  4. Retention form
  5. Creating the forms during preparation
  6. Typical mistakes and their consequences
  7. Key points

Introduction

Resistance form and retention form are the second and third of Black's steps of cavity preparation, built into the cavity together with the outline (topic 2). Resistance form lets the tooth and the restoration withstand chewing forces without breaking; retention form keeps the restoration from being dislodged. Both are designed for the restorative material, and fractured cusps, broken amalgams and lost fillings usually trace back to a mistake in one of them.

Definitions and the forces involved

Resistance form is the shape and placement of the cavity walls that best enable both the restoration and the tooth to withstand, without fracture, masticatory forces delivered mainly along the long axis of the tooth.

Retention form is the shape of the cavity that prevents the restoration from being displaced or lifted out by tipping or lifting forces.

A restoration has to withstand three kinds of force:

  • Compressive forces along the long axis during biting – the largest; well tolerated when the floors are flat and the material is thick enough.
  • Lateral (tipping) forces on cusp inclines and in excursive movements – they bend the cusps outwards and rock a restoration that has no flat seat.
  • Tensile (lifting) forces, e.g. from sticky food, and forces that pull the proximal part of a Class II filling out of its box (proximal displacement).

Amalgam is brittle and several times stronger in compression than in tension, so it breaks in thin sections and at sharp angles. The tooth breaks where cusps and marginal ridges have been thinned or undermined. Sturdevant divides both forms into primary features, cut in the initial stage of preparation with the outline, and secondary features (grooves, coves, pins, slots, bonding), added in the final stage after caries removal and pulp protection.

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