The philosophies regarding functional impressions relate to how the secondary (or definitive) impression is recorded and how the underlying oral mucosa and tissues are managed during the process. These approaches dictate whether the tissues are captured in their resting state, under general pressure, or under strategically distributed pressure.
The sources detail three main types of impression philosophies used in complete denture fabrication:
1. Mucostatic Impression Philosophy
The mucostatic impression philosophy aims to capture the oral mucosa in its state when it is not displaced by external forces.
- Definition: The state of the oral mucosa as recorded when not displaced by external forces.
- Advantages: Dentures constructed from these impressions conform closely to the shape of the mucosal surface, and physical retention is optimal when the denture is not under load.
- Disadvantages: A major limitation is that the resulting occlusal loads will be uneven, and it is inherently difficult to obtain a truly static (or true mucostatic) impression.
2. Functional Impression Philosophy
The functional approach aims to record the tissue state under a deliberate load.
- Definition: The tissue state is recorded using a custom tray under a “functional” load.
- Principle: This approach aims for all tissues to be loaded equally, irrespective of their inherent ability to bear load.
3. Selective Pressure Impression Philosophy
The selective pressure philosophy is a hybrid approach, applying load strategically.
- Definition: The tissue state is recorded under load in a custom tray that provides maximum coverage but incorporates relief in specific areas.
- Mechanism: This technique involves providing wax relief over certain areas of the impression cast during custom tray fabrication (e.g., over the incisive papilla and central palate, or over flabby ridges).
- Advantages: This method allows for a more even displacement under occlusal load.
- Disadvantages: It may result in less retention when the teeth are apart.
Clinical Context and Evidence
It is important to note that a review by Carlsson GE in 2013 found no controlled studies supporting an advantage associated with any of these impression factors—mucostatic, functional, or selective pressure impressions—or factors like border moulding or post dam placement.
Secondary impressions (which utilize these philosophies) are performed in Visit 2 of the conventional denture fabrication schedule, using a custom (special) tray fabricated from the anatomical models (diagnostic casts) produced during Visit 1. The custom tray for secondary impressions is often designed to provide maximum coverage to minimise the pressure applied to the underlying tissues.