A System of Design
When designing partial dentures, it is important to consider all aspects of the design in order to ensure that the final denture is stable, aesthetic and functional. In order to do this, we have a system of design which can be followed to ensure you don’t miss any components of the denture. This post will briefly look at each part of this system and explain a little about it
The sections include:
- Indirect Retention
The saddles are the parts of the denture that are going to house the teeth. They can be bounded or unbounded. Things that need to be considered at this point:
- Which teeth are you going to replace?
- Are you going to replace like-for-like? It is often more efficient and comfortable to replace molars with premolars
- How are the saddles going to relate to existing teeth?
This isn’t strictly part of the system, but it’s needed in order to determine the rest of the design. When the model is cast up for your patient, this then needs to be placed on a model surveyor. The surveyor has a vertical arm that comes down and contacts the model. This process will be covered in more detail in another post. Various instruments are used with the surveyor including a lead marker and analysing rod. The aim is to analyse the model in the horizontal plane, and alternative planes, to identify undercuts for your denture to use and the path of insertion.
The path of insertion is the path taken by the denture from first tooth contact until it is fully seated.
Support is preventing the denture from sinking in towards the underlying soft tissues. This is aiming to help protect the underlying mucosa. Partial dentures can either be:
- Tooth-borne – where the force is directed down the long axis of the teeth via rests
- Tissue-borne – where adequate support is achieved by enlarging the footprint as much as possible to distribute the load on the denture on to the soft tissues
- Mixed – the majority of partial dentures have both tooth and tissue-borne components
Rests can be occlusal, cingulum or incisal. The state of the abutment tooth has to be considered first to see what force it can tolerate. Rests also provide a number of other functions including indirect retention, positioning the denture correctly and distribution of load.
Retention is preventing displacement of the denture AWAY from the mucosa. This is usually achieved by clasps (but also sometimes by altering the path of insertion). Clasps can either be:
- Gingivally approaching clasps – usually found at the front of the mouth and are more aesthetic
- Occlusally approaching clasps – usually found towards the back of the mouth and avoids the gingival margin
Clasps provide direct retention and prevent this displacement. Only the final third of the clasp engages the undercut.
When something like a clasp is placed around a tooth, during disengagement from the undercut, the clasp will apply a lateral force on the tooth. Over time, this will cause the tooth to tip and weaken. Therefore, something is needed on the opposite side of the tooth the reciprocate this force i.e. a bar or a plate. So reciprocation is balancing the sideways force on a tooth. The reciprocating plate must be in contact with the tooth in order to function properly.
Bracing is preventing sideways and anteroposterior movement of the denture. Often this occurs automatically through the extension of the denture itself. Bracing can become difficult in free-end saddle cases, where there isn’t much material present. In these situations, extending the footprint of the denture can prove to be beneficial.
Connectors are the components of the denture that join everything up, which can be minor or major. Minor connectors join the smaller components to the saddles, while major connectors join the saddles up and complete attachment between the two halves on the arch.
There are a huge number of connectors including:
- Lingual bars
- Lingual plates
- Sublingual bar
- Palatal plate
- Palatal bar
Connectors also help in terms of support, bracing and indirect retention. Connectors need to be hygienic, rigid and tolerable to the patient.
Indirect retention prevents tipping/rotation of the denture about a fulcrum. Indirect retention is most necessary in situations such as Kennedy Class I, II and IV cases, where the main components of the denture are restricted to one part of the denture. This creates an axis which the denture wants to tip over (as shown by the image below). Therefore, components need to be placed on the opposite side of this axis to stabilise it. Indirect retention can be provided by connectors, clasps and rests.
The system of design is a method that is used to help design a partial denture in an ordered manner to ensure all components and features are covered. It is a simple and complete method to follow for partial denture design.