Partial Denture Design

• Classification of partial dentures:

  • Kennedy classification
  • Always classified by the distal-most saddle (or the lowest classification number)
  • 4 main groups - based on positioning of the saddles
  • Class 1 = bilateral free-end saddles
  • Class 2 = unilateral free-end saddle
  • Class 3 = bounded posterior saddle (tooth abutments on either side of the edentulous span)
  • Class 4 = anterior bounded saddle
  • First example  = class 2, mod 1 because it's got a unilateral free-end saddle and 1 other modification
  • Second example = class 3, mod 2; bounded posterior saddle & 2 other modifications

  
• Material choices for partial dentures:

  • A partial denture should, where possible, gain support from both the hard and soft tissues.
  • A cobalt chrome denture allows this. There are occasions where a completely acrylic denture is indicated, such as inappropriate/inadequate support from remaining teeth or if the denture is to be provided as an immediate denture.
  • Cost is often stated as a reason for not providing patients with cobalt chrome dentures - but in the long term it's a better option.
• Support and stability of partial dentures:

  • A large proportion of a partial denture's support & stability (especially for free-end saddles) will involve extending the denture into the full denture-bearing anatomy
  • You should therefore aim to cover the full denture bearing area including the sulcus like we do for complete dentures. The exceptions are very small bounded saddles where the sulcus is relatively insignificant.

  
• Periodontal assessment before partial dentures:

  • A full periodontal assessment should have been carried out before the denture is designed
  • BPE is a minimum, with APPROPRIATE recent radiographs showing the degree of bony support available, and whether there is any pathology associated with the standing teeth
• Primary impressions:

  • The primary impressions record the full denture-bearing area
  • You will often need to use a supportive material to carry your alginate into difficult-to-reach places
  • This is made slightly more difficult by the fact that you will be using dentate trays - there is significant potential to allow trays to tip if they are not supported in the edentulous spans. You will also miss important anatomical areas.

  
• Preliminary registration for partial dentures:

  • Once primary impressions have been obtained, wax preliminary registration blocks should be constructed to allow you to record the relationship of the opposing natural teeth.
  • The models are then articulated.
  • You should check this articulation whilst designing your denture to ensure that there is adequate room for your framework inter-occlusally.
  • Natural tooth contact should be the same with or without the blocks.
• Eyeballing the casts:

  • The casts should be detachable from the articulator through a process known as split-cast mounting
  • It's important for each denture arch to remove the cast and eyeball its characteristics in terms of:
  • Ridge undercuts
  • Guide planes
  • Dead spaces
  • This will help you to determine whether you would like your denture to insert into the mouth along a different axis to the occlusal plane. This is known as a path of insertion.

  
• The design process:

  • Many texts will, at this point, tell you to survey the casts formally and then come up with a design.
  • Practically there is no real need YET, and it is useful to come up with what you would consider to be an 'ideal' design based on your eyeballing of the cast.
  • This makes the assumptions that you have adequate undercuts to clasp teeth and that guide planes or undercuts are favourable - but you can check this later.

  
• Saddles (support from alveolar ridges):

  • Prescribing a saddle does not always mean prescribing the replacement of teeth
  • A large saddle will provide a lot of support and bracing especially in a class 1 or 2 denture
  • Your ability to extend into these areas depends on how comprehensive your primary impression was (and subsequent major impression will be)
  • We aren't taking a major impression yet in case we need to make some tooth modifications based on our design
• Rests:

  • Rests transmit occlusal loads from the denture down the long axis of the supporting teeth. They should ideally be placed on each end of a saddle.
  • They also deflect blood away from the saddle-abutment junction

  
• Rests:

  • Sometimes it is not appropriate to prepare a rest (the removal of sound tooth tissue or damage to existing restorations outweighs the benefit of having a rest in that area) and so clinical judgement should be used

  
• Preliminary registration for partial dentures:

  • You should check this articulation carefully and use it whilst designing your denture to ensure that you don't have any occlusal interferences and that there is adequate room for your framework inter-occlusally.
• Rests (support from the teeth): aim to 'rest' each end of the saddles.

  
• Clasps:

  • Clasps provide direct retention for the denture
  • Ideally two clasps: right & left, anterior and posterior
  • Clasps will only work where there is undercut in relation to the path of natural displacement (and ideally the path of insertion)

  
• Reciprocation:

  • Clasps need some form of reciprocation otherwise they will displace teeth or cause jiggling forces
  • Reciprocation can take the form of a clasp arm, in a circumferential or 3-arm clasp
  • Reciprocation can take the form of a rigid connector, for example when a major connector is extended right up to the gingival margin

  
• Indirect retention:

  • Prevents rotation/tipping of the denture about the clasping axis
  • Two well-chosen clasps will set up an axis of rotation between the clasp tips
  • Indirect retention = design elements of the denture that resist rotation about that axis
  • They can be rests, major/minor connectors and saddles. It is important to consider indirect retention on both sides of the axis
  • The further away from the axis the indirect retention, the better the mechanical advantage.

  
• Indirect retention:

  • It's often best to opt for an asymmetrical clasping axis, as this increases the likelihood that one of the indirect retainers is already in a useful position - antero-posteriorly and contra-laterally opposing clasps
  • Indirect retention is often a problem with free-end saddles and ultimately is determined by the type of major connector and whether rest elements can be placed onto the anterior teeth (we will come back to this next time)

  
• Bracing:

  • Bracing elements allow the denture to resist lateral forces whilst it is fully-seated
  • Rigid components placed against stable vertical surfaces and the lateral aspects of ridges will provide good bracing
  • Rotation can be resisted by incorporating rigid elements that are joined by the rigid major connector or by rigid reciprocating elements

  
• Major connectors:

  • It's important to consider aspects such as: tooth spacing, aesthetics, interferences, frenal attachments, clinical crown height, and prognosis of remaining teeth

  
• Major connectors:

  • Upper major connectors can take the form of:
  • Solid connector
  • Hollow or ring-style connector
  • Strap connector
  • Horse-shoe connector
  • The final choice comes down to pt preference + clinical judgement
  • Anything but a solid connector will lose significant:
  • Rigidity
  • Cohesive and adhesive forces
  • Bracing from the palatal vault
  • However, some pts who have had dentures before really like an open design
  • Severe denture intolerance may warrant a horseshoe design but don't go down this route lightly
  • Lower major connectors in pic

  
• Connectors, bracing and reciprocation:

  • Finally check whether your optimal design is as simple as possible
  • You should then take your cast and survey it to make sure that your chosen optimal design will work
• Surveying:

  • Stary by surveying your edentulous ridges and clasping teeth at the path of natural displacement (perpendicular to occlusal plane)
  • If you have little undercut, you may need to add some composite buccally. Conversely you may need to consider lowering a survey line if the undercut is too aggressive
  • Regardless of whether you now change the path of insertion, there needs to be common undercut to both planes. Use a different colour to mark undercut at the path of insertion.
• Surveying:

  • Check whether you wish to tilt the cast to engage undercuts, guide planes or improve aesthetics
  • A heels down tilt will often improve aesthetics anteriorly
  • Once you have chosen a new path of insertion, you should re-survey the teeth to check for common undercut
  • You must then mark on the cast your chosen path of insertion using vertical lines
• Flexible dentures:

  • Such as Valplast, Sunflex - you will see patients who have had these provided
  • Pts like them because there are often no visible clasps, and the denture clicks into place under divergent undercuts
  • Personally I think these have the potential to be more damaging than even pure acrylic dentures, because the load is not shared between hard and soft tissues, and they allow differential loading
  • But we need some good evidence to show this