7 Prosthetics and gerodontology

Chapter 7

Prosthetics and gerodontology

Laura Mitchell

David A. Mitchell

Lorna McCaul

Contents


Relevant pages in other chapters Bridges—treatment planning and design, p. 260; occlusion, p. 238; acrylic and other denture materials, p. 640; casting alloys, p. 630; impression materials, p. 626.



Principal sources and further reading M. R. Y. Dyer 1989 Notes on Prosthetic Dentistry, Wright. J. C. Davenport 1988 A Colour Atlas of Removable Partial Dentures, Wolfe. J. F. McCord 2002 Missing Teeth: A Guide to Treatment Options, Churchill Livingstone. D. W. Bartlett 2004 Clinical Problem Solving in Prosthodontics, Churchill Livingstone.


Treatment planning

Reasons for prosthetic replacement of missing teeth

Restore aesthetics.
↑ masticatory efficiency.
Improve speech.
Preserve or improve health of the oral cavity by preventing unwanted tooth movements.
Improve distribution of occlusal loads.
Space maintenance.
Prepare patient for complete dentures.

Disadvantages of prosthetic replacement

↑ plaque accumulation/changes in composition.
Damage to soft tissues and remaining teeth, exacerbated by poor denture design and/or lack of patient care.

Treatment options for the partially dentate mouth

No replacement of missing teeth. If the benefits of a prosthesis do not outweigh the disadvantages, then replacement is C/I. An occlusion with first premolar to first premolar present in each jaw (shortened dental arch) is usually functionally adequate. Poorly controlled epilepsy is a C/I to dentures.
Bridges (p. 260) are preferable for short bounded spans in well-motivated patients.
Removable partial dentures. Indicated for patients with satisfactory OH and whose remaining teeth have an adequate prognosis, or as a training/interim appliance prior to F/F.
Complete immediate dentures. These are indicated for patients who have already mastered wearing a partial denture and whose remaining teeth have a poor prognosis.
Extraction of the remaining teeth and provision of a denture after healing has occurred. Avoid if possible as considerable guesswork is involved in the subsequent denture and the chances of the patient coping successfully are ↓.
In the older, partially dentate patient it is important to assess whether the patient is likely to retain some functional teeth for the remainder of their life-span. If this is improbable, some advocate providing F/F dentures while the patient is still young enough to adapt.

Treatment planning for partial dentures

NB It is important to enquire about previous denture history (just because a patient is not wearing a denture does not mean that they have not had one) and assess the reasons for failure or success. If a patient produces an extensive collection of unsuccessful dentures, unless there is an obvious and easily remedied fault, it is probably wiser to assume that you are unlikely to succeed where so many have failed and refer the patient for a specialist opinion.

Relief of pain and any emergency treatment.
History and exam, including a thorough clinical and radiographic assessment of remaining teeth and edentulous areas.

Unless immediate dentures planned, extract any teeth with poor prognosis.
OH and periodontal treatment.
Preliminary design of partial denture.
Carry out restorative treatment required.
Modify design if necessary and commence prosthetic treatment (p. 304).

Treatment planning for complete dentures

Relief of pain and any emergency treatment, including temporary modification of existing dentures, if indicated.
History and exam.
Investigation and treatment of any systemic problems.
Removal of pathological abnormalities (e.g. retained roots), and pre-prosthetic surgery, if required.
? rebase (p. 318), copy (p. 326), or construct new dentures (p. 308).

image Discussing with the patient the limitations of dentures prior to their construction is more likely to be viewed as explanation, whereas leaving it until after fitting the dentures will be seen as making excuses!

Principles of removable partial dentures

Definitions

Saddle

That part of a denture which rests on and covers the edentulous areas and carries the artificial teeth and gumwork.

Connector (major and minor)

Joins together component parts of a denture.

Support

Resistance to vertical forces directed towards mucosa.

Retainers

Components which resist displacement of denture.

Indirect retention

Resistance to rotation about clasp axis by acting on the opposite side to the displacing force.

Fulcrum axis

Axis around which a tooth- and mucosa-borne denture tends to rock when saddles are loaded.

Bracing

Resistance to lateral movement.

Guide planes

Two or more parallel surfaces on abutment teeth used to limit path of insertion, and improve retention and stability.

Survey line

Indicates the maximum bulbosity of a tooth in the plane of the path of withdrawal.

Free-end saddle

An edentulous area posterior to the natural teeth.

Stress-breaker

A device allowing movement between saddle and the retaining unit of partial denture.

Gum-stripper

A tissue-borne partial denture which can ‘sink’.

Dysjunct denture

Has complete separation between tooth- and mucosa-borne parts.

Swinglock denture1

Has a labial retaining bar or flange which is hinged at one side of the mouth and locks at the other.

Sectional denture

Made in two or more sections which are then fixed together with screws or other devices.

Classification

Kennedy

Describes the pattern of tooth loss (Figure 7.1):

I Bilateral free-end saddles.
II Unilateral free-end saddle.
III Unilateral bounded saddle.
IV Anterior bounded saddle, only.

Any additional saddles are referred to as modifications (except Class IV), e.g. Class I modification 1 has bilateral free-end saddles and an anterior saddle.

display

Fig. 7.1 Kennedy classification.

Craddock

Describes the denture type:

1) Tooth-borne.
2) Mucosa-borne.
3) Mucosa- and tooth-borne.

Acrylic versus metal dentures

Approximately 75% of the dentures provided in the UK have an acrylic connector and base. Although metal bases are generally preferred because the greater strength of metal permits a more hygienic design, an acrylic base is indicated for:

Temporary replacement, e.g. following trauma or in children.
Where there is inadequate support from the remaining teeth for a tooth-borne denture.
When additions to the denture are likely in the near future.

However, where financial constraints C/I a metal base, attention to the following may avoid the production of a gum-stripper:

Wide mucosal coverage to provide maximum support.
Keep base clear of the gingival margins wherever possible.
No interdental extensions of acrylic.
Point contact and wide embrasures between natural and artificial teeth.
Labial flanges for extra retention and bracing.
Additional support from wrought SS rests.

Components of removable partial dentures

Saddles

can be made entirely of acrylic or have a sub-framework of metal overlaid by acrylic.

Rests

are an extension of the denture onto a tooth to provide support &/or prevent over-eruption. Occlusal rests are used on posterior teeth (usually over either the mesial or distal marginal ridge and fossa) and cingulum rests on anterior teeth. Rests may be wrought or cast; the latter is preferred for strength and fit.

Clasps

provide direct retention by engaging the undercut portion of a tooth (Figures 7.2 and 7.3). The action of a clasp must be resisted either by a non-retentive clasp arm above the maximum bulbosity of the tooth or by a reciprocal connector. Clasps can be classified by their position (occlusally approaching or gingivally approaching) or by their construction and material.

display

Fig. 7.2 Occlusally approaching three-arm clasp.
One arm is the bracing reciprocal arm
One arm is the retentive component
One arm is the occlusal rest

display

Fig. 7.3 Gingivally approaching T clasp

Cast

(cobalt chrome) clasps are stiff, easily distorted and liable to #. However, provided they are limited to undercuts of 0.25mm, the advantage of being able to cast them as an integral part of a denture framework offsets these drawbacks.

Wrought

clasps are usually attached by insertion into the acrylic of a saddle. SS is the most commonly used alloy, but gold clasps are more flexible and easily adjusted (and distorted).

The stiffer the wire the smaller the undercut that can be engaged. This can be offset by reducing the diameter of the wire to ↑ flexibility (but ↑ the likelihood of #) or by increasing the length of the clasp arm (e.g. gingivally approaching clasp). Cast cobalt chrome can be too stiff for occlusally approaching clasps on premolar teeth. The actual design used depends upon:

Depth of undercut: 0.25mm—cast cobalt chrome; 0.5mm—SS wire; <0.75mm—wrought gold.
Position of undercut on tooth and relative to saddle, e.g.:

High survey line: gingivally approaching clasp or modify tooth shape by grinding.
Diagonal survey line, (a) sloping down from saddle: gingivally or occlusally approaching (ring or recurved) clasp; (b) sloping up from saddle: gingivally or occlusally (circumferential) approaching clasp.
Medium survey line: as above.
Low survey line: modify tooth shape, e.g. with resin composite.
Position of tooth. Gingivally approaching clasps are less conspicuous and are therefore preferred for anterior teeth.
Occlusion: adequate inter-occlusal space should be present or created for a clasp arm to cross a contact point between two natural teeth, to prevent occlusal disruption.
Shape of sulcus: fraenal attachments and alveolar undercuts may prevent use of gingivally approaching clasps.
Periodontal health: reduced periodontal support requires more flexible clasps to avoid overload.
Material of denture base. Cast clasp arms are easily cast as part of the framework but for acrylic dentures wrought clasps are more usual.

Connectors

In addition to joining parts of the denture together, the connector can also contribute to support and retention.

P/– connectors

Table_Image

–/P connectors

Lingual bar should only be used if there is >7mm between floor of mouth and gingival margin to give 3mm clearance from gingivae. Does not contribute to indirect retention. Usually cast. C/I if incisors are retroclined. If insufficient space can use sublingual bar.
Sublingual bar lies horizontally in anterior lingual sulcus, but opinions differ as to patient tolerance. More rigid than lingual bar.
Lingual plate is well tolerated and provides good support, bracing, and indirect retention if used in conjunction with rests but covers gingival margins. Can be made of cast metal or acrylic.
Continuous clasp is really a bar which runs along the cingulae of the lower anterior teeth and is usually used in conjunction with a lingual bar. Poorly tolerated.
Dental bar is similar to continuous clasp, but of ↑ cross-sectional area and without lingual bar. Useful for teeth with long clinical crowns. Provides support and indirect retention. May not be well tolerated.
Buccal/labial bar is indicated when the lower incisors are retroclined.

Figures 7.2 and 7.3 show the two most commonly used types of clasp.

Removable partial denture design

P/P design is carried out after assessment of the patient and with reference to any previous dentures (Figure 7.4). A set of accurately articulated study models is essential.

display

Fig. 7.4 Sectional partial denture.

Surveying

Objectives:

Establish path of insertion.
Define those undercuts which may be used to retain denture.
Define those undercuts which require blocking out prior to finish.

If the path of insertion is at 90° to the occlusal plane insertion of the denture will be straightforward; however, where the teeth are tilted or few undercuts exist, an angled path of insertion may be advantageous. Which provides more resistance to displacement during function is controversial.

A survey line can then be marked on the teeth to indicate their maximum bulbosity in the plane of the path of withdrawal. This is done using a dental surveyor.

Design

1. Outline saddles

Usually straightforward. If <½ tooth width or if in doubt of the need to replace a missing tooth, omit.

2. Plan support

Support can be tooth only, mucosa only, or both. Tooth-borne support (occlusal and cingulum rests) should be used wherever possible, as teeth are better able to withstand occlusal loading and support will not be compromised following resorption. Tooth and mucosa support are inevitable with large or free-end saddles and where plate designs are used. Tissue-only support should be utilized when no suitable teeth are available, and is less damaging in the upper than the lower arch, because of the palatal vault.

Need to assess the role of the denture, length of the saddles, the amount of support required (? denture opposed by natural or artificial teeth), and the potential of remaining teeth to provide support (root area in bone), before a final decision is made.

3. Obtain retention

Retention can be:

Direct

e.g. clasps, guide planes, soft tissue undercuts, or precision attachments. Of these, clasps are the most commonly used. The best arrangement is to use three clasps as far away from each other as possible. Guide planes help to establish a precise path of insertion and withdrawal. Need be only 2–3mm in length, reducing reliance on clasps.

Indirect

This is derived by placing components so as to resist ‘rocking’ of the denture around direct retainers, e.g. by the position of clasps and rests and the type of connector. Particularly important with free-end and large anterior saddles.

4. Assess bracing required

Bracing is provided by the connector, maximum saddle extension, and the reciprocal arms of clasps. Elimination of occlusal interferences ↓ need for bracing.

5. Choose connector

After consideration of above. Is there space in the occlusion to accommodate the chosen connector? Where possible the connector should be cut away from the gingival margins.

6. Reassess ?

as simple as possible ?aesthetic.

Instructions to technician

Should include written details and diagram. Where some confusion may arise over the precise position of a component it may be helpful to mark this directly on the cast. Computer aided design programs are now available.

Some design problems

The lower bilateral free-end saddle (Class I)

This presents a particular problem because of a lack of tooth support and retention distally, small saddle area compared to force applied, and distal leverage on abutment tooth in function (which ↑ with resorption). Possible solutions include:

Maximize indirect retention by placing rests and clasps on mesial aspect of the abutment tooth and using lingual plate design.
Using a muco-compressive impression of saddle area to ↓ displacement in function. The altered cast technique.
Use fewer, smaller teeth and maximize base extension.
RPI system for distal abutment teeth. Mesial Rest, distal guiding Plate and mid-buccal I bar. During function the saddle moves tissue-ward and rotates around the mesial rest. The plate and I bar are constructed in such a way as to disengage from the tooth and avoid potentially harmful loading.
Stress-breaker design (advantages more theoretical than practical).
Use precision attachments (beware of overloading abutments).

Class IV

Can sometimes avoid unsightly clasps anteriorly by the use of:

A flange engaging a labial alveolar undercut.
A rotational path of insertion1 utilizing rigid minor connectors that are rotated into proximal undercuts anteriorly.
Inter-proximal undercuts, which may allow minimal display of clasps—‘hidden clasps’.
An acrylic spoon denture held in place by the tongue.

Multiple bounded saddles

A horseshoe design, which utilizes guide planes for retention, may be indicated.

Clinical stages for removable partial dentures

1. Assessment and treatment plan,

p. 294.

2. Take first impressions

These are usually taken using alginate in a stock tray. For free-end saddles modify the tray first with compound or silicone putty.

3. Record occlusion

If ICP is obvious the occlusion can be recorded conventionally (p. 240) at the same visit as first impressions. If ICP is not obvious, wax record blocks will be required and a separate visit. Where there are no teeth in occlusal contact, the steps involved are the same as for recording the occlusion for F/F (p. 312). If there is an occlusal stop, but insufficient standing teeth to produce a stable relationship of the casts, the procedure is as follows:

Determine the OVD and mark the position of two index teeth with pencil.
Define the occlusal plane using the record block on which this is easiest, e.g. tooth to tooth, tooth to retromolar pad.
Check the record blocks in the mouth, using the mark on the index teeth as a guide, and adjust blocks if necessary.
Record occlusion with bite-recording paste.
Check the relationship of the index teeth on the articulated casts corresponds to that in the mouth.

4. Mounted casts are surveyed and denture designed,

p. 202.

5. Tooth preparation

may be required to:

Accommodate rest seats. Rests need to be >1mm for strength, therefore if insufficient room in occlusion to accommodate this bulk, tooth reduction is required.
Establish guide planes.
Modify unfavourable survey line, e.g. ↓ bulbosity.
Increase retention, e.g. by the addition of resin composite to create undercuts.

6. Record second impressions

using a special tray. Alginate is the most commonly used material, but elastomers are preferable for deep undercuts. It is helpful to have a wax try-in before the framework is made. This enables you to confirm tooth position so that the retentive elements for the acrylic are placed appropriately.

7. Try-in of framework

Check extension, adaptation, and position of clasp, and rests. If casting does not fit, use of correcting fluid may reveal which areas to relieve.
Check upper and lower separately for OVD and occlusion, and then together.
Major faults: repeat second impressions.
Minor faults: adjust at finish.
Re-record occlusion, if required.
Select tooth mould and shade.
Altered cast technique, if required.

8. Try-in of waxed denture

Check position of denture teeth.
Check flange extensions/thickness.
Check OVD and occlusion.
Check aesthetics with patient and only proceed when patient is satisfied.
Prescribe post-dam relief areas and management of undercuts.

9. Finish

Once any fitting surface roughness is eliminated, the dentures are tried in separately, adjusting undercuts and contacts as required. The extension, occlusion, and articulation are then adjusted if necessary. Give the patient written and verbal instructions, and a further appointment.

Rebasing P/P

Acrylic mucosa-borne dentures can be rebased at the chair-side with self-cure materials, but difficulty may be experienced in removing the denture in the presence of undercuts, and the materials are generally inferior to the original denture base. Alternatively, P/P can be rebased in the laboratory by means of a technique similar to that used for F/F (p. 318). Alternatively, make a new denture. For cast metal dentures an impression can be recorded of saddle area using an elastomer or ZOE, whilst holding denture by the framework. In all cases care must be taken to avoid the introduction of occlusal errors, e.g. ↑ OVD.

Immediate complete dentures

When the remaining teeth have a poor prognosis management depends upon whether the patient is already a partial denture wearer or not.

Rx alternatives for patients with no previous denture experience

Extract remaining teeth, wait 6 months for resorption to slow, and then construct F/F dentures. A recipe for disaster!
Extract majority of posterior teeth leaving sufficient only to maintain OVD and occlusal relationship, and then make immediate complete dentures when resorption has slowed.
Provide partial denture and allow patient to adapt before progressing to an immediate complete denture. The best solution.

Rx alternatives for partial denture wearer

A ‘creeping partial denture’ to which teeth are added as required. This allows a gradual progression towards edentulousness and is preferable for the elderly patient.
Immediate complete denture. This has the advantage that the form and position of the natural teeth can be copied and is said to promote better healing and reduce resorption, but frequent adjustments and early replacement are necessary.
Overdenture (p. 328); may retain alveolar bone.

Types of immediate complete denture

Flanged Either full or part (extended 1mm beyond maximum bulbosity of ridge).
Open face No flange, artificial teeth sit over (or just into) the socket of natural predecessor.

Flanged dentures are preferable as they afford better retention and make subsequent rebasing easier. However, where a deep labial undercut exists into which it would be impossible to extend a flange, the choice is either surgical reduction or an open-face denture. Most patients choose the latter.

Clinical procedures

1) Assessment Warn the patient about the effects of resorption and the need for early rebasing/replacement.
2) Primary impressions (as for P/P, p. 304).
3) Secondary impressions in alginate or silicone.
4) Recording occlusion. Where there are sufficient posterior teeth remaining a wax wafer should suffice, and this can be taken at the same visit as impressions are recorded. Otherwise, record blocks will be required.
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Jan 5, 2015 | Posted by in General Dentistry | Comments Off on 7 Prosthetics and gerodontology
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