Design Flashcards
What is the availability for removable partial dentures on the NHS?
Pay bands = cobalt chrome denture is in the £250 band and they cost way more than that to make!
What do we need to design a removable partial denture?
- Assessment
- Primary impression
- Surveyed articulated casts
Which assessments need to be carried out before designing a removable partial denture?
Extra oral, intraoral, soft tissues, perio status, X-rays, teeth present
Need to give diagnosis and treatment plan
Following assessment what needs to be done?
Treatment planning and mouth preparation (cannot put dentures into caries, periodontal disease or infection of the gingivae)
What are the hygienic principles partial denture design should incorporate?
- Avoid unnecessary coverage of gingival tissues
- components should be kept at least 3mm away from gingival margins where possible (Every’s principles = space so the saliva can wash away plaque)
- create spaces for natural cleansing and OH measures
What are the design principles?
- Dentures in themselves do not automatically increase caries and perio disease -> poorly designed and maintained prostheses can (even the best designed dentures increases the complexity of oral hygiene regimes for the patient)
- a prosthesis may aggravate existing conditions i.e. lichen planus
- dont want to make teeth looser
- avoid unnecessary gingival coverage
- tooth supported is ideal
- simplicity of design
- full extension of mucosal supported saddles
What are the stages of partial denture production?
- Indications/justification
- Complexity
- Design (in conjunction with patient = informed consent)
- Preparation (look at patients mouth, stabilised periodontal disease/ caries & adjust teeth to make it fit better)
- maintenance (see if principles are wrong i.e. loose tooth, cracked fillings etc.)
If you are seeing a patient who in the future is going to need a denture then what should happen?
Should plan indirect restorations in conjunction with the removable partial denture (cast restorations should not be constructed without a final denture design)
i.e. rest seats, undercuts and milled guid planes can be incorporated
= improved fit, retention and reduces perceived bulk of denture for the patient
LEARN THIS BY HEART:
What is the design process (6 core elements)?
- Saddles/teeth to be replaced (classification)
- Support
- Retention
- Reciprocation
- Anti-rotation/indirect retention
- Major connector
How do we decide the number of teeth to be replaced?
We replace teeth for aesthetics and function rather than to make up numbers no need for 7 to 7 replacement
-> may consider replacing 7’s if the posterior tooth has an opposing tooth and already replacing an anterior tooth
N.b. if the patient wants something what do you have to do?
Explain why thats not recommended or give them what they want
What are the different types of support?
Tooth-borne, Mucosa-borne or tooth and mucosa borne
What is support?
The resistance of movement towards the denture (stops it sinking into the soft tissue)
What are the different types of tooth support?
Cingulum rest, Occlusal rest & incisal rest
When using mucosal support what needs to happen?
Maximum extension (n.b. this is against principles of design so need to compromise) -> for uppers extend to hamular notch tuberosities and for lowers extend to retromolar pad = this is to spread out the forces of mastication
What is direct retention?
Resistance away from the tissues
What types of mechanical retention are there?
Clasps & precision attachments
What is the path of insertion and how is it involved in retention?
The plane the denture is put in by
The path of displacement = gravity so path of insertion cannot be the same as path of displacement otherwise the denture will fall out = so angle the path of insertion differently
What are the forms of physical retention?
- Neuromuscular control (n.b. why those who have had a stroke may find it much more difficult to keep a denture in)
- Saliva = produces a seal = helps hold it in
Ideally how many retainers do we want per denture if possible?
Only two
Which type of denture is most common?
Acrylic (for every one cobalt chrome denture 5 are constructed from acrylic)
n.b. the more teeth you are missing the less likely you are to have a cobalt chrome denture
What are the advantages of an acrylic denture?
- cheap
- relatively easy to construct
- easy to modify
What are the disadvantages of acrylic dentures?
- weak material (thin sections fracture)
- non rigid
- requires bulk for strength
- potential soft tissue damage
What are the potential damages caused by a partial denture?
Periodontal breakdown, plaque and oral hygiene, coverage of gingival margin (if come away too much, denture will snap) and occlusal forces
What is the potential damage caused by a free end posterior denture with a plate behind the anterior teeth?
The gum stripping effect (dont call it this) but it pishes the mucosa away from the lingual side of the anterior teeth causing recession)
What are the every principles?
- point contact between adjacent standing and artificial teeth
- wide embrasures
- no occlusal interferences
- 3mm gingival clearance
- correct denture extension with accurate fit and the polished surfaces to assist muscular control
- distal stop
What are the palatine fovea?
Opening of some salivary ducts
What are the different classification systems?
- Kennedy classification = the distribution of missing teeth
- Craddock = by support (tooth, mucosal, tooth and mucosal)
What is a kennedy class I?
Bilateral free end saddle
What is a kennedy class II?
Unilateral free end saddle
What is a kennedy class III?
Bounded saddle
What is a kennedy class IV?
An anterior bounded saddle that crosses the midline
What features does a Kennedy class I denture have?
- tooth and mucosal borne (differential movement inevitable)
- unless a ‘transitional denture’ then needs tooth support to prevent ‘gum stripping’ regardless of material
- needs optimal coverage of tissues in saddle areas, tooth support if possible and indirect retention to minimise rotation
What features does a Kennedy class II denture have?
Features and problems of class I & III - can be the most challenging with maximum denture base extension on free end saddle and correct direct retention
What features does a Kennedy Class III denture have?
- tooth supported with mucosal support only on long spans / decreased perio support
- direct retention is important
What features does a Kennedy Class IV have?
Class I in reverse = falls out when chewing
What are the kennedy classifications based on?
The most posterior edentulous area
What do we do when there is a more anterior saddle?
We call it a Kennedy class * modification number of extra saddles
What is support?
Resistance to vertical forces towards the mucosa
How do we plan which support should be used?
Determine which type is required for an individual case
- the state of the mouth indicates a long life span every effort should be made to have tooth supported
What are the advantages to mucosal borne dentures?
- Cheap
- Can be added to (acrylic)
- Preparation for completed dentures
- Periodontal disease (further tooth loss)
- Young children
What are the advantages of tooth borne dentures?
- Force are directed through the periodontal ligament
- If designed correctly = no damage to denture foundation
- Well tolerated
What happens when a tooth is put under excessive forces?
- Bone resorption
- Mobility of teeth
What should be considered when planning support
- The root area of abutment teeth (lower incisors only have 1 very thin root, too much force = wobbles out but molars, premolars and canines can be loaded without worry)
- Saddle extension
- Force expected on saddles
When do we survey?
Before we take secondary impressions
- After primary impressions have been poured and articulated
= used to design the dentures
= makes sure its suitable clinically
Why do we survey?
- To determine the path of insertion
- Determines the hard and soft tissue undercuts you are going to use (wanted & unwanted) = determines which material we can use
What is a surveyor?
A fixed horizontal rod that is held perpendicular to a horizontal platform = survey line for that path of insertion
Which materials can be placed in undercut or out of the undercut?
- Any rigid part of a denture must be designed to lie outside of the undercut area (only flexible parts may be designed to go into the undercut)
Which part of a retentive clasp is flexible?
The terminal 1/3rd of a retentive clasp are is flexible
Which materials can class be made of and what size of undercut does each need (3)?
Very rigid... Cobalt chrome = 0.25 mm Stainless steel = 0.5 mm Gold = 0.75 mm ...most flexible
Which material has the highest modulus of elasticity?
The most rigid = cobalt chrome
Which factors influence the path of insertion?
- Retention
- Guide surfaces
- Aesthetics
- Dead space interferences
Which Kennedy classifications require the most tilted path of insertion?
Kennedy class 1 and 4 to lock into undercuts
Which has better aesthetics a Kennedy class 1 or 4 denture?
Kennedy class 4 because bulbous canines = no black triangles
What is resistance?
Is opposition to the denture moving AWAY from supporting tissues during function
What are guide surfaces?
A series of surfaces parallel to each other and the path of insertion (smooth bulbosities = frictional contact assists in overall retention)
= ensure that dentures can be inserted and withdrawn along the selected path of insertion & reduces dead space
= 2-3 mm long
Which aesthetics need to be considered with removable partial dentures?
- Don’t want unsightly anterior clasps
- When replacing lost anterior teeth want to eliminate unsightly gaps between replacement and abutment teeth
- Path of insertion
- Labial flange
What is a dead space?
Any unwanted undercut areas between the survey line on the surface of the abutment tooth next to the partial denture framework or other teeth enclosed in the framework
= stagnation areas
= deliberate enlargement of space = hygienic open saddles
What should the path of insertion avoid?
- Interferences
- A compromised appearance
- A poort distribution of retentive undercuts
What are the objectives of design?
- The denture should be easily inserted and removed
- resist dislodging forces
- be aesthetically pleasing
- avoid creation of undesirable food traps (should add any damage)
- minimise plaque retention
- choose the optimum path of insertion for the denture
- to choose the design material and position of the clasps
What are the different specific components of a removable partial denture?
Saddles, rests, retainers (direct and indirect) & a major connector
What is a saddle?
Part of the denture that rests on and covers the tissues of the alveolar ridge
- can be made out of two types of material (most of the time it will be acrylic)
What features does a saddle need?
- Extension of the saddle
- Material of the fit surface of the saddle
- Design of the occlusal surface of the saddle i.e. no point replacing if no opposing tooth to posterior saddle
- Relationship of the saddle to the abutment tooth
Why is extension of the saddle important?
- needs to be as far as possible, upper = hamular notch, lower = retromolar pad
- polished surface must be in the “neutral zone”
- replaces tooth, gum and supporting tissue
What is the neutral zone?
Area of minimum contact where tongue, lips and cheek rest
What is a point contact?
Where saddle touches abutment tooth
What is an open contact?
Where saddle does not touch abutment tooth
Why is the material of the fit surface of a saddle important?
Can be acrylic or metal (usually acrylic so can add to it if bone resorption occurs & other advantages)
- metal fit surfaces are v. expensive, difficult to make, cannot be altered -> would only use if there was insufficient interocclusal space (acrylic fractures in small sections)
Why is design of the occlusal surfaces of a saddle important?
When mucosal or tooth and mucosal support is planned reduce the number and size of occlusal surface to reduce the force on the underlying mucosa during mastication i.e. no point replacing the UL7 if no LL7 (makes more technically difficult -> should communicate this with the patient)
Why is the relationship of the abutment tooth to the saddle important?
Closed vs. open design
= better to have open design when aesthetics is not an issue i.e. posterior teeth
What are rests?
A metal projection as part of the metal framework attached to the denture that extends onto the surface of a tooth (only really found on cobalt chrome dentures)
What sort of retention do rests provide?
Support = resistance to the forces towards the mucosa
What is the main function of a rest?
Support
What are the other functions of rests?
Indirect retention, deflect food away from saddle abutment junction, improve occlusal contacts when used as an onlay
What are the 3 types of rest?
= Occlusal (on molars or premolars)
= Incisal (on mandibular canine)
= Cingulum rest (on maxillary incisors)
(n.b. placement does not have to be symmetrical on left and right)
What do rests sit in?
Rest seats (on the tooth surface)
What are the essential features of a rest?
- Rigid
* Non-interference with existing occlusion
Which surface of the tooth must a rest be placed upon with a free end saddle?
Usually placed mesially on the tooth to prevent theoretical overloading and improved stress distribution (may pull tooth over)
What is retention?
Displacement away from the tissue (i.e. muscles)
What are retainers?
Components that prevent movement of the partial denture away from the tissue
When are retainers described as ‘direct’?
When applied to the abutment teeth to prevent withdrawal of the denture along its chosen path of insertion
When are retainers described as ‘indirect’?
When applied to teeth at a distance from a possible axis of rotation of a denture
What are the different forms of retention?
- Mechanical
- Muscular control
- Physical forces (saliva)
Give some examples of direct retainers:
- Clasps (grip hold of teeth)
Molars = occlusally approaching (wraps around tooth & made of cobalt chrome)
Premolars & canines = gingivally approaching (if cobalt chrome it needs quite a long length, adv flexibility 12 mm, if placed as occlusally approaching its not long enough on these smaller teeth)
Why is it a bad idea to put a direct retainer (even a gingivally approaching clasp) on an incisor?
Aesthetically bad!
What is the essential design criteria for an effective clasp?
- Flexible retentive arm
- Reciprocation
- Encirclement
- Passivity
Which factors affect clasp retention?
- Material of clasp
- Cross section of clasp
- Length of clasp
- Depth of undercut (0.25 mm cobalt chrome, 0.5 mm stainless steel, 0.75 mm wrought gold)
- Clasp design
How do we determine which clasp to use?
- Position of undercut (last 1/3/ of clasp that engages the undercut otherwise it fractures)
- Amount of bone support
- Length of clasp
- Appearance
Which other mechanical devices can be used?
Magnets, precision attachment and implants
What does RPI stand for?
R= rest P = proximal plate I = gingivally approaching I bar
When is RPI used?
Only in free end saddles in the mandible under masticatory load
What is a more practical approach to use than RPI?
Use a displacement impression technique
When are indirect retainers required?
In free end saddle situations where displacement occurs as a rotation of saddles away from the tissues e.g. a Fulcrum acid through the clasp tips BUT not at the expense of overly complicating the denture
What type of retainers can clasps be?
Both direct and indirect (simultaneously!)
What is bracing?
Opposes forces from the clasp to stop orthodontic movements of the tooth (resists horizontal or lateral forces WHEN DENTURE IS FULLY SEATED)
= achieved with the rigid 2/3rd portion of the clasp arm or the plate
What is reciprocation?
Resistance to horizontal or lateral forces WHEN DENTURE IS ON THE MOVE OVER THE MAXIMUM BULBOSITY OF THE TOOTH = prevents orthodontic tooth movement
What is key about the saddle?
Extension and number of teeth
What is key about support?
Mucosal broad extension
What is key about retention?
Design of clasp depends on morphology and perio status of tooth
What is a connector?
Links the saddles components of the partial denture together
What is a major connector?
Links the saddles to the denture
What is a minor connector?
Joins components such as rests and clasps to the saddle and major connector
What properties should a major connector have?
- Rigid to void flexion and distortion
- Vertical support and soft tissue protection
- Indirect retention
- Promote patient comfort
- Linking of saddle areas
What are the different types of maxillary major connectors?
- Rings
- Horse shoe
- Palatal strap
- Palatal bar
- Palatal coverage
What is the difference between a palatal strap and bar?
Pretty much the same but the bar is thinner but bulkier
When is a ring indicated?
Multiple bounded saddles
What is the advantage of a ring connector?
Patients say they can taste better (can feel the temp of the food)
What is the disadvantage of a ring connector?
Some people dont like the feel of it
What are the advantages of a horse shoe connector?
- Strong connector
- Useful for missing anterior teeth
- Relieves palate
- Gains some support from tissues
(especially handy for a retcher)
What are the disadvantages of a horse shoe connector?
- Can deflect and distort in free end saddle cases (must be rigid)
- Bulk of metal on anterior border can be a problem for patients
- No retention from palate
What are the advantages of a palatal strap?
- Thin and versatile
- can be in two planes = increased rigidity
- well tolerated by patients
- Good tissue support due to increased surface area
What are the disadvantages of a palatal strap?
- Must be at least 8mm in width for strength
- patients can be sensitive to strap border positioning (anterior border not on rugae, posterior border anterior to the junction between the hard and soft palate)
What are the disadvantages of a palatal bar?
- Bulky (stands out from palate)
- Narrow gaining little support from palatal tissues
- If anterior to second premolar region may impede speech
What are the advantages of a complete palate?
- Ultimate rigidity and support
- Generally the most comfortable for patients
What are the disadvantages of a complete palate?
- Extensive coverage of tissues
- Oral hygiene and denture hygiene issues
- Really difficult to adjusted (i.e. get a good post dam etc)
What are the maxillary major connector issues?
- Borders must be at least 3mm from gingival margins or extended onto the palatal tooth surfaces
- borders should blend with anatomy
- if perio support of teeth is compromised a connector which gains more tissue support should be used
- if perio support is good a palatal strap or bar may be used
Technician should ensure: thickness of metal should be uniform, all borders that contact son tissues should be beaded, these lines should be less distinct as they approach the gingivae
