4. RPI Flashcards
What aspects of denture support/retention/stabilization are demonstrated in each aspect of the teeth
- Occlusal third
- Middle third
- Gingival third
- Occlusal third= support (rests)
- Middle third = Stabilization (reciprocation arms, and plates)
- Gingival third= retention
Indications for infrabuldge clasps
- Esthetics
- Distal extension
- Decreased caries incidence
What is the clasp that is suprabuldge and stress releasing
combination clasp
List all the infrabuldge clasps
- RPI (I-bar)
- T-bar (or clasp)
- 1/2 T-bar or modified T-clasp
Describe the movement of the distal extension partial and abutment during occlusal loading with a suprabuldge clasp
Fulcrum= distal occlusal rest
- Distal extension moves down and mesial toward the ridge
- Clasp moves up engaging the undercut
- Forces move the abutment tooth distally
Describe the movement of the distal extension partial and abutment during occlusal loading with an RPI clasp
Fulcrum is at the mesial occlusal rest
- rotation occurs at the mesial occlusal rest (ball-and-sock like)
- Downward movement of the distal extension –> downward and mesial movement of the clasp –> disengagement of the undercut on the abutment tooth
- No torquing forces on the tooth
Describe the movement of the distal extension partial and abutment during occlusal loading with a wrought wire clasp
Flucrum is at the distal occlusal of the abutment tooth
-Wrought-wire clasp is flexible and is capable of bending in many different directions (compared to half-round clasp arm)
What are the two forms of reciprocation on a combination clasp
- Reciprocal arm
- Plate
What are the two ways to incorporate a WW clasp into a partial
- Solder it to the framework
- Incorporation into the acrylic
What are the three components of the RPI accemblt
- I- bar
- Proximal plate
- Mesial rest
Describe the design and mechanics of the mesial rest
- Ball and socket rest design*
- Allows for true rotation through the rest when toward the distal extension
- Moves the rotational axis anteriorly
Describe where the proximal plate extends occlusally and lingually
Occlusally= Extends ~1mm occlusal to the gingival portion of the guide plane
Lingually= Extends to the DL line angle of the tooth
Why does relief wax need to be placed gingival to the inferior portion of the guide plane
When the distal extension moves toward the tissue the framework will release into the area of gingival relief
The guide plane is prepped from _ to _
marginal ridge to the junction of the middle and gingival thirds
Decreasing the amount of surface area of contact between the proximal plate and the guide plane results in
more even distribution of functional force between the abutment and ridge
The GP is prepped to what width
2/3 the width of the cusps
For the I-bar the lowest point of contact is… the highest point of contact is…
Lowest= undercut Highest= HOC
The I bar is placed where relative to the HOC
Mesial or mid buccal
The I bar is placed (mesial/distal) to the rotational axis
distal (because remember the fulcrum in an I bar is a MESIAL occlusal rest!)
The I bar moves in what two directions upon loading of the distal extension
gingivally and mesially
What serves as reciprocation for an I-bar
- remember there is no reciprocation arm*
- Minor connector of the mesial rest
- Procimal plate (wraps to the dl line angle)
What the the three points of contact that provide encirclement for an I-bar
- I-bar (buccal)
- Proximal plate (distal)
- Minor connector of mesial rest
To obtain encirclement the clasps must surround the abutment teeth a minimum of _ degrees
190
Advantages of RPI assembly
- Stress release (doesn’t put stress on abutment during chewing)
- Mesial rest placement (places all forces along the long axis of tooth and toward the mesial where there is an adjacent tooth)
- Minimizes gingigval trauma
- Direction of forces (toward the mesial)
- Prevents posterior movement of framework (due to mesial rest)
- Decreased caries risk
- Esthetics
What happens when the distal extension is loaded with an RPI… where are the forces directed to
- Clasp rotates through mesial rest
- Proximal plate moves into the gingival relief areaa
- I-bar moves gingivally and mesially
Forces are directed
- Along the long axis of the abutment
- Posterior residual ridge
What happens when you increase the height of the PP occlusally
- Transfers more stress to the tooth (less on the ridge)
- Fulcrum shifts from the mesial rest to the PP (assembly no longer rotates as planned)
Advantages of the mesial rest are
- Allows for rotation (resulting in movement of the claps gingivally and mesially thus less torquing forces on the tooth)
- Directs the forces along the long axis of the tooth
- Drives abutment mesially into the adjacent tooth
What would happen if an I-bar was paired with a distal rest
- I-bar would engage the undercut upon distal extension loading
- Forces on abutment would cause movement distally
With a RPI the support of the denture relies more on (abutment teeth/ridge)
ridge
Explain how RPI reduces gingival trauma
- Relief space prevents compression of distal gingiva by the proximal plate
- Less horizontal components allow more normal stimulation
Posterior (Distal) movement of the framework is prevented by what two areas
- I-bar mesial to the mid-buccal HOC
- Minor connector contacting at the MO embrassure
Why does RPI –> decreased caries susceptibility
-Less tooth coverage
_% of people can’t see the gingival third of a maxillary and mandibular cuspid
Maxillary= 44% Mandibular= 93%
What are the disadvantages of RPI design
- Increased force on the ridge
- Increased gingival coverage
- Decreased stability (due to less horizontal components)
- Need to control tooth contours is exacting
- May need to crown abutments
Contraindications for RPI
- Insufficient undercut (lingually tipped tooth or no MB undercut)
- Revere rotation of abutment (prevents correct placement of mesial rest and negates proper placement of rotational axis)
- Severe mesial inclination of abutment (prevents creation of guide plane)
- Height of FOM <8 mm requiring a lingual plate
What would requiring a lingual plate rather than a lingual bar contraindicate the use of an RPI complex
because the lingual plate negates the principle stress release as the rotational axis moves from the rest to the area of plate contact
-Thus the plate becomes the area of reciprocation not the pp.
What is a condition where a lingual plate and RPI may be used
When the I bar is used for esthetics
Contraindications for all infrabuldge clasps
- Insufficient attached gingiva
- Inadequate vestibular depth (severe soft tissue undercuts or buccal tilt of abutment tooth)
