RPD design retention Flashcards
what is retention
a term used to describe the resistance of a denture to lifting away from the tissues (vertical dislodging forces)
how can retention be achieved
mechanical means
muscular forces
physical forces
how is retention achieved mechanically
through clasps
how is retention achieved through muscular forces
the action of surrounding musculature on the shape of the non-impression surface (polished surface) of the denture
how is retention achieved through physical forces
using forces of adhesion, cohesion, surface tension and atmospheric pressure on the impression surface of the denture
what is adhesion
surface forces of saliva on denture and mucosa
what is cohesion
forces within saliva, viscosity
what does atmospheric pressure allow
resistance to displacing forces
what is direct retention
resistance to vertical displacement of the denture
what is indirect retention
resistance to rotational displacement of denture
for direct retention, where do we want it to be
close to the base and adjacent to the abutment teeth meaning we want the clasp tips to be as close to the saddle as we can
what are guide planes
they are parallel surfaces which sometimes occur naturally
how do guide planes provide long term retention
They do not reform during insertion/removal/loading and as long as you maintain the space (no drifting) then the frictional contact is kept and the denture is retained for longer as guide planes give long term retention. They are hard to find naturally and hard to make with a bur.
how does a clasp provide mechanical retention
A clasp has a metal arm that when in position contacts the tooth preventing removal of the denture base
where is a clasp placed
To be effective as a retainer the clasp is placed in the undercut
what are the two types of clasps
occlusal approaching
gingival approaching
what is reciprocation
As part of the clasp assembly each clasp unit will have an opposing element to the retentive clasp arm to prevent pressure from the clasp acting on the tooth
how does the other arm provide reciprocal action
The reciprocal action is provided by another arm on the otherside of the tooth; this arm lies on the survey line and is called the reciprocal arm. As it lies on the survey line it provides a balancing force but gives no retention
why should a clasp not be designed without reciprocal action
A lone clasp arm places unacceptable lateral stresses on a tooth during insertion, removal and function, really an intermittent orthodontic appliance
what is the magic number for retainers
3
wat is an RPI system
Used in a free end saddle
Prevents stress on last abutment tooth
what are the 3 components of an RPI system
rest
proximal plate
gingival approaching I-bar clasp
what is a rest (RPI)
put on the mesial of the tooth. It prevents any distal tipping of the tooth when occlusal forces act on the posterior saddle
what is a proximal plate (RPI)
undercut must be present below the distal plate allowing it to slide into the undercut relieving pressure from the tooth when masticatory forces act on the denture
what does the I-bar clasp do in RPI
provides retention and disengages from the tooth when occlusal forces are placed on the free end saddle. The clasp should be positioned on or anterior to the long axis of the tooth and this is important to ensure that displacement is achieved.
what happens if the clasp is very close to the rest in indirect retention
If the clasp is very close to the rest then the free end saddle will pivot
where do we need indirect retention
look at plane of retainer closest to the saddle area
perpendicular to that
