RPD design retention

Question 1

what is retention

Answer 1

a term used to describe the resistance of a denture to lifting away from the tissues (vertical dislodging forces)

Question 2

how can retention be achieved

Answer 2

mechanical means
muscular forces
physical forces

Question 3

how is retention achieved mechanically

Answer 3

through clasps

Question 4

how is retention achieved through muscular forces

Answer 4

the action of surrounding musculature on the shape of the non-impression surface (polished surface) of the denture

Question 5

how is retention achieved through physical forces

Answer 5

using forces of adhesion, cohesion, surface tension and atmospheric pressure on the impression surface of the denture

Question 6

what is adhesion

Answer 6

surface forces of saliva on denture and mucosa

Question 7

what is cohesion

Answer 7

forces within saliva, viscosity

Question 8

what does atmospheric pressure allow

Answer 8

resistance to displacing forces

Question 9

what is direct retention

Answer 9

resistance to vertical displacement of the denture

Question 10

what is indirect retention

Answer 10

resistance to rotational displacement of denture

Question 11

for direct retention, where do we want it to be

Answer 11

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

Question 12

what are guide planes

Answer 12

they are parallel surfaces which sometimes occur naturally

Question 13

how do guide planes provide long term retention

Answer 13

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.

Question 14

how does a clasp provide mechanical retention

Answer 14

A clasp has a metal arm that when in position contacts the tooth preventing removal of the denture base

Question 15

where is a clasp placed

Answer 15

To be effective as a retainer the clasp is placed in the undercut

Question 16

what are the two types of clasps

Answer 16

occlusal approaching

gingival approaching

Question 17

what is reciprocation

Answer 17

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

Question 18

how does the other arm provide reciprocal action

Answer 18

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

Question 19

why should a clasp not be designed without reciprocal action

Answer 19

A lone clasp arm places unacceptable lateral stresses on a tooth during insertion, removal and function, really an intermittent orthodontic appliance

Question 20

what is the magic number for retainers

Answer 20

3

Question 21

wat is an RPI system

Answer 21

Used in a free end saddle

Prevents stress on last abutment tooth

Question 22

what are the 3 components of an RPI system

Answer 22

rest
proximal plate
gingival approaching I-bar clasp

Question 23

what is a rest (RPI)

Answer 23

put on the mesial of the tooth. It prevents any distal tipping of the tooth when occlusal forces act on the posterior saddle

Question 24

what is a proximal plate (RPI)

Answer 24

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

Question 25

what does the I-bar clasp do in RPI

Answer 25

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.

Question 26

what happens if the clasp is very close to the rest in indirect retention

Answer 26

If the clasp is very close to the rest then the free end saddle will pivot

Question 27

where do we need indirect retention

Answer 27

look at plane of retainer closest to the saddle area

perpendicular to that