RPD Retention

Question 1

What is meant by the term ‘direct retention’?

Answer 1

Resistance of a denture to VERTICAL displacement

Question 2

How is direct retention achieved in RPDs?

Answer 2

1. Mechanical (clasps)
2. Muscular forces (action of musculature on shape of denture)
3. Physical forces (adhesion, cohesion, pressure, friction = altered path of insertion)

Question 3

What does the efficiency of a clasp depend on?

Answer 3

1. Material (CoCr, gold, Ti, wrought SS)
2. Thickness and shape
3. Length (min. 15mm to engage in 0.25mm undercut for molar)

Question 4

What is meant by the term ‘indirect retention’?

Answer 4

Resistance of a denture to ROTATIONAL displacement

Question 5

In regards to clasps, what is meant by the term ‘reciprocation’?

Answer 5

An opposing element to the retentive clasp which prevents pressure from the retentive clasp acting on the tooth

Question 6

What is an ideal pattern of retention? (placement of clasps +/- indirect retention)

Answer 6

Triangular pattern

BUT…
One clasp on each side of arch sufficient depending on saddle length

DIRECT = 3 clasps
INDIRECT = 2 clasps, 1 indirect retainer

Question 7

What are the main methods for direct retention of acrylic dentures?

Answer 7

1. Musculature forces

2. Physical forces

Question 8

What is an ‘RPI system’?

Answer 8

Stress relieving clasp system used in free-end saddle designs

Prevents stress on last standing abutment tooth

Usually used in lowers

Question 9

What components of the RPD can by utilised as an indirect retainer?

Answer 9

• Major connector
• Minor connects
• Rests
• Saddle
• Denture base

Question 10

Where should an indirect retainer ideally be placed?

Answer 10

As far away from clasp axis as poss

i.e. perpendicular to clasp axis (terminal end of clasps)

Question 11

How does an RPI system relieve stress on the abutment tooth?

Answer 11

Load onto saddle causes rotation at mesial rest -> plate and i-bar rotates downwards + mesially –> disengages tooth –> no stress on tooth