materials

Question 1

what are the different materials available for posts

Answer 1

1) METAL
- made of noble alloy for CPC
- stainless steel, cocr, co-ni for prefab posts
- great for strength
- failure likely to be catastrophic
- prefab posts have higher MOE (means it is stiffer), and thus it is more rigid and resists bending better

2) TITANIUM
- hgihly biocompatible
- but soft and mecahnically inferior
- poor strength can be improved by alloying but at the expense of biocompatibility

3) CERAMIC
- made of zirc oxide but are of low toughness (brittle)
- prone to fracture and should not be used

4) FIBER
- made of CR structures which are bundles of stretched aligned glass (quartz) or carbon fibers embedded in epoxy/ acrylic resin matrix
- mostly fail by debonding but is reversible

Question 2

advantages of fiber posts over metal

Answer 2

1) resins lend themselves to bonding so can theorectically create a monobloc

2) fiber posts MOE is lower compared to metal and hence they have reduced stess concentration in the root
- less likely to fracture, more likely to deform

3) adhesion to dentine + its approximate match in MOE means that post shape become secondary

4) metal posts may corrode but fiber based wont

5) if re tx is needed, fiber posts can be conveniently removed

Question 3

guidelines for post length and diameter

Answer 3

LENGTH
- should be at least equal but preferably eceeding height of clinical crown

• post should extend >4mm apical to bone crest or at least 0.5x of root length inside the alveolar bone
• should be as long as possible without disturbing the apical seal

DIAMETER
- does not exceed 1/3 of root width at its apical end

• ensure at least 1mm of root dentine remains around the entire length of the post

Question 4

how to achieve retention form for posts

Answer 4

1) POST SHAPE
- parallel sided posts are more retentive than tapered posts
- need snug fit of apical 2-3mm

2) PASSIVE VS ACTIVE
- threaded posts that engage radicular dentine are the most retentive
- but not recommended due to stress concentration

3) POST SURFACE TEXURE
- serrated or roughened post is more retentive than a smooth one

4) POST LENGTH
- relationship of retention with post length is not linear but increse length does increase retention

5) POST DIAMETER
- increasing post diameter to increase retention is not recommended due to minimal retentive gain and weakeneing of root unnecessarily

6) luting agent
- adhesive resin luting agents have the potential to improve performanes of retention

Question 5

how to improve resistance form for posts

Answer 5

1) stress distribution
- one of the functions of post & core restorations is to improve resistance to laterally directed forces by distributing them over as large an area as possible

• post design should enable stresses to be distributed as evenly as possible
• because the MOE of fiber posts and dentine is similar hence its like a monobloc

2) based on anatomic location
- greatest stress concs are found at shoulder margin (especially interproximally) and at the apex of the root and so dentine should be conserved in these areas if possible

3) post length
- stress reduced as post length incresae

4) shape of posts
- parallel side posts may distribute stress more evenly than tapered posts which may have a wedging effect

5) cementation
- high stress can be generated during insertion, particularly with smooth parallel sided posts without vents for cement escape

6) rotational resistance
- prep geometry should prevent a post with circular cross section from rotating during function
this is usually not a problem as remainng coronal tooth structure is sufficient to prevent rotation
- if coronal dentine is lost, small groove can be placed in canal at bulkiest part of root (usually lingual) for anti rotation