Post Endodontic Restoration-sem8 / orthodontic courses by Indian dental academy

POST-ENDODONTIC RESTORATION*************************************************************INTRODUCTION

WHY DO WE NEED TO RESTORE ENDODONTICALLY TREATED TEETH?

FUNCTIONS

WHEN DO YOU PROVIDE A POST-ENDO RESTORATION?

BASIC COMPONENTS OF A POST-ENDO RESTORATION

THE POST

THE CORE

THE FINAL RESTORATION

BIOMECHANICAL PRINCIPLES OF TOOTH PREPARATION

CLINICAL TECHNIQUES

CONCLUSION

INTRODUCTION

Root canal therapy cannot be summarized by saying, “fill it, shut it, forget it”. The final

restoration over an endodontically treated tooth is as important or probably even more

important than the actual root canal therapy itself.

The endodontically treated tooth constitutes a unique subset of teeth requiring

restorations because:

First and foremost, the tooth structure has already been weakened by previous

episodes of caries, trauma, restorative procedures and tooth preparations.

An accountable percentage of structural integrity is lost because of the root canal

therapy itself due to access cavity preparation and canal instrumentation leading

to increased flexing of the tooth and ultimately its fracture.

Some authors quote an alteration in the composition of the dentin such as a

change in the collagen cross linking, dehydration etc. Of course, whether such a

change is actually detrimental to the prognosis of the treatment is questionable as

studies have not been very conclusive about it.

Moreover such teeth are esthetically and physically compromised requiring

special kinds of restoration.

Finally and most importantly, the RC treated tooth has got an impaired

neurosensory feed- back mechanism because of the lack of pulpal tissue i.e. the

protective property of ‘proprioception’ is lost. This renders the tooth more

vulnerable to fracture under normal masticatory forces. So, a person can un-

intentionally bite too hard on the RC treated tooth compared to a vital tooth,

which can lead to its fracture and failure.

All these points emphasis the need of a wisely designed, accurate and strong

restoration over the endodontically treated tooth, which would not only protect the tooth

but also prevent any form of microleakage. Coronal leakage or orthograde contamination

via saliva has now been sited as the prime cause of endodontic failures.

Hence the functions of a post-endodontic restoration are:

- Providing a coronal seal.

- Providing protection to the remaining tooth structure.

- Restoring the esthetics and functions of the tooth.

Treatment planning for the post-endodontic restoration should start, ideally

speaking, prior to the commencement of the RCT itself. An extraction should be carried

out if the restorability and periodontal status of the tooth is questionable instead of

spending unnecessary time and expenses on it. If this is not the case and the tooth is

salvageable, then the following principles should be kept always in mind:

1. Preservation of remaining tooth tissue by conservative restorative design

compatible with acceptable aesthetics and function.

2. Reduction of stress and its favorable distribution within the remaining tooth tissue

by proper design and selection of materials.

Now, an important question before us is when to restore after endodontic treatment. Most

authors agree that since the success rate of RCT is pretty high at 80-95%, an arbitary

period of a couple of weeks is sufficient before providing a permanent restoration. A

permanent restoration may be delayed if the endodontic treatment has:

- Doubtful prognosis requiring possible re-treatment.

- Presence of sinus

- Tenderness to palpation /percussion

Basic components of the post-endodontic restoration are:

- The post

- The core

- The final restoration

Simply put, the post helps in the retention of the core, the core helps in the replacement

of the missing tooth structure and the final restoration helps in restoring the form and

function of the tooth and protects the remaining tooth structure from fracture.

Not all teeth require a post or a crown. Some require all 3 components while some may

need just an access seal.

THE POST

Definition:

The post can be defined as a relatively rigid restorative material placed in the

canal of a non-vital tooth.

Function:

The post has got 2 main functions:

- Retention of core

- Protection of the tooth by dissipation of forces along its long axis to the

surrounding periodontal tissues and the alveolar bone.

Ideal properties:

- Retentive to the tooth and the core

- Strong and provide protection to the tooth

- Radiopaque

- Biocompatible

- Inert

- Economical

- Esthetic wherever required

Indications

- Extensive coronal tooth structure loss/ >50% loss of tooth structure including

important landmarks such as marginal ridges, cingulum, reciprocating walls etc.

- Teeth which are going to be used as abutments for FPD’s, overdentures etc

- Teeth exhibiting extensive wear or patients having any parafunctional habits

which can cause excessive amounts of load on the tooth.

- Teeth that are going to become a part of a periodontal splint.

- For the retention of a complex restoration.

Contraindications:

Besides the opposites of the above points, certain other contraindication include:

- Endodontically treated teeth having a questionable prognosis requiring possible

re-treatment.

- Teeth having minimal canal dentin

- Teeth having unusual anatomy such as extreme curvatures and fragile roots.

Classification:

Numerous classifications have been quoted by several authors, which are more or less

similar to each other. The following is a simple way of classifying posts based on their

features:

1. Fabrication: Custom cast

Prefabricated

2. Materials used: Metallic

Non-metallic

3. Esthetics: Esthetic

Non-esthetic

4. Retention mode: Active / threaded

Passive/ cemented

5. Shape: Parallel

Tapered

Parallel with apical taper

6. Surface configuration: Smooth

Serrated

Threaded

7.Shank configuration: Solid

Hollow

Split

8. Venting: Vented

Non-vented

9. Resiliency: Rigid

Flexible

Features:

The posts, as mentioned in the classification, come in various diameters, lengths, tapers,

sizes, surface configurations, shank configurations etc. The choice of an individual post

selection is purely based on the operator’s discretion and the type of case before him.

Types of posts:

Posts can broadly be of 2 types – custom cast or prefabricated. Custom cast posts are

those posts which are made by taking an impression of the canal space either directly or

indirectly and casting a post which exactly replicates the canal space. The prefabricated

post is available commercially in predetermined dimensions, length and contours and

hence does not replicate the canal space. Some major differences between the 2 are:

The prefabricated metal post can be of 2 types based on their mode of retention

- Passive post or the cemented post which rely on a cementing media for

their retention

- Active post or the threaded post which actively thread into the dentin to

attain retention.

The non-metallic prefabricated posts can be either esthetic posts or non-esthetic posts.

THE CORE

Definition: The core can be defined as a restorative material placed in the coronal portion

of the tooth to replace the missing coronal tooth structure.

The core can be anchored to the tooth either via its extension onto the

coronal aspect of the tooth or via an endodontic post.

Ideal properties:High compressive strength

- Dimensional stability

- Should not deform plastically under loading

- Retentive to post, tooth and if possible to the overlying crown as well

- Should be easy to manipulate and place

- Should set rapidly

- Biocompatible

- Inert

- Economical

- Esthetic wherever indicated

Core materials: Advantages Disadvantages

Cast core High strength

No concern regarding

delamination from the post

Expensive

Time consuming

Casting inaccuracies

Amalgam High compressive strength

High modulus of elasticity

Easy manipulation and

placement

Stable to thermal and

functional stresses

Unesthetic

Cannot be bonded to

Tendency to discolour

adjacent gingiva

Tendency to corrode

Low early strength –

preparations cannot be done

immediately

Composite Esthetic

Bondable

Adequate strength

Command set- preparation

can be done immediately

Microleakage due to

polymerization shrinkage

Dimensionally unstable

Tendency to deform

plastically and thus cannot

be used in high stress areas

GIC Chemically adhesive to the

tooth

Anticariogenic

Low fracture resistance

Early moisture sensitivity

MMGIC -----------//-----------------

Used when>50% of tooth

structure is intact

Very brittle

RMGIC Properties lie in between

composites and GIC

Anticariogenic

More adhesive than GIC

Decreased moisture

sensitivity

Decreased microleakage

Tendency to expand in

presence of moisture- can

lead to fracture of overlying

all-ceramic crowns.

Pin-retained cores:

At times posts cannot be placed in the root due to anatomical factors or

due to the non-irretrievability of a silver cone obturation. In such situations, pins can be

used to gain an adequate amount of retention. Sometimes pins can also be used along

with a post to gain an auxiliary mode of retention.

Amalcore:

Nayyar and Walton have described a technique wherein the coronal 2mm of gutta-

percha is removed and amalgam condensed into it to attain adequate amount of retention

for the core. Studies have shown favorable results with this type of technique.

FINAL RESTORATION :

The final component of a post-endodontic restoration is the final or the definitive

restoration. As mentioned before, the final restoration helps to

Protect the remaining tooth structure from fracture

Protect the entire unit from microleakage

Restore the esthetics and functions of the tooth

Posterior teeth:Because of the vertical forces experienced by a posterior tooth, as a

general rule all endodontically treated posterior teeth require a full occlusal coverage

restoration. This could be a metal / a ceramo-metal crown or it could be an onlay.

Anterior tooth:Anterior teeth usually experience shear forces. Depending on the

structural integrity and the amount of discolouration of the tooth of the tooth, a

permanent restoration for an anterior tooth can take any of the following forms:

Teeth with posts = Jacket crowns-all-ceramic/ ceramo-metal

Teeth without posts but discolored = Laminate veneer /JC

Intact teeth with no discoloration = Composite for the access seal

PREPARATION PROCEDURES

Biomechanical principles governing placement of post-endodontic restorations:

CONSERVATION OF TOOTH STRUCTURE

Canal: Care should be taken to remove minimum amount of tooth structure from

within the canal space to prevent the fracture of the root either during cementation

or function. If the cross section of the tooth is considered as a ring, then the

strength of the ring is directly proportional to R4 – r4 where R= radius of the tooth

and r= radius of canal i.e the strength is drastically effected by a substantially

sized post. Thus, the canal should be enlarged just enough so as to seat a post

passively yet accurately at the same time ensuring retention and rigidity for the

post. Ideally, the post diameter= 1/3 the diameter of the tooth or the post should

have a minimum of 1mm of sound dentin surrounding it.

Coronal tooth structure: As much of the coronal tooth structure should be

preserved to avoid any kind of stress build-up especially at the narrowest

circumference of the tooth which is the cervical 1/3 of the tooth. This prevents

any chance of a fracture, which can occur during function.

Time and again, it is emphasized that a minimum of 2mm of tooth structure is

compulsory for the success of a post endodontic restoration. Why is this so? It is because;

this remaining vertical height of tooth is necessary to accommodate the ferrule or the

extra-coronal brace, which is going to be provided by the subsequent full coverage metal

crown/coping.

What is a ferrule?

A ferrule is defined as a 3600 metal collar of the crown or coping

surrounding the parallel walls of the dentin extending coronal to the cervical line of the

preparation resulting in an elevation of resistance form for the tooth.

Stresses in radicular dentin get concentrated at the narrowest circumference of the tooth.

If no ferrule is provided then constant forces on the tooth can lead to a vertical splitting of

the root during function. Providing an encasement of binding of the gingival 2mm of the

axial walls of the preparation reduces the incidence of fracture of the tooth by

reinforcement and dissipation of forces that concentrates at the narrowest circumference

of the tooth. Thus, a ferrule:

- Helps to bind the remaining tooth structure together

- Prevents root fracture during function

- Prevents stress concentration at gingival margin

To be successful, the ferrule must encircle a minimum of 2mm of vertical wall of sound

tooth structure between the cervical finish line and gingival extent of the core. In case the

remaining tooth structure is inadequate, then periodontal crown lengthening surgery or

orthodontic extrusion is necessary.

This effect can easily be understood by a simple example :

When a nail is hammered into a wooden peg, the latter tends to split along its long axis. If

a metal ring is placed to brace the impact, no such splitting occurs.

According to Wein and Ingle, if adequate tooth structure is present, a 450 bevel can be

placed in the occlusal aspect of the preparation to allow for an extension of the core

around the axial walls. This extra metal collar of the core gives extra resistance

to fracture and also acts as a stop against over seating / wedging action of the post and

core.

If inadequate, crown lengthening surgery or orthodontic extrusion should be done.

RETENTION FORM :

Post retention can be defined as the ability of a post to resist vertical dislodging

forces.

This is affected by:

- Preparation design

For circular canals = parallel walls

For elliptical canals = restricted taper of 60-80

- Post design:

Active > passive parallel> passive tapered

- Post length:

ore the length, more the retention

Ideally, length of post = Crown Length or at least 2/3-3/4 the

length of root

- Post diameter:

ore the diameter more is the retention but more is the fracture

susceptibility as well.

Ideally d < 1/3 cross- sectional d of root

- Surface:

Rough, serrated, abraded, notched, grooved or threaded posts provide more

retention than smooth. According to Nergiz, roughening the canal surface via

notching or grooving will enhance the retention of the prefabricated posts.

Luting cements: Studies have shown that resin cements are the most retentive

followed by Zn PO4, Type II GIC, RMGIC and finally polycarboxylate cements

- Number of posts: In multiple rooted teeth, more than 1 post can be used to

increase retention and retain core especially in severely broken down teeth.

Some considerations for posterior teeth:

- Relatively long / circular posts are to be avoided in curved ribbon shaped canals.

Short posts in divergent canals provide better retention.

- A cast core can be made in sections (split casting) with different paths of with

drawl

- The widest canal is selected for the major post and short auxiliary post spaces are

prepared in the other canals with same path of withdrawl

RESISTANCE FORM:

It is defined as the ability of the post tooth to withstand lateral and rotational

forces.

Resistance to stresses : is provided by -

1. Ferrule effect

2. Minimum internal preparation

3. Conservation of dentin apicaly and cervically as these are high stress

zones

4. Increase in post length

5. Avoiding sharp angles

6. Preferring parallel over tapered posts as the former distributes stresses

more evenly

7. Preferring vented parallel posts over smooth parallel posts (tapered posts

are self -venting)

8. Stresses induced by threaded posts can be reduced by backing off ½ a turn

9. Cement layer results in more even stress distribution.

Rotational Resistance : is provided by -

- If sufficient coronal tooth structure is present by the vertical / axial wall.

- If insufficient then –

A groove / notch at the orifice of canal in the region of max bulk is placed. Then

is called as anti-rotational notch/groove / element or keyway.

An auxillary can be placed

A cavity can be prepared in the post and adjacent tooth and amalgam can be

condensed

In posterior teeth a post in another canal can be used to provide resistance to

rotation.

CLINICAL TECHNIQUES:

1. Removal of endodontic filling material

> Silver cone

– Remove and reobturate with GP

- Earlier sectional silver cone obturation was suggested done by pre-

notching 3mm above the cone and twisting off the coronal portion

after placement. This should be clearly avoided as chances of

dislodgement and apical leakage is very high

> Gutta-percha

The timing for removal of GP is still controversial. It was previously believed

that 24 hours to a week delay was required to avoid disturbing the seal. Recent reports

reveal that adequately condensed GP can be removed immediately.

a) Non-carrier based obturation (lat/warm vertical/ injectable): GP can be removed

via :

- Heat using pluggers / system B /Touch ‘n heat.

- Chemical using solvents like chloroform, xylene, halothane turpentine etc.

- Mechanical using rotary instruments like GG drills, Peeso reamers, Parapost drills

NiTi rotary –instruments.

Of all these methods, fastest, safest and easiest method is the mechanical method. If

immediately removal is needed, then heat is the preferred method to avoid

distributing the seal.

b) If a carrier based obturation like Thermafil was planned, then pre-notching of

carrier 3-4mm above the apical extent can be done and the coronal portion twisted off

after insertion.

A radiograph is taken to confirm adequate removal of GP.

2. Post selection: as mentioned before depends on root morphology remaining

coronal tooth structure, occlusal forces and esthetic concerns.

3. Enlargement of canal: Is done to remove undercuts and prepare the canal to

receive an appropriately sized post. As mentioned before the enlargement should

not be more than 1/3rd the diameter of the root with at least 1mm of sound dentin

surrounding the canal.

4. Coronal tooth structure preparation

- Remove undermined enamel

- Prepare as though tooth was undamaged

- Prepare finish line at-least 2mm gingival to core to attain adequate ferrule design

- A contra bevel can be placed at junction of future core and tooth to provide an

additional metal collar from the core. This helps to:

Brace tooth against fracture

Provides a vertical stop against overseating

Reduces wedging effect

- At times because of excessive flaring / then dentinal walls due to caries extension a

post can be cemented prior to crown preparation to protect the weakened tooth

structure

Another approach is to fill the canal space with composite around a Luminex light

transmitting post, which transfers light to the composite but does not bond to it. The

post space thus created is treated routinely using peeso or low speed drills.

- If anti-rotational key way is required, a fracture 170 bur is used to make a groove /

notch extending 4mm into the canal to a depth of 0.6mm at the canal orifice at its

bulkiest portion

- Eliminate all sharp angles

5. Post Fabrication:

Custom –cast:

a) Direct Procedures

- Select sprue formers [wood/ plastic], which slide easily into canal till apical end

without binding.

- After lubricating the canal, coat sprue former with inlay wax, place in canal and

acquire a with inlay wax, place in canal and acquire a proper impression of the

canal space.

- Form a core with additional wax in the shape of the final tooth preparation

- This procedure can be done using cold cure acrylic instead of wax.

b) Indirect Procedures:

- An orthodontic ‘J’ shaped wire is verified for loose fit full length in the canal and

coated with an adhesive

- Canal is lubricated.

- Fill canal with elastomeric impression material

Using a lentulospiral

- Seat the wire and syringe in more impression material and seat the tray

- Pour the cast and fabricate the post and core on the cast done in the direct method

Investing and Casting:

Some important points to remember is:

- 1-2 cc of extra water is added to the investment a liner is omitted. This is done to

increase the casting shrinkage, which would result in a slightly smaller post that

does not bind to the canal and provides space for the cement. A tight fit could

cause root fracture.

A provisional restoration: Made of an orthodontic wire within the canal a temporary

acrylic JC should be provided to the patient.

CEMENTATION:

- All temporary materials are removed

- Canal is dried irrigation with EDTA and NaOCl

- Fit of post and core is checked with light pressure

- If it binds air abrade the post, reinsert, check for shiny spots and relieve them.

- Finish polish the core

- Place vertical groove if needed on the post to serve as an escape vent for the

cement

- Coat as canal with cement using a lentulo spiral

- Coat post with cement and slowly place the post and core into place.

For a prefabricated post, an appropriate post with adequate length and diameter is

selected. The length is adjusted so as to leave 2-3mm of it supragingival for core

retention. The core is built up over this.

If cementation is done resin cement is planned then the method is:

- Mix and place primer in dried canal

- Mix resin cement and coat the post with it

- Do not place paste in canal as it will set almost instantly an contacting primer

- Place post in canal and apply pressure for one-minute light cure it.

- Apply oxyguard for 3 minutes and wash it off

An advancement in the fiber reinforced post system is the poly ethylene woven ribbon

fibers or the Ribbond fiber post. Developed by Frielich, it involves the packing of several

strips of resin saturated Ribbond fibers into a conditional and primed post space along

with dual-cure resin cement. A core of composite is fabricated over this.

The all ceramic post and core are fabricated by:

Slip casting

Copy milling

Two piece technique = Zr post + copy milled ceramic core

Heat pressed technique = Zr post + pressed glass ceramic core

Even though endodontically treated teeth with their restorations have excellent prognosis

failures do occur a failure rate of 5-17% has been sited for post and core restry. These are

mainly due to:

1. Root fracture

2. Post fracture

3. Secondary caries

4. Periodontal disease

Careful case selection, adherence to Biomechanical principles, appropriate post selection

and meticulous maintenance of oral hygiene can prevent these problems.

CONCLUSION:

“ SO NEAR…BUT YET SO FAR”

From a simple piece of wire in 352AD, to the wooden post of the 18 th century to the

esthetic post of today, history has borne witness to the relentless efforts made to secure an

artificial tooth to the root. Judging by the pace at which research is moving, combining

material science and understanding with dental relevance, that day is not far when one

can provide an accurate and complete dental rehabilitation to an endodontically treated

tooth in minimal chair-side time with maximum satisfactory results. Here’s looking

forward to a 100% success rate for endodontics.