BIOCOMPATIBILITY OF RESTORATIVE MATERIALS

Dorlands Illustrated Medical Dictionary defines the term

‘BIOCOMPATIBILITY’ as “being harmonious with life and not having

toxic or injurious effects on biologic function”.

In general, biocompatibility is measured on the basis of:

Localized cytotoxicity

Systemic responses.

Allergenecity.

Carcinogenicity.

Based on these criteria, the requirements for dental material

biocompatibility include the following:

1) It should not be harmful to the pulp and soft tissues.

2) It should not contain toxic diffusible substances that can be released

and absorbed into the circulatory system.

3) It should be free of potentially sensitizing agents that are likely to

cause an allergic responses.

4) It should have no carcinogenic potential.

Currently, a new document is being developed that

will meet international needs which is entitled “PRECLINICAL

1

EVAULATION OF BIOCOMPATIBILITY OF MEDICAL DEVICES

USED IN DENTISTRY – TEST METHODS”.

In a broad sense, a BIOMATERIAL can be defined

as any substance, other than a drug, that can be used for any period as a

part of a system that creates/replaces any tissue, organ / function of the

body.

Thus, when dentists purchase a material, they should

know if it is safe, and if it is safe, how safe it is relative to other

materials.

To evaluate the biocompatibility of materials various

tests are carried out.

A BRIEF NOTE ON PULPAL REACTIONS

Dentin protects the pulp and owes its vitality and its

sensitivity to stimulation of the dental pulp.

This intimate relationship has far reaching clinical

implications.

The nature of pulp reaction that follows peripheral

injury of the dentin.

1. Depends on the nature of causative

agent.

2

2. Its proximity to the pulp.

3

i.e. Attrition; abrasion i.e. Progressive i.e Iatrogenic

and Erosion Dental caries Injuries

PULP

RESPONSE: Mild Irritation Severe Irritation May result in

- Reparative dentine - Inflammation - Inflammation

of pulp of pulp

As pulp inflammation can also be iatrogenic in origin “Do Not

Harm” is a basic principal that should be followed by all members of

the health profession; as it is ironic that in attempting to correct the

damage caused by dental caries.

- Iatrogenic pulp injury can develop:

1. during the preparation of a tooth for

restoration.

2. during the insertion of the restorative

material.

3. it can be due to inherent irritational

properties of the material; either

a. clinical components of the

material.

b. injurious products generated

during setting of the material.

PULPAL REACTION can also be caused by Bacteria

4

a. Either residual bacteria left behind in the cavity.

b. Or by the bacteria that gain access to the cavity after restoration as a

result of microleakage.

Before restoration; while cavity preparation, a simple rule should be

followed.

“NEVER CUT DRY”

1. Avoid heat generation

by the use of H2O coolant

2. Dentin dessication.

Coming to the restorative materials; depending on their chemical nature

can be grouped as:

MATERIAL RESTORATIVE MATRIALS

NON-METAL RESTORATIVE MATERIALS

1. Amalgam.

2. Direct Filling Gold.

DENTAL CASTING ALLOYS

3. Technique Alloy (Gold)

4. Base Metal Alloys

1. Different Restorative Materials.

2. Acrylic.

3. Composite.

4. Porcelain.

1) Amalgam

Conventional Amalgam Restorations are considered

inert / mildly irritating to the pulp.

5

Mercury itself does not seem to contribute to any

pulp response.

It was suggested that physical forces during insertion

of the amalgam is a major factors responsible for greater

responses rather than the toxic, chemical, or thermal properties

of amalgam.

However, if reparative dentin is already present or if

a cement is placed prior to the insertion of the restoration there is

only little reaction to the conventional forces.

SOREMARK and ASSOCIATES (1968) Showed

that Hg reached the pulp in humans after 6 days if no cavity liner

was used.

The rate of diffusion of Hg into enamel and dentin

was inversely related to the degree of mineralization (It was

found that areas in dentin near the amalgam had a high Hg

content).

Thus older patients – there is less penetration of Hg ions.

Non-vital tooth – there is less even less penetration of Hg ions

(because the H2O component in E and D not reduced).

6

It was found that, the discolouration of tooth was

caused by Zn / Sn and not Hg when it gets corroded (instead it

was found that Hg repenetrates the Amalgam and reacts freely

with previously unreacted alloy particles.

HALSE (1975) confirmed these findings; using

human teeth.

Lichenoid reactions representing a long-term effect

in the oral mucous membrane adjacent to Amalgam restoration is

quite often.

Buccal mucosa and lateral borders of the tongue.

For many years a controversy has raged over the

biocompatibility of amalgam restoration because of the presence

of elemental Hg.

The symptoms of chronic Hg poisoning (element)

are:

Weakness Insomnia

Fatigue Irritability

Anorexia Shyness

Weight loss Tremors in the extremities

The signs and symptoms of methyl Hg poisoning

(sea food).

- Ataxia (gait disturbances).

- Paresthesia of extremities, lips and tongue.

7

- Constriction of visual fields (tunnel viscous).

Few patients react to extremely small amounts of Hg

with the signs and symptoms of:

1. Mercury poisoning.

2. Multiple sclerosis.

3. Epilepsy.

Dentists diagnosed this condition as “MICROMERCURIALISM HYPERSENSITIVITY”.

Accepted Hg levels:

Patient with amalgam – the over Hg level

- Normal 0.7ng/ml

Sea food per week

- 2.3 to 5.1ng/ml

2) DIRECT FILLING GOLD

The pulp responses from the insertion of cohesive

and compacted gold are also associated with condensation,

whether with land instruments / with mechanical pneumatic

instruments.

The responses develop when the condensation

occurs over freshly cut dentinal tubules, but not when dentinal

tubules are lined with pre-operatively formed reparative dentin

induced from previous episodes of disease / restorative

procedures.

8

Apparently it was found that; DG that is compacted

properly into sound tooth structure produces only a minimal pulp

response.

Under extremely rare conditions (1:1 million);

patients who have been sensitized to gold restorations with:

1. burning sensations.

2. lichenoid lesions of the oral mucosa.

3. generalized systemic reactions.

9

8.5% of the female patients had presented:

a. lesions of oral lichen planus.

b. Burning mouth syndrome.

Because of its (Ag) high thermal conductivity

patients experience POST-OPERATIVE SENSITIVTY. For a

woman, who reports that she is allergic to certain metals, the

following 3 options can be pursued if.

1) After a thorough medical history that includes questions on

dermatologic reactions to coins, jewellery / dental metals, we

can conclusively identify the ALLERGEN as the component

of a GOLD BASED / PALLADIUM BASED / BASE

METAL ALLOY (on trial basis).

2) If, the patient states that “she is allergic to gold alloy”; (this

situation is highly unlikely, because the incidence is less than

1% compared with an allergy potential of 10%for women to

Ni under extraoral conditions.

3) If, the patient c/o allergenicity to all metals and if out

examination fails to identify the most probable allergen, the

patient should be referred for medical diagnosis

(dermatologist / allergist).

10

DENTAL CASTING ALLOYS

We have to select alloys based on individual patients specific,

functional, and economic requirements There is no one alloy suitable

for all applications for e.g.: certain base metal alloys contain Be, Ni, Co

and Cr and the biocompatibility of each metal varies to different degrees of

tissue tolerance.

1) BERILLIUM : To date there have been no documented cases of Be

toxicity of dental origin.

- However, under uncontrolled conditions, when inhalation of

dust and fumes can be anticipated, the presence of Be

constitutes a recognized health hazard.

- It may result in

Acute form Chronic form

Responses

vary from contact dermatitis

to severe chemical

pneumatics

Symptoms

range from coughing, chest

pain and general weakness to

pulmonary dysfunction

11

A high mortality rate of dental technicians was

found due to the inhalation of Be vapour – which resulted in

lung cancer and death.

Therefore when grinding of Be containing alloys,

there should be adequate local exhaust ventilation.

2) NICKEL : Epidermiologic studies on workers in non-dental

industries have identified Ni and Ni compound as carcinogenic.

The major hazardous route is aspiration.

There is no experimental evidence that Ni

compounds are carcinogenic when administered by oral /

cutaneous routes.

It causes dermatitis (contact) because it is a potential

sensitizing agent and in sensitized patients intra-orally.

1. Burning and tingling sensation during

the first 24 hours and

2. later exhibited a slight erythematous

reaction in the mucosa.

However, there is no correlation found between the

incidence of Ni hypersensitivity and the presence intra-orally of

Ni alloy restorations.

12

Co alloys – have a potential for 1) dermatologic and

2) systemic effects that may result from patient and personnel

exposure to cobalt alloys.

Although, allergic reactions may be of some

concern, the toxicity potential of Co-Cr alloys appear to be

insignificant.

Palladium has also some allergic potential but

known patients of this metal allergy have not shown any reaction

in the mucosa, when a study was carried out.

NON-METALLIC RESTORATIVE MATERIALS

A major cause of iatrogenic pulp injury is

“CHEMICAL IRRITATION” caused by Restorative Materials.

DCNA classified.

The restorative materials into 3 major groups according

to their irritation qualities.

GROUP I GROUP II GROUP III

1) LOW IRRITATIONAL POTENTIAL

- Zn OE

- Polycarboxylate

- GIC

2) MODERATE IRRITATIONAL POTENTIAL

- Zn phosphate

3) HIGH IRRITATIONAL POTENTIAL

- Silicate cements

- Resins

13

GROUP I

ZINC OXIDE EUGENOL

Has low irritational potential.

Its pH, when freshly mixed is 7.

ZnO and E have an-OBTUNDENT EFFECT on pulp.

Eugenol inhibits the synthesis of prostaglandin (it should be

recalled that) are among the.

1) Chemical mediators of inflammation.

2) Contribute to pain sensation in areas of injury.

HYGROSCOPIC QUALITIES OF ZOE may result in:

1) Withdrawal of fluid from the pulp through dentinal tubules.

[Thus relieving pressure on sensory nerve ending of the pulp].

It has ANTIBACTERIAL property.

However; placed on an exposed pulp.

ZnOE – does not stimulate reparative dentinogenesis on the contrary; it

elicits a low-grade inflammatory response.

The low irritational potential of ZnOE makes it ideal as a

negative control in studies that evaluate pulp reaction to restorative

material.

14

ZINC POLYCARBOXYLATE:

It is remarkably INNOCUOUS despite a pH of 1.7 of the

polyacrylic acid liquid of the cement (this is due to the rapid rise of

the pH during setting of the cement).

The large molecular size of the polyacrylic acid and its

Tendency to form complexes with proteins would limit its

diffusion through the tissues.

In this regard polycarboxylate cements are equivalent to ZOE

cements.

POST-OPERATIVE sensitivity effects are negligible for

both cements.

GLASS IONOMER CEMENT

The pulp response of GIC is bland.

This blandness is attributed to the absence of strong acids and

monomers.

Polyacrylic acid and related polyacids are weak and possess

higher molecular weights that may limit their diffusion through the

DT to the pulp.

However studies of Pameyir and Stanley (1984) showed that

when anhydrous GIC was permitted to set:

15

1. Under pressure (continuous) - simulating crown cementation

2. pulp abscesses.

3. response occurred when RD thickness was 0.5nm / less.

4. intense haemorrhage

When the RDT was nearly 1mm – the set GIC caused a mean

inflammatory cellular pulp response of 1.67° which exceeded the

acceptable response level of 1.5° (Duralay study).

(Thus this study showed the importance of RDT in determining the

pulp response to GIC luting agents).

GIC, appear to be pulp irritants only when used as luting

agents.

Therefore it was recommended that small dab of CH be

applied only to areas of extensive crown preparations whenever

RDT was 1mm of the pulp before the cementation procedure was

carried out.

(This provided the required pulp protection to the critical areas without

decreasing the overall adhesion benefits of the GIC).

Lately, dentin adhesives that seal DT and infiltrate etched dentin

are being used in addition to CH.

GROUP II:

16

ZINC PHOSPHATE CEMENT

It has an irritational potential intermediate to ZnOE and

silicate cement.

As a base – it is not highly toxic.

As a luting agent – on pressure, causes – a WIDESPREAD 3

– dimensional lesion involving all the coronal pulp tissue - as the

phosphoric acid within the mix of Zn phosphate cement is forced in

the DT and after ¾ days.

An young tooth with wide – open DT is more susceptible to

such an intense inflammatory response compared to an older tooth

which has sclerotic / RD (that blocks DT and prevents the acids

from reaching the pulp).

The pH of the cement 3 minute after mixing is 3.5; the pH

rapidly increases thereafter, approaches neutrality in – 24 hours.

Thus, damage to the pulp occurs during the first few hours after

insertion of the cement.

This damage can be prevented by:

1. Application of appropriate varnish, DBA. Line thin

was of CH which eliminates 90% of the severity of

the adverse pulp response.

17

GROUP III:

SILICATE CEMENT

It has high irritational potential.

Being of its high-potential, it is used as an ideal material for

the control in studies that evaluate pulp reactions to restorative

material.

The pH is below 3, at the time of insertion; and the pH

remains below neutrality even after 1 month.

A CH base provides adequate pulp protection from quality of

the cement.

RESINS : (TOXICOLOGY)

There is no indication that commonly used acrylic resins

produce systemic effects in humans.

The amount of residual monomer in processed polymethyl

(methacrylate) is extremely low.

The oral mucosa and underlying tissues function as barriers

that significantly diminish the volume of monomer reaching the

blood stream.

Residual monomer that reaches the blood stream is rapidly

HYDROLYZED to methacrylic acid and excreted (It is estimated

18

that the half-life of methyl metacrylate in circulating blood in 20 to

40 minutes).

Clinical experience indicates that true allergic reactions to

acrylic resins seldom occurs in oral cavity.

Theoretically, such reaction (toxic and allergic) could occur after

contact with the polymer, residual monomer, benzoyl peroxide

hydroquinone, pigments etc.

The allergic reactions are dose dependent.

The surface monomer is completely eliminated after 17 hours

of storage in H2O.

Clinically, most patients reported denture-induced SORE

MOUTH which on evaluation indicates tissue irritation which is

generally related to unhygiene conditions / trauma caused by ill

fitting prosthesis.

Repeated / prolonged contact with monomer may result in

CONTACT DERMATITIS.

This condition is commonly experienced by dentists and

dental personnel involved in manipulation of acrylic resins.

Because of this possibility, dental personnel should refrain

from handling such materials with ungloved hands.

19

The high concentration of monomer in freshly mixed resins

may produce local irritation and serious sensitization of the fingers.

Finally, inhalation of monomer vapour may be detrimental.

Therefore the use of monomer should be restricted to well ventilated

areas.

COMPOSITE RESINS

This material whether conventional / microfilled;

autopolymerizing / photo activated (UV/VL) are found

IRRITATING to the pulp.

(a) CHEMICALLY CURED RESIN COMPOSITES:

The addition of fillers to the direct filling, CCRC in the

1960s and 70s did not reduce their potential for creating severe pulp

responses.

The filled resin; if not properly LINED, still cause

CHRONIC PULPITIS for an indefinite time even in cavities of

ordinary depth (Depth thickness of approximately 1mm).

This potential for irritating the pulp persisted because CIRC’s

still required the use of matrix pressure to enhance adaptation to the

cavity walls during polymerization.

20

The response of pulp to composite restorations may take

several days to 3 weeks to develop a massive pulp lesion.

Some moderate to severe degrees of pulp response could be

expected no matter which proprietary CCRC is used.

Thus, a thin coating of a hard-setting, Ca(OH)2 cement was

recommended for deep cavity preparations and over areas of all

freshly cut dentin before any composite material were placed.

Lichenoid reactions to Resin Based composites – on long-term

effect

(b) VISIBLE LIGHT CURED RESIN COMPOSITES

It is important to obtain as complete a polymerization as

possible through the entire composite restoration to minimize pulp

response.

The level of the pulp response in deep cavity preparations is

more because more chances of incomplete curing of the resin which

permits an even higher concentration of residual unpolymerized

monomers to reach the pulp.

A question can be raised now “which component of

composite elicits pulp injury?”

21

In a study of pulp reactions to 8 components of composite

resins, none of the components tested elicited significant pulp

injury.

It appears likely that reactive radicals generated during the

polymerization of the resin are responsible for pulp injury.

Pulp injury by resin restorations can be totally abrogated

through the application of a hard setting Ca(OH)2 base beneath the

resin.

With proper light curing technique with incremental layering

composite that were previously quite toxic to the pulp have become

less so with the elimination of the need for matrices and pressure to

gain acceptable adaptation.

CONDITIONING (ETCHING) AGENTS

As the resin restoration leak badly, acid etch technique was

developed to:

(a) Improve marginal seal between cavity and

restoration.

Acid etching of enamel is safe.

1. Provided a protective base of Ca (OH)2 is applied over

exposed dentin prior to etching.

22

Acid etching of dentin:

1. Markedly increases its permeability.

2. Removes the amorphous smear layer over cut dentin

(which plugs the orifices of the DT).

3. It demineralizes the peritubular dentin resulting in

increased tubular diameter. Such patent tubules

provide easy access of irritants to the pulp.

- Studies suggest that only the surface of the dentin 10-µm

depths) needs to be modified and not its deeper layers.

Conditioning techniques that are associated with weaker

acids, shorter periods of application, and the elimination of

rubbing and scrubbing procedures produce a minimal pulp

response and satisfactory bonding.

BONDING AGENTS

Bonding agents do not appear to be toxic.

Between 1975 and 1992, some studies demonstrated that

bonding agents helped.

1. Reduce the expected pulp responses induced by the

subsequent placement of the more toxic resin-based

composite materials.

23

Lee pharmaceutical cooperation in 1975

- had produced an “ADHESION BOOSTER” (used either

alone with enamelite, a resin composite / in conjunction with

50% phosphoric acid) that reduced the pulp responses, even

though the acid evidently removed the smear layer and

opened the tubules.

The resin primer is applied that infiltrate the demineralized

dentin surface (smear layer and tubules) and the exposed collagen

mesh to form a hybrid layer. On this layer a bonding resin is placed

and cured. This plugging of the DT’s prevents the penetration of

toxic components to the pulp from subsequently placed resin-based

composite restorations.

RESIN BASED COMPOSITE CEMENTS (DUAL-CURE):

In 1992 Pameyor and Stanley, found that:

Only when the dual-cure resin cement received no visible

light energy did the average pulp response level exceed the accepted

level of biocompatibility and resmbled pulp responses similar to

CCRC’s.

The increase exposure time to visible light is not harmful to

pulp tissue.

24

CAVITY VARNISH AND LINERS

Cavity varnish produces a positive effect on the reduction of

pulpal irritation.

This effect is because of the reduced infiltration of irritating

fluids through marginal areas.

The varnish also prevents penetration of corrosion products

of amalgams into the D.T.

It cannot be used under composite and GIC restoration.

Cavity liners like Ca(OH)2, GI and ZOE are used for

accelerating the formation of reparative dentin in deep cavities.

LUTING CEMENTS AND THEIR APPLICATIONS

1) For retentive small single tooth castings / 3-unit FPD’s.

Polycarboxylate (has minimal pulp irritation and lack of

postoperative sensitivity).

ZnP, GIC, IRM.

2) Long span FPD’s – Zn phosphate / GIC.

3) Sensitive teeth relieving cast restoration – Zn polycarboxylate /

RZOE (blandness).

25

4) Cast restoration in extremely caries active patients – GIC (cause of

F released through the postoperative sensitivity.

5) Porcelain veneers / inlays – Resin cements and liners.

6) Porcelain JC / Dicor – ZnP / Resin – Non irritation to pulp and

sufficient strength and added colour

7) P and C cementation – GIC (Increased strength anticariogenic and

increased flow).

8) Continually dislodged cast – GIC / Resin (with protect).

9) Castings cemented in a wet field – reinforced ZOE (lack of

sensitivity to moisture).

PROTECTIVE BASES

-Ideal bases should be well:

1) Tolerated by the pulp.

2) Should stimulate reparative dentinogenesis. (in case an undetected

microscopic exposure of pulp exists).

3) It should provide adequate protection of the pulp from irritant

component of the restorative material.

4) Abundant effect.

26

5) Antibacterial (to eliminate bacteria in residual carious dentin when

used with IPC).

6) Adequate compressive strength (to withstand forces incident to the

condensation of the material).

7) Low thermal and electric conductivity (to protect the pulp from

thermal shock and electrogalvanism).

8) Should exhibit low acid – solubility and etch tend (in case if the acid

comes in contact with the material).

For i.e.

1. For metallic restoration like amalgam.

a. A hard setting Ca(OH)2 and reinforced ZOE.

2. For DGF

a. Zn phosphate cement will provide a

stronger base, however; Ca(OH)2 liner /

cavity varnish should be applied prior to

the insertion of the cement – to protect

the pulp from chemical irritation.

PULP CAPPING (In Deep Cavities)

Numerous materials have been investigated as pulp capping

agents.

27

However, Ca(OH)2 preparations have best withstood the test

of time.

There has been a widespread concern among dentists that

Ca(OH)2 events a persistent stimulating effect on the pulp that

results in eventual obliteration of pulp.

Some also believed that it causes persistent inflammation of

the pulp.

The exact mechanism by which CH generates a dentinal

bridge is not clear but its.

Caustic action associated with its high pH (11 to 13), when

solubilized and its reduction of superficial necrosis.

Were assumed to be the factors responsible for stimulation of

secondary dentin formation.

MICROLEAKAGE

Brannstrom and colleagues (1971; 74) have proposed that

infection caused by penetration of microorganism from marginal

leakage around the restoration.

And the residual microorganisms left on the cavity floor

cause a greater threat to the pulp than is the toxicity of

restorative material.

28

When cavities are deep – Bacteria + Irritation potential of

restorative materials are responsible for pulpal irritation.

Bergenholtz (1982) pointed out that although

microorganisms may contribute to pulp responses beneath

reactions; they appear to be unable to sustain a long-standing

irritation to the pulp.

Unless recurrent caries develops under a clinically –

defective restoration; the dentin permeability to bacteria

decreases over time, allowing the pulp to heal.

This may partially explain why pulp remain VITAL in most

restored teeth.

Consequently pulp devitalization occurs due to:

1. Mechanical injury during cutting.

2. Toxicity of restorative material.

3. Action of bacteria.

Earlier high irritating solution were used for sterilization of

the cavity.

But the need for cavity sterilization is questionable.

Because:

1. Dentin has shown to resist bacteria invasion.

2. and deep cavities are usually given a protective base

of Ca(OH)2.

29

Both of which have shown ANTIBACTERIAL

PROPERTIES

A safe and effective measures for cavity cleansing is simply

rinsing the cavity with warm H2O.

30