short presentation on mechanical properties of dental materials

1. MECHANICAL PROPERTIES OF DENTAL
MATERIALS
PRESENTED BY :
BHUPENDRA RIZAL
MDS 1ST YEAR

2. OUTLINE OF THE PRESENTATION
• INTRODUCTION
• WHAT ARE MECHANICAL PROPERTIES ?
• STRESS
• STRAIN
• MECHANICAL PROPERTIES BASED ON ELASTIC DEFORMATION
• MECHANICAL PROPERTIES OF TOOTH STRUCTURE
• STRENGTH PROPERTIES
• MASTICATION FORCE AND STRESS
• OTHER MECHANICAL PROPERTIES
• SUMMARY AND CONCLUSION
• REFERENCES

3. INTRODUCTION
• IN THE ORAL ENVIRONMENT, RESTORATIONS ARE SUBJECTED TO
HEAVY MASTICATORY FORCES. THESE FORCES ACT ON TEETH AND/
OR MATERIAL PRODUCING DIFFERENT REACTIONS THAT LEAD TO
DEFORMATION, WHICH CAN ULTIMATELY COMPROMISE THEIR
DURABILITY OVER TIME.
• IT IS IMPORTANT TO INTRODUCE SOME CONCEPTS THAT ARE
EXTREMELY RELEVANT TO UNDERSTAND THE PERFORMANCE
PRESENTED BY SUCH MATERIALS UNDER SPECIFIC TEST
CONDITIONS.

4. WHAT ARE MECHANICAL PROPERTIES ?
• MECHANICAL PROPERTIES ARE DEFINED BY THE LAWS OF
MECHANICS, THAT IS, THE PHYSICAL SCIENCE THAT DEALS WITH
ENERGY AND FORCES AND THEIR EFFECTS ON THE BODIES.
• IMPORTANT FACTOR IN DESIGN OF DENTAL PROSTHESIS IS
STRENGTH, A MECHANICAL PROPERTY OF THE MATERIAL THAT
ENSURE IT SERVES ITS INTENDED FUNCTION EFFECTIVELY SAFELY
AND FOR REASONABLE TIME PERIOD.
• THESE PROPERTIES ARE EXPRESSED MOST OFTEN IN UNITS OF
STRESS AND STRAIN.

5. STRESS
• WHEN AN EXTERNAL FORCE IS APPLIED TO ACT ON A BODY, AN
INTERNAL FORCE, EQUAL IN MAGNITUDE AND OPPOSITE IN
DIRECTION TO THE APPLIED FORCE IS SET UP IN THE BODY.
• THIS INTERNAL RESISTANCE TO THE EXTERNAL FORCE IS CALLED
“STRESS”
• DENOTED BY “S” OR “Σ”
• DESIGNATED AS FORCE PER UNIT AREA (Σ=N/M²)
• COMMONLY STRESS IS REPORTED IN TERMS OF MEGA PASCALS.
WHERE, PASCAL = 1 N / M².

6. TYPES OF STRESS
DEPENDING ON TYPE OF
FORCE APPLIED:
• COMPRESSIVE
• TENSILE
• SHEAR
• FLEXURAL

7. TENSILE STRESS
 TENSILE STRESS OCCURS WHEN 2
SETS OF FORCES ARE DIRECTED
AWAY FROM EACH OTHER IN THE
SAME STRAIGHT LINE.
 ALSO WHEN ONE END IS
CONSTRAINED AND THE OTHER END
IS SUBJECTED TO A FORCE AWAY
FROM THE CONSTRAINT.
 IT IS CAUSED BY A LOAD THAT
TENDS TO STRETCH OR ELONGATE A
BODY.

8. COMPRESSIVE STRESS
 Compressive stress occurs when 2
sets of forces are directed towards
each other in the same straight line.
 Also when one end is constrained and
the other end is subjected to a force
towards the constraint.
 It is caused by a load that tends to
compress or shorten a body.

9. SHEAR STRESS
 Shear stress occurs when 2 sets of
forces are directed parallel to each
other but not along the same
straight line.
 A shear stress tends to resist the
sliding of one portion of a body over
another.
 Shear stress can also be produced
by a twisting or torsional action on a
material.

10. FLEXURAL STRESS
 FORCE PER UNIT AREA OF A MATERIAL
THAT IS SUBJECTED TO FLEXURAL
LOADING (BENDING).
 A SHEAR STRESS TENDS TO RESIST THE
SLIDING OF ONE PORTION OF A BODY
OVER ANOTHER.
 A FLEXURAL FORCE CAN PRODUCE ALL THE
THREE TYPES OF STRESSES IN A
STRUCTURE, BUT IN MOST CASES
FRACTURE OCCURS DUE TO THE TENSILE
COMPONENT.

11.  FLEXURAL STRESSES
PRODUCED IN A THREE-UNIT
FIXED DENTAL PROSTHESIS.
 FLEXURAL STRESSES
PRODUCED IN A TWO-
UNIT CANTILEVER
BRIDGE.

12. STRAIN
 IT IS THE CHANGE IN LENGTH PER UNIT LENGTH, IS THE RELATIVE
DEFORMATION OF AN OBJECT SUBJECTED TO STRESS.
IT MAY EITHER BE ELASTIC OR PLASTIC
ELASTIC STRAIN IS REVERSIBLE
PLASTIC STRAIN REPRESENTS PERMANENT DEFORMATION

13. 1. ELASTIC STRAIN DEFORMATION 2. PLASTIC STRAIN DEFORMATION

14. STRESS STRAIN RELATIONSHIP
• EACH TYPE OF STRESS IS CAPABLE OF PRODUCING A
CORRESPONDING DEFORMATION IN THE BODY.
• THEREFORE, STRESS AND STRAIN ARE NOT INDEPENDENT AND
UNRELATED PROPERTIES, BUT THEY ARE CLOSELY RELATED AND MAY
BE SEEN AS CAUSE AND EFFECT.
• THE RELATIONSHIP OF STRESS AND STRAIN IS OFTEN USED TO
CHARACTERIZE THE MECHANICAL PROPERTIES OF MATERIALS.

15. HOW TO REDUCE ?
• LIKE FOR EXAMPLE IN FIXED PARTIAL DENTURE WITH LONG BRIDGE
SPAN WE TRY TO REDUCE THE BRIDGE SPAN OR USE A NON RIGID
TYPE OF CONNECTOR .
• IN CASE OF CAST PARTIAL DENTURE WE GIVE STRESS BREAKERS IN
ORDER TO REDUCE THE STRESS ON PROSTHESIS.
• IN CASE OF COMPLETE DENTURE WE TRY TO COVER THE DENTURE
BEARING TISSUE AS MUCH AS POSSIBLE SO THAT THE STRESS
CONCENTRATION IS DISTRIBUTED EVENLY AND NOT CONFINED TO
AN AREA.

16. 1. ELASTIC MODULUS
• IT DESCRIBES THE RELATIVE STIFFNESS OR RIGIDITY OF A MATERIAL
• IF THE TENSILE STRESS OR COMPRESSIVE STRESS BELOW THE
PROPORTIONAL LIMIT IS DIVIDED BY ITS CORRESPONDING STRAIN VALUE,
A CONSTANT OF PROPORTIONALITY IS OBTAINED THAT IS KNOWN AS
THE ELASTIC MODULUS OR MODULUS OF ELASTICITY OR YOUNG’S
MODULUS
• DESIGNATED BY LETTER “E”.
• THE ELASTIC MODULUS HAS A CONSTANT VALUE THAT DESCRIBES THE
MATERIAL RELATIVE STIFFNESS.
• MECHANICAL PROPERTIES BASED ON ELASTIC DEFORMATION

17. PLASTIC
THE SLOPE OF THE CURVE IS THE ELASTIC MODULUS
P
O
E
• O IS STRAIN
OFFSET FROM
ORIGIN
• P IS THE
PROPORTIONAL
LIMIT
• E IS THE VALUE
OF ELASTIC
MODULUS
• S IS THE
ULTIMATE
TENSILE
STRENGTH
S
ELASTIC PLASTIC

18. SCHEMATIC ILLUSTRATION OF A PROCERDURE TO CLOSE AN OPEN MARGIN OF
AMETAL CROWN BY BURNISHING WITH A ROTARY INSTRUMENT

19. CLINICAL SIGNIFICANCE:
• THE METAL FRAME OF A METAL-CERAMIC BRIDGE SHOULD HAVE A
HIGH STIFFNESS. IF THE METAL FLEXES, THE PORCELAIN VENEER
ON IT MIGHT CRACK OR SEPARATE. SUCH A MATERIAL WOULD
POSSESS A COMPARATIVE HIGH MODULUS OF ELASTICITY.
• A POLYETHER MATERIAL HAVE GREATER STIFFNESS THAN ALL
OTHER ELASTOMERIC IMPRESSION MATERIALS. THUS A GREATER
FORCE IS NEEDED TO REMOVE A IMPRESSION TRAY FROM
UNDERCUTS IN MOUTH.

20. ELASTIC MODULUS OF MATERIALS

21. CAN BE MEASURED BY DYNAMIC METHOD.
ULTRASONIC LONGITUDINAL AND TRANSVERSE WAVE
TRANSDUCERS AND APPROPRIATE RECEIVERS ARE USED.
THE VELOCITY OF SOUND WAVE AND DENSITY OF MATERIAL ARE
USED TO CALCULATE ELASTIC MODULUS.
A VALUE OF 0.3% IS TYPICAL.

22. 3. FLEXIBILITY
 ABILITY OF A MATERIAL TO RETURN TO ITS ORIGINAL FORM INDICATES
ITS ELASTICITY, BUT THE STRAIN TAKING PLACE AT ELASTIC LIMIT IS
KNOWN AS FLXIBILITY.
 FLEXIBILITY IS BENDING CAPACITY.
 THE MAXIMUM FLEXIBILITY IS DEFINED AS A FLEXURAL STRAIN THAT
OCCURS WHEN THE MATERIAL IS STRESSED TO ITS PROPORTIONAL
LIMIT.
IN CASE OF DENTAL APPLIANCES AND RESTORATIONS, A HIGH VALUE FOR
ELASTIC LIMIT IS A NECESSITY FOR THE MATERIALS FROM WHICH THEY
ARE FABRICATED BECAUSE THE STRUCTURE IS EXPECTED TO RETURN TO
ITS ORIGINAL SHAPE AFTER IT HAS BEEN STRESSED AND THE FORCE IS
REMOVED.
SUCH AS IN CASE OF INLAYS OR AN IMPRESSION MATERIALS.

23. 4. RESILIENCE
• DEFINED AS THE AMOUNT OF
ENERGY ABSORBED WITHIN A
UNIT VOLUME OF A STRUCTURE
WHEN IT IS STRESSED TO ITS
PROPORTIONAL LIMIT.
• THE PROPERTY IF OFTEN
DESCRIBED AS “SPRING BACK
POTENTIAL.”

24.  ELASTOMERIC SOFT LINERS ABSORB CONSIDERABLE AMOUNTS OF
ENERGY WITHOUT BEING PERMANENTLY DISTORTED WHEN
STRESSED AND THE ENERGY STORED IS RELEASED WHEN THE
MATERIAL SPRINGS BACK TO ITS ORIGINAL SHAPE AFTER REMOVAL
OF THE APPLIED STRESS. THEREFORE, THESE MATERIALS ACT AS
CUSHION BETWEEN THE HARD DENTURE BASE AND SOFT TISSUES
TO REDUCE MASTICATORY FORCES TRANSMITTED BY PROSTHESIS
TO THE UNDERLYING TISSUES.

25. • IF LARGE PROXIMAL STRAINS ARE DEVELOPED DURING
COMPRESSIVE LOADING, A PROXIMAL INLAY MIGHT ABSORB THE
ENERGY AND CAUSE EXCESSIVE MOVEMENT OF THE ADJACENT
TOOTH WHEN THE ABSORBED ENERGY IS RELEASED.
• HENCE THE RESTORATIVE MATERIAL SHOULD EXHIBIT A
MODERATELY HIGH ELASTIC MODULUS AND RELATIVELY LOW
RESILIENCE.

26. • CLINICAL SIGNIFICANCE :
• RESTORATIVE MATERIALS SHOULD WITHSTAND HIGH STRESSES
AND SHOW MINIMUM DISTORTION OR SHOULD HAVE MINIMUM
FLEXIBILITY.
• IMPRESSION MATERIALS SHOULD HAVE LARGE FLEXIBILITY OR
ELASTIC DEFORMATION TO WITHDRAW THROUGH SEVERE
UNDERCUTS WITHOUT PERMANENT DEFORMATION.
• MAXILLOFACIAL MATERIALS AND SOFT DENTURE RELINERS
SHOULD HAVE HIGH FLEXIBILITY.

27. 5. POISSON'S RATIO
• WHEN A TENSILE FORCE IS APPLIED ALONG ONE AXIS TO PRODUCE
ELONGATION, COMPRESSIVE STRAIN IS PRODUCED AT RIGHT ANGLES,
PROPORTIONATELY.
• WITHIN ELASTIC RANGE THE RATIO OF LATERAL TO THE AXIAL
• STRAIN IS CALLED POISSON'S RATIO.
• DENTAL MATERIALS HAVE POISSON'S RATIO
• VALUE IN RANGE OF 0.3 TO 0.5.

28. MECHANICAL PROPERTIES OF TOOTH STRUCTURE
• THE PROPERTIES OF ENAMEL ALSO VARY SOMEWHAT WITH ITS POSITION
ON THE TOOTH, CUSPAL ENAMEL IS STRONGER THAN ENAMEL ON OTHER
SURFACES.
• ALSO THE PROPERTIES VARY ACCORDING TO HISTOLOGICAL STRUCTURE,
ENAMEL ARE STRONGER UNDER LONGITUDINAL COMPRESSION RATHER
THAN SUBJECTED TO LATERAL COMPRESSION
• ON THE OTHER HAND PROPERTIES OF DENTIN APPEAR TO BE INDEPENDENT
OF STRUCTURE, REGARDLESS OF DIRECTION OF COMPRESSION.
• DENTIN IS CONSIDERABLY STRONGER IN TENSION 50MPA THAN ENAMEL
10 MPA

30. STRENGTH PROPERTIES
• STRENGTH IS THE STRESS THAT IS NECESSARY TO CAUSE EITHER
FRACTURE (ULTIMATE STRENGTH) OR A SPECIFIED AMOUNT OF PLASTIC
DEFORMATION (YIELD STRENGTH)
• THE STRENGTH OF A MARETIAL CAN BE DESCRIBED BY ONE OR MORE OF
THE FOLLOWING PROPERTIES:
1. PROPORTIONAL LIMIT
2. ELASTIC LIMIT
3. YIELD STRENGTH OR PROOF STRENGTH
4. ULTIMATE TENSILE STRENGTH, SHEAR STRENGTH, COMPRESSIVE
STRENGTH AND FLEXURAL STRENGTH
5. FRACTURE STRENGTH AND FATIGUE STRENGTH

33. MASTICATION FORCES AND STRESS
• BITING STRESS DURING MASTICATION ARE DIFFICULT TO MEASURE BECAUSE
OF THEIR DYNAMIC NATURE.
• THE HIGHEST BITING FORCE RECORDED IS 4337 N (975LBS) AVERAGE VALUE
IS ABOUT 756 N (170 LBS)
• HIGHER IN MALES THAN IN FEMALES AND GREATER IN YOUNG ADULT THAN
IN CHILDREN
• MODULUS OF RESILIENCE OF DENTIN IS GREATER THAN THAT OF ENAMEL SO
ABSORBS IMPACT ENERGY BETTER.
• ENAMEL IS BRITTLE WITH HIGH MODULUS OF ELASTICITY, LOW
PROPORTIONAL LIMIT IN TENSION AND A LOW MODULUS OF RESILIENCE,
HOWEVER SUPPORTED BY DENTINE WITH SIGNIFICANT ABILITY TO DEFORM
TEETH FRACTURES SELDOM UNDER NORMAL OCCLUSION.

34. • SINCE MOST OF MASTICATORY FORCES ARE COMPRESSIVE IN
NATURE, IT IS IMPORTANT TO INVESTIGATE MATERIALS UNDER
THIS CONDITION.

35. OTHER MECHANICAL PROPERTIES
1. TOUGHNESS :
IT IS THE AMOUNT OF ELASTIC AND
PLASTIC DEFORMATION ENERGY REQUIRED
TO FRACTURE A MATERIAL.
IT IS THE MEASURE OF ENERGY REQUIRED TO
PROPAGATE CRITICAL FLAWS. TOUGHNESS
INCREASES WITH INCREASE IN STRENGTH
AND DUCTILITY.

36. 2. FRACTURE TOUGHNESS :
• OR CRITICAL STRESS INTENSITY IS A MECHANICAL PROPERTY THAT
DESCRIBE THE RESISTANCE OF A BRITTLE MATERIAL TO
CATASTROPHIC PROPAGATION OF FLAWS UNDER AN APPLIED
STRESS.
• IT IS AN INDICATION OF THE AMOUNT OF STRESS REQUIRED TO
PROPAGATE A PREEXISTING FLAW.

37. 3. BRITTLENESS :
• IT IS THE RELATIVE INABILITY OF A MATERIAL TO
SUSTAIN PLASTIC DEFORMATION BEFORE FRACTURE
OF A MATERIAL OCCURS.
• AMALGAMS CERAMICS AND COMPOSITES ARE
BRITTLE AT ORAL TEMPERATURE(5-55°C) THEY
SUSTAIN LITTLE OR NO PLASTIC STRAIN BEFORE
THEY FRACTURE.
• HOWEVER A BRITTLE MATERIAL IS NOT
NECESSARILY WEAK .COBALT-CHROMIUM PARTIAL
DENTURE HAVE % ELONGATION OF LESS THAN 1.5%
BUT AN ULTIMATE TENSILE STRENGTH OF 870 MPA

38. 4. DUCTILITY:
• THE ABILITY OF A MATERIAL TO SUSTAIN A LARGE PERMANENT
DEFORMATION UNDER A TENSILE LOAD BEFORE IT FRACTURES. EG A WIRE
THAT DRAWN INTO LONG THIN WIRE IS DUCTILE.
IMPORTANCE OF DUCTILITY IN DENTISTRY:
• CLASPS CAN BE ADJUSTED, ORTHODONTICS APPLIANCES CAN BE PREPARED,
CROWNS OR INLAYS CAN BE BURNISHED IF THEY ARE PREPARED FROM
ALLOYS OF HIGH VALUES OF PERCENTAGE ELONGATION.(GIVES AN
INDICATION OF THE WORKABILITY OF AN ALLOY )

39. 5. MALLEABILITY :
• THE ABILITY OF A MATERIAL TO SUSTAIN CONSIDERABLE PERMANENT
DEFORMATION WITHOUT RUPTURE UNDER COMPRESSION, AS IN
HAMMERING OR ROLLING INTO A SHEET, IS TERMED MALLEABILITY.
• GOLD IS MOST DUCTILE AND MALLEABLE AND SILVER STANDS THE
SECOND.
• PLATINUM IS THIRD MOST DUCTILE AND COPPER IS THIRD MOST
MALLEABLE.

41. 6. HARDNESS :
HARDNESS : IS THE RESISTANCE OF THE MATERIAL TO SCRATCHING,
INDENTATION OR PENETRATION.
IT IS A SURFACE PROPERTY NOT RELATED DIRECTLY TO ANY OTHER
MECHANICAL PROPERTY I.E. STRONG OR STIFF MATERIALS ARE NOT
NECESSARY HARD.
HARDNESS CAN’T BE SEEN OR CALCULATED FROM STRESS-STRAIN CURVED
BUT ONLY BY USING ONE OF THE FOLLOWING: BRINELL, KNOOP, VICKERS,
ROCKWELL AND SHORE A HARDNESS TEST.

42. 1. BRINELL HARDNESS TEST
• A STEEL BALL IS PRESSED INTO THE SURFACE OF THE MATERIAL UNDER A
SPECIFIED LOAD. THE LOAD IS DIVIDED BY THE AREA OF THE SURFACE OF
THE INDENTATION. THUS, THE SMALLER THE INDENTATION THE LARGER
THE HARDNESS NUMBER BECOMES, AND THE HARDER THE MATERIAL IS.
THIS TEST IS USED TO DETERMINE THE HARDNESS OF THE METALLIC
MATERIALS. IT IS EXPRESSED IN B.H.N

43. • 2. ROCKWELL HARDNESS TEST :
ROCKWELL HARDNESS TEST IS SIMILAR TO BRINELL TEST IN THAT
STEEL BALL OR CONE IS USED. INSTEAD OF MEASURING THE
DIAMETER OF THE INDENTATION, THE DEPTH IS MEASURED
DIRECTLY BY A DIAL GAUGE ON THE INSTRUMENT.

44. 3. VICKER HARDNESS TEST
VICKER HARDNESS TEST A DIAMOND SQUARE – BASED PYRAMID (CONE) IS
USED. THE VICKER’S HARDNESS NUMBER IS DETERMINED BY DIVIDING THE
LOAD BY THE AREA OF INDENTATION WHICH IS SQUARE AND NOT ROUND
AS IN THE BRINELL TEST.

45. 4. KNOOP HARDNESS TEST :
KNOOP HARDNESS TEST USES A DIAMOND CONE DESIGNED TO GIVE
AN INDENTATION HAVING A LONG AND A SHORT DIAGONAL(7 : 1).
THE LOAD MAY BE VARIED OVER A WIDE RANGE, FROM ONE GM TO
MORE THAN A KG, SO THAT VALUES FOR BOTH HARD AND SOFT
MATERIALS CON BE OBTAINED. IT IS EXPRESSED IN K.H.N.

46. • 5. SHORE A HARDNESS TEST :
• THE HARDNESS TESTS DESCRIBED PREVIOUSLY CANNOT BE USED TO
DETERMINE THE HARDNESS OF THE RUBBERS, SINCE THE INDENTATION
DISAPPEARS AFTER THE REMOVAL OF THE LOAD. AN INSTRUMENT CALLED
A SHORE A IS USED IN THE RUBBER INDUSTRY TO DETERMINE ITS
HARDNESS. THE INDENATOR IS ATTACHED TO A SCALE THAT IS
GRADUATED FORM 0 TO 100. IF THE INDENTOR COMPLETELY PENETRATES
THE SAMPLE, A READING OF 0 IS OBTAINED, AND IF NO PENETRATION
OCCURS, A READING OF 100 RESULTS.

47. CLINICAL SIGNIFICANCE
• 1) DENTURE – WEARING PATIENTS MUST TAKE CARE NOT TO BE
AGGRESSIVE DURING THE CLEANING OF THEIR DENTURES BY USING
BRUSHES WITH HARD BRISTLES.
• 2)HARDNESS IS AN IMPORTANT PROPERTY TO CONSIDER FOR MODEL AND
DIE MATERIALS ON WHICH CROWN AND BRIDGE WAX PATTERNS ARE
MADE, BECAUSE A SOFT SURFACE MAY BECOME SCRATCHED, AFFECTING
THE ACCURACY OF THE FINAL RESTORATION.

48. SUMMARY AND CONCLUSION
• IT IS VERY IMPORTANT TO KNOW THE PROPERTIES OF MATERIAL WE USE
IN DENTISTRY. THIS WILL ENABLE US TO SELECT A MATERIAL THAT WILL
HAVE PROPERTIES CLOSE TO THAT OF NATURAL TOOTH STRUCTURE. ALSO
WE WILL BE ABLE TO BETTER UNDERSTAND AND SELECT MATERIALS FROM
THE WIDE RANGE THAT ARE COMING IN THE MARKET.
• WHILE DESIGNING A DENTAL APPLIANCE OR A RESTORATIVE
MATERIAL, IT SHOULD HAVE ADEQUATE MECHANICAL PROPERTIES TO
WITHSTAND THE STRESS AND STRAIN CAUSED BY THE FORCES OF
MASTICATION.
• ALL THE METHODS MUST BE EMPLOYED TO MINIMIZE STRESS
CONCENTRATION SO THAT THE RESTORATIVE MATERIAL OR THE
APPLIANCE IS IN HARMONY WITH THE DIFFERENT TYPES OF FORCES
OCCURRING IN THE ORAL CAVITY.

49. REFERENCES
• PHILLIPS SCIENCE OF DENTAL MATERIALS 11TH EDITION
• CRAIG’S RESTORATIVE DENTAL MATERIALS 13TH EDITION
• S. MAHALAXMI MATERIALS USED IN DENTISTRY 1ST EDITION