https://userupload.net/u3964jam31yq Biomechanics comprises all kinds of interactions between tissues and organs of the body and the forces acting on them. Biomechanics comprises the response of the biologic tissues to the applied loads. The basic principles of biomechanics must be respected when doing oral implants (or) else the case may fail. The primer of oral implant biomechanics should familiarize the clinician with key issues to be confronted when using oral implants. To a large extent, the biomechanical consideration for implants follows simple mechanical rules based on the leverage principles. By considering the patient's functional behaviour, limiting the extension of the prosthesis and controlling the occlusal pattern and contacts, possible overload situations can be minimized.

1. BIOMECHANICS OF DENTAL
IMPLANTS
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2. CONTENTS
 Introduction
 Definition of Biomechanics, Mass, force and weight
 Loads applied to dental implants
 Forces and components of forces
 Three types of forces
 Stress
 Stress-strain relationship
 Biting forces
 Predicting forces on oral implants
 Stiffness of teeth and implant

3.  Force delivery and failure mechanism
 Moment loads
 Clinical moment arms
 Fatigue failure
 The biomechanical response to loading
 A scientific rationale for dental implant design
 Character of the applied forces
 Functional surface area
 Biomechanics of frameworks and misfit
 Treatment planning based on biomechanical risk factors
 Conclusion
 List of reference

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6. Biomechanics
Definition:
The discipline of bioengineering,
which applies engineering principles to
living systems.

7. MASS, FORCE AND WEIGHT
Mass – A property of matter, is the degree of gravitational attraction
the body of matter experiences.
Unit – kg : (lbm)
FORCE (SIR ISAAC NEWTON 1687)
Newton’s II law of motion
F = ma
Where a = 9.8 m/s2
Mass – Determines magnitude of static load
Force – Kilograms of force (Newton)
WEIGHT
Is simply a term for the gravitational force acting on an object
at a specified location.

8. LOADS APPLIED TO DENTAL IMPLANTS
In function – occlusal loads, vary in magnitude,
frequency, duration depending on parafunctional habits
Absence of function – Perioral forces
Horizontal loads
Mechanics help to understand such physiologic and non
physiologic loads and can determine which t/t renders
more risk.

9. FORCES AND FORCE COMPONENTS
 Forces may be described by magnitude, duration, direction,
type and magnification
 Forces on implant -‘Vector quantities’
(magnitude & direction)
 Direction – dramatic influence
(longevity)
Break down of 3D forces into their component parts -
‘vector resolution’
 Point of action of a vector
VECTOR
F / F Magnitude F

10. F = 44.5 N at pt B
 Analysis - vector resolution
 Co-ordinate system
 Angles that the F vector makes with co-ordinate axes,
resolution of F into its 3 components is possible
i.e. Fx, Fy & Fz
F = F2
x + F2
y + F2
z
Cos2
θx + Cos2
θy + Cos2
θz = 1
 Lateral as well as vertical components are acting at the same
time
Not || to direction of long axis of implant

11. Vector addition : More than one force FR = F1 + F2 + F3
MOMENT / TORQUE
Tend to rotate a body Units – N.m; N.cm, lb.ft ; oz.in
Eg :
In addition to axial force, there is a moment on the implant which is
equal to magnitude of force times (multiplied by) the perpendicular
distance (d) between the line of action of the F and center of the
implant

12. THREE TYPES OF FORCES
 Compressive
 Tend to push masses towards each other
 Maintains integrity of bone – implant interface
 Accommodated best
 Cortical bone strongest in Comp
 Cements, retention screws, implant components and bone –
implant interfaces – well acc Comp F
 Dominant

13. Tensile Shear
↓ ↓
Pull object apart Sliding
Distract / disrupt bone implant interface
Shear – most destructive, cortical bone is weakest
 Cylinder implants – highest risk for shear forces
require coating
 Threaded / finned implants
 Impart all 3 force types
 Geometry of implant

14. STRESS
The manner in which a force is distributed over a surface is
referred as mechanical stress
γ = F/A
Goal - Even distribution of mechanical stress in the implant
system and contiguous bone
Force magnitude
Reducing magnifiers of force
1. Cantilever length
2. Crown height
3. Night guards (bruxism)
4. Occlusal
material( impact)
5. Overdentures (night)
Functional cross sectional area
1. Number of implants
2. Implant geometry

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16. DEFORMATION & STRAIN
 Applied load – deformation
 Deformation and stiffness of implant material
 Interface
 Ease of implant manufacture
 Clinical longevity
 Concept of strain – key mediator of bone activity
Implant
Tissue
Strain = deformation per unit length

17. STRESS – STRAIN RELATIONSHIP
Load – versus – deformation curve; stress - strain curve
Prediction of amount of strain experienced by the material under
an applied load.
↑In stress
↓
↑ In stiffness difference
↓
↑ Relative motion
↓
Interface is more affected
Viscoelastic bone can stay in contact
with more rigid titanium more
predictably when the stress is low
Modulus of elasticity
tnalpmI | biologic tissue
↓
Lesser the relative motion

18. STRAIN
Controlling applied stress Changing density of bone
Strength Stiffness
 Greater the strength stiffer the bone
 Lesser the stiffness greater the flexibility (soft bone)
 Difference in stiffness is less for CpTi & D1 bone but more for D4
bone
 Stress reduction in such softer bone
 To reduce resultant tissue strain
 Ultimate strength
 Hook’s law
Stress = Modulus of elasticity x strain
γ = E.ε

19. BITING FORCES
 Axial component of biting forces : (100 – 2500 N) / (27 – 550 lbs)
 It tends to increase as one moves distally
 Lateral component - 20 N (approx.)
 Net chewing time per meal = 450 sec
• Chewing forces will act on teeth for = 9 min/day
• If includes swallowing = 17.5 min/day
• Further be increased by parafunction
Provides indication of minimum time /day that teeth (implants) are
bearing load due to mastication and related events

20. PREDICTING FORCES ON ORAL IMPLANTS
Problems :
 To compute the loading on the individual supporting abutment
when more than two implant supported prosthesis
COMPLICATING FACTORS
Nature of mastication
•Chewing – frequency
sequence
•Biting – strength
favoured side
•Mandibular movements
Nature of Prosthesis
•Full / partial
•Tissue – supported
Vs
Implant – supported
•No. & location
•Angulation
Properties
•Elastic moduli
•Stiffness
•Connection
•Deformability

21. Two implants supporting a cantilever portion of a prosthesis
P = Force
a = Cantilever length
b = Dist. Between two implants
 If beam is in static equilibrium – sum of forces and sum of
moments are zero.
 Σ Fy = 0 ; -F1 + F2 – P = 0Σ mQ = 0; -F1b + Pa = 0
 Here, F1 = (a/b)P F2 = (1 + a/b) P
 In most clinical situations a/b = 2.
 So, F1 = 2P and F2 = 3P
 Newton’s 3rd
law of motion
Implant 2  compressive load Implant 1  tensile load

22. FOUR IMPLANTS SUPPORTING A FRAMEWORK
(BRANEMARK SYSTEM)
LIMITATIONS
1. Does not predict forces on all 4 implants
2. Overestimation of loads on 2 implants close to the point of F
3. Based on theory of rigid body statics
‘Skalak model’
Can predict the vertical and horizontal force components on
implants supporting a bridge

23. • Bridge and bone are rigid
• Implants and/or their connections to bridge and/or bone elastic
Purely vertical force Purely horizontal force
Counterbalanced by distribution of N no. of implants so, there
will be both vertical and horizontal forces on each implant

24. • 4 or 6 implant – symmetrically distributed in the arc of 112.50
with radius of mandible radius to 22.5 mm
• Arc of 112.50
= interforaminal dist. (approx)
• Single vertical force of 30N acts at a position defined by θ = 100
(So, how to predict the vertical forces on each implant)
F< 30 N Magnitude of force is |||

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26. • Forces on remaining 4 implants become much larger than in
original 6 implant case
• Condition can be worsened if 4 implants are placed in a line across
the anterior mandible.
• As, ratio a/b is very large as b (interimplant distance) is very
small.
• Implant angulation.

27.  Implant 1 at 300
angulation.
 Offaxis loading – detrimental to the system.
 Cannot be solved by Skalak or Rangert model.
 Finite element modelling or analysis.
 Properties of the prosthesis
 Positioning and angulation of implants
 Properties of interfacial bone can be accounted to FE

28. Skalak modle –
• Prosthesis is infinitely rigid
• Acrylic and metal alloy bridge – flexible
Concentrating forces on the implants nearest to loading point
• Complicated if implant system - unequal stiffnesses
as stiffest implant will generally take up most of the load

29. STIFFNESS OF TOOTH AND IMPLANT
 Prosthesis supported by teeth and implants-different mobility
 Neither Rangert nor Skalak model specifically deal with
differencing mobility
 A way to approach this problem is
1. Displacement in any direction
• Unidirectional force but displacement in many direction
• Secondary effect
2. Application of constant force
• Increase in displacement slowly with time
↓
Creep
Not significant with implants – fibrous tissue
3. Intrusive tooth displacement is not always Linear
– usually bilinear
4. Net stiffness (implants) > natural tooth

30. FORCE DELIVERY AND FAILURE MECHANISM
 Manner of application of force – dictates the failure
 Moment loads
 Interface breakdown
 Bone resorption
 Screw loosening
 Bar / bridge fracture
Clinical moment arms

31. 1) Occlusal height
 Working and balancing occlusal contacts
 Tongue thrusts, perioral musculature
 Force component along vertical axis – no effect
 Initial moment load at crest
↓ In div A ↑ Div C and D
↑ Crown height
Faciolingual axis Mesiodistal axis

32. 2) Cantilever length
Vertical axis force components-large moments
Lingual force component - also exists
Force applied directly over the implant–no moment

33. OCCLUSAL WIDTH
 ↑ Moment arm for any offset occlusal load
 Narrow occlusal table - ↓ faciolingual tipping
Moment loads Crestal bone loss
Increases occlusal height
Occlusal ht. moment arm
↑ Faciolingual micro
rotation or rocking
More crestal bone loss
Failure if biomechanical
environment is not
corrected

34. FATIGUE FAILURE
Dynamic cyclic loading condition
1) Biomaterials
 High stress – few loading cycles
 Low stress – infinite loading cycles
 Endurance limit-stress level below which-loaded indefinetly
 Ti alloy > CpTi.
2) Geometry
 Dictates degree - resists bending and torque
 Lateral loads – fatigue fracture
 4th
power of the thickness difference
 Weak link – Difference inner and outer diameter of screw and
abutment screw space

35. 3) Force magnitude
 Reduction of applied load - (stress)
 Higher loads on posteriors
 Moment loads - eliminate
 Geometry for functional area - optimize
 Increase No. of implants
4) Loading cycles
 Fatigue failure - No. of loading cycles
 Elimination of parafunction
 Reduce occlusal contacts

36. BIOMECHANICAL RESPONSE TO LOADING
 High degree of variation as a function of load – direction, rate and
duration
 Direction of load
Orthotropic Isotropic Transversely isotropic
 Mandible (transversly isotropic) Arch of it having stiffest
direction orientation
 Mandible acts - long bone molded into a curve beam
 Inferior border more compact bone
 Inter forminal part – increase quality of trabecular bone

37. RATE OF LOADING
 McElhaney – strain rate dependence of bone
 Higher strain rate, bone acts – stiffer and stronger
 Also noted, bone fails at higher strain rate, but with less
allowable elongation when compared to lower strain rate
 Brittle
Duration of loading
 ‘Carter and Caler’- # caused by mechanical stress
 Creep (time-dependent loading) + cyclic / fatigue loading
Anatomic location and structural density also has got influence

38. ANATOMIC LOCATION
 Edentulous mandible – Trabecular bone continuous with cortical
shell
 FEM show – cortical bone – dissipation of occlusal loads
 Muscle loads on mandible – Dorsoventral shear, twisting,
transverse
 Anterior mandible – large moment loads – buccolingal flexure
 Posterior mandible – higher bite force
 Density and ultimate compressive strength of posterior trabecular
bone (↓)
 Large, multirooted molars – dissipate occlusal loads

39. Qu et al – 65% higher stiffness for trabecular bone of mandible
when bounded by cortical plates
Structural density
Qu et al – Mechanical properties of mandibular trabecular bone i.e.
Elastic modulus and ultimate strength.
47% - 68% > in anterior compared to posterior
Premolars = molars.
Scientific rationale for dental implant design
Transfer of load to surrounding biologic tissue. Two factors are
1) Character of applied load 2) Functional surface area
Character of forces applied to dental implant depend on ----
Magnitude, duration, type, direction and magnification

40. FORCE MAGNITUDE
A) Physiology vs design :
 Normal physiology - Limits magnitude of force for a
engineered design
 Magnitude of forces -----
Parafunction > Molar > Canine > Incisors
1000 lb 200 lb 100 lb 25-35 lb
 ↓ density ↓ forces

41. B) Biomaterial selection :
Silicone, HA, carbon – High biocompatibility
Low ultimate strength
Titanium and its alloy – Excellent biocompatibility
Corrosion resistance
Good ultimate strength
 Closest approx. to stiffness of surgical bone substitutes
 6 times more stiff
C) Failures :
Vitreous carbon implant Al2O3 ceramic implant
Modulus of elasticity Ultimate strength
Ultimate strength Modulus of elasticity

 


42. FORCE DURATION
A) Physiology vs design
 Duration of bite forces on the dentition has wide range
 Ideal condition < 30 min/day
 Parafunction – several hours
B) Implant body design
 Endurance limit 1 ½ times < ultimate tensile strength
 Fatigue – more critical especially in parafunction
 Off axis, cyclic loading – failure of implant
 Root form implant – not specifically designed to withstand
cyclic bending loads.

43. FORCE TYPE
A) Physiology
 Bone – Strongest in compression
30% weaker in tensile
65% weaker in shear
 Endosteal root-form implants – pure shear - unless
Incorporation of surface features to transform shear
to resistant forces
B) Implant body design
 Smooth cylinder - Titanium / HA
 Integrity of interface - shear strength of HA-to-bone bond

44. THREADED IMPLANTS
 Buttress comparable to V-shaped
 V-shaped 10 times greater shear (square / power)
 Caution in D3 and D4 bone
Failure
 Smooth shear surface implant – inadequate load
transfer
V-shaped
SquareButtress

45. FORCE DIRECTION
A) Physiology
 Constraint - Positioning of root form implants suitable for axial loading –
anatomy of jaws
 Undercuts – further limit
 Usually occur on facial aspect except
 Submandibular fossa
 Angled to the lingual
 Bone is strongest when loaded along its long axis.
 300
offset load : 11% ↓ compressive st.
25% ↓ tensile
B) Implant body design
 Vulnerable crestal bone region – designed to place perpendicular to
occlusal plane

46. FORCE MAGNIFICATION
 Extreme angulation
 Parafucntion
 Cantilevers and crown heights – levers
 Density α strength
D4 bone 10 times weaker than D1 bone
 Thus resultant force will be magnified when placed in softer
bone
Exceeds the capability of any
dental implant to withstand
physiologic loads

47. SURFACE AREA
 Normal anatomy of jaws – limits size and configuration
Bone volume (external architecture)
 Anatomic location and degree of bone resorption
Width : 6-8 mm in anterior  4 mm implant
> 7 mm in posteriors  5 mm implants
 ↑ Implant width  anterior to posterior
Height :
Anterior mandible > anterior maxilla > post mandible > post maxilla
Hence, ↑ occlusal forces ↓ in bone height
Bone quality (internal architecture)
 35% failure rate in D4 bone
 Poor quality, porous bone - ↑ed clinical failure
 No. of implants, design with greater surface area - stress

48. SURFACE AREA OPTIMIZATION
Implant macrogeometry :
 Smooth sided cylindrical implants
 Ease in surgical placement
 Greater shear at interface
 Smooth sided tapered implants
 Component of compressive force  delivered to the bone-implant
interface.
 Taper - delivery of force
 Taper < 300
 Threaded implants
 Ease of surgical placement
 Greater functional surface area – compressive loads
 Limits micro-movement during healing

49. IMPLANT WIDTH
 Branemark – 3.75 mm
 ↑ Implant width - ↑ functional surface area
 4 mm implant 33% greater surface area
 Diameter appropriate to ridge width
 Teeth width 6 – 12 mm
 Similar implant width  bending resistance  inadequate
strain to bone  resorption
 Crestal bone anatomy constraints  less than 5.5 mm width
implant

50. THREAD GEOMETRY
 Parameters – thread pitch, shape and depth
Thread pitch
 Number of threads per unit length
 Fine pitch  ↑ threads  ↑ surface area / unit length
 Fewer threads  easy to bone tap

51. Thread shape
V-thread design – ‘fixture’ – fixating metal parts and not for
load transfer
Buttress thread optimized for – pullout loads

52. Thread depth
= Major diameter – minor diameter
 Conventional implant – uniform
 Can be varied in the region of highest stress – increase functional surface
area
 Reverse taper in minor diameter
Increased depth
Dramatic ↑ in functional surface area

53. IMPLANT LENGTH
 Length ↑ - total surface area ↑
 If longer implants used
 Need for – nerve repositioning – mandible
Sinus graft – maxilla
 Does not benefit the primary regions of increased stress –
crestal bone region
 Greater stability under lateral loading
 Not necessarily better
 Minimum implant length – for each bone density depending on
the width & design
 Softer bone – greater the length suggested

54. CREST MODULE CONSIDERATIONS
 Transosteal region from the implant body and characterized as
a region of highly concentrated mechanical stress
 Not ideally designed for load bearing
 Smooth parallel sided crest module – shear
 Angled crest module ( > 200
) surface texture – lead
to compressive component

55. CREST MODULE : Slightly larger than outer diameter
Reasons ---
Crest module seats fully over the implant body
osteotomy – deterrent to ingress of bacteria &
fibrous tissue
Seal created by larger crest module – provides
greater initial stability of the implant
 surface area - stress at crestal region.

56. APICAL DESIGN CONSIDERATION
 Most root form implants – circular
 Do not resist torsional / shear forces (single tooth implant)
 Antirotational feature – hole or vent
 Flat sides or grooves along the body or apical region
 Apical end should be flat instead of pointed
Advantages Disadvantages
Bone can grow in and resist
torsional forces
Increases surface area
May fill with mucus or
fibrous tissue

57. BIOMECHANICS OF FRAMEWORKS AND MISFIT
Frameworks :
 Metal framework for full arch prosthesis can fracture
 More towards the cantilever section
Reasons :
1) Overload of cantilever
Unlikely to occur – typical prosthetic alloy.
2) Metallurgic fatigue under cyclic loads
Prevention – substantial cross sectional area of framework
near the root of cantilever
– 3-6 mm

58.  Inevitable dimensional
inaccuracies
 Assuming that the misfit is
not too severe the
framework may appear to
fit well - ‘Passive fit’
 Misfitting framework can
cause loads on implant even
before any bitting force is
applied
FRAMEWORK MISFIT

59. TREATMENT PLANNING BASED ON BIOMECHANICAL
RISK FACTORS
 Design of final prosthetic reconstruction
 Anatomical limitation
Geometric risk factor
1) No. of implants less than no. of root support
 One implant replacing a molar – risk.
 1 wide – platform implant / 2 regular implants
 Two implants supporting 3 roots or more – risk
 2 wide – platform implants
2) Wide – platform implants
 Risk – if used in very dense bone
3) Implant connected to natural teeth

60. 4) Implants placed in a tripod configuration
 Desired  counteract lateral loads
5) Presence of prosthetic extension
6) Implants placed offset to the center of the prosthesis  in
tripod arrangement, offset is favorable
7) Excessive height of the restoration

61. OCCLUSAL RISK FACTORS
 Force intensity and parafunctional habit
 Presence of lateral occlusal contact
 Centric contact in light occlusion
 Lateral contact in heavy occlusion
 Contact at central fossa
 Low inclination of cusp
 Reduced size of occlusal table

62. BONE IMPLANT RISK FACTORS
 Dependence on newly formed bone
 Absence of good initial stability
 Smaller implant diameter
 Proper healing time before loading
 4 mm diameter minimum – posteriors
Technological risk factors
 Lack of prosthetic fit and cemented prosthesis
 Proven and standardized protocols
 Premachined components
 Instrument with stable and predefined tightening torque

63. WARNING SINGS
 Repeated loosening of prosthetic / abutment screw
 Repeated fracture of veneering material
 Fracture of prosthetic / abutment screws
 Bone resorption below the first thread

65. LIST OF REFERENCES
 Dental implant prosthetics – Carl E. Misch.
 Esthetic implant dentistry – Patric Palacci.
 Osseointegration in oral rehabilitation – Naert et al.
 Principles and practice of implant dentistry – Charles Weiss,
Adam Weiss.
 Tissue – integrated prosthesis. Osseointegration in clinical
dentistry – Branemark, zarb, Albrektsson
 Implant & restorative dentistry – Gerard M. Scortecci
 Implant dentistry 2000; 9 (3) : 207-218.
 JPD 2002 ; 88 : 604-10.
 IJOMI 1992 ; 7 : 450-58.
 JPD 2000 ; 83 : 450-55.