Biomechanics implants/certified fixed orthodontic courses by Indian dental academy

BIOMECHANICS OF DENTAL
IMPLANTS
INDIAN DENTAL ACADEMY
Leader in Continuing Dental Education
www.indiandentalacademy.com

CONTENTS
 Introduction
 Loads applied to dental implants
 Mass, force and weight
 Forces and components of forces
 Three types of forces
 Stress
 Stress-stain relationship
 Biting forces
 Predicting forces on oral implants
 Stiffness of teeth and implant
 Models for predicting forces on prosthesis supported by
teeth and implants

 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

LOADS APPLIED TO DENTAL IMPLANTS
 In function – occlusal loads
 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.
MASS, FORCE AND WEIGHT
Mass – A property of matter, is the degree of gravitational attraction
the body of matter experiences.
Unit – kgs : (lbm) www.indiandentalacademy.com

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
WEIGHT
Is simply a term for the gravitational force acting on an
object at a specified location.

FORCES AND FORCE COMPONENTS
 Magnitude, duration, direction, type and magnification
 ‘Vector quantities’
 Direction – dramatic influence
Break down of 3D forces into their component parts -
‘vector resolution’
 Point of action of a vector
VECTOR
F / F Magnitude F

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

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

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

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

STRESS
The manner in which a force is distributed over a surface is
referred as mechanical stress
γ = F/A
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
4. Occlusal material
5. Overdentures
Functional cross sectional area
1. Number of implants
2. Implant geometry

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

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

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

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 minimum time /day that teeth (implants) are bearing load
due to mastication and related eventswww.indiandentalacademy.com

PREDICTING FORCES ON ORAL IMPLANTS
Problems :
 To compute the loading on the individual supporting abutment
 More than two implant supporting 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

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

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

• 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 implantwww.indiandentalacademy.com

• 4 or 6 implant – symmetrically distributed in the arc of 112.50
with radius of mandible at 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 |||www.indiandentalacademy.com

• 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.

 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 FEwww.indiandentalacademy.com

Skalak modle –
• Prosthesis is infinitely rigid
• Acrylic and metal alloy bridge – flexible
Concentrating forces on the implants nearest to loading point
• Unequal stiffnesses
• Stiffest implant will generally take up most of the loadwww.indiandentalacademy.com

STIFFNESS OF TOOTH AND IMPLANT
 Prosthesis supported by teeth and implants.
 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
3. Intrusive tooth displacement is not always Linear
– usually bilinear
4. Net stiffness > natural tooth

PROSTHESIS SUPPORTED BY TEETH AND IMPLANTS
• Use of FEA
• Concept of IME
eg: F = 100 N
Natural tooth = 30% when paired with an implant without IME
= 38% when IME is incorporated
• Rationale for use of IME
Effectiveness in clinical situations have to be checked
• Rangert et al
Equal sharing of forces by tooth and implant so, need for IME
in an osseointegrated implant is questionable.

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

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

2) Cantilever length
Vertical axis force components
Lingual force component
Force applied directly over the implant
4 or 6 implant case
 Exact cantilever length
 2-3 premolars
 6 instead of 4 implants
A-P spread
↑ A-P spread ↓ the resultant loadwww.indiandentalacademy.com

MISCH
• Amount of stress applied to system
• Generally –
• Distal cantilever – not be > 2.5 times of A-P spread
• Patients with parafunction – not to be restored by cantilever
• Square arch form - ↓ A-P spread - ↓ cantilever
• Tapered arch form – largest A-P spread – largest cantilever
design.

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

FATIGUE FAILURE
Dynamic cyclic loading condition
1) Biomaterials
 A plot of applied stress vs no. of loading cycles
 High stress – few loading cycles
 Low stress – infinite loading cycles
 Endurance limit
 Ti alloy > CpTi.
2) Geometry
 Resists bending and torque
 Lateral loads – fatigue fracture
 4th
power of the thickness difference
 Inner and outer diameter of screw and abutment screw space

3) Force magnitude
 Reduction of applied load - (stress)
 Higher loads on posteriors
 Moment loads
 Geometry for functional area
 No. of implants
4) Loading cycles
 No. of loading cycles
 Elimination of parafunction
 Reduce occlusal contactswww.indiandentalacademy.com

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  Arch of it having stiffest direction orientation
 Long bone molded into a curve beam
 Primary loads = occlusal ? Flexural
 Inferior border more compact bone
 Inter forminal part – increase quality of trabecular bonewww.indiandentalacademy.com

RATE OF LOADING
 McElhaney – strain rate dependence
 Higher strain rate – stiffer and stronger
 Bone fails at higher strain rate, but with less allowable
elongation
 Brittle
Duration of loading
 ‘Carter and Caler’
 Creep (time-dependent loading) + cyclic / fatigue loading
Anatomic location and structural density also has got influence

ANATOMIC LOCATION
 Edentulous mandible – Trabecular bone continuous with cortical
shell
 FEM – cortical bone – dissipation of occlusal loads
 Attention to trabecular bone mechanical properties
 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 (↓)
 Large, multirooted molars

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
Magnitude, duration, type, direction and magnificationwww.indiandentalacademy.com

FORCE MAGNITUDE
A) Physiology vs design :
 Limits magnitude of force for a engineered design
 Function of anatomic region and state of dentition
Parafunction > Molar > Canine > Incisors
1000 lb 200 lb 100 lb 25-35 lb
 ↓ density ↓ forces

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 bone
 6 times more stiff
C) Failures :
Vitreous carbon implant Al2O3 ceramic implant
Modulus of elasticity Ultimate strength
Ultimate strength Modulus of elasticity

 


FORCE DURATION
A) Physiology vs design
 Duration of bite force
 Ideal condition < 30 min/day
 Parafunction – several hours
B) Implant body design
 Endurance limit 2 ½ times < ultimate tensile strength
 Fatigue – more critical especially in parafunction
 Off axis, cyclic loading
 Bending loads in buccolingal plane
 Root form implant – not specifically designed to withstand
cyclic bending loads.www.indiandentalacademy.com

 Components moment of inertia
 Apical extension of the abutment screw within the implant
body
 Crest-module around an abutment screw
 (ODR)4
– (IDR)4
 Small ↓ in wall thickness is significant
 OD ↑ by 0.1 mm – 33% ↑ in strength
ID ↓ by 0.1 mm – 20% ↑ in strength
 Prosthesis / coping screw – ↓ moment of inertia
 Screw breakage – long term advantage
 Failure – Morgan et al

FORCE TYPE
A) Physiology
 Bone – Strongest in compression
30% weaker in tensile
65% weaker in shear
 Endosteal root-form implants – pure shear
 Incorporation of surface features
 Titanium / HA
 Shear strength of HA-to-bone bond

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 – inadequate load transfer
V-shaped
SquareButtress

FORCE DIRECTION
A) Physiology
 Positioning of root form implants suitable for axial loading
 Undercuts – further limit
 Usually occure on facial aspect except
 Submandibular fossa
 Angled to the lingual
 Bone is strongest when loaded along its long axis.
 300
offset load : 15% ↓ compressive
25% ↓ tensile
 Vulnerable crestal bone region

FORCE MAGNIFICATION
 Extreme angulation
 Parafucntion
 Cantilevers and crown heights – levers
 Indication for ↑ functional surface area
 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

SURFACE AREA
 Normal anatomy – 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

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  Taper
 Taper < 300
 Threaded implants
 Ease of surgical placement
 Greater functional surface area – compressive loads
 Limits micro-movement during healing

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

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

Thread shape
 V-thread design – ‘fixture’ – fixating metal parts and not for load
transfer
 Buttress thread – pullout loads
 Dental implants  load transmission  intrusive
Square / power thread

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

IMPLANT LENGTH
 Length ↑ - total surface area ↑
 Bicortical stabilization
Eg: Anterior mandible – adequate height, greater density and less
occlusal forces
 Simply does not need longer implant
 D3 and D4 bone – posterior region, less available bone
 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 lengthwww.indiandentalacademy.com

CREST MODULE CONSIDERATIONS
 Transosteal region from the implant body and characterized as
a region of highly concentrated mechanical stress
 Not ideally designed for stress
 Smooth parallel sided crest module – shear
 Angled crest module (∠ > 200
) surface texture
 Slightly larger than outer diameter – 4 reasons
 Polished collar (0.5 mm) – perigingival area
 Longer polished collar – shear loading – crestal bone loss
 Bone is often lost to first thread

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

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
– 3-6 mm

GOLD SCREWS AND ABUTMENT SCREWS
 Metal framework is held onto the abutments by screw joints,
in which gold screw is torqued into the abutment screw.
 Screw joint’s main function is to clamp the gold cylinder and
attached framework onto the abutment cylinder.
 Tensile force on gold screw and abutment screw &
compressive clamping force on titanium abutment cylinder.
 Two forces are equal and opposite – desired situation.
 Joint clamping force is called preload.
 External applied force > preload = opening of screw joint.

 Inevitable dimensional
inaccuracies
 ‘Passive fit’
 Misfitting framework can
cause loads on implant even
before any bitting force is
applied
FRAMEWORK MISFIT

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 – plat from 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 teethwww.indiandentalacademy.com

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

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

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 prostheses
 Proven and standardized protocols
 Premachined components
 Instrument with stable and predefined tightening torquewww.indiandentalacademy.com

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

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.

THANK YOU

Journal club
By
Dr. Jagan Mohan Reddy
Date : 10.12.2005 Saturday 9.30 am

Biomechanics implants/certified fixed orthodontic courses by Indian dental academy

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