Biomechanics of dental implants/certified fixed orthodontic courses by Indian dental academy

Biomechanics of implants
Contents:



Introduction.



Loads applied to dental implants.



Mass, force and weight.



Types of forces.



Stress, strain relationship.



Force delivery and failure mechanisms.



Fatigue failure.



Scientific rationale for dental implant design.



Single tooth implant and biomechanics.



Cantilever prosthesis and biomechanics.



Biomechanics of frame works and misfit.



Treatment planning based on biomechanical risk factors.



Conclusion.



References.

Page 1


INTRODUCTION:
Biomechanics comprises of all kinds of interactions between tissues
and organs of the body and forces acting on them. It’s the response of the
biologic tissues to the applied loads.
Dental implants function to transfer load to surrounding biological
tissues. Thus the primary functional design objective is to manage
(dissipate and distribute) biomechanical loads to optimize the implant
supported prosthesis function.
Definition
Process of analysis and determination of loading and deformation of
bone in a biological system.
Natural tooth Vs Implant:

Natural tooth
1. Natural tooth is anchored in to

Implant
1. Implant is rigidly fixed by

the bone by flexible periodontal

functional ankylosis.

ligament.
2.

The

around

periodontal
the

natural

ligament
tooth

significantly reduces the amount
of stress transmitted to the bone

2. The concentration of stresses
mainly occurs at the crestal
region.

Page 2

and

facilitates

even

force

distribution.
3. The pdl acts as viscoelastic
shock

absorber

serving

to

3. The implant is fixed and rigid.

decrease the magnitude of stress
to the bone.
4. The precursor signs of a
premature contact or occlusal

4. These initial reversible signs

trauma on natural teeth are

and symptoms of trauma donot

usually reversible and include

occur with implants.

signs of cold sensitivity, wear
facets, pits, drift away and tooth
mobility.
5. This condition often helps in

5. The magnitude of stress may

the patient seeking professional

cause bone microfracture, bone

treatment by occlusal adjustment

loss which ultimately leads to

and

mechanical failure of implant

a

reduction

in

force

magnitude in force magnitude

components.

which further reduces the stress
magnitude.
6. The elastic modulus of a tooth
is closer to the bone than any of

6. The implant materials differs by

the currently available dental

5-10 times from the surrounding

implant biomaterial. The greater

bone structure.
Page 3

the flexibility difference between
the two materials, the greater the
potential
generated

relative
between

motion
the

two

surfaces at the endosteal region.
7. Implants deliver a slow dull
7. The proprioceptive information

pain that triggers a delayed

relayed by teeth and implants also

reaction if any.

differs in quality. Natural teeth
deliver a rapid, sharp, high
pressure

that

triggers

proprioceptive mechanism.

8. Where as the bone loading
around an implant is performed by

8. The surrounding bone of

the dentist in a much more rapid and

natural teeth is developed slowly

intense fashion.

and gradually in response to
biomechanical loads.
9. Lateral forces in implants
9. A lateral force on natural tooth
is dissipated rapidly away from

concentrates

at

the

crestal

region.

the crest of bone toward the apex
of the tooth.

Page 4

CHARACTER OF FORCES APPLIED TO DENTAL IMPLANTS:
Excess loads on an osseointegrated implant may result in mobility
of supporting device and excessive loads also may fracture an implant
component or body. The internal stresses that develop in an implant
system and surrounding biological tissues under imposed load may have a
significant influence on the long term longevity of the implants in vivo. A
goal of treatment planning should be to minimize and evenly distribute
mechanical stress in implant system and contiguous bone.
LOADS APPLIED TO DENTAL IMPLANTS:
o In function – occlusal loads
o Absence of function – Perioral forces
 Horizontal loads
o 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)
FORCE (SIR ISAAC NEWTON 1687):


Newton’s II law of motion
Page 5

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
MOMENT / TORQUE:
The force which tends to rotate a body. Units – N.m; N.cm, lb.ft ; oz.in
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.

Page 6


FORCES ACTING ON THE IMPLANTS:
Three types of forces acting on the dental implants


Compressive



Tensile



shear
Compressive:

i)

Tend to push masses towards each other.

ii)

Maintains integrity of bone – implant interface.

iii)

Accommodated best.

iv)

Cortical bone is strongest in compression.

v)

Cements, retention screws, implant components and bone – implant
interfaces can accommodate greater compressive forces than tensile or
shear forces.
Page 7

vi)

Hence compressive forces should be Dominant in implant
prosthetic occlusion.

TENSILE FORCES

SHEAR FORCES

↓

↓

Pull objects apart

Sliding

 Distract / disrupt bone implant interface.
 Shear forces are most destructive, cortical bone is weakest to
accommodate shear forces.
 Cylinder implants –in particular are

highest risk for shear forces at

the implant tissue interface unless an occlusal load directed along the long
axis of the implant body.
 They require a coating to manage the shear forces to manage the shear
forces through a more uniform bone attachment.
 Threaded / finned implants impart a combination of all three types of
forces at the interface under the action of single occlusal load. This
Page 8

conversion of a single force in to three types of forces is controlled by the
implant geometry.
STRESS:
The manner in which a force is distributed over a surface is referred
as mechanical stress.
γ = F/A
The magnitude of stress depends on two variables:
-

force magnitude.

-

cross sectional area over which the force is dissipated.
Force magnitude may be decreased by reducing magnifiers of force that
are:

1.

Cantilever length

2.

Crown height

3.

Night guards

4.

Occlusal material

5.

Over dentures
Functional cross sectional area may be optimized by:
1. increased by Number of implants
2. Selecting an Implant geometry that has been designed carefully to
maximize the functional cross sectional area.

Page 9

DEFORMATION & STRAIN:
 A load applied to a dental implant may induce deformation of the implant
and surrounding tissues
 Deformation and stiffness of implant material may influence
A.

Implant tissue Interface

B.

Ease of implant manufacture

C.

Clinical longevity

STRESS – STRAIN RELATIONSHIP:



A relationship is needed between the applied stress that is imposed on
the implant and surrounding tissues and the subsequent deformation.



The load values by the surface area over which they act and the strain
experienced by the object produces a stress strain curve.
Page 10



The slope of the linear portion of the curve is referred to as the modulus
of elasticity and its value indicates the stiffness of the material.



The closer the modulus of elasticity of the implant to the biological
tissues, the less the relative motion at the implant tissue interface.
Once a particular implant system is selected the only way for an
operator to control the strain experienced by the tissues is to control the
applied stress or change the density of bone around the implant.



Greater the strength stiffer the 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
 Lower Ultimate strength


Hook’s law
Stress = Modulus of elasticity x strain
γ = E.ε
BITING FORCES:

 Axial component of biting force: (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
Page 11

•

Chewing forces will act on teeth for = 9 min/day

•

If includes swallowing = 17.5 min/day

•

Further be increased by parafunction
FORCE DELIVERY AND FAILURE MECHANISM:

 The manner in which forces are applied to the dental implant restorations
within the oral environment dictates the likelihood of system failure.
 An understanding of force delivery and failure mechanisms is critically
important to the implant practitioner to avoid costly and painful
complications.


The moment or torque is the product of the force

magnitude multiplied by the perpendicular distance from the point of
interest to the line of the action of the force.

 Moment loads are destructive in nature and may result in:
Interface breakdown
Bone resorption
Page 12

Screw loosening
Bar / bridge fracture
A total of six moments may develop about the three clinical coordinate
axes:
- occlusoapical
- faciolingual
- mesiodistal
These moment loads induce microrotations and stress concentrations at
the crest of the alveolar ridge at the implant to tissue interface , which
lead inevitably to crestal bone loss. Three clinical moment arms in
implant dentistry
- occlusal height
- cantilever length
- occlusal width

Page 13

Minimization of each of these moment arms is necessary to prevent
unretained restorations, fracture of components, crestal bone loss or
complete implant system failure.
1) Occlusal height:
- Occlusal height serves as the moment arm for force components directed
along the faciolingual axis:
- working or balancing occlusal contacts, tongue thrusts or peri oral
musculature, and the force components directed along the mesiodistal
axis.
- force components along the vertical axis is not affected by the occlusal
height because there is no effective moment arm.
- in division A bone initial moment load at the crest is less than in
division C or D bone because the crown height is greater in Cand D.
2) Cantilever length:
 Large moments may develop from vertical axis force components in
prosthetic environments designed with cantilever extensions or offset
loads from rigidly fixed implants.
 A Lingual force component may also induce a twisting moment about the
implant neck axis if applied through a cantilever length.
 Force applied directly over the implant does not induce a moment load or
torque because no rotational forces are applied through an offset distance.

Page 14

 Antero posterior spread is the distance to the center of the most anterior
implant and the most distal aspect of the posterior implants.
 The greater the A-P spread the smaller the resultant loads on the implant
system from cantilevered forced because of the stabilizing effect of the
antero-posterior distance.
According to MISCH
 Cantilever length is determined by the 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 involves smaller A-P spreads between splited implants
and should have smaller length cantilever.
 Tapered arch form – largest A-P spread – larger cantilever design.
3). Occlusal width:
Wide occlusal tables increase the moment arm for any offset
occlusal loads. Faciolingual tipping (rotation) can be reduced significantly
by narrowing the occlusal tables or adjusting the occlusion to provide
more centric contacts.
A vicious destructive cycle can develop with moment loads and result
in crestal bone loss.

Page 15

Moment loads

Crestal bone loss
Increases occlusal height

Failure if biomechanical
environment is not corrected
Occlusal ht. moment arm

More crestal bone loss

↑ Faciolingual micro
rotation or rocking

FATIGUE FAILURE:
Fatigue failure is characterized by Dynamic cyclic loading conditions,
four factors significantly influence the fatigue failure.

1) Biomaterials
2) Geometry
3) Force magnitude
4) Loading cycles
1) Bio materials:


Fatigue behaviour of biomaterials is characterized to a plot of applied
stress vs no. of loading cycles



High stress – few loading cycles



Low stress – infinite loading cycles

Page 16

Ti alloys exhibits a higher endurance limit compared with



commercially pure titanium (Cp Ti)
2) Macro geometry:
The geometry of an implant influences the degree to which it can



Resists bending and torque


Lateral loads also causes fatigue fracture



The fatigue failure is related as 4th power of the thickness difference
Also affected by the difference in Inner and outer diameter of screw



and abutment screw space
3) Force magnitude:
The magnitude of loads on dental implants reduced by careful
consideration of arch position


Higher loads on posteriors



Limitation of Moment loads



Geometry for functional area



Increasing the No. of implants

4) Loading cycles


Reducing the No. of loading cycles



Elimination of parafunction



Reducing the occlusal contacts
SCIENTIFIC RATIONALE FOR DENTAL IMPLANT DESIGN
Page 17

 Dental implants function to transfer of load to surrounding biologic
tissues.
 Thus the primary functional design objective is to manage (dissipate and
distribute) biomechanical loads to optimize the implant supported
prosthesis function.
 Biomechanical load management depends on two factors that are
1) Character of applied load.

2) Functional surface area

 Forces applied to dental implant characterized in terms of Magnitude,
duration, type, direction and magnification.
FORCE MAGNITUDE:


The magnitude of biting force varies as a function of
anatomic region and state of dentition. The magnitude of force is greater
in molar region and lesser in canine region.



Higher magnitude demands increased bone density and
Influence the selection of biomaterials.



Materials such as silicon hydroxyapatite and carbon are
characterized by lesser ultimate strengths even though they are highly
compatible with the biological tissues.



In

contemporary

applications,

these

materials

are

considered for use as coatings applied to stronger substrate materials.


Silicone, HA, carbon has- High biocompatibility
- Low ultimate strength

 Titanium and its alloy – Excellent biocompatibility
Page 18

- Corrosion resistance
- Good ultimate strength
- Closest approx. to stiffness of bone
FORCE DURATION:
 The duration of bite forces on dentition has a wide range under ideal
conditions; the total time of those brief episodes is less than 30 minutes
per day.
 Patients who exhibit bruxism, clenching or other parafunctional habits
may have their teeth in contact several hours each day.
 The endurance limit or fatigue strength is the level of highest stress
through whish a material may be cycled repetitively without failure. The
endurance limit of a material is often less than one half its ultimate tensile
strength.
 The ability of implants and abutment screws to resist fracture from
bending loads is related directly to the moment of inertia of the
component.
 This parameter is a function of the cross sectional geometry of the
component.
 Implant bodies are particularly susceptible to fatigue fracture at the
apical extension of the abutment screw within the implant body or at
the crest module around abutment (eg: with an internal hexagon)
 The formula for the bending fracture resistance in these conditions is
related to the outer diameter radius to the fourth power minus the inner
diameter radius to the fourth power.

Page 19

 The wall thickness of the implant body in this region controls the
resistance to fatigue failure. Even a small increase in wall thickness
results in a significant increase in bending fracture resistance because
the dimension is multiplied to a power of four.
TYPE OF FORCE:


Three types of forces may be imposed on dental implants within the
oral environment
-Compression
-Tension
-Shear



Bone is strongest when loaded in compression. 30% weaker when
subjected to tensile forces and 65% weaker when loaded in shear



A smooth sided implant may be called a cylinder design, and this
cylinder implant body result in essentially a shear type of force at the
implant to bone interface. Thus this body geometry must use a
microscopic retention system by coating the implant with titanium plasma
spray or hydroxyl apatite



If the hydroxyapatite resorbs from infection or bone remodeling, the
remaining smooth sided cylinder is severely compromised for healthy
load transfer to the surrounding tissues



A threaded implant may use microscopic and macroscopic design
features to load the bone in compression and tensile loads



Threaded implants have the ability to transform the type of force
imposed at the bone interface through careful control of the thread
Page 20

geometry. Thread shape is particularly important in changing force type at
the bone interface


Thread shapes in dental implant design include square, v shape and
buttress



Under axial loads to a dental implant a v thread face (typical of
paragon, 3i and Nobel Biocana) is comparable to the buttress thread and
has a 10 times greater shear component of force than a square or a power
thread



A reduction in shear load at the thread to bone interface reduces the risk
of overload; which is particularly important in compromised D3 and D4
bone. A threaded implant also may have a surface condition such as
hydroxyapatite, TPS or other roughed surface.

FORCE DIRECTION:


The anatomy of the mandible and maxilla places significant constraints
on the ability to surgically place root form implant suitable for loading
along their long axis.



Bony undercuts further constrain implant placement thus force
direction. Most of all undercuts occur on the facial aspects of the bone,
with the exception of the submandibular fossa in posteroior mandible.
Hence implant bodies often are angled to the lingual to avoid penetrating
the facial undercut during insertion.



As the angle of the load increases, the stresses around the implant
increases, particularly in the vulnerable crestal bone region. As a result all
implants are designed for placement perpendicular to the occlusal plane.

Page 21

This placement allows a more axial load to the implant body and reduces
the amount of crestal loss.
FORCE MAGNIFICATION:
There are various factors which can magnifies the forces on dental
implants


Surgical placement resulting in extreme angulation of the implant



Para functional habits



Cantilever and crown height



Increase in functional area



Increased density of the bone



Increase in implant number decreases cantilever length and limits
the force magnifier.

FUNCTIONAL SURFACE AREA:


Functional surface area is defined as the area that actively serves to
dissipate compressive and tensile non shear bonds through the implant to
bone interface and provides initial stability of the implant following
surgical placement.



The total surface area may include a passive area that does not
participate in load transfer.



Functional surface area also plays a major role in addressing the
variable implant to bone contact zones related to bone density.

Page 22



D1 bone, is the densest bone found in the jaws is also the strongest
bone and provides an intimate contact with a threaded root form implant
at initial implant loading.



D4 bone has the weakest biomechanical strength and the lowest contact
area to dissipate the load at the implant to bone interface.



Thus an improved functional surface area per unit length of the implant
is needed to reduce the mechanical stress to this weak bone.



Implant macrogeometry and implant width are two important design
variables for optimizing surface area.

IMPLANT MACROGEOMETRY:


The macro design or shape of an implant has an important
bearing on the bone response.



Growing bone concentrates preferentially on protruding elements
of the implant surface, such as ridges, crests, teeth, ribs or the edge of
threaded surface.



The shape of the implant determines the surface area available
for stress transfer and governs the initial stability of the implant.



Smooth sided cylindrical implants provide ease in surgical
placement, however the bone to implant interface is subjected to
significantly larger shear conditions.



A smooth sided tapered implant allows for a component load to
be delivered to the bone implant interface, depending on the degree of
taper, however the greater the taper of smooth sided implant the less the
overall surface area of the implant body.

Page 23



Threaded implants with circular cross sections provide for ease
of surgical placement and allow for greater functional surface area
optimization to transmit compressive loads to bone implant interface.



A smooth surface cylinder depends on a coating or
microstructure for load transfer to bone.

IMPLANT WIDTH:


An increase in implant width adequately increases the area over which
occlusal forces may be dissipated.



Wider root form designs exhibit a greater area of bone contact than
narrow implants of similar design because of an increase in
circumferential bone contact.



The larger the width of the implant the more it resembles the
emergence profile of the natural tooth.



The increased width of implants 6-12 mm also enhances the bending
fracture resistance. But the crestal bone anatomy most often constrains
implant width to less than 5.5mm.

THREAD GEOMETRY
Threads are designed to maximize initial contact enhance surface area and
facilitate dissipation of stresses at the bone- implant interface.
Functional surface area per unit length of the implant may be modified by
varying three thread geometry parameters
-

thread pitch

-

thread shape
Page 24

-

thread depth

THREAD PITCH:



Thread pitch is defined as the distance measured parallel with its axis
between adjacent thread forms or the number of threads per unit length in
the same axial plane or on the same side of the axis.



The smaller the pitch (finer) the more threads on the implant body for a
given unit length, and thus the greater surface area per unit length of the
implant body.



If force magnitude increase or bone density decreases one may decrease
the thread pitch to increase the functional surface area.



Some of the current popular designs which have different pitches.



The distance between pitches:
Page 25

ITI Implant – 1.5mm
Sterioss

- 0.8mm

Nobel biocare,zimmer, 3i & life core – 0.6mm
Biohorizons - 0.4mm
-the fewer the threads , the easier to bond or insert the implant.
THREAD SHAPE:



Thread shapes in implant geometry (dental implant designs include
square, Vshape and buttress.



The V shape thread design is called a fixture and is primarily used
for fixating metal parts together not load transfer.



The buttress thread shape was designed initially for and is optimized
for pullout loads.



The square or power threaded provides an optimized surface area
for intrusive, compressive load transmission.



The shear force on a V threaded face (typical of Zimmer, 3i and
Nobel biocare) is about 10 time greater than the shear force on a square
thread.
Page 26

THREAD DEPTH:


The threaded depth refers to the distance between the major and minor
diameter of the thread.



the greater the thread depth, the grater the surface area of the implant if
all the other factors are equal.

IMPLANT LENGTH:


As the length of an implant increases so does the overall total surface
area.



D1 bone is the strongest and densest bone of the oral environment. The
strength of the bone and the intimate contact between the bone and
implant provide resistance to lateral loading. Bicortical stabilization is not
needed in D1 bone because it is already a homogenous cortical bone.



A long implant in D2 or D3 bone in the anterior mandible may cause
increased surgical risk, since attempting to engage the opposing cortical
plate and preparing a longer osteotomy may result in overloading of the
bone.



In poor quality D3 and D4 bone functional surface area must be
maximized to distribute occlusal loads optimally, the placement of longer
implants in posterior regions require surgical modifications like nerve
repositioning, placement of sinus grafts in maxillary posterior regions.



The shorter and smaller diameter implants had lower survival rates than
their longer or wider counter parts.

CREST MODULE CONSIDERATIONS:

Page 27

 Crest module of an implant body is the transosteal region from the
implant body and characterized as a region of highly concentrated
mechanical stress.


Slightly larger than outer diameter, thus the crest module seats fully
over the implant body osteotomy, providing a deterrent for the ingress of
bacteria or fibrous tissue.



The seal created by the larger crest module also provides for greater
initial stability of the implant following placement.



Polished collar (0.5 mm) – perigingival area, provides for a desirable
smooth surface close to the perigingival area.



Longer polished collar – shear loading – crestal bone loss



Bone is often lost to first thread, because the first thread changes the
shear force of the crest module to a component of compressive force in
which bone is strongest.

APICAL DESIGN CONSIDERATIONS:
Round cross sectional implants do not resist torsional shear
forces when abutment screws are tightened hence anti rotational feature is
incorporated usually in the apical region of the implant body, with a hole
or vent. Bone can grow through the apical hole and resist torsional loads
applied to the implant. The apical hole region may increase the surface
area available to transmit compressive loads on the bone.
The disadvantage of the apical hole occurs when the implant
is placed through the sinus floor or becomes exposed through a cortical
plate. The apical hole may fill with mucous and become a source of
Page 28

retrograde contamination. Another anti rotational feature of implant body
may be flat sides or grooves along the body or apical region of the
implant body.
The apical end of each implant should be flat rather than
pointed, this allows for the entire length of the implant to incorporate
design features that maximize desired strain profiles.
Progressive Loading
Misch (1980) proposed that
Gradual increase in occlusal load separated by a time interval to allow
bone to accommodate.
Softer the bone à increase in progressive loading period.
Protocol Includes,


Time



Diet



Occlusal Contacts and occlusal material



Prosthesis Design

Time:

Page 29

Two surgical appointments between initial implant placement and stage
II uncovery may vary on density.


D1

-

3 Months



D2

-

4 Months



D3

-

5 Months



D4

-

6 Months

Diet:



Limited to soft diet – 10 pounds
Initial delivery of final prosthesis-21 pounds
Occlusal Material:
Initial step – no occlusal material placed over implant
Provisional – Acrylic – lower impact force
Final - Metal / Porcelain
Occlusion:



Initial

-



Provisional -

Out of occlusion



Final

At occlusion

-

No occlusal contact

Prosthesis Design:
First transititional – No occlusal contact
Page 30

No cantilever
Second transititional -

Occlusal contact
With no cantilever

Final restoration

- narrow occlusal table and cantilever with implant

protective occlusion guidelines.

SINGLE TOOTH IMPLANTS:


Single tooth implants require good bone support and control of
harmful effects of occlusal levers that are not parallel to the long axis of
the implant.



The prosthesis must be designed to allow good oral hygiene, with
easy access to inter proximal surfaces and the retaining screw.



A molar can be replaced with two standard diameter implants or one
wide implant.



This type implant is contraindicated for larger spaces because the
masticatory and occlusal forces to the most distal or mesial portions will
be harmful.



To avoid excessive loads, the implant must be centered in the
edentulous space during placement.

Page 31


ANTERIOR SINGLE TOOTH RESTORATIONS:


The anterior

single tooth restoration is achieved using a standard

diameter implant, which is preferred over a narrow implant because it
provides a larger surface for osseo integration


Generally the use of wide implants in this area is not advocated because
it may compromise good esthetic results.



To avoid levers that may be produced during parafunction in centric
and eccentric positions, its recommended that the implant supported
restoration be left out of occlusion.
SHORT SPAN FIXED PARTIAL DENTURE:
The construction of a 3 unit particularly cantilever fixed
partial dentures require a posterior triangular zone of occlusal surface
between the supporting implants.
The chances of overloading the implants are far less and
this provides a better long term prognosis, because it offers a wider
active zone while also achieving good occlusal load in relationship to the
axes of the implants. the use of wide implants to support cantilever fixed
partial dentures improves the prognosis further, especially in those cases
where only two wide implants are needed compared to three of standard
diameter. wide implants allow for an increased occlusal surfaces in these
circumstances.
Page 32

The proximity of anatomical features such as the
mandibular canal or the maxillary sinus limit the use of long implants. In
the presence of adequate bucco lingual bone width these limitations ca be
managed with the use of wide implants.
CANTILEVER FIXED PARTIAL DENTURE:


It results in greater torque with distal abutment as fulcrum.



May be compared with Class I lever arm.



May extend anterior than posterior to reduce the amount of force
It depends on stress factors



Parafunction



Crown height



Impact width



Implant Number
The design of cantilever fixed partial dentures is dependent on the
occlusal forces that can be elicited at the free end of the denture and the
length and width of the implants selected.

CASE 1:


A case with two implants placed for the lateral incisor and the canine
with a free end central incisor.



Two implants of adequate length are required.



The cantilever tooth should avoid contacts on the central incisors
during protrusion, lateral excursions and maximum intercuspation.
Page 33


CASE II:


When the implants serve as support for the central and lateral incisors
with a free end canine, the occlusal configuration should provide group
function during lateral movements and avoids loading of canine.



If it’s not possible lateral guidance may be provided by the central and
lateral incisors avoiding any contact with the canine.

CASE III:
 When two implants are placed unilaterally at the site of two maxillary
premolars, the free end canine must be left out of occlusion.
Page 34

CASE IV:


Molar replacements achieve best results with a three Implant supported
fixed prosthesis providing premolar morphology to the restorations.



The length of the implants influences the outcome of treatment



Due to the enormous occlusal loads in the second molar area the use of
a free end fixed prosthesis is contra indicated.

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
Page 35

– 3-6 mm

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 form 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
4) Implants placed in a tripod configuration


Desired à counteract lateral loads

Page 36

5) Presence of prosthetic extension
6) Implants placed offset to the center of the prosthesis à 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

Page 37



Lack of prosthetic fit and cemented prostheses



Proven and standardized protocols



Premachined components



Instrument with stable and predefined tightening torque

WARNING SIGNS:
–

Repeated loosening of prosthetic / abutment screw

–

Repeated fracture of veneering material

–

Fracture of prosthetic / abutment screws

–

Bone resorption below the first thread

CONCLUSION:
Biomechanics is one of the most important
consideration affecting the design of the frame work for an implant bone
prosthesis. It must be analyzed during diagnosis and treatment planning as
it may influence the decision making process which ultimately reflect on
the implant supported prosthesis.

REFERENCES

1.

Dental implant prosthetics – Carl E. Misch

Page 38

2.

Principles and practice of implant dentistry – Charles Weiss, Adam
Weiss.

3.

Tissue – integrated prosthesis. Osseointegration in clinical dentistry
– Branemark, zarb, Albrektsson

4.

Oral rehabilitation with implant supported prosthesis -Vincente

5.

ITI dental implants- Thomas G.Wilson

Page 39

Biomechanics of dental implants/certified fixed orthodontic courses by Indian dental academy

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