Contents:
þ Introduction.
þ Loads applied to dental
implants.
þ Mass, force and weight.
þ Types of forces.
þ Stress, strain
relationship.
þ Force delivery and failure
mechanisms.
þ Fatigue failure.
þ Scientific rationale for
dental implant design.
þ Single tooth implant and
biomechanics.
þ Cantilever prosthesis and
biomechanics.
þ Biomechanics of frame
works and misfit.
þ Treatment planning based
on biomechanical risk factors.
þ Conclusion.
þ References.
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
|
Implant
|
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1. Natural tooth is anchored in to
the bone by flexible periodontal ligament.
2. The periodontal ligament around
the natural tooth significantly reduces the amount of stress transmitted to
the bone and facilitates even force distribution.
3. The pdl acts as viscoelastic
shock absorber serving to decrease the magnitude of stress to the bone.
4. The precursor signs of a
premature contact or occlusal trauma on natural teeth are usually reversible
and include signs of cold sensitivity, wear facets, pits, drift away and
tooth mobility.
5. This condition often helps in
the patient seeking professional treatment by occlusal adjustment and a
reduction in force magnitude in force magnitude which further reduces the
stress magnitude.
6. The elastic modulus of a tooth
is closer to the bone than any of the currently available dental implant
biomaterial. The greater the flexibility difference between the two
materials, the greater the potential relative motion generated between the
two surfaces at the endosteal region.
7. The proprioceptive information
relayed by teeth and implants also differs in quality. Natural teeth deliver
a rapid, sharp, high pressure that triggers proprioceptive mechanism.
8. The surrounding bone of natural
teeth is developed slowly and gradually in response to biomechanical loads.
9. A lateral force on natural tooth
is dissipated rapidly away from the crest of bone toward the apex of the
tooth.
|
1. Implant is rigidly fixed by
functional ankylosis.
2. The concentration of stresses
mainly occurs at the crestal region.
3. The implant is
fixed and rigid.
4. These initial reversible signs
and symptoms of trauma donot occur with implants.
5. The magnitude of stress may
cause bone microfracture, bone loss which ultimately leads to mechanical failure
of implant components.
6. The implant materials differs by 5-10 times from the
surrounding bone structure.
7. Implants deliver a slow dull
pain that triggers a delayed reaction if any.
8. Where as the bone
loading around an implant is performed by the dentist in a much more rapid
and intense fashion.
9. Lateral forces in implants
concentrates at the crestal region.
|
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):
Ø Newton’s II law of motion
F
= ma
Where
a = 9.8 m/s2
Ø Mass – Determines
magnitude of static load
Ø 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:
Ø Magnitude, duration,
direction, type and magnification
Ø ‘Vector quantities’
Ø 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.
FORCES ACTING ON THE IMPLANTS:
Three types of forces acting on the
dental implants
þ
Compressive
þ
Tensile
þ
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.
vi) Hence compressive forces should be Dominant in
implant prosthetic occlusion.
TENSILE
FORCES SHEAR FORCES
¯ ¯
Pull objects apart Sliding
Ø Distract / disrupt bone
implant interface.
Ø Shear forces are most
destructive, cortical bone is weakest to accommodate shear forces.
Ø 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.
Ø They require a coating to
manage the shear forces to manage the shear forces through a more uniform bone
attachment.
Ø Threaded / finned implants
impart a combination of all three types of forces at the interface under the
action of single occlusal load. This 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.
g = 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.
DEFORMATION & STRAIN:
Ø A load applied to a dental
implant may induce deformation of the implant and surrounding tissues
Ø Deformation and stiffness
of implant material may influence
A. Implant tissue Interface
B. Ease of implant manufacture
C. Clinical longevity
STRESS – STRAIN RELATIONSHIP:
v A relationship is needed
between the applied stress that is imposed on the implant and surrounding
tissues and the subsequent deformation.
v The load values by the
surface area over which they act and the strain experienced by the object
produces a stress strain curve.
v 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.
v 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.
Ø Greater the strength
stiffer the bone
Ø Difference in stiffness is
less for CpTi & D1 bone but more for D4 bone
Ø Stress reduction in such
softer bone
§ To reduce resultant tissue
strain
§ Lower Ultimate strength
Ø Hook’s law
Stress
= Modulus of elasticity x strain
g = E.e
BITING FORCES:
Ø Axial component of biting
force: (100 – 2500 N) / (27 – 550 lbs)
Ø It tends to increase as one
moves distally
Ø Lateral component - 20 N
(approx.)
Ø 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
FORCE DELIVERY AND FAILURE MECHANISM:
v The manner in which forces
are applied to the dental implant restorations within the oral environment
dictates the likelihood of system failure.
v An understanding of force
delivery and failure mechanisms is critically important to the implant
practitioner to avoid costly and painful complications.
v 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.
Ø Moment loads are
destructive in nature and may result in:
Interface breakdown
Bone resorption
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
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.
§ 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.
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:
v
Fatigue behaviour of biomaterials
is characterized to a plot of applied stress vs no. of loading cycles
v
High stress – few loading cycles
v
Low stress – infinite loading
cycles
v
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
Ø
Reducing the No. of loading cycles
Ø
Elimination of parafunction
Ø
Reducing the occlusal contacts
SCIENTIFIC RATIONALE FOR DENTAL
IMPLANT DESIGN
v Dental
implants function to transfer of load to surrounding biologic tissues.
v
Thus the primary functional design
objective is to manage (dissipate and distribute) biomechanical loads to
optimize the implant supported prosthesis function.
v
Biomechanical load management
depends on two factors that are
1) Character of applied load. 2) Functional surface area
v 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
- 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.
§ 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 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. 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.
§
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:
v
The macro design or shape of an
implant has an important bearing on the bone response.
v
Growing bone concentrates
preferentially on protruding elements of the implant surface, such as ridges,
crests, teeth, ribs or the edge of threaded surface.
v
The shape of the implant determines
the surface area available for stress transfer and governs the initial
stability of the implant.
v
Smooth sided cylindrical implants
provide ease in surgical placement, however the bone to implant interface is
subjected to significantly larger shear conditions.
v
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.
v
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.
v
A smooth surface cylinder depends
on a coating or microstructure for load transfer to bone.
IMPLANT WIDTH:
v
An increase in implant width
adequately increases the area over which occlusal forces may be dissipated.
v
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.
v
The larger the width of the implant
the more it resembles the emergence profile of the natural tooth.
v
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
-
thread depth
THREAD PITCH:
v
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.
v
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.
v
If force magnitude increase or bone
density decreases one may decrease the thread pitch to increase the functional
surface area.
v
Some of the current popular designs
which have different pitches.
v
The distance between pitches:
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:
v
Thread shapes in implant geometry
(dental implant designs include square, Vshape and buttress.
v
The V shape thread design is called
a fixture and is primarily used for fixating metal parts together not load
transfer.
v
The buttress thread shape was
designed initially for and is optimized for pullout loads.
v
The square or power threaded
provides an optimized surface area for intrusive, compressive load transmission.
v
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.
THREAD DEPTH:
v
The threaded depth refers to the
distance between the major and minor diameter of the thread.
v
the greater the thread depth, the
grater the surface area of the implant if all the other factors are equal.
IMPLANT LENGTH:
v
As the length of an implant
increases so does the overall total surface area.
v
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.
v
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.
v
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.
v
The shorter and smaller diameter implants
had lower survival rates than their longer or wider counter parts.
CREST MODULE CONSIDERATIONS:
Ø Crest
module of an implant body is the transosteal region from the implant body and
characterized as a region of highly concentrated mechanical stress.
Ø
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.
Ø
The seal created by the larger
crest module also provides for greater initial stability of the implant
following placement.
Ø
Polished collar (0.5 mm) –
perigingival area, provides for a desirable smooth surface close to the
perigingival area.
Ø
Longer polished collar – shear
loading – crestal bone loss
Ø
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 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,
Ø
Time
Ø
Diet
Ø
Occlusal
Contacts and occlusal material
Ø
Prosthesis
Design
Time:
Two surgical appointments between initial
implant placement and stage II uncovery may vary on density.
Ø
D1 - 3
Months
Ø
D2 - 4
Months
Ø
D3 - 5
Months
Ø
D4 - 6
Months
Diet:
Ø
Limited
to soft diet – 10 pounds
Ø
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:
Ø
Initial
- No occlusal contact
Ø
Provisional
- Out of occlusion
Ø
Final - At
occlusion
Prosthesis Design:
First transititional
– No occlusal contact
No
cantilever
Second transititional -
Occlusal contact
With no cantilever
Final restoration - narrow occlusal table and
cantilever with implant protective occlusion guidelines.
SINGLE TOOTH IMPLANTS:
v
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.
v
The prosthesis must be designed to
allow good oral hygiene, with easy access to inter proximal surfaces and the
retaining screw.
v
A molar can be replaced with two
standard diameter implants or one wide implant.
v
This type implant is
contraindicated for larger spaces because the masticatory and occlusal forces
to the most distal or mesial portions will be harmful.
v
To avoid excessive loads, the
implant must be centered in the edentulous space during placement.
ANTERIOR SINGLE TOOTH RESTORATIONS:
v
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
v
Generally the use of wide implants
in this area is not advocated because it may compromise good esthetic results.
v
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.
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:
Ø
It results in greater torque with
distal abutment as fulcrum.
Ø
May be
compared with Class I lever arm.
Ø
May
extend anterior than posterior to reduce the amount of force
It depends
on stress factors
Ø
Parafunction
Ø
Crown
height
Ø
Impact
width
Ø
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:
v
A case with two implants placed for
the lateral incisor and the canine with a free end central incisor.
v
Two implants of adequate length are
required.
v
The cantilever tooth should avoid
contacts on the central incisors during protrusion, lateral excursions and
maximum intercuspation.
CASE II:
v
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.
v
If it’s not possible lateral
guidance may be provided by the central and lateral incisors avoiding any
contact with the canine.
CASE III:
v When
two implants are placed unilaterally at the site of two maxillary premolars, the
free end canine must be left out of occlusion.
CASE IV:
v
Molar replacements achieve best
results with a three Implant supported fixed prosthesis providing premolar
morphology to the restorations.
v
The length of the implants
influences the outcome of treatment
v
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:
Ø
Metal framework for full arch
prosthesis can fracture
Ø
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
TREATMENT PLANNING BASED ON
BIOMECHANICAL RISK FACTORS
Ø
Design of final prosthetic
reconstruction
Ø
Anatomical limitation
Geometric risk factor
1) No. of implants less than no. of
root support
Ø
One implant replacing a molar – risk.
§ 1
wide – plat form implant / 2 regular implants
Ø
Two implants supporting 3 roots or
more – risk
§ 2
wide – platform implants
2) Wide – platform implants
Ø
Risk – if used in very dense bone
3) Implant connected to natural
teeth
4) Implants placed in a tripod
configuration
Ø
Desired à
counteract lateral loads
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:
Ø
Force intensity and parafunctional
habit
Ø
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
Ø
Dependence on newly formed bone
Ø
Absence of good initial stability
Ø
Smaller implant diameter
§
Proper healing time before loading
§
4 mm diameter minimum – posteriors
Technological risk factors
Ø
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
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
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