biomechanics of dental implants

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.


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

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

1. Implant is rigidly fixed by

functional ankylosis.

2. The concentration of stresses

mainly occurs at the crestal

region.


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

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.


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.

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):

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

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

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.


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


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:

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.


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

g = E.e

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


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


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:

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

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


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


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


- 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:

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.


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


- 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:


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.


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:


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


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:


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 - No occlusal contact

Provisional - Out of occlusion

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:

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.


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.


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.


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.


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


– 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

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

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

biomechanics of dental implants

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