Scientific rationale fordental implant design

Presented by:Dr.Piroz Givehchian

Supervised by: Dr. Mansour

Rismanchian

And Dr.saied Nosouhian

Dental of implantology

Dental implants research center

Isfahan university of mediacal

science

 

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Treatment planning sequence for implant dentistry design of final restoration ( position and number of teeth replaced and

type of prosthesis ) patient force factor bone density key implant positions and additional implant number implant size available bone in the edentulous sites

› When it is not present, modification of treatment is necessary:

1. Bone augmentation

2. consideration of optional implant locations usually with additional implants, increase in implant size

3. optimization of implant design 2

implant designed to answer cause of implant failure :

1. implant surgery

2. bacterial plaque complication

3. loading condition

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Body design for surgical ease of placement tend to have tapered , short implant body press_fit insertion

Cylinder or press-fit implant were popular in 1980s: › has a friction-fit insertion and have less risk of pressure necrosis› has no need to bone tap (even in dense bone)

but after 5 years of loading , they include more of loss of crestal bone and implant failure :› fatigue overload condition› harmful shear loads on bone causing large bone turnover rates› less bone_implant contact (BIC) percent› higher risk of overload failure

surgical success rate is usually higher than 98% regardless of implant size or design , so designing an implant for surgical

ease dose not appear to be the most important aspect to reduce the incidence of complications

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Design to reduce the plaque-related complication

smooth crest module:• easier to clean• collects less plaque

problem with this philosophy:• smooth crest module is initially placed below

the crest of the bone• encourage marginal bone loss from the

extension of biological width after uncovery and from shear forces after occlusal loading

• so this plaque-reducing design feature increases the peri-implant sulcus depth

• Therefore 5

Implant body designed for: ease of surgery

reduce plaque-related and peri-implantitis

Dose not the most common complications observed in implant

prostheses

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Most implant body complications in literature:1. early implant failure after loading 2. marginal bone loss before loading but after exposure of the implant3. marginal bone loss after the loading of the implant-bone

interface

Goodacare 1981-2003 (soft bone)Misch 1981-2004 (<10mm)

Implant body designs should attempt to primarily address the primary causes of complication

Factors that address the loading condition of the implant after implant are placed in function

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Implant design related to occlusal forces

primary functional design objective is to manage (dissipate and distribute) biomechanical loads to optimize the implant-supported prosthesis function

load management is dependent: 1. Character of applied force2. functional surface area which the load is dissipated

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Force type and influence on implant design 3 type of forces be imposed on implants:

Compression tension shear

bone is strongest when loaded in compression, 30% tensile 65% shear

implant has macroscopic body design and a microscopic component.› Microscopic features : important during initial healing

and initial loading period

› Macroscopic design : important during early loading and mature loading periods 9

Smooth-sided, cylindrical implants:› Ease in surgical placement› larger shear condition in bone-implant interface

smooth-sided, tapered implants:› Greater the taper, greater the component of compressive load

delivered › taper cannot be greater than 30 degrees:

body length to be reduced immediate fixation and initial healing to be reduced less overall surface area and less initial stability

tapered threaded implants:› provide some surgical advantage during insertion› lesser surface area of tapered implant increase the amount

of stress at crestal portion› thread at apical half are often less deep because of the outer

diameter continues decrease and limit the initial fixation 10

smooth cylinder implant results in shear load at interface, so must rely on a microscopic retention system for initial loading period:› Roughening or coating, acid etch, mechanical etch, coat with plasma

spray or HA› the quality of coating as absolutely paramount in such application

surface condition of an implant may enhance BIC and adhesion qualities at initial healing

surface coating don’t permit compressive loads to be effectively transmitted to bone cells because the micro features of the coating are too small for cells to be loaded in compression

so:

surface area_bone contact percentage is greater during initial healing but the functional surface area is most dependent on

macroscopic design of implant body 11

Watzek et al

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0

20

40

60

80

100

nonthreadedcylindricalimplants

branmarksystem

implants

% of BIC

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Bolind et al

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Bolind et alImplant body design was more important

than the surface condition of the implant for crestal bone loss and overall BIC after loading

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Force direction and influence on implant body design bone is weaker when loaded under an angled force

greater the angle of load, the greater the stresses

… implant body long axis should be perpendicular to the curve of

wilson and curve of spee

axial alignment places less shear stress on the overall implant system and decrease the risk of complications as screw loosening and fatigue fractures

So:

Virtually all implant are designed for placement perpendicular to occlusal plan

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implant body design with threaded features have ability to convert occlusal loads into more favorable compressive loads at bone interface

under axial load to an implant-bone interface:› buttress or squre-shaped thread would transmit

compressive forces to bone( BioHorizons, BioLok , Ankylosis )

› V-tread face angle (Zimmer , LifeCore, 3i and some Noble Biocare) is comparable to the reverse buttress thread (Noble Biocare) because of the similarity in the inferior portion of thread face angle

the thread shape (macroscopic design) is independent from the surface coating (microscopic design)

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Implant body: functional versus theoretical surface area

to reduce stress, the force must decrease or the surface area must increase

functional surface area: the area that serves to dissipate compressive load to implant-bone interface

functional thread surface area: portion of the thread that participates in compressive load transmission under occlusal load

total theoretical surface area: “passive” area on implant that dose not participate in load transfer or has a feature so small, bone cannot adapt to load transfer › plasma spry coating provide up to 600% more total surface area for BIC

› size of each plasma particle is less than 8µm and the 120µm bone cell dose not receive a transfer of mechanical stress from this feature

› amount of actual BIC that can be used for compressive loading may be less than 30% of total theoretical surface area 19

functional surface area is beneficial to reduce the mechanical stress to bone› most stress to implant-bone interface in D1 to D3 bone is in crestal 5

to 9 mm of implant› design of body in coronal 9mm is most important to appropriately

distribute occlusal stress 20mm V-shaped or reverse buttress threaded implant may have more total

surface area compared with 13mm square threaded but functional area may be higher in the 13mm implant because of thread geometry

Functional surface area plays major role in addressing variable initial BIC zone related to bone density › D4 bone has weakest biomechanical strength and lowest BIC area to

dissipate the load at implant-bone interface › the functional surface area requirements would increase from a

minimum for implants in D1 bone to maximum for implants in D4 bone

Functional S.A may be increased by diameter of an implant› however the higher bite force and lower bone density of posterior

regions may not always be able to be addressed adequately by only implant diameter

› Ivanoff 1999 20

as results:Implant geometric body design , length , bone density are

related to the functional surface area

an ideal treatment plan:Implant length of 12mm or greater with 4mm

diameter for most anterior and 5mm or greater in molar regions

when ideal implant size cannot be insert, the alternative may be to increase the surface area of implant by modifying implant body design

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Implant body geometry versus occlusal load

The design of implant not only governs the initial stability of the implant but as important determines the BIC percent and location of contact available for effective load transfer to the bone after occlusal loading

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when bone did not fully occupy the threads, greater bone volume was observed on the lower aspect compared with upper aspect of square threads

bone bridge was found from one square thread to another

after dynamic loading the bone implant density was grater on the bottom of the thread face angle less on top of thread

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Duyck et al

during a lateral load, the strain was more concentrated at tip of thread and was decreased from the exterior to interior region of thread › at tip of each of thread, bone was resorbed› at depth of tread, bone was maintained

bone contact was least at tip of each thread ( the highest strain occurs)

bone was greatest under the thread face angle ( bone is loaded more on compressive)

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Hollister and kohn

Scientific rational for implant design

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histological description of bone: lamellar, woven , composite, and bundle bone

Lamellar bone: most organized , highly mineralized , strongest

of bone type , called load-bearing bone Woven bone: unorganized , less mineralized, less strength,

called immature bone

microstrain levels 100 times less than ultimate strength of bone may be responsible for remodeling rate because the bone cell membranes are able to act as a mechanosensory system in bone› load applied on bone >>> microstrain in cells >>> energy

to open ion membrane channels

Bone remodeling rate (BRR), : › is time needed for new bone to replace the existing bone and

allows for adaptation of bone to its environment › percentage or volume of new bone within a specific time period › lamellar bone forms at rate of 1 to 5 µm each day› woven bone forms at rate of more than 60 µm each day

26Adapted widow

interface remodeling allows a viable bone interface to form between the dental implant and original bone after the implant has been surgically inserted

higher BRR is directly related to an increase in the amount of woven bone formation

so BRR may be directly to strength of implant interface and degree of risk for implant-bone interface

higher risk is related to higher turnover rate , because the bone is less mineralized, less organized and less strange at the interface

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before implant insertion: › bone is usually mature, lamellar bone

surgical trauma:› cause the bone to repair, with primarily woven bone formation

by end of 4 months:› osteoblasts have deposited about 70% of mineral found in

mature vital bone have reformed lamellar bone› remaining 30% of mineral deposition occurs during secondary

mineralization over the next 8-month period after bone has healed and the implant is then loaded:

› interface again remodels as influenced by its local strain environment

› if woven bone forms as a result of mechanical loading. Its called reactive woven bone and very similar in structure and properties to the repair woven bone from surgical trauma

long term maintenance of implant depends on:› continues remodeling of interface › allows new bone to replace bone, that may have sustained

microfractures or fatigue as a result of cyclical loading

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Microdamage act as key step in signaling increases in remodeling and replacing of skeletal tissue

microdamage in cortical bone surrounding screw-type implants has been reported during both insertion and pullout force

amount of microdamage was related to the thread design of implant

strong association between microdamage, osteocyte apoptosis and subsequent bone remodeling

threaded implants with axial tensile loading have higher remodeling rates and less mineralized bone than implants didn't receive a load after healing

non axial loaded implants have a greater BRR compare with axially loaded

so: implant design, direction of load, surface condition may affect the bone at interface and bone turn over

rate

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thread geometry› Designed to: maximize initial

contact ,enhance surface area , facilitate dissipation of load at bone-implant interface

› three geometric thread parameters: thread pitch thread shape thread depth

crest modulation considerations

apical design consideration 30

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Thread pitch Distance measured parallel between adjacent thread

1.5mm in ITI 0.6mm in Zimmer, Biomet 3I

Smaller pitch, the more threads on body for given unit length and thus greater surface area per unit length

higher BIC was observed with the implant with greater thread number.

pitch has most significant effect on changing the surface area› when an ideal implant length cannot be planned without bone

augmentation

pitch may be used to help resist the forces to bone with poorer quality

if force magnitude is increased , implant length is decreased , bone density decreased, the thread pitch be decreased to increase thread number and increase functional surface area

surgical ease of implant placement is related to thread number› fewer thread , easier to insert ( in denser bone)

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thread number may be affected by implant crest module design› extended smooth crest module , reduced

thread number to support occlusal load

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implant bodies with double or triple thread leads› this term related to manufacturing process

and don’t increase functional area

› uses one or tow or three cutting blades to manufacture thread

› at 1 rpm inserting implant, one thread lead implant insert a distance of one thread and double-thread insert two thread into bone

› if revolution per minute is double(30rpm versus 15rpm), both implant thread into bone at same rate

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Thread shape square, v-shape , buttress , and reverse buttress Thread shape has application for loading conditions

› In reverse buttress the force transfer for occlusal loads to bone is similar to v-shaped

› square or power thread provides an optimized surface area for intrusive, compressive load transmission

› buttress thread may also load with primarily a compressive load transfer

thread shape may contribute to initial healing phase of osteointegration› V-shaped and reverse buttress thread shapes had similar BIC

percent and similar torque values to remove the implant after initial healing

› square thread had higher BIC percent and greater reverse torque

face angle of the thread or plateau in an implant body modify direction of occlusal load› face angle of v-shaped is 30 degree off the long axis and square

thread be perpendicular to long axis› occlusal loads in axial direction be compressive at bone

interface in square or plateau design but can be converted to higher shear load in v-shaped design

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Thread depth the distance between the major and minor diameter of thread

In conventional implants uniform thread depth throughout the length

tapered implant has a similar minor diameter , but outer diameter decreased in relationship to taper , so depth decreased toward the apical

tapered implant:› Decreased thread depth in apical› less overall surface area › less ability to fixate the bone in apical at initial

insertion› has less functional surface area may result in

higher stresses especially in shorter implant 39

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the greater thread depth, the greater surface area of implant

thread depth may be modified relative to diameter and overall surface area

as implant becomes wider the depth of thread may be deeper without decreasing the body wall thickness between the inner diameter and abutment screw space

more shallow thread depth, the easier it is to thread the implant in dense bone and less likely bone tapping is required

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Bioengineering of an implant design

when implant act as functionary unit for a prosthesis. An elevated BRR is an ongoing response adjacent to implants› BRR higher than 500% per year

in bone immediately adjacent (5mm) to V-shape threaded implant

› BRR 50% in the regions distant from interface

› Bone at interface of implants in this report is likely in the mild overload zone

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if implant design is bioengineering, so that loading will produce a microstrain in adapted window zone› it should maintain lamellar bone at interface› similar BRR adjacent to and distal to implant interface› a square-thread design be used on body of implant, showed

40_50% BRR and was same as observed in the bone away from the implant interface

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Crest module considerations

crest module: transosteal region, which extends from the implant body and often incorporates the antirotation components of abutment implant connection

it has:› Surgical influence› Biological width influence› loading profile consideration› prosthetic influence

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During surgical phase the crest module design benefits the crest implant interface It should be slightly larger than outer thread diameter of

body› Seals completely the osteotemy

› Provide barrier and deterrent for ingress of bacteria or fibrous tissue during initial healing

› greater initial stability of implant following placement, because it compresses the crest bone region

› increases surface area ,can further decreases stress at crest region

› increases platform of abutment connection with stress

reduction to abutment screw during lateral loading platform dimension is more critical to reduce the stress applied to

abutment screw than height or depth of antirotational hex 46

Crest module with a smooth collar : the concept of this designing is for a reduction of plaque

accumulation and improved hygiene› when bone loss occurs on the implant crest module, the gingival

sulcus depth can not be accessed by hygiene procedures after crestal bone loss

“formation of biological width” and “ occlusal overload” have increased bone loss risk when smooth metal is placed below the bone

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Herman et al

a biological width of at least 0.5 has been reported apical to abutment-implant connection regardless of implant crest module design :› a 0.5 mm collar length may provide for desirable smooth

surface close to perigingival area

› microgap between implant crest module and abutment may reduce when polished surface are approximate

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Crest module design and its relation to occlusal loading

most of occlusal stress occurs at the crestal region of an implant design

Smooth, parallel-sided crest module› will increase the risk of bone loss after loading› smooth metal promotes shear stresses in bone interface› smooth metal doesn't encourage bone cell contact before loading

amount of bone loss in machined (smooth) coronal region, is directly related to length of smooth crest module

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A roughened surface: › maintains the bone through the biological width cycle, but may

lost the marginal bone during occlusal load condition

› a rough crest module may not be sufficient to stop crestal bone loss once implant is loaded

› Its common clinical observation that bone is often lost to first thread after loading regardless of manufacture type or design

› magnitude of crestal bone loss is often directly related to distance between the crest module and first thread Most bone loss on the internal hex crest module design with extended

cylinder crest module

bone loss stop at first thread:› it changes the shear load created by crest module to compressive

loading52

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crest module design that incorporate, angled geometry, grooves to crest module ,surface texture, increases bone contact and impose beneficial compressive component and decrease risk of bone loss

Astra and BioHorizons

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prosthetic feature of crest module may affect the implant design:

In an internal hex implant :› implant body is lower in profile and easier to cover with tissue› antirotational feature is often deeper in body so › body diameter at crest module is reduced› threads on outside of implant can not be designed at or above

the antirotational feature› greater smooth metal and shear forces are observed above the

first thread

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Apical design considerations

it is most often tapered to permit the implant to seat in the osteotemy before implant engages the crestal region › patient not need to open his

mouth› benefit in posterior regions

most root form are circular in cross section› a round drill to prepare a round

hole corresponding to implant body

› don’t resist torsion/shear forces

antirotational feature is incorporated into the implant body:› usually apical region (hole or

vent) › flat sides or grooves along the

body or apical

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hole and vent feature:› bone can grow through them and resist torsional loads› increased surface area to transmit compressive loads to bone› when implant placed through sinus or become exposed, it fill with

mucus and become source of retrograde contamination or fill with fibrosis tissue

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recess areas allow bone filling from cutting threads to fill area

decreased the angle of cutting thread along the apical portion and then less torque is required to thread implant

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flat and grooved regions: bone grow against on them and helps resist torsional

loading

help to enhance the self tapping aspect of an implant design

recess areas allow bone filling from cutting threads to fill area

decreased the angle of cutting thread along the apical portion and then less torque is required to thread implant

Apical end of implant should be flat :› pointed geometry has less surface area› raising the stress› if the perforation is occurred , a sharp apex may irritate the

soft tissue

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Body material related to fracture

risk of implant body fracture in early to intermediate period :› for implant 3.75mm in diameter is

1%› abutment screw fracture risk is 2%› and prosthetic screw risk is 4%

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Goodacare et al

element of implant body that influence the fracture risk include :› biomaterial› size› design

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magnitude of force is 200Ib in molar region, less in canine area (100Ib), and least in anterior incisor(25 to 35Ib) and 1000Ib in parafunction

many biomaterial are unable to withstand the type and magnitude of parafunctional loads› ceramic , HA

Titanium and titanium alloy:› biocompatibility › with its highly active titanium oxide layer, they are well tolerated by local

tissue› good mechanical and physical properties› they represent the closest approximation to stiffness of bone (almost 6 time

stiffer)Ti-6Al-4V Grade 4 Grade 3 Grade 2 Grade 1 Property 930 550 450 345 240 Tensile strength

860 483 380 274 170 yield strength

113 103 103 103 103 Modulus of elasticity62

Implant design failures related to biomaterial and force magnitude

two example of implant failures related to biomaterial choice:

vitreous carbon implant:› optimized modulus of elasticity with out attention to ultimate

strength › composed of smooth tapered carbon body with an internal

stainless steel post› microcrack in body developed and pathway of biological fluids

was introduced to internal stainless steel post › post have corrosion and release of metallic ions to tissues and

then tissue inflammation be occurred

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ceramic implant:› optimized ultimate strength without adequate attention to

modulus of elasticity › brittle nature and susceptibility of failure in tension and shear

load› very stiff ceramic carried a disproportionate amount of load

and interfacial bone was moved into disuse atrophy

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Force duration and implant body design

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mechanical failure are due to : static load and/or fatigue loads

implant body and components are prone to fatigue fracture. with incidence between 1%to 4% after 5 to 10yo

Increase fracture risk was observed with:› Grade 1 Ti implant body used› treatment plan has higher stress› parafunctional forces › force type, direction of load › Geometric design

fatigue strength:› level of highest stress a material may be repetitively cycled with

out failure› less than one half its ultimate tensile strength › is more critical factor rather than ultimate strength 66

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Implant body size and design relation to fracture

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Therefore an implant or component 2 times as wideis 16 times more resistant to fracture.

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Considering the same equations, it can be shownthat an abutment screw, which has a smaller crosssectionalarea than an implant (typically about 2 rnrn,}

is more susceptible to fracture. This is particularly truewhen the abutment screw comes loose and bears a large,disproportionate component of a transverse load to theocclusal surface.

ability of implant and components to resist fracture is directly related to component’s moment of inertia

solid cylinder fracture resistance is equal to radius to the 4th power› wider diameter implant may be used when offset loads or greater

stress condition exist› abutment screw, is more susceptible to fracture

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annulus portion of implant: space in implant body blew the

abutment screw this permit the receptor site to be

machined and allow the screw to tighten without “ bottoming out”

wall thickness of implant body in region blew abutment screw controls the resistance to fatigue fracture

when bone loss occurs to annulus, implant body fracture is imminent

abutment screw length is an important implant design issue and should be as long as possible

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crest module may be designed to have a space around the abutment screw:› this space increase risk of

fracture at this location

external hex has space above the implant body:› slightly higher risk of

fracture in abutment internal hex has space in implant

body:› has an increased risk of

fracture at crest module of implant

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Thank you

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Dr. Pirooz Givehchian