3- surface configurations of dental implants.pdf

8/11/2019 3- Surface configurations of dental implants.pdf

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Periodontology

2000 Vol.

17 1998 36-46

Printed in Denm ark. All rights reserved

C o p y r i g h t M u n k s g a a r d 1 9 9 8

PERIODONTOLOGY 2000

I SSN 0906-6713

Surface configurationsof

dental implants

JAN

EIR IKELLINGSEN

The use and development of implants for restoring

single or multiple loss of teeth has been through sev-

eral phases to reach the optimal goal of having a per-

manent, artificial anchoring of fixed bridges or

crowns in the upper and lower jaw. This period,

mainly based on trial-and-error approaches, has led

to the development of different implant materials,

designs and treatment techniques that have not al-

ways led to the results expected or desired.

More needs to be known about the optimal situ-

ation of the connection between an artificial ma-

terial and the tissues what type of material that

gives the best tissue response and what type of sur-

face is preferred by the bone cells or the cells in the

soft tissue.

If

this is known, the response of the bone

or soft tissue can be predicted when the implants are

installed into the jaws.

Today's implant materials function well when the

bone quality is good and especially when there is

bicortical anchorage. However, in many cases the

situation is not optimal and regions have cancellous

bone and a thin cortical lamellae. Although some re-

Table 1. Different types of materials used in the

body as biomaterials

Gold

Cobalt-chromium alloys

Stainless steel

Titanium

Zirconium

Niobium

Tantalum

Hydroxyapatite

Bioglasses

Tricalcium phosphate

Carbon

Polymers

A 1 2 0 3

cent reports (52, 66,

72,

85) indicate that good clin-

ical results can be achieved even after shorter heal-

ing time using the conventional implants, most re-

search (1, 2, 4 18-21) shows that a relatively long

healing period without any stress is needed to

achieve a tight bony contact with the implant ma-

terial. The healing time, however, depends on the

bone quality, and one aspect of current research is to

try to determine the best treatment modalities with

reduced bone quality, particularly in the maxillary

region.

Another aim of biomaterial research is to improve

the bone quality after implantation by introduction

of active bone-inducing substances. Structures on

the implant surface or in the tissues may also have

such an effect.

The current situation concerning the clinical use

of implants is based on basic research and animal

and clinical trials that have been performed at many

institutions worldwide during the last

3

decades. The

result of this activity is that treatment with implants

is now generally accepted in dentistry if the treat-

ment is done according to some accepted principles:

cylindrically shaped implants, preferably with

threads, and the surgical and the subsequent pros-

thodontic procedures performed according to an ac-

cepted protocol.

However, with all respect to the research per-

formed in this field, the state of the art today is

mainly a result of trial-and-error approaches to op-

timizing already known materials and methods.

Scientifically based engineering of new materials to

reach a specific goal is rare. Most biomaterials used

in implantology are not specifically developed for

this purpose but were available before they were

used as biomaterials. Examples of these materials

are listed in Table

1.

The materials form a heterogen-

eous group consisting of metals, precious and non-

precious, oxide forming, corrosive, more or less non-

corrosive, ceramics and non-metals and non-cer-

amics. The effects of these materials in bone

36


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Ellinasen

sults obtained with this type of implants and is not

based to the same extent on systematic biomechan-

ical research and knowledge of the optimal bio-

mechanical connection between the implants and the

bony structures. Following an implantation, the im-

plant goes through several phases in which stability

and the transfer of loads are important. To obtain op-

timal healing of the bone close to the implant surface,

stability of the implant bed is important

(22, 73,

88).

Introduction of threads to the cylindrically implant

improves the implant stability significantly

(41).

Threaded cylindrical implants help the surgeon in

placing the implant exactly in the pre-drilled cavity,

and due to the threads the implant is stable and fixed

during the healing process. This probably has clinical

consequences. Screw-shaped implants have been

found to be in closer contact with bone than are cylin-

drical implants without threads

(24).

The observation

of improved bone healing around screw-shaped im-

plants has also been confirmed by Albrektsson et al.

(5)

in a long-term clinical trial. Different types of

thread profiles have been used on the cylindrical im-

plants but all with the same goal: to establish a stable

fixation between the implant and the bony tissue and

to improve load transfer.

Introduction of threads on cylindrical implants

leads to a change in the distribution of the stress ap-

plied to the implant compared with cylindrically

shaped implants (80).A cylindrically shaped implant

will largely transfer the stress to the apical part with

axially directed loads and probably to the neck and

apical part with horizontally directed loads. When

bone implants are designed, attention should be di-

rected towards distributing the loads evenly in the

bone. This will reduce the possibilities for overloading

the bone, and according to Wolff's law, induce bone

remodeling

or

bone formation. Both excessive and in-

sufficient stress have been suggested to promote bone

resorption at the neck region (30,811.

Overloading was suggested as a main causal factor

behind loss of Brinemark (Nobel Biocare AB, Gote-

borg, Sweden) implants (68). The same group found

improved results with longer implants compared

with shorter ones, further indicating that over-

loading is a major factor in the loss of dental im-

plants.

An

even distribution of loads in the bone thus

increases the load-carrying capacity of the implant.

Although the bone structure differs between differ-

ent types of cancellous bone and cortical bone areas,

studies have been performed to identify an optimal

thread profile. Frandsen et al. (41) observed an in-

creasing retention with increasing screw diameter

and increasing thread length when testing the hold-

ing power of four different screws implanted in can-

cellous bone of cadaveric femoral heads.

Most cylindrically threaded implants have a

thread profile similar to machined screws. It has

been argued that this is not an optimal profile in

bone. Other groups have therefore constructed im-

plants with different thread profiles and a different

angle of the thread flank (Fig.

2)

(82). It is debated

whether these changes in thread profiles have any

real clinical significance. Both systems with different

thread types have demonstrated long-term clinical

success

(1, 20).

In a recent thesis, Hansson et al. (50) present an-

other threading profile with minute threads with a

depth of only 0.1 mm (Fig. 3). These authors argue

that this type of threads have improved capacities to

carry load compared with the regular type of threads,

such as those on Branemark implants

(50).

These findings are in accordance with recent

findings by Wong et al. (94) that demonstrated im-

proved push-out strength for implant surfaces with

many small peaks compared with a surface with

high, but few peaks.

Surface microstructure

Fig.

3.

Example of an implant with a combination

of

microthreads and conventional threads: Astra Tech ST

Molndal, Sweden)

Another important factor of the surface configur-

ation is the microstructure of the implant surface.


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Surface configurations of dental implants

This can vary considerably depending on the surface

treatment of the implant. Variation of the surface

microstructure has been reported to influence the

stress distribution, retention of the implants in bone

and cell responses to the implant surface.

Gross et al. (45) investigated the in vivo responses

to pellet-blasted or flame-sprayed cylinders made of

titanium and Ti,+414V lloy, and furthermore to ti-

tanium rods flame sprayed with hydroxyapatite,

which were implanted into rabbit femurs. The

authors concluded that, independent of the implant

chemistry, each implant should have a micro-rough-

ness that

allows

fixation of trabeculae and conse-

quently a transmission of forces. The implants with

rough surfaces had improved bone response, with

bone trabeculae growing in a perpendicular direc-

tion to the implant surface.

An improved retention in bone has also previously

been reported after implantation of rough-surfaced

implants.

Several authors have discussed the dimension of

the ideal roughness that would provide increased re-

tention and an improved bone response. The rough-

ness can be considered on different levels: macros-

tructural, microstructural and ultrastructural, and

roughness on these different levels probably has dif-

ferent effects on the living tissues. It has been estab-

lished in the literature based on several studies that,

to gain complete growth of bone into a materials

irregularities, these need to be at least

100

pm in size.

Growth of bone into cavities or pores of this size will

give a mechanical interlocking of the material with

bone. This was demonstrated by Bobyn et al. (12) in

studying cobalt-based alloys with pore sizes of 50-

400 pm, Bone ingrowth was also observed by Cle-

mow

et al.

(29)

when this group studied porous coat-

ed

Ti l,V

femoral implants with pore sizes ranging

from 175 to 235 pm.

The optimal pore size to obtain bone ingrowth

might also depend on the material. Breme et al. (14)

stated that titanium alloy implants had an optimal

pore size of 100 pm. Increased removal torque values

were, however, also found when

using

titanium

po-

rous-coated screws of stainless steel with a pore size

of 10-40 pm. These small pores do not allow a maxi-

mal ingrowth of bone but may give increased reten-

tion based on mechanical interlocking. This has

also

been stated by Predecki et al. (67) in a study of bone

ingrowth to titanium and aluminum implants with

channels with diameters of 95-1000 pm. This group

reported the fastest bone ingrowth into channels

with diameters of 500-1000 pm, and no ingrowth

smaller scale was, however, found to be important

for integration of the bone with the implant surface.

These findings indicate that other mechanisms, not

purely based on mechanical interlocking, determine

the reactions between bony tissue and biomaterials.

Although surface roughness on a micrometer scale

gives some retention due to bone ingrowth,

in vitro

cell studies indicate that this property of the surface

influences the function of the cells, the matrix depo-

sition and the mineralization (13, 64, 78). Cells seems

to be sensitive to microtopography and appear to be

able to use the morphology of the material for orien-

tation and migration

116,

25 , 2 6 . The maturation of

the cells also affects the response to the surface

roughness, which is in agreement with earlier obser-

vations that indicated that chondrocytes are affected

differently by local factors such as vitamin

D

and

transforming growth factor

p

depending on the

stages of maturation of the cells

13,

76-78, 84).

Microtopography may therefore be one factor that

influences the differentiation of mesenchymal cells

into fibroblasts, chondrocytes or osteoblasts. Based

on these studies, the authors hypothesized that

osteogenesis may be favored by vascular ingrowth,

whereas a limited vascular ingrowth may induce

chondrogenesis.

Implants exhibiting a micro-roughness on their

surface have been tested and used both in animal

studies and in human patients. Buser et al.

(20)

in-

vestigated the correlation between different surface

structures and the bone response, as measured by

bone-to-implant contact. The surfaces investigated

in that study included titanium plasma-sprayed ti-

tanium, sandblasted and acid-etched titanium as

Fig.

4.

Scanning electron micrograph

with

high resolution

x503) of the surface of a machined, threaded implant

fNobel Biocare Mark 111

nto the 95-pm channels. Surface roughness on a

.~

_

39


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Fig. 5. Scanning electron micrograph with high resolution

x503) of the surface

of

a titanium plasma-sprayed

threaded implant

IT1

Bonefit)

well as hydroxyapatite-coated titanium. The authors

reported a positive correlation between increased

roughness and bone contact measured by histologi-

cal techniques. Titanium surfaces may also be blas-

ted with T i 0 2 particles to give micro-roughness. In

a clinical study in dogs, the biological response to

implants with this surface was compared with ma-

chined titanium. The authors reported significantly

higher removal torque values for the blasted im-

plants

(44)

(Fig.

4-6).

The ideal surface roughness for bone implants on

a micrometer scale probably depends on the distri-

bution of cortical or cancellous bone and on the

level of loading to the implants. Nevertheless, an op-

timal surface roughness has been proposed based on

experimental studies. In a series of studies, Wenneb-

erg et al. systematically investigated the effect of sur-

face roughness of implants and the response in rab-

bit bone (90-93). The implant surfaces were char-

acterized by profilometry as well as with a three-

dimensional laser technique. Titanium implants

with four different surface structures were created by

blasting with A12 3or Ti02 in addition to the ma-

chined surface. The surfaces were blasted with 25-

pm particles of

T i 0 2

or 25-pm, 75-pm or 250-pm par-

ticles of A1203.The authors concluded that implants

with a surface roughness of Sa 1 to 1.5 pm seemed

to be at an optimal roughness with regard to reten-

tion in bone as well as bone-to-implant contact as

measured by histomorphometry. This optimal sur-

face was created by blasting with 25- to 75-pm par-

ticles and resulted in surface roughness of Sa=0.83

and Sa=1.29 respectively.

A

rougher surface as

created by 250-pm particles led to a surface rough-

ness of Sa=2.11 and did not result in an improved

bone response. Implants treated with the 25-pm par-

ticles had significantly more bone-to-implant con-

tact than implants treated with the 250-pm particles

when the three best threads were considered.

No

sig-

nificant differences could, however, be detected

when the authors recorded the bone-to-implant

contact to all threads on the screw implants. Blasting

the titanium implants with 250-pm particles did not

improve the retention of the implants in bone.

Al-

most identical removal torque values were recorded

when the implants blasted with 25-pm particles and

250-pm particles were removed. The observation of

an improved response of bone to implants with a

surface roughness of Sa 1.0-1.5 pm is also in accord-

ance with observations by von Recum van Kooten

(89) that reported excellent tissue attachment with-

out signs of inflammation when implanting filter

membranes with pore sizes of 1-3 pm. Based on

these results, it seems that the benefit of increasing

roughness on a micrometer scale reaches a maxi-

mum level between 1.0 and 1.5 pm. Above this level

no further positive response in the bone can be ex-

pected. This observation indicates that the findings

from in

vitro

cell experiments that bone cells are ru-

gofile and respond positively with increased matrix

deposition and mineralization are also true in an in

vivo situation. This may be because the less rough

surfaces to a certain extent stimulate the bony tissue

through loading, which in return responds with in-

creased bone growth. Another, or additional inter-

pretation of the findings of Wennerberg et al. (90-

93) could be that the rugofile bone cells recognizes

the surface prepared by the course particle, as a

Fig.

6.

Scanning electron micrograph with high resolution

X503) of the surface of a titanium dioxide-blasted

threaded implant Astra Tech TiO-blast)

40


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urface configurationsof dental implants

smooth surface, whereas the 25-pm particles creates

a rough surface that is identified by the osteoblasts

(Fig.

7).

Surface ultrastructure

Although micro-roughness seems to be an important

characteristic for tissue response to biomaterials,

there are also observations that indicate a biological

response to irregularities on the nanometer level.

Larsson et al. 59-61) studied the biological effect of

changing the oxide thickness of titanium implants

from an electropolished level, to thick oxide layers

formed by anodization. By this treatment the surface

changes from an amorphous metal surface with a

noncrystalline oxide to a polycrystalline metal sur-

face with a crystalline oxide layer. Analysis of these

surface at a high resolution level demonstrated that

the new surface was heterogeneous with mainly

smooth areas of thick oxide but separated with po-

rous regions on a nanometer level. This observation

of an increased roughness after anodization of ti-

tanium was in line with earlier transmission electron

microscopic studies demonstrating increased pore

sizes with increased oxide thickness (58). Implants

with this thick, heterogeneous oxide seemed to have

a slightly improved response in bone, particularly in

the first weeks after implantation. This difference

could, however, not be observed after longer healing

periods. This is in accordance with earlier obser-

vations by Ellingsen Videm

36)

that did not show

any significant correlation between oxide thickness

on titanium implants and the bony response. The

latter observation was measured by push-out experi-

ments and histomorphometry, after 8-week healing

of titanium implants in rabbits. These authors ar-

gued that the outermost molecular layers of the bio-

material are the important part for the bone re-

sponse, because the surface chemistry

of

these

layers is exposed to the tissue. Morphological

changes on a nanometer level may introduce ad-

ditional effects to the tissue response which, in turn,

can further improve the bone healing.

The surface chemistry of the

implants

The chemical properties

of

the biomaterial surface

play an important role for the tissue responses

elicited by the material. This is at least one main rea-

Fig. 7. Bone cells exposed to a medium rough and a very

rough surface. The rugofile bone cells may recognize the

very rough surface right) as

a

smooth surface, whereas

the medium rough surface left) is recognized as a trough

rough surface by the osteoblasts.

son why the tissues responds differently to different

materials. A material with a surface that is accepted

by the tissue seems to exhibit improved integration

with bone, either due to passive growth, leading to a

tight connection between implants and bone, or by

stimulation that probably leads to a bone-implant

bonding. This is probably the case with the two main

materials used in dental implants, hydroxyapatite

and titanium. The calcified parts of the bone con-

sists of hydroxyapatite (or rather carbonated apa-

tite), and introducing this substance as an implant

material often gives favorable responses in the bone.

The chemical structure of the hydroxyapatite is,

however, important, and small changes in this sur-

face chemistry may have biological consequences.

This was observed in a recent study 11) investigat-

ing the osteoblastic responses to synthetic hydroxy-

apatite powders supplied by different manufac-

turers. Although both powders was shown to have a

phase-pure hydroxyapatite structure as indicated by

X-ray diffraction analysis, the calcium-to-phosphate

ratio deviated significantly from the stoichiometric

value for hydroxyapatite; one was rich in calcium

and the other slightly calcium deficient. There were

also small differences in impurities of carbon, so-

dium, silica and alumina. The authors observed that

one material seemed to be inferior to the other

based on the way that the osteoblasts reacted. The

cells cultured on this material had half the amount

of alkaline phosphatase and DNA as the cells cul-

tured on the other hydroxyapatite material. It seems

justified to do similar experiments

in

viva

In some elegant studies, Hanein et al.

(47-49)

demonstrated that cell adhesion

is

sensitive to vari-

41


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Ellingsen

ations in the surface organization of the material on

the atomic level. This group has tested the adhesion

of epithelial A6 cells to chemically equivalent but

structurally distinct phases or surfaces of calcium

(R,R)-tartrate tetrahydrate crystals. The observations

demonstrated that the outermost molecular layers of

a surface influence the interaction between the cells

and the biomaterial.

The biological effects of modifying the biomaterial

surface have also been elaborated by our group

31,

35-38). In an attempt to study the effect of the oxide

layer of titanium on calcium-phosphate precipita-

tion, titanium-dioxide(TiOz) and powder of oxidized

and nonoxidized titanium were introduced into an

in uitro nucleation test system 31). In this system

we found that titanium powder enhances calcium-

phosphate nucleation only after prolonged pre-in-

cubation in an aqueous buffer, or after autoclaving.

These treatments enhance the growth of the oxide

layer. Calcium phosphate crystals could be identified

on the surface

of

the oxidized titanium powder.

X-

ray diffraction analysis of the precipitates revealed

the pattern of poorly crystalline hydroxyapatite. This

observation indicated that the oxide content, or

structure, is required for titanium to act as a nu-

cleation substrate. Even more effective nucleation

was observed when pure TiOZ was used as a nu-

cleation substrate. The presence of serum proteins,

as in the body, will probably mask the reaction sites

and interfere with the capacity of these substances

to induce nucleation. This effect was observed in the

present study, but to a lesser extent when pure TiOa

was present as nucleation substrate. The nucleation

capacity and formation of calcium phosphate pre-

cipitates is related to the biocompatibility of ti-

tanium, and enhanced nucleation capacity may in-

dicate improved biocompatibility.

The biological activity of the TiOZ probably also

influences the protein adsorption to titanium. In an

in uitro study, serum proteins seemed to adsorb to

titanium dioxide by the same mechanisms as to hy-

droxyapatite through calcium binding

35).

The

surface characteristics of TiOZprobably change from

an anionic to a cationic state by the adsorption of

calcium to the surface. This will subsequently in-

crease its ability to adsorb acidic macromolecules,

such as albumin, a property demonstrated for hy-

droxyapatite (9,

10).

Manipulation of the surface oxide layer of ti-

tanium results in modification of the surface prop-

erties of the biomaterial with consequences for its

biological properties. This was demonstrated in a

study in which TiOZwas pretreated with lanthanum

37).Lanthanum ions (La3+)are known to have high

affinity for binding sites usually occupied by cal-

cium. Lanthanum pretreatment of TiOz alters the

properties of this substance, as shown by both in ui

tro and in uiuo tests. The total capacity for adsorp-

tion of serum proteins increased, and the amount of

albumin adsorbed to TiOZ pretreated with lantha-

num was shown to be more than five times increased

compared with untreated TiOz. Marked effects by

this modification were also observed in uiuo when

TiOz was implanted subperiosteally on the rats

sculls. Untreated TiOZ resulted in no adverse reac-

tions from the tissues, as demonstrated by a tight

connection between the TiOz powder with new bone

formation after 4 weeks and no indication of bone

resorption. Implantation of the lanthanum-pre-

treated TiOz resulted in the formation of a layer of

fibrous tissue with mononuclear cells that separated

the implant material from the bone. A similar ten-

dency was also observed when metallic titanium im-

plants were pretreated with lanthanum and im-

planted in rabbits and evaluated by the use of a

push-out test. The lanthanum-pretreated implants

had a significantly looser fit than untreated titanium

implants.

The mechanisms that lead to bone-bonding or to

a firm connection between the biomaterial and bone

are not completely known. The increased adsorption

of proteins to lanthanum-treated TiOZ means that

these proteins may therefore well contain more in-

hibitors of mineralization than the proteins that ad-

sorb to pure TiOz. Proteoglycans are important

modulators in the mineralization of bone and may

interact with mineral crystallites as an important

stage in the control of mineral growth

33, 87).

Fluoride ions have documented activity in bone.

This element is known to form fluoridated hydroxy-

apatite or fluorapatite with improved crystallinity

and better resistance to dissolution than hydroxy-

apatite 8, 42). Fluoride also enhances the incorpor-

ation of newly formed collagen into the bone matrix

and increases the rate of seeding of apatite crystals

as well as increasing trabecular bone density and

stimulating osteoprogenitor cells number in uitro (6,

79). The alkaline phosphatase activity, which is an

indication of the bone formation, was also elevated

after introduction of fluoride in an in uitro study

40).Titanium fluoride forms a stable layer, a glaze,

when applied onto tooth surfaces

(86).

This stable

layer is assumed to consist of titanium, which shares

the oxygen atoms of phosphate on the hydroxyapa-

tite surface, giving a covalently bonding between ti-

tanium and the hydroxyapatite.

4


8/11

Surface configurations of dental implants

titanium-bound oxygen and forms a titanium-fluor-

ide compound. When fluoride-modified titanium

implants are implanted into bone, the surfaces are

exposed to phosphate from the bone at a neutral pH.

This will make possible a reaction in which the oxy

gen in phosphate replaces the fluoride and binds to

titanium to create a covalently binding between

bone and titanium. The fluoride ions released by this

process may thus catalyze the new bone formation

in the surrounding tissue through the bone-stimu-

lating mechanisms discussed above (Fig.9). Fluoride

has also an inhibiting effect on the proteoglycan and

glycosaminoglycan adsorption to hydroxyapatite,

which may further improve the bonding to bone (34,

46, 71). ~~~~~~~l~~~~~nd

g~ycosam~nog~ycans

re

known to inhibit mineralization 39).

The surface qualities are

Of

utmost importance in

establishing of a reaction between the implant and

the tissues. This concerns the surface structure as

well as its chemical and biological properties. Much

attention has been focused on the importance of the

macrostructure of the implants for establishing re-

tention in the bone. More attention will probably be

focused in the future on the biological effects of the

surface structure on the microstructural and ultra-

structural levels as well as on the surface chemistry

of the implants. Progress in these fields based on

knowledge of the biological effects may provide im-

plants with improved tissue response and clinical

performance in the future.

Fig. 8. Scanning electron micrograph of a fluoride-modi-

fied implant after the push-out procedure. The implant is

partly right side) covered by bone that is firmly fixed to

the implant surface, which indicates bonding between the

titanium implant and bone.

References

Fig. 9. A possible mechanism between the fluoride-modi-

fied titanium and bone. Oxygen in phosphate may replace

the fluoride and bind to titanium to create a covalently

binding between bone and titanium. The fluoride ions

which are released by this process may thus catalyze the

new bone formation in the surrounding tissue.

When nonthreaded smooth surfaced titanium im-

plants with a fluoride-modified oxide layer were in-

stalled into rabbits, significantly increased retention

in the bone was observed after a push-out test pro-

cedure (38). This was observed after a 4-week heal-

ing period, but an even more pronounced effect was

observed after an 8-week healing period. Scanning

electron microscopic analyses of the retrieved im-

plants revealed that the fluoride-coated implants

were partly covered by bone that had fractured in-

ternally in the bone and was firmly k e d to the im-

plant surface (Fig. 8). By a surface modification of

TiOz

with fluoride, the fluoride probably replaces the

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