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Dental implants: Introduction to dental implant

Article  in  British dental journal · August 1999

DOI: 10.1038/sj.bdj.4800222

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PRACTICEdental implants

BRITISH DENTAL JOURNAL, VOLUME 187, NO. 3, AUGUST 14 1999 127

Introduction to dentalimplantsRichard Palmer1

Implants have been used to support dentalprostheses for many decades, but they have notalways enjoyed a favourable reputation. Thissituation has changed dramatically with thedevelopment of endosseous osseointegrateddental implants. They are the nearest equiva-lent replacement to the natural tooth, and aretherefore a useful addition in the managementof patients who have missing teeth because ofdisease, trauma or developmental anomalies.There are a number of dental implant systemswhich offer predictable long-term resultsbacked by good scientific research and clinicaltrials. In the first place it may be helpful to clar-ify some of the commonly used terms inimplant dentistry (Table 1).

Success criteriaIt is important to establish success criteria forimplant systems, and for implants to be testedin well controlled clinical trials. The minimumsuccess criteria proposed by Albrektsson et al.(IJOMI 1986; 1: 11) is set out in Table 2.

The most obvious sign of implant failure ismobility. However, some of the criteria inTable 2 apply to the overall requirements of animplant system, but are not as useful whenjudging the success of individual implants. Thisis well illustrated by considering the radi-ographic criteria. Bone remodelling occurs in

the first year of function in response to occlusalforces and establishment of the normal dimen-sions of the peri-implant soft tissues (See Part2). The ‘ideal’ bone level is usually judgedagainst a specific landmark on the implant(such as the implant/abutment junction) and itmay differ therefore between implant systems(fig. 6). Subsequently the bone levels are usuallymore or less stable, and small changes such as0.2 mm per annum are impossible to measurewith conventional radiographs. These specifiedchanges therefore do not apply to individualimplants but to mean (average) changes mea-sured across a large number of implants. Forexample, a detectable change of 1mm or moremay occur at very few implants in contrast tothe majority which remain unchanged or in asteady state. It is also difficult to stipulate whatlevel of change in an individual implant over agiven period of time would constitute failure. Arapid change in bone level may be followed by along period of stability. On the other hand, pro-gressive or continuous bone loss is a worryingsign of impending failure. An implant withmarked loss of bone may therefore be judged as‘surviving’ rather than ‘successful’.

Implants placed in the mandible (particu-larly anterior to the mental foramina) enjoy ahigher success rate than the maxilla (approxi-mately 95% success for implants in the

Professor of Implant Dentistry andPeriodontology, Guy's Kings and StThomas' Medical and Dental School,London SE1 9RT© British Dental Journal 1999; 187: 127–132

This first part of a newseries outlines thesalient aspects ofosseointegration,implant design andother factors whichcontribute to successfultreatment.

1

Basic terminology in implant dentistryTable 1

Osseointegration A direct structural and functional connection between ordered, livingbone and the surface of a load-carrying implant (Albrektsson et al. ActaOrthopaedica Scand 1981; 52:155 (fig. 1).

Endosseous dental implant A device inserted into the jaw bone (endosseous) to support a dental pros-thesis. It is the ‘tooth root’ analogue and is often referred to as a ‘fixture’(fig. 2).

Implant abutment The component which attaches to the dental implant and supports theprosthesis. A transmucosal abutment (TMA) is one which passes throughthe mucosa overlying the implant. A temporary or healing abutment maybe used during the healing of the peri-implant soft tissue before the defini-tive abutment is chosen (fig. 3).

Abutment screw A screw used to connect an abutment to the implant.Single stage implant surgery Surgical placement of a dental implant which is left exposed to the oral

cavity following insertion. This is the protocol used in non-submergedimplant systems (fig. 4).

Two stage implant surgery Initial surgical placement of a dental implant which is buried beneath themucosa and then subsequently exposed with a second surgical proceduresome months later. This is used in submerged implant systems (fig. 5).

In this part, we willdiscuss:• Success criteria• Basic guide to

osseointegration:➤ Biocompatibility and

implant design➤ Bone factors➤ Loading conditions➤ Prosthetic

considerations

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mandible compared with 85 to 90% for themaxilla with systems such as Branemark, 5 years after loading). An example of the lowestrecorded success rates are for short implants(7 mm) used in the maxilla to support overden-tures, especially when the implants are notjoined together. A few studies have now shownthat the overall mean failure rate in smokers isabout twice that in non-smokers. Smokersshould be warned of this association andencouraged to quit the habit. It should also benoted that reported mean failure rates are notevenly distributed throughout the patient pop-ulation. Rather, implant failures are more likelyto cluster in certain individuals.

Basic guide to osseointegrationFigure 1 shows an histological section of a tita-nium screw threaded implant which has beenin function in bone for 1 year. There is veryclose apposition of bone over most of theimplant surface. It has been proposed that thebiological process leading to and maintainingosseointegration, is dependent upon a numberof factors which include:

Biocompatibility and implant designImplants made of commercially pure titaniumhave established a benchmark in osseointegra-tion, against which few other materials com-pare. Related materials such as niobium are ableto produce a high degree of osseointegration

Figure 1. Histological section of an implantwith bone growing in intimate contact with thesurface. The dense bone which contains asmall medullary space fills the area betweentwo thread profiles which are 0.6 mm apart.

Figure 2. Three different designs ofendosseous implants being inserted intoprepared sites within the jaw bone.Scanning electron micrographs of theimplants are shown in Figures 7 to 9.Figure 2a is a machined threadedimplant of the Branemark design (NobelBiocare). Figure 2b is an Astra STimplant which has a microthreadedcoronal portion, a macro-threadedapical portion and the surface has beenblasted with titanium oxide. Figure 2c isan ITI Straumann implant which has asmooth transmucosal collar, a macro-threaded body and a plasma sprayedsurface.

a

c

b

Implant design parameters• Implant length• Implant diameter• Implant shape• Surface characteristics

Prosthetic considerations• The type of prosthetic

reconstruction• The occlusal scheme• The number, distribution,

orientation, and designof implants

• The design and proper-ties of implant connectors

• Dimensions and locationof cantilever extensions

• Patient parafunctionalactivities

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and in addition, successful clinical results arereported for some titanium alloys and hydrox-yapatite coated implants. More recentlyresorbable coatings have been developed whichaim to improve the initial rate of bone healingagainst the implant surface and then resorbwithin a short time frame to allow establish-ment of a bone to metal contact.

The implant design has a great influence oninitial stability and subsequent function. Themain design parameters are:• Implant length – implants are generally avail-

able in lengths from about 6 mm to as muchas 20 mm. The most common lengthsemployed are between 8 and 15 mm whichcorrespond quite closely to normal rootlengths.

• Implant diameter — most implants areapproximately 4 mm in diameter. At least3.25 mm in diameter is required to ensureadequate implant strength. Implant diametermay be more important than implant lengthin the distribution of loads to the surroundingbone. Implant diameters up to 6 mm are avail-able, which are considerably stronger, but theyare not so widely used because sufficient bonewidth is not so commonly encountered.

• Implant shape — hollow-cylinders, solid-cylinders, hollow screws or solid screws arecommonly employed shapes which aredesigned to maximise the potential area for

osseointegration and provide good initialstability (figs 7a–9a). Even minor alterationsin the size and pitch of threads can enhancethe latter property. Screw shaped implantsalso offer good load distribution characteris-tics in function.

• Surface characteristics — the degree of sur-face roughness varies greatly between differ-ent systems. Surfaces which are machined,grit-blasted, etched, plasma sprayed andcoated are available. Figures 7b to 9b showthe characteristics of these surfaces viewedwith the scanning electron microscope,showing considerable increases in potentialsurface area. The optimum surface mor-phology has yet to be defined, and some mayperform better in certain circumstances. By

Suggested minimum success criteria for dental implants*Table 2

1. An individual, unattached implant is immobile when tested clinically.2. Radiographic examination does not reveal any peri-implant radiolucency.3. After the first year in function, radiographic vertical bone loss is less than 0.2 mm per annum.4. The individual implant performance is characterised by an absence of signs and symptoms such as

pain, infections, neuropathies, paraesthesia, or violation of the inferior dental canal.5. As a minimum, the implant should fulfil the above criteria with a success rate of 85% at the end of a

5 year observation period and 80% at the end of a 10 year period.

*after Albrektsson et al. IJOMI 1986; 1:11

Figure 3. Various forms of implantabutment are illustrated. Figure 3ashows ball abutments which areused to support overdentures.Figure 3b shows abutments whichare used to support individualcrowns in ‘single toothrestorations’. The crowns arecemented on the parallel sidedhexagon. Figure 3c shows fourconical shaped abutments whichare used to support a bridgesuperstructure. In this the bridgewould be screwed to the abutmentsrather than being cemented. Figure3d shows some simple cylindricalhealing abutments which are usedduring the healing phase of themucosa before definitive abutmentsare selected.

a b

c d

Figure 4. An implant of the ITIStraumann type has beeninserted and left protrudingthrough the mucosa in a onestage surgical procedure. Awide screw has been placedon the top to protect the inneraspect of the implant until adefinitive abutment isconnected.

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increasing surface roughness there is thepotential to increase the surface contact withbone but this may be at the expense of moreionic exchange and surface corrosion. Bacte-rial contamination of the implant surface willalso be affected by the surface roughness if itbecomes exposed within the mouth.

Bone factorsThe stability of the implant at the time of place-ment is very important and is dependent uponbone quantity and quality as well as implantdesign. The edentulous ridge can be classifiedin terms of shape and bone quality (fig. 10).Following loss of a tooth the alveolar boneresorbs in width and height. In extreme casesbone resorption proceeds to a level which isbeyond the normal extent of the alveolarprocess and well within the basal bone of thejaws. Radiographic determination of bonequantity and quality is considered in Part 5 andprocedures which can be used to augment bonein Part 8. The most favourable quality of jawbone for implant treatment is that which has a

well formed cortex and densely trabeculatedmedullary spaces with a good blood supply.Bone which is predominantly cortical may offergood initial stability at implant placement butis more easily damaged by overheating duringthe drilling process, especially with sites morethan 10 mm in depth. At the other extreme,bone with a thin or absent cortical layer andsparse trabeculation offers very poor initialimplant stability and fewer cells with a goodosteogenic potential to promote osseointegra-tion. Success is highly dependent upon a surgi-cal technique which avoids heating the bone.Slow drilling speeds, the use of successive incre-mentally larger sharp drills and copious salineirrigation aim to keep the temperature belowthat at which bone tissue damage occurs(around 47°C for 1 minute). Further refine-ments include cooling the irrigant and usinginternally irrigated drills. Methods by whichthese factors are controlled are considered inmore detail in Part 6 (Basic Implant Surgery).Factors which compromise bone quality areinfection, irradiation and heavy smoking. Theeffects of the latter two are a result of a diminu-tion of the vascular supply to the bone whichcompromises the healing response, a featurewhich has been well described in the healing offractures.

Loading conditionsFollowing installation of an implant it isimportant that it is not loaded during the earlyhealing phase. Movement of the implantwithin the bone at this stage results in fibroustissue encapsulation rather than osseointegra-tion. This has been compared to the healing ofa fracture where stabilisation of the bone frag-ments is very important to prevent non-union.In partially dentate subjects it is desirable toprovide temporary/provisional prostheseswhich are tooth supported to avoid earlyimplant loading. However, in patients whowear mucosally supported dentures it is gener-ally recommended that they should not beworn over the implant area for 1 to 2 weeks.This also helps to prevent breakdown of thesoft tissue wound. Systems such as Branemarkhave advised leaving implants unloadedbeneath the mucosa for around 6 months inthe maxilla and 3 months in the mandible,mainly because of differences in bone quality.However, these are largely empirical guide-lines, and bone quality and implant stabilitywill vary greatly between individuals, jaws andsites within jaws. Currently there is no accuratemeasure which precisely determines the opti-mum period of healing before loading cancommence. Bone quality can be assessed bymeasuring the cutting torque during prepara-tion of the implant site. The stability of animplant and increasing bone-to-implant con-tact has been quantified using resonance fre-

Figure 5. Exposure of twoimplants which have beenburied beneath the mucosafor a period of 6 months.Bone has grown over the topof them and this needs to beremoved before a healingabutment is connected.

Figure 6. A periapical radiograph ofa single tooth implant. The bonecontacts the implant up to the mostcoronal thread. An abutment screwwhich is more radio-opaque can beseen connecting the abutment to theimplant. The crown is all porcelainand is cemented to the abutment. Inthis system (Branemark) thelandmark for measuring the bonelevel from is the junction betweenimplant and abutment.

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quency analysis. This newly developed non-invasive research tool measures the stiffness ofthe implant at the bone interface. In some cir-cumstances it has been shown that immediateloading is compatible with subsequent suc-cessful osseointegration, providing the bonequality is good and the functional forces can beadequately controlled. The latter may involveplacing an adequate number of implants andconnecting them together as soon as possiblewith a rigid framework. However, these latterprotocols should be considered experimentalat the present time, and there is much data tosupport the more cautious approach advo-cated by Branemark in ensuring a high level ofpredictable implant success. Some systemsemploy a single stage approach in which theimplant is installed so that it protrudesthrough the overlying mucosa (ie non-sub-merged), although avoidance of early loadingis equally critical. Following the recommendedhealing period (around 3 months) abutmentsare connected to the implant to allow con-struction of the prosthesis. This protocoltherefore avoids further surgery to uncover theimplants. The loading of the implant sup-ported prosthesis is a further important con-sideration which will be dealt with in thefollowing section.

Prosthetic considerationsCarefully planned functional occlusal load-ing will result in maintenance of osseointe-gration and possibly increased bone toimplant contact. In contrast, excessive load-ing may lead to bone loss and/or componentfailure. Clinical loading conditions arelargely dependent upon:

The type of prosthetic reconstructionThis can vary from a single tooth replacementin the partially dentate case to a full archreconstruction in the edentulous individual.Implants which support overdentures maypresent particular problems with control ofloading as they may be largely mucosal sup-ported, entirely implant supported or a com-bination of the two.

The occlusal schemeThe lack of mobility in implant supported fixedprostheses requires provision of shallow cuspalinclines and careful distribution of loads in lat-eral excursions. With single tooth implantrestorations it is important to develop initialtooth contacts on the natural dentition and toavoid guidance in lateral excursions on theimplant restoration. Loading will also dependupon the opposing dentition which could benatural teeth, another implant supported pros-thesis or a conventional removeable prosthesis.Surprisingly high forces can be generatedthrough removable prostheses.

Figure 7. This shows a scanning electronmicrograph of a Branemark/NobelBiocare implant. Figure 7a shows thebasic thread design and figure 7b ahigher power view of the machinedsurface.

a b

Figure 8 shows a scanning electronmicrograph of an Astra ST implant. Theconical neck has a microthread and theapical part a coarser self tappingthread (fig. 8a). Figure 8b shows ahigher power view of the blasted (Tio-blast) surface at the samemagnification as figure 7b.

a b

Figure 9 shows a scanning electronmicrograph of an ITI Straumann solidscrew implant. The polishedtransmucosal neck is clearly demarcatedfrom the plasma sprayed body (fig. 9a).The thread has a coarser pitch than theimplants shown in figures 7 and 8.Figure 9b shows the plasma sprayedsurface at the same magnification asfigure 7b and 8b. The increase in surfacearea is considerable.

a b

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The number, distribution, orientation, anddesign of implantsThe distribution of load to the supporting bonecan be spread by increasing the number anddimensions (diameter, surface topography,length) of the implants. The spacing and 3-dimensional arrangement of the individualimplants will also be very important. The so-called ‘tripod’ arrangement of three implants isrecommended in situations of high load, suchas replacement of molar teeth in the partiallydentate individual.

The design and properties of implant connectorsMultiple implants are joined by a cast or milled

framework. A rigid connector provides goodsplinting and distribution of loads betweenimplants. It is equally important that the con-nector has a passive fit on the implant abut-ments so that loads are not set up within theprosthetic construction.

Dimensions and location of cantilever extensions. Some implant reconstructions are designedwith cantilever extensions to provide function(and appearance) in areas where provision ofadditional implants is difficult. This may becaused by practical or financial considerations.Cantilever extensions have the potential to create high loads, particularly on the implantadjacent to the cantilever. The extent of theleverage of any cantilever should be consideredin relation to the anteroposterior distancebetween implants supporting the reconstruc-tion. The cantilever extension should not exceedthis length and the cross sectional design shouldbe adequate to prevent flexing.

Patient parafunctional activitiesGreat caution should be exercised in treatingpatients with known parafunctional activities.Excessive loads may lead to loss of marginalbone or component fracture.

These factors will be considered in moredetail in Parts 4 and 7.

ConclusionThere are a great many factors to take intoaccount to ensure predictable successfulimplant treatment. There is no substitute formeticulous attention to detail in all of theseareas. Failure to do so will result in higher fail-ure rates and unnecessary complications.

Figure 10 shows examples of dental panoramic tomograms ofedentulous jaws. Both show extensive resorption of themaxillary ridge. There is far less resorption of the mandible infigure 10a than figure 10b. In the latter case there is reasonablebone volume in the anterior mandible but resorption close to thelevel of the inferior dental canal in the posterior part.

a

b

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