Dental implants: Teeth and implants

PRACTICEdental implants

BRITISH DENTAL JOURNAL, VOLUME 187, NO. 4, AUGUST 28 1999 183

Teeth and implantsRichard Palmer1

Clinicians who use dental implants in the treat-ment of their patients require an understandingof the nature of osseointegration and theimportant fundamental differences betweendental implants and natural teeth. The maincomparisons are summarised in Table 1 andillustrated in figure 1 which shows a singletooth implant and the adjacent natural teeth.The tooth originally formed within the jawsand erupted through the overlying mucosa in acomplex series of biological events that are byno means fully understood. The implant on theother hand was surgically placed within the jawbone, and is one of the few prosthetic devicesthat has been shown to successfully and perma-nently breach the surface epithelium with min-imal or no complications.

Gingiva versus periimplant soft tissuesIn healthy teeth the gingival margin is locatedon enamel. The gingival margin is scallopedand forms a shallow sulcus at the tooth surface.The gingiva rises between the teeth to form theinterdental papillae, which are complex struc-tures. Between the anterior teeth the papillaeare pyramidal structures with the attachmentof the gingivae following the contour of thecement enamel junction (fig. 2). In the molarregions, the buccal and lingual papillae at nat-ural tooth embrasures are separated by the ‘col’,an area of gingivae which forms a slight dipbeneath the contact point. A complex array of

gingival connective tissue fibres form welldefined bundle groups:• Interdental fibres• Dento-gingival fibres• Circular fibres• Alveolar crest fibres.

Many of these fibres are inserted into the rootcementum between the alveolar crest andcement enamel junction, and are thereforedependent upon the presence of natural teeth.

In the case of an implant, a transmucosal ele-ment (an abutment, neck of the implant or therestoration) protrudes through the overlyingmucosa which heals and adapts around it with-out a cementum attachment. The collagenfibres within the periimplant mucosa run par-allel to the abutment with no insertion into theabutment surface. There have been descrip-tions of more ordered fibre arrangements inrelation to transmucosal implant surfaceswhich have a rougher surface (such as plasmaspraying). In this situation some fibres appearto run at right angles to the implant surface, butthere is no good evidence of an attachmentmechanism. However a rough abutment sur-face does have potential negative properties,

Professor of Implant Dentistry andPeriodontology, Guy's Kings and StThomas' Medical and Dental School,London SE1 9RT© British Dental Journal 1999; 187: 183–188

An osseointegratedimplant restorationmay closely resemblea natural tooth.However, the absenceof a periodontalligament andconnective tissueattachment viacementum, results infundamentaldifferences in theadaptation of theimplant to occlusalforces, and thestructure of the gingival cuff.

2

Fig. 1a. Clinical photograph of a single toothimplant replacing the upper left lateral incisor.The porcelain fused to metal crown appears toemerge from the gingiva with interdental tissuewhich appears very similar to normal papillae

Fig. 1b Radiograph of the single tooth implantand adjacent teeth. The bone contacts theimplant surface with no intervening radiolucentspace which would be observed if there werefibrous tissue encapsulation. The bone marginis coincident with the implant/abutmentjunction. The adjacent teeth have a normalperiodontal ligament space

In this part, we willdiscuss:• Gingiva versus

periimplant soft tissues

• Periodontal ligament versus osseointegration

• Periodontitis and peri-implantitis

184 BRITISH DENTAL JOURNAL, VOLUME 187, NO. 4, AUGUST 28 1999


such as increased corrosion potential andmicrobial contamination if it becomes exposedwithin the oral cavity.

The papillae which form around a singletooth implant may be supported by collagenfibres attached to the adjacent natural teeth.However, in cases where there are adjacentimplants rather than teeth, the formation ofsoft tissue papillae is less predictable and theirform is dependent upon the presence of an ade-quate thickness of soft tissue, bone height,implant spacing and careful contouring of thecrown profiles to encourage the appearanceand maintenance of a papillary form (fig. 3).The soft tissue between multiple posterior unitimplants is more likely to have a flat contourbut again may be influenced by soft tissuethickness and crown morphology.

Junctional epitheliumIn healthy teeth the junctional epithelium(fig. 4) is attached to enamel by hemidesmoso-mal contacts and a basal lamina-like structureformed by the epithelial cells. The biologicalattachment mechanism is now thought to bemediated through particular adhesins or inte-grins, which are fundamental in cell to celladhesion as well as cell to matrix adhesion. It iswell established that a junctional epithelium

Healthy teeth versus healthy implantsTable 1

Healthy teeth Healthy implants

Gingival sulcus depth Shallow in health Dependent upon abutment length and restoration margin

Junctional epithelium On enamel On titanium

Gingival fibres Complex array inserted No organised collagen into cementum above fibre attachment –crestal bone parallel fibres

Crest of bone 1 to 2 mm apical to CEJ According to implant design eg at or about first thread in threaded implants or at the level of change in surface morphology

Connective tissue Well organised collagen Bone growing into close attachment fibre bundles inserted as contact with implant

Sharpey’s fibres into surface: oxide layer/alveolar bone and bone proteoglycancementum and collagen

Physical characteristics Physiologic mobility caused Rigid connection to bone - by viscoelastic properties as if ankylosedof the ligament

Adaptive characteristics Width of ligament can No adaptive capacity to alter to allow more allow mobility.mobility with increased Orthodontic movementocclusal forces impossible

Proprioception Highly sensitive No ligament receptorsreceptors present withinthe periodontal ligament

Fig. 2 A histological section of an interdentalspace between two teeth. The enamel has beenremoved by the demineralisation process. Thejunctional epithelium outlines the enamel spaceand terminates at the level of the rootcementum. The interdental bone septum issituated just below the cement enamel junction(in health 1–2 mm) and there is a welldeveloped transeptal fibre arrangement. Thereis a small inflammatory infiltrate in the gingivalconnective tissue at the top of the papilla

Gingiva versus periimplant soft tissues• Junctional epithelium• Biological width• Probing depth

examination



will also form on root surface cementum, den-tine and various dental materials includingimplant components (fig. 5). A normal junc-tional epithelium can be regenerated fromadjacent oral mucosa/gingiva following exci-sion, and the new junctional epithelium isindistinguishable from that which previouslyexisted. It is thought that the properties of thejunctional epithelium are dictated by the influ-ence of the underlying connective tissue, thepresence of an inflammatory infiltrate and thepresence of a tooth/implant surface to which itadheres (rather than the inherent properties ofthe epithelial cells). The junctional epitheliumhas a particularly high turnover and is perme-able to both the ingress of substances and tocomponents of the immune and inflammatorysystem. It is therefore well equipped to dealwith the problems of a breach in the epithelialintegrity caused by an emerging tooth orimplant. The junctional epithelium may befound on the implant itself or on the abutment.This will be because of differences in thedesigns of implants, the biological require-ments of the attachment of the soft tissue cuffand the level of the junction between abutmentand implants.

Biological widthIn teeth, the concept of the biological width iswell established, in that a zone of attached con-nective tissue separates the underlying alveolarbone from the apical termination of the junc-tional epithelium (fig. 6a). The connective tissue zone is about 2 mm wide and the lengthof the junctional epithelium about 1.5 mm. Figures 6b and c show two different designs ofimplants and the corresponding biologicalwidth. In the first case the implant design istypical of a submerged (two stage) system suchas the Branemark. After 1 year of function thebone margin is usually located at the firstthread. The junctional epithelium (1.5 mm to2 mm apicocoronal width) is located on theabutment, and a zone of non-arranged connec-tive tissue of about 1mm to 2 mm in widthintervenes. The join between abutment andimplant head is located within this zone. Incontrast the non-submerged (single stage)implant (typical of the ITI Straumann type) isplaced so that its roughened surface is placedwithin bone, but the smooth neck which is anintegral part of the implant performs the func-tion of the transmucosal element. The junc-tional epithelium is therefore routinely locatedon the implant, and the implant/abutment joinis located coronal to this level. It has been pos-tulated that the join within the submerged(two stage) system may influence the level ofsoft tissue attachment and biological width.This may be caused by micromovementbetween the two components or by allowingmicrobial penetration of the microgap between

Fig. 5 A histological section ofthe soft tissue cuff excisedfrom around an implant. Anon-keratinised sulcular andjunctional epithelium ispresent and is very similar tothat which exists aroundteeth. The collagen fibrebundles are not so wellorganised as there is noattachment to theabutment/implant surface

Fig. 4. A histological section ofjunctional epithelium at anatural tooth. It terminates atthe cement enamel junctionand was attached to theenamel by hemidesmosomesand a basal lamina-likestructure. Collagen fibres areinserted into the cementumand radiate into the gingivalconnective tissue

Figure 3a. Two hexagonalabutments used to supportsingle implant crownsemerging through a cuff ofgingiva. The space aroundthem has been created by alarger healing abutmentwhich has been replaced bythe hexagonal abutment. Thegingival tissue between theabutments has a form whichresembles a normal papillabut is flatter and is notsupported by a normalgingival fibre arrangement

Fig. 3b The porcelain fused tometal crowns have beencemented onto the abutments.The emergence of the crownsfrom the soft tissue produces anatural looking appearance



implant and abutment. At present the theoreti-cal differences between the two types do notreveal any major differences at the histologicallevel or in their clinical performance.

Probing depth examinationPeriodontal probing of natural teeth is animportant part of any dental examination. It iswell established that the probe penetrates thejunctional epithelium to some degree in health,and that this penetration increases in the pres-ence of inflammation. Under these latter cir-cumstances the probe is stopped by the mostcoronal intact gingival connective tissue fibres,about 2 mm from the bone. The situationaround the dental implant is different and thesulcus depth is very much dependent upon thethickness of the soft tissue cuff. Probing depthsaround implants are generally deeper thanaround teeth, but penetration of the soft tissueat the base of the sulcus occurs to a similardegree with the probe tip finishing short of thebone margin by about 2 mm. The informationgained from probing around implants is ofquestionable value and many clinicians do not

recommend probing, preferring to rely on radi-ographic assessment of bone levels. In addition,digital pressure on the external surface of theperiimplant soft tissue may elicit signs ofinflammation such as bleeding or suppuration.

Periodontal ligament versusosseointegration

Periodontal ligamentThe periodontal ligament is a complex struc-ture, about 0.1 to 0.2 mm in width, providingsupport to the teeth in a viscoelastic manner(fig. 7). The ligament comprises collagen fibreswhich are embedded as Sharpey’s fibres in theroot cementum and the alveolar bone, togetherwith the blood supply and connective tissueground substance which provide the other keyelements to the supporting mechanism. Theperiodontal ligament has a sensitive proprio-ceptive mechanism which can detect minutechanges in forces applied to the teeth. Forcesapplied to the teeth are dissipated throughcompression and redistribution of the fluid ele-ments as well as through the fibre system.Forces transmitted through the periodontal lig-ament can result in remodelling and toothmovement as seen in orthodontics or in thewidening of the ligament and an increase intooth mobility in response to excessive forces(eg occlusal trauma). The periodontal liga-ment is therefore capable of detecting andresponding to a wide range of forces.

OsseointegrationThe precise nature of osseointegration at a mole-cular level is not fully understood. At the lightmicroscopical level there is a very close adapta-tion of the bone to the implant surface (fig. 8). Atthe higher magnifications possible with electronmicroscopy, there is a gap ( about 100 NM inwidth) between the implant surface and bone.This is occupied by an intervening collagen richzone adjacent to the bone and a more amor-

Fig. 6 a,b,c The biological width of the dentogingival junction in (a) teeth and (b) around implants typical of the Branemark system,and (c) the non-submerged ITI implant system. S= sulcus which is approximately 0.5 to 1 mm deep; JE = junctional epithelium whichis about 1.5 to 2 mm in apicocoronal width; CT = Connective tissue zone (1 to 2 mm in width) in which the fibres are attached to rootcementum in teeth but run parallel to the implant surface; A = abutment — The abutment to implant junction is situated beneath thesoft tissue in the Branemark system; C = smooth transmucosal collar of the IT system

(a) (b) (c)

Fig. 7 A histologicalsection of a tooth root,periodontal ligament andalveolar bone. Theperiodontal ligament isinserted into thecementum and the laminadura as Sharpey’s fibres.The viscoelastic propertiesof the ligament give thetooth a degree of mobilityand the ligament is ableto respond to increasedforces by remodellingprocesses

Periodontal ligamentversus osseointegration• Periodontal ligament• Osseointegration

SJE

CTApical extent of JEBone margin



phous zone adjacent to the implant surface.Bone proteoglycans may be important in the ini-tial attachment of the tissues to the implant sur-face, which in the case of titanium implantsconsists of a titanium oxide layer, which has theproperties of a ceramic. Osseointegration is notan absolute phenomenon and can be measuredas the proportion of the total implant surfacethat is in contact with bone. Greater levels ofbone contact occur in cortical bone than in can-cellous bone, where marrow spaces are oftenadjacent to the implant surface. The degree ofbone contact may increase with time and func-tion. When an implant is first placed in the bonethere should be a close fit to ensure stability. Thespace between implant and bone is initially filledwith blood clot and serum/bone proteins.Although great care is taken to avoid damagingthe bone, the initial response to the surgicaltrauma is resorption, which is then followed bybone deposition. There is a critical period in thehealing process at around 2 weeks post implantinsertion when bone resorption will result in alower degree of implant stability than thatachieved initially. Subsequent bone formationwill result in an increase in the level of bone con-tact and stability. This has been demonstrated inunloaded implants in the early healing periodand over longer time periods following loadingof the implant. Thus osseointegration should beviewed as a dynamic process in which boneturnover occurs, but not as the same adaptiveprocess that occurs within the ligament of nat-ural teeth. Osseointegration is more akin to anankylosis, where the absence of mobility and no intervening fibrous tissue capsule is the signof successful integration. Under these circum-stances there is no viscoelastic damping systemalthough proprioceptive mechanisms mayoperate within bone and associated oral struc-tures. Forces are distributed to the bone andmay be concentrated in certain areas, particu-larly around the neck of the implant. Somedesigns, particularly those with threads, may dis-sipate the forces more effectively. Excessive forcesapplied to the implant may result in remodellingof the marginal bone ie apical movement of thebone margin with loss of osseointegration. Theexact mechanism of how this occurs is notentirely clear but it has been suggested thatmicrofractures may propogate within the adja-cent bone. This type of bone loss caused byexcessive loading may be slowly progressive to apoint where there is catastrophic failure of theremaining osseointegration or fracture of theimplant. Fortunately, either eventuality is rare.Excessive forces are usually detected prior to thisstage through radiographic marginal bone lossor mechanical failure of the superstructureand/or abutments (See Part 10).

It has been shown however, that well con-trolled forces result in an increase in the degreeof bone to implant contact and remodelling of

adjacent trabecular structures to dissipate theforces. Adaptation is therefore possible, thoughosseointegration does not permit movement ofthe implant in the way that a tooth may beorthodontically repositioned. Therefore theosseointegrated implant has proved itself to bea very effective anchorage system for difficultorthodontic cases, and may be used as an alter-native anchorage system to head gear. The factthat the implant behaves as an ankylosed unitalso restricts its use to individuals who havecompleted their jaw growth (fig. 9). Placementof an osseointegrated implant in a child willresult in relative submergence with growth ofthe surrounding alveolar process during nor-mal development. It is therefore advisable todelay implant placement until after growth iscomplete.

Periodontitis and peri-implantitisIt is quite possible that bacteria which areimplicated in periodontitis, such as Porphry-romonas gingivalis, are also the majorpathogens in destructive inflammatory lesionsaround implants (peri-implantitis). There is

Fig. 9 An ankylosed toothfollowing trauma. Damageto the periodontal ligamenthas led to a boney ankylosisand resorption. The tooth hasno detectable mobility andhas not developed into anormal vertical position withthe adjacent teeth. In thisrespect it is behaving like anosseointegrated implant. Anosseointegrated implantshould not be placed in achild until growth is complete

Fig. 8a A histological section throughan osseointegrated screw shapedimplant which has been in place for6 months. Bone is in close appositionover a large proportion of thesurface

Fig. 8b A higher power view of an areaof figure 8a showing bone filling thethread profiles and contacting theimplant surface without a visible gap (at this magnification), except for asmall area of marrow space



therefore a possibility of colonisation or infec-tion of the implant surfaces from pre-existingperiodontopathic bacteria. The destruction ofthe supporting tissues of teeth and implantshave many similarities but there are importantdifferences caused by the nature of the support-ing tissues (see earlier). This is particularlynoticeable with the different patterns of tissuedestruction observed. Peri-implantitis affectsthe entire circumference of the implant result-ing in a ‘gutter’ of bone loss filled with inflam-matory tissue extending to the bone surface(fig. 10). In contrast, periodontitis-affected

teeth commonly have irregular loss of support-ing tissues, often confined to proximal surfacesand resulting in complex infrabony defects. Inaddition, for the most part the periodontal tis-sues are capable of ‘walling off ’ the inflamma-tory lesion from the alveolar bone andperiodontal ligament with a zone of fibrous tis-sue. It would seem probable that destructiveinflammatory lesions affecting both teeth andimplants have stages in which the diseaseprocess is more rapid (burst phenomenon) fol-lowed by periods of relative quiescence. Theincidence of peri-implantitis would appear tobe low, but can result in rapid destruction of themarginal bone and is difficult to differentiatefrom bone loss because of excessive forces. Thisproblem is dealt with in Part 10.

ConclusionModern osseointegrated implants are a usefulalternative to natural teeth. There are funda-mental differences between them, and anunderstanding of the attachment mechanismsof hard and soft tissues and their responses tothe harsh environment of the oral cavity isessential to the dental surgeon who is involvedin providing this form of treatment.

Fig. 10 An exposed implantfollowing destruction of themost coronal bone by aninflammatory infiltrate. Therewas a plaque inducedinflammation caused byretention of cement at thecrown abutment junctionwhich was situatedsubgingivally

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Dental implants: Teeth and implants