Case Report – Tooth-implant-supported Bridges as a Treatment Option

24 October 2014


Tooth vs implantis a common subjectfordiscussion,which appears to be justified in many indications. However, nothing is simply "black and white" and when choosingtheoptimum treatment concept, many factors need to be considered. For example, the toothimplant-supported bridge can promote long-term therapeutic success and retention of the anatomic structures. The author describes this prosthetic treatment option.

Author: Dr Bernd Siewert

Developmentsinthefieldofimplantsystems and materials have resulted in implant treatment being available to nearly all patients. Particularly in dentistry in the elderly and those withoutteeth,thereareprostheticimplanttreatmentconceptsavailablewhichareconvincingin their simplicity. However, in terms of the best possible retention of the existing anatomic structures, the strategic increase of pillars as a differential therapy to extraction needs to be considered in the treatment planning. In this respect, the tooth-implant-supported bridge is successfullyusedbymanytreatmentteams.The following article discusses advantages of this treatment type and, taking the example of a patient case, demonstrates the convincing possibilities of a relatively new frame material in prosthetics.Untilnow,metalalloysorzirconium oxide have been used predominantly for toothimplant-supported bridges. However, PEEK frames have recently started to be fabricated in CAD/CAM-assisted processes. The highly pure PEEK material Juvora Dental Disc (from Juvora Dental Ltd.) assuresthis process-optimised materialmachining and thereby the efficient fabrication of high-precision and tension-free tooth and/orimplant-supported restoration.

Arguments for the tooth-implantsupported structure

Tooth-implant-supported bridges are defined as fixed or conditionally removable denture constructionsthatcombineatleastonetoothandone implant. 10 Compared with purely implant-supported bridges, treatment with a tooth-implantsupported structure results in reduced surgical and financial expense, due to the reduced number of implantsthat need to be inserted

In addition, retention of the teeth meets the requirements of a treatment concept that is as minimally invasive as possible. Thismeans bone resorption (which occurs after extraction)can be prevented. Also with regard to wearing comfort and transfer offorce on the jawbone, a tooth-implant-supported bridge is advantageous. With a purely implant-supported structure, the periodontal receptors are lacking and therefore only fewpressurereceptorsinthebonegeneratefeedback. The tactile sensitivity is therefore low and functionalchewing exertionscan be passed on in an uncontrolled manner and cause overloading ofthe bone.5,15 The inclusionof periodontally healthy teeth in the prosthetic treatment can help retain the natural tactile sensitivity and tactile reflexes.

Adversary or ally? Healthy tooth and implant

The physiological mobility of a periodontally healthyanchortoothcan,inthecaseofaxialloading,cause anintrusionpathintothe alveoliof approximately 50 to 150µm. 8 On the other hand, an osseointegrated implant, which has fused with the bone without a periodontal fibre device, has no or minimal mobility of max. 10µm. 1,3 As a consequence of these differing characteristics of tooth and implant, mechanical and biological complications are expected, such as screw loosening, frame fractures, overloading of the bone and/or periimplant bone lesions. With regard to this, Lindh et al. for example discussed the questionofwhetherhealthyteethshouldbeextracted in order to avoid combining tooth and implant. 6 However,thiscontradictstherequirementforthe bestpossibleretentionoftheanatomicstructures andshouldonlybeconsideredafterthoroughdifferential therapeutic deliberations.

Tooth-implant-supported bridges: Construction principles

The mobility of a tooth comparedwith its physiological mobility is significantly reduced by the rigid joining with the implant as part of a toothimplant-supported bridge, 11 which makes the possible complications described above appear ad absurdum. Furthermore, the "mobility" of a natural tooth can only be established in the presence of a high and slow-acting force; however, during "normal"chewing actions, impulse-like forcesoccur. 7Theelasticdeformationsof the suprastructure should also be taken into account in the therapeutic considerations. For example, through the modulus of elasticity of the PEEK material, the construction can be given a damping effect. These shock-absorbingpropertiescancompensateforthedifferences in mobility between the natural tooth and the implant. In terms of attachment type, there are two different variantsforthe tooth-implant-supported bridge: definitively cemented and removable under certain circumstances. 2 In the case described below,the removable variantwaschosen. The natural anchor teeth were previously crowned with frame caps (similar to the double crown technology) and then the PEEK suprastructure (Juvora Dental Disc, Juvora Dental Ltd.) was temporarily cemented on the anchor teeth (semi-permanent cementation) and screwed on the implants. In addition to the repair possibilities, the removable nature of the suprastructure also offers periodontal prophylactic benefits and possibleaccesstotheimplant.Inordertoprevent secondary caries, a definitive cementation is advocated, which is why frame caps (double crowns)were used as a base.

There are two advantages of the double crown as part of the tooth-implant-supported bridge construction: first, the protection of the natural anchortooth, by avoiding decementation and/or secondary caries; and secondly, the expandability, because if an anchortooth islost,the restoration can easily be reworked.

Patient case: Initial situation

Fig.1 and 2:Initialsituation.Insufficient,metal-ceramic denture in the upperfront tooth region,which until nowserved as an anchorage for a combination prosthesis.

The 58-year-old patient consulted the practice withaninsufficientdenture intheupperjaw. The combination prosthesis was anchored on residual teeth 13, 12, 11, 21, 23 using a metal-ceramic denture (Fig. 1 and 2). The bridge frame was broken between 11 and 12 and ceramic spalling was observed. The masseter musclewasstrongly developed. This diagnosis pattern led to the conclusion that the patient was a "clencher". In our practice, we have observed a worrying increase in clenching in all age groups over 17 years. The patientscome to uswith symptoms, but the majority of them are not aware of the habit of clenching.Unconsciousclenchingcantakeplace day and night. Compared with normal chewing, clenching can exert fourto five times more force on the dental arch. 9 The long force-retaining cyclewhichoccursduring clenchingdoesnotoccur with the physiologicalchewing action. The habit ofclenching is not triggered by a dysfunction in theorganofmastication,butispurelycerebralin origin.

Clenching can be triggered by stress and other factors over the years,constantly or in episodes. Evenapreviouslyasymptomaticpatientcansuddenly manifest an episode of severe clenching. The organ of mastication suffers severely from this pathological chewing pressure. In terms of thisissue,adenturewithpropertiesthatdampen the chewing pressure offers additional safety, unlike rigid and hard dentures.

The panoramic radiograph of the initial findingsshowed root-treated anchorteethwithout periapicalshadows,sufficientosseousanchoring of the teeth and a stable periodontium. In the lower jaw, the patient had a closed row of teeth with a number of metal-ceramicbridges.Acervicalcarieswasdiagnosed on the mesial root of untreated tooth 38. Tooth 46 showed a bone defect in the root bifurcation. The toothwas extracted during the treatment and replaced by an implant with an occlusally screwed zirconium oxide crown and a glued-in titaniumbase

Fig. 3:Drilling template forthe navigated insertion of five implantsin the upperjaw/ lateral tooth region. - Fig. 4: The controlX-ray after insertion of the five implants

Planning and implant treatment

The patient wanted an aesthetic, fixed solution with a palate-free design. For the reasons mentioned, the author decided to retain the anchor teeth as an anchoring element for the new restoration. Bone was available for five implantsinthelateraltoothregion. In order to use the autochthonous boneoptimallyfortheinsertionofthe implants (reduced bone availability), a 3-D X-ray (DVT) was the method of choice for the implantplanning. The data setwasimported into the planning software and the implants werevirtuallyinsertedinanoptimumpositionin tooth positions 18, 14, 24, 25 and 28. A drilling template (Fig. 3 and 4) fixed on the bridge was used for the planning, with which the implants (BlueSky, bredent) were inserted in the jaw using the template as a guide 

The transgingival method was selected for the healing,forwhichtheimplantsintoothpositions 18 and 28 were fitted intraoperatively with a definitive multi-unit. The advantage of this is that thepostsdonotneedtoberemovedagainduring treatment and the soft tissue is not therefore exposed to additional irritation (screwing in and out). 

In the planning, a post with a 17-degree incline was deemed optimal for tooth position 18, and a 0-degree post for tooth position 28. The implantsin regions14, 24, and 25 were fitted with flathealingposts.Inordertobeabletoservethe patientsatisfactorily in the comingmonths and, at the same time, prevent loading of the implants, the present model-cast prosthesis was cut free around the area of the implant exit and lined to remain soft(Fig. 5).

Fig. 5: The situation twoweeks afterinsertion of the implants. Therewas excellentwound healing following the minimally-invasive procedure. – Fig. 6: For making an impression of the anchorteeth—following restoration—amodified combination ofcopperring/silicone and alginate impressionwas used. – Fig. 7: Zirconiumoxide capsweremade forthe anchor teeth using the CAD/CAM-assisted process.

Start of the prosthetic phase

The patient’s definitive prosthetic treatment began one year later than planned for reasons on the patient’s part. Such delays in the treatment process can bring advantages as well as disadvantages, which was the case here and whichwill be explained below. However,there was one serious negative consequence of the unusually long provisional phase. A caries and deep gingival pocket had developed on tooth 13. The tooth showed high mobility and could not be considered retainable and was therefore extracted. The residual tooth situation in the upper jaw was strengthened with madeto-measure titanium root pins and new stump reconstructions were created with a self-curing hybrid composite (paste-paste composite, MerzDental). Thenamildlypronouncedchamfer preparation of the teeth was carried out as well as an impression of the situation. For the impressionoftheanchorteeth(manufactureof zirconium oxide caps), a combination (modified by the author) ofcopper ring/silicone and alginate over-impression material was used (Fig. 6). Over the past 25 years, this procedure has proven to be reliable, simple, atraumatic and precise in subgingival impressions. Any potential sulcus bleeding with this method doesnothaveanegativeeffectontheprecision of the model. 

 Fig. 8 and 9: After a trial inmouth of the zirconiumoxide caps, the impression ismade using an open impression tray. The impression postswere set in themouth using lightcuring composite.

Applying the prosthesis 

The technical dental difficulty came from the need to manufacture a bridge that encompassed the jaw and that was also removable under certain circumstances. In the author's practice, screw connections for tooth-implant-supported bridges have proven to be an optimum concept over the past few years. For the natural anchor teeth in the fronttooth region,thin zirconiumoxide caps,cutto 1 degree,weremade using double crowntechnology (Fig.7).After a fitting inthepatient's mouth, an open impression traywas used to make an impression of both the caps and the impressionpostsappliedontheimplantsandfixatedwith plastic(Fig. 8 to 10). The dental technician created the master model from a modelling plastic(Fig.11), in orderto prevent any difference in fit occurring through gypsum expansion. After aligning the zirconium oxide caps to the model, the laboratory implantswere fittedwith the correspondingabutmentsandthesituationwasdigitalised by the laboratory scanner. The STL data set included all relevant model data (zirconium oxide caps, implant posts,soft tissue) forthe virtualconstruction of the frame

For the decision regarding the optimum frame material, the unusually longwearing time of the provisional treatment proved advantageous to both the patient and the treatment team. Whereas until recently, a casting process(metal alloy) or compression moulding process (highperformance polymer) needed to be used for tooth-implant-supported fixed dentures, nowadays a highly pure PEEK material (Juvora Dental Disc, JuvoraDental Ltd.)can be used forthe computer-assisted manufacturing of the frame. The process-optimised material machining enables the precise, tension-free and efficient manufacturing of the dental restoration. This was to be taken advantage of in thiscase aswell. 11,12 

Fig.10: Impression forcreating the master model. – Fig.11: The master model made from modelling plastic andwith gingival mask. – Fig.12: Example image (removable clip prosthesis): ThePEEKmaterial(JuvoraDentalDisc)isofferedas anindustriallymanufacturedmillingblank forCAD/CAM-assistedmachining.Forthemillingblank,themanufacturer(Juvora Dental Ltd.) usesthe pure formofPEEK-OPTIMA(Invibio),whichhas beenanestablished productinhumanmedicine for decades.–Fig.13 and 14: The virtualframe constructionwith andwithout gingivalmask

Why PEEK?

The advantage of PEEK as a frame material for tooth-implant-supportedbridgesis,amongother things, its ability to compensate forslight imprecisionsbetweenthemodelandoralsituation.Particularly in a tooth-implant-supported construction, the reliable and tension-free fit of the frame is very important. In this respect, it should be noted that aCAD/CAM-manufactured restoration does not automatically mean a good fit. Many variablescanaffecttheresult(gypsumexpansion, shrinkage ofthe impression silicone, etc.), and, in the author's opinion, imprecisionsresulting from thesecannotbefullyavoidedevenwithhigh-precision, accurate working methods. A high-performance polymer such as PEEK has the advantage ofcompensating forsmall discrepancies.

PEEK has a modulus of elasticity similar to spongybone, aswell as ahighbreaking strength. Fig.13 Fig.14 Fig.10: Impression forcreating the master model. – Fig.11: The master model made from modelling plastic andwith gingival mask. – Fig.12: Example image (removable clip prosthesis): ThePEEKmaterial(JuvoraDentalDisc)isofferedas anindustriallymanufacturedmillingblank forCAD/CAM-assistedmachining.Forthemillingblank,themanufacturer(Juvora Dental Ltd.) usesthe pure formofPEEK-OPTIMA(Invibio),whichhas beenanestablished productinhumanmedicine for decades.–Fig.13 and 14: The virtualframe constructionwith andwithout gingivalmask. Fig.10 Fig.11 Fig.12 The material used in this case is available as industrially manufactured milling blanks for CAD/CAM-assisted machining (Fig. 12). For the milling blanks, the manufacturer uses the pure form of PEEK-OPTIMA (Invibio), which has been an established material for decades in human medicine. No colour additives, additives, processing aids orsimilar are added to the material. Due to itssemi-crystalline structure, PEEK is not brittle, but rather has a bone-like elasticity. The good physical and chemical properties of PEEK are explained by the composition of the polymerfrom a chain of aromaticrings. PEEK isideal for the fabrication of conditionally removable restorations, for example fortooth-implant-supported bridges. Deformations or small imprecisionsin the fit of the frame can be compensated for, thanks to the physiological elasticity of the material. The shock-absorbing properties have an impact-sparing effect on the implants and bone around the implants. In addition, the low specific weight makesit possible to create, even with large-volume bridge frames (e.g. implantsupported dentures), a lightweight denturewith a totalweight of15 grams.

Construction and manufacture of the frame

After importing the STL data into the modelling software, the dental technician constructed the suprastructure (Fig.13),whichwasto be screwed on the implants in tooth positions 18, 28, 14, 24 and 25, and semi-permanently cemented on the zirconium oxide caps of the anchorteeth. During the construction, it is important to observe the minimum thicknesses specified by the manufacturer and to create rounded geometries (or smooth transitions). Naturally, this must also be observed during the planning. Grooves must be avoided, as these are potential breaking points. In orderto guarantee appropriate stability of the frame and, at the same time, use the very good properties (biocompatibility, tissue-compatibility, low plaque formation) of the PEEK material, the basal and palatal parts of the structure were designed fully anatomically in this case (Fig. 13 and 14).

It proved a good idea to machine out the screw channels up to the occlusalsurface in PEEK and apply the veneers around these. As a result, the channel was adapted exactly to the screw head and cannot be contaminated by the residues of the veneer plastic. This also guarantees a tightening of the screw which can be palpably checked. The beige colour of the PEEK material prevents any potentially compromising effect of the screw channel. The concern that PEEK mightnotwithstandthescrewingpressureisnot shared by the author. As a high-performance polymer, PEEK meets the requirements of a screw connection in every respect; the initial torque does not need to be reduced. The author has used PEEK in this design, and in individual crowns, in clinical applicationsfor more than a year and to date he has not observed any screw loosenings.

Fig.15and16:Theframe fitsonthemodelfully tensionfreewithoutany subsequentwork andispreparedfortheveneers. Theocclusalcontactsinthefront toothregionaremadefromPEEK (fullyanatomic),whichprevents fractureorspallingofthe veneers.

The construction data were loaded onto the milling machine software and the frame wascut fromthePEEKblank (JuvoraDentalDisc).Subsequent work was limited to detaching the construction from the blank and smoothing the edges. The frame fitted on the working model withoutsubsequentwork and the veneerscould be added; teeth 17 and 27 were implemented as full crowns (Fig. 15 and 16). For this, industrially prefabricatedPMMAshellswereused(visio.lign, bredent). The shells, which are stable in shape andcolour,havebeenusedbytheauthorformore than six years and he has not yet observed any discolouration.ThePEEKframeandveneerplasticwere joined using chemicalconditioning.

Integration

Fig.17 and18: Finished tooth-implant-supported bridge onaPEEKframe. The basal and palatal parts aswell asteeth17 and 27were designed fully anatomically. The optimumposition of the implantsin relation to the bridge frame and thusthe ideal occlusal exit points of the screwchannelswasthanksto the three-dimensional planning. – Fig.19: Front viewof the restoration veneeredwith industrially prefabricated PMMA shells.

In addition to the aesthetically clean result, the restoration produced impressed due to its absence of any metalstructures and itslight weight, and it was accepted immediately by the patient(Fig.17to 19). Aftercementing the zirconium oxide caps(Fig. 20and21),thestructurewastestedinthemouthand then fixed in place after checking all relevant parameters (aesthetics, function, phonetics). The bridgewasscrewedontotheimplantsandthescrew channelswereclosedwithalight-curingcomposite. The double crowns in the front tooth region were sealedwithatemporary cement(Fig. 21to23).

Fig. 20 and21: The situationatthe timeofthedefinitive integrationofthe implant-toothsupportedbridge. The anchorteethwere crownedwithzirconiumoxide andthe tooth-implantsupported bridgewassemi-permanently cemented in place. The bridgewasscrewed onto the implants.

Conclusion

The patient was treated with a fixed and palate-free denturewith the best possible retention of structures. The teeth present were included in the treatmentconcept to counteract resorption of the jaw bone. A fixed structure is preferable in a tooth-implant supported bridge,whereas a removable structure undercertain circumstances allowsfor extraoral repair and expansion, as well as regular professional cleaning. In order to avoid the risk of secondary caries, the toothsupported part of the tooth-implant-supported bridge should be cemented. 1 Nevertheless, in order to fabricate a structure that could be removed under certain circumstances, the natural anchor teeth were fitted with crown caps in the case described here, and the bridge was semi-permanently cemented onto these. Zirconiumoxidewas used for the caps, as this has excellent biological properties. The suprastructure was made from PEEK material using CAD/CAM technology and veneered with a composite. The advantages of the chosen framework material include, among other things, the bone-like elasticity, as a result of which small imprecisionsin fitcan be compensated for, the high tissue compatibility and corrosion-free characteristics. The construction concept described (tooth-implantsupported bridge) has been proven as a prosthetic option and it gains additional safety and relevance from the possibility of being able to make the frame from PEEK material using CAD/CAM technology. 13,14

Fig. 22: The definitively integrated restoration afterfourmonths of beingworn (without prophylaxis). - Fig. 23: The finalX-ray

contact

Dr. Bernd Siewert
Clinica Somosaguas
Calle Aquilon, 2 Local 7/8
28223 Madrid, Spain
Tel.: +34 91 5188101
Siewert@dental-med.com