implants - international magazine of oral implantology

industry report _ TRIPOD procedure I thecaseofNobelGuide,accuratepositioningdepends on the patient’s ability to bite reproducibly and pre- cisely, with even gingival thickness and consistency, andassumesthatboneshowsasimilardegreeofhard- nessatdifferentscrew-retentionsites.Unfortunately, as recently reviewed by Schneider etal.6 and detailed by Valente et al.7 , the deviation between entry point and orientation consistently differs between the plannedandactualpositionoftheimplants.Thisgen- erallyaccountsfortheresultsobtainedbyguidesused inflaplesssurgery.Otherfailurefactorsmayberelated to poor cooling ability during the drilling procedure.8 Ascitedabove,inaccuraciesmayarisefromthepo- sitioningoftheguideorofthepatient,orberelatedto theradiologicaltechniqueitself.Inthecaseofflapless surgery,thepositionoftheguideisconditionedbythe thickness and consistency of the underlying soft tis- sue,aswellasthepatient’sabilitytobitepreciselyina replicable manner. In addition, there is always some degreeofpatientmovementduringtheCTscan,which canhardlybecontrolled,aninaccuracytermeda‘me- chanical artefact’. Of course, any study performed on cadavers or models cannot reproduce this particular radiological aspect.9, 10 Other inaccuracies are related to the radiological equipment itself and include geo- metric, hardening and threshold artefacts. Geometric artefacts are related to the ability of software to re- construct a 3-D space based on the serial addition of 2-Dimagesthatarefilteredbythesoftware.11,12 Hard- eningartefactsareduetothedifferentdensitiesofad- jacentobjects.AnX-raybeamiscomposedofindivid- ual photons with a range of energies. As the beam passes through an object, it becomes ‘harder’, that is, its mean energy increases because the lower-energy photons are absorbed more rapidly than higher-en- ergy photons.13 The last significant artefact, the digi- talartefact,isduetothesegmentationmasksthatare used to obtain volumes. In order to obtain a mask, an interval of radiodensity is defined by choosing the Hounsfield values at both ends of the tissue(s) under interest. By using this method, an area of lower or greater density can be discarded and missed in the fi- nal volume. This may be particularly true when digi- tally producing a surgical template based on hard or softtissue.Finally,imagesproducedbyavailabletech- niques are too unreliable to be used directly for this type of treatment. We propose a new protocol in this article with the aim of reducing inaccuracies in terms of reliability, aesthetics and function. TRIPOD:Descriptionofanewclinicaltechnique Initially, a treatment plan is performed to ade- quately evaluate a case, propose alternate solutions anddecidewhetherthepatientisasuitablecandidate forafullyimplant-supportedmaxillarybridge.Thisre- quiresafirstassessmentthatincludesapossiblewax- up and a radiographic stent for visualising the crown position on the CT scan, as well as an evaluation of a potential need for bone- and soft-tissue augmenta- tionprocedures.Patientsoftenpresentwiththeirown cement-retainedbridgeworkonnaturalteethinplace that,whenadequate,maybeusedasareferenceguide for implant placement. It is essential to evaluate the implantsitewithinthemaxillaryboneprecisely.Inor- dertoperformthesemeasurements,aPositioningTRI- PODandaComputingTRIPODneedtobedetermined. Theterm‘PositioningTRIPOD’isusedtodenotethe selected pre-existing three fixed points (Fig. 2) in the mandible or maxilla, which can be based on: _teeth that are stable enough to support the surgical guide during surgery; _implants placed in posterior areas; _temporarymini-implantsthatwillberemovedatthe end of surgery. ThechoiceofappropriatebasesforthePositioning TRIPOD is critical for its accuracy. Owing to its com- pressibility, soft gingival tissue has to be avoided. Problems with remaining teeth may arise due to ad- vanced periodontal disease causing excessive mobil- ity. In some cases, temporary mini-implants are used, but often the amount of maxillary residual bone is so reducedthattheseimplantsonlyinterferewithdefin- itive implant placement. Nevertheless, they may be useful when no other alternative is available. Anec- dotal cases in which there is sufficient bone for tem- porary and definitive implants at the same time have been reported, but are rare. The best choice is to use posteriorly placed implants before inserting anterior implants.Inthiscase,anextremelyprecisepositioning Fig. 2_The Positioning TRIPOD is based on a temporary implant (a) and two residual teeth (b & c). Fig. 3a_Radiographic template fixed on the Positioning TRIPOD (a, b & c) with standardised X-ray opaque resin pins (d, e & f). I 31implants3_2011 Fig. 2 Fig. 3a b b c c a a d e f