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20 I I clinical technique _ TRIPOD The maxillary plaster model is trimmed to leave spaceforabutmentanaloguesandplasterispouredto fill this open space after the impression tray has been secured to the trimmed model (Fig. 11). The modified model simultaneously shows two parts: the first part corresponding to the initial impression and the other corresponding to the second impression (Fig. 12). The provisionalprosthesesarefittedtothemodelandoc- clusionisvalidated.Whenthislaboratoryphaseisover, the protective caps are removed, and the prostheses are screwed into position (Figs. 13a & b). If done well, occlusaladjustmentsshouldbeminimal,evenperhaps none being required. Temporary caps on VARIOmulti arefilledwithtemporarylight-curedmaterialtoclose the screw channel and the patient is advised to treat thetemporarybridgeworkscarefully. Suturesareremovedaftertendays.Aestheticsisre- evaluatedthreemonthsaftersurgery,beforeinitiating the final prostheses, owing to subsequent loss of tis- sue volume. Additional temporary bridgework is often required to ensure that the final aesthetic will be ade- quatebeforeproceedingwiththedefinitiveprostheses. _Discussion TherearemultipletechnicalbenefitsoftheTRIPOD procedure.Precisionimplantplacementisachievedby removingpositionalandmechanicalartefacts,partic- ularly when the actual surgical guide is screwed onto stableimplants.Inotherwords,thereisnomovement evoked by a bite variation or tissue differences, and if thepatientmovesduringtheCTscan,theguidemoves with the anatomical structures. However, there is no way to conquer geometric, hardening or digital arte- facts. There is still room for a small degree (<1 mm) of freedom in implant placement and, if necessary, final correctioncanbedoneaftertheinitialdrillingwiththe 2.8mmdrill.Thisresultsinamaximumfreedomofap- proximately0.7mmindiameterforafinalimplantsite withadiameterof3.5mm.However,consideringthat the last drill at the centre is just half of this value, this freedom corresponds radially to 0.35 mm, providing an opportunity to adapt the implant site preparation to anatomical conditions slightly. This distance of 0.35 mm is sufficiently important to become particu- larly significant for leaving some buccal bone, but it is still sufficiently small to be handled by the dental technicianforidealprostheticscrewplacement.Nev- ertheless,theinitialimplantplacementcannotexceed thislimit,whichrequiresverypreciseinitialdrillingand an additional step to verify that the surgical guide is actuallysuitableforuse. Comparedwithflaplesstechniques,openflapsur- gery not only allows the visual capacity opportunity for controlling bone site preparation, but also retains precious keratinised tissue that is important for both marginal tissue stability and volume. The patient’s re- action to this procedure, with its associated pain and discomfort,stillhastobeexaminedinfuturestudies. Another benefit of this procedure is that sterility is maintained throughout the surgery, since all ma- terials used can be sterilised, which is not the case with common guides such as NobelGuide or the SAFE SurgiGuide,whicharebothmadeofastereolithicresin and are currently not capable of undergoing sterilisa- tion.Inaddition,theprecisionoftheprocedureallows the impression tray to remain unmodified—and thus sterile—throughoutthesurgery. YongandMoy8 statethatimplantlossintheirstudy wasprobablyprimarilyrelatedtotheabsenceofproper cooling ability with NobelGuide use, since most of the late implant failures involved long implants in cases in which the guide was used directly at the gingival con- tact.Indeed,onlytherearpartofthedrill(thusfarfrom the tip) can be cooled efficiently, and this probably makes the cooling procedure ineffective. In contrast, during the TRIPOD procedure described, the guide is placedonthegingivaatthetimeoffabrication,leaving an open space for cooling at the time of the open flap surgery.Inaddition,thebonebecomesvisible,whichal- lowsthepractitionertovisualisethedepthmarksofthe drillrightatthecrestalridge,makingtheinstrumenta- tion less expensive and easier, as no special drill with mechanical depth limitation is required. Site prepara- tion may be modified through piezoelectric bone sur- gery, since this device can grind bone on a particular wall from the previous drilling, in contrast to conven- tional drilling, which grinds all walls from the previous drilling, with a preference for softer tissue, which re- sultsinfacialboneperforation.Insomesituations,one Fig. 11_Second impression secured to reduced initial model. Fig. 12_Modified model: the yellow part corresponds to the initial impression; the pink part was poured at the time of surgery. CAD/CAM 3_2011 Fig. 11 Fig. 12