CAD/CAM - international magazine of digital dentistry

Fig. 2_The Positioning TRIPOD is based on a temporary implant (a) and two residual teeth (b & c). Fig. 3a_The radiographic template fixed on the Positioning TRIPOD with standardised X-ray opaque resin pins. Fig. 3b_The Computing TRIPOD. Fig. 4_The position of standardised X-ray opaque resin pins allows the calculation of implant coordinates. I 17 clinical technique _ TRIPOD I cad/cam 3_2011 less surgery, the position of the guide is conditioned by the thickness and consistency of the underlying soft tissue, as well as the patient’s ability to bite precisely in a replicable manner. In addition, there is always some degree of patient movement during the CT scan, which can hardly be controlled, an in- accuracy termed a “mechanical 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 geometric, hardening and threshold artefacts. Geometric artefacts are re- lated to the ability of software to reconstruct a 3-D spacebasedontheserialadditionof2-Dimagesthat are filtered by the software.11,12 Hardening artefacts areduetothedifferentdensitiesofadjacentobjects. An X-ray beam is composed of individual photons witharangeofenergies.Asthebeampassesthrough anobject,itbecomesstronger,thatis,itsmeanenergy increases because the lower-energy photons are ab- sorbed more rapidly than higher-energy photons.13 Thelastsignificantartefact,thedigitalartefact,isdue to the segmentation masks that are used to obtain volumes. In order to obtain a mask, an interval of ra- diodensity is defined by using the Hounsfield values at both ends of the tissue(s) under interest. By using this method, an area of lower or greater density can bediscardedandmissedinthefinalvolume.Thismay be particularly true when digitally producing a sur- gical template based on hard or soft tissue. Finally, imagesproducedbyavailabletechniquesaretooun- reliable 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: Description of a new clinical technique Initially,atreatmentplanisperformedtoevaluate a case adequately, propose alternate solutions and decide whether the patient is a suitable candidate for a fully implant-supported maxillary bridge. This requires a first assessment that includes a possible wax-up and a radiographic stent for visualising the crown position on the CT scan, as well as an evalua- tion of a potential need for bone- and soft-tissue augmentation procedures. Patients often present with their own cement-retained bridgework on natural teeth in place that, when adequate, may be used as a reference guide for implant placement. It is essential to evaluate the implant site in the maxil- lary bone precisely. In order to perform these meas- urements, a Positioning TRIPOD and a Computing TRIPOD need to be determined. The term “Positioning TRIPOD” is used to denote theselectedpre-existingthreefixedpoints(Figs.2a–c) inthemandibleormaxilla,whichcanbebasedon: _teeth that are sufficiently stable to support the surgicalguideduringsurgery; _implantsplacedinposteriorareas; _temporary mini-implants that will be removed at theendofsurgery. Fig. 2 Fig. 3a b b c c a a d e f Fig. 4Fig. 3b