Dental Tribune United Kingdom Edition

planned and actual position of the implants. This generally accounts for the results obtained by guides used in flapless surgery. Other failure factors may be related to poor cooling ability during the drilling procedure.8 As cited above, inaccuracies may arise from the positioning of the guide or of the patient, or be related to the radiological tech- nique itself. In the case of flapless 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 repli- cable manner. In addition, there is always some degree of patient movement during the CT scan, which can hardly be controlled, an inaccuracy termed a ‘mechan- ical artefact’. Of course, any study performed on cadavers or models cannot reproduce this particu- lar radiological aspect.9, 10 Other inaccuracies are related to the radiological equipment itself and include geometric, hardening and threshold artefacts. Geometric ar- tefacts are related to the ability of software to reconstruct a 3-D space based on the serial addition of 2-D images that are filtered by the software.11, 12 Hardening arte- facts are due to the different den- sities of adjacent objects. An X-ray beam is composed of individual photons with a range of energies. As the beam passes through an object, it becomes ‘harder’, that is, its mean energy increases be- cause the lower-energy photons are absorbed more rapidly than higher-energy photons.13 The last significant artefact, the digital artefact, is due to the segmenta- tion masks that are used to ob- tain volumes. In order to obtain a mask, an interval of radiodensity is defined by choosing the Houns- field 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 final volume. This may be particularly true when digitally producing a surgi- cal template based on hard or soft tissue. Finally, images produced by available techniques 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: Description of a new clinical technique Initially, a treatment plan is per- formed to adequately evaluate a case, propose alternate solutions and decide whether the patient is a suitable candidate for a fully im- plant-supported maxillary bridge. This requires a first assessment that includes a possible wax-up and a radiographic stent for visu- alising the crown position on the CT scan, as well as an evaluation of a potential need for bone- and soft-tissue augmentation pro- cedures. 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 within the maxillary bone precisely. In order to perform these measure- ments, a Positioning TRIPOD and a Computing TRIPOD need to be determined. The term ‘Positioning TRI- POD’ is used to denote the select- ed 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 • Temporary mini-implants that will be removed at the end of surgery The choice of appropriate bases for the Positioning TRIPOD is critical for its accuracy. Owing to its compressibility, soft gingival tissue has to be avoided. Prob- lems with remaining teeth may arise due to advanced periodontal disease causing excessive mo- bility. In some cases, temporary mini-implants are used, but often the amount of maxillary residual bone is so reduced that these im- plants only interfere with defini- tive implant placement. Never- theless, they may be useful when no other alternative is available. Anecdotal cases in which there is sufficient bone for temporary and definitive implants at the same time have been reported, but are rare. The best choice is to use posterior-placed implants before inserting anterior implants. In this case, an extremely precise positioning is not required since the large volume of the corre- sponding teeth provides some de- gree of freedom to the laboratory technician designing the prosthe- ses. These posterior areas often require some bone reconstruc- tion (such as sinus lift or onlay bone grafts), thereby prolonging time to loading. The correspond- ing implants will then ensure not only the most precise positioning for radiographic templates and surgical guides, but also for the occlusal guide and impression tray, since all these parts will be screw-connected to these previ- ously placed and osseointegrated implants. In order to transfer the planned implant position from the planning software to the surgical guide, a Computing TRIPOD is necessary. This Computing TRI- POD is made with three SKYplanX reference pins (Bredent) placed on the radiographic template with the reference plate (Fig 3a). The patient is scanned with the ra- diographic template fixed on the Positioning TRIPOD. The position of the standardised X-ray opaque reference pins is detected by the software, building the Comput- ing TRIPOD (Fig 3b), and used to calculate the implant coordinates (Fig 4). This data is then set in the transfer table (Fig 5a) to place the drill sleeves accordingly and transfer the radiographic tem- plate into a surgical guide (Fig 5b). Some days prior to the full- arch surgery, once an adequate TRIPOD has already been planned and initial implants placed, an initial impression (Fig 6) will be taken for the model to prepare the impression tray, oc- clusal guide, surgical guide from the radiographic template, as well as the provisional prostheses. The surgical guides are produced in sterilisable resin with radiopaque sleeves (DéPlaque). Special at- tention is given to the impression tray that will extend to all maxil- lary surfaces, but room for the impression material is exclusively limited to the planned implant sites. They must be ready at the time of surgery. On the day of the surgery, the practitioner begins by reducing all remaining crowns that would interfere with the surgical guide, which is then placed on teeth or preferably screwed onto previ- ously placed implants forming the Positioning TRIPOD (Fig 7). A CT is performed to verify all drilling page 19DTß Figs. 1a–c Precision positioning of dental implants is mandatory for adequate abutment and screw placement 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 stand- ardised X-ray opaque resin pins (d, e & f) Fig. 3b The Computing TRIPOD Fig. 4 The position of standardised X-ray opaque resin pins allows the calculation of implant coordinates Fig. 5a The drill sleeves being placed in the radiographic template with the transfer table Fig. 5b The implant coordinates for the transfer table Fig 1a Fig 1b Fig 1c Fig 2 Fig 3a Fig 3b Fig 4 Fig 5a Fig 5b November 19-25, 201220 Implant Tribune United Kingdom Edition