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CAD/CAM - international magazine of digital dentistry

special _ guided implantology I demonstrated. This is because, thus far, interpola- tion has only shown a reorientation of the optical limits for both cone beam and MSCT.8, 9 _Errors in sleeve placement CT is also responsible for errors in sleeve place- ment inside the surgical guide. These errors are caused by an inescapable approximation in the CT resolutionlimits.CTcannotexceeditsMTFlimit,and this should be considered during planning and data transfer. There can be repercussions on the sleeve place- ment inside the surgical guide, both for smooth or threaded sleeves. Sleeve position and axis are pa- rameters associated with this procedure, and the distance to the ridge and adjacent teeth, as well as thesleeveaxis,shouldbeconsidered.However,from a practical perspective, they have no relevant influ- enceonthisprocedure,butthelimitsgivenbythese parameters are sufficient for the production of a surgicalguide.Furthermore,theyrespectthestruc- turesadjacenttotheimplantsite,forexampleplates and vascular adjacent structures, IANs, sinuses, nasal cavities, pterygopalatine fossae, mental foramina and adjacent roots. Owing to the technical production limits of CT, the sleeve position in the surgical guide tends to be inaccurate, regardless of the technique applied (STL or stone surgery). _Evaluationofdata-transfertechniques As for data transfer in the course of producing a surgical guide, the chosen technique should result inthesleevebeingplacedinthecentreofthepalate bone. In order to decide between CAD/CAM and stone surgery for this process, a cadaver study may helpincomparingandevaluatingthevarioustech- niques on the market. Inordertoproverepeatability,eachcadavermust bescannedseveraltimes.Eachscanshouldconsider the protocol of a different company or manufac- turer. The corresponding surgical guides should be tested on the same cadaver in order to evaluate the precisionofeachtechniqueinplacingthesleevesin the centre of the bone, according to position and axis. Surgical kits should fit into the mouth and I assume that the axis should respect the palate’s anatomy. Furthermore, drilling and implant place- ment should be avoided in order to prevent inac- curacy errors other than those derived from using smooth sleeves. Likewise, a repeated scan for super- impositionisnotofanyuse. Mathematicallyspeak- ing, a system can be considered reliable if its re- peatability can be confirmed. In the cadaver study, the cadaver should therefore be tested to fit several repeated surgical guides. A similar technique pro- posed by Al-Harbi, in which the accuracy of the sleeveaxisisassessedviaCMM(coordinatemeasur- ing machine) and laser techniques, also appears promising.10 ThestudybyBouSerhaletal.isbasedonacadaver study, but once again, the cadaver was scanned ac- cordingtoasuperimpositionprotocol.11 Butwhyex- pect to obtain more information from a second CT scan if we know that CT can be imprecise? There are many articles on the reliability of CT and its corre- spondencetotheanatomicaltruth,suchasthestud- ies by Lou et al.,12 Brown et al.13 and Damstra et al.14 However, these publications appear to restrict theirinteresttothescannedfiduciallandmarkmeas- urements and record an error between 0.1 and 0.5 mm for 2-D CT. It is therefore my opinion that thesestudiesfailtodistinguishsourcesoferrorsuch as the MTF limit and smooth sleeves by concen- trating on the superimposition of two low-quality 3-D images. I 09CAD/CAM 2_2012 Fig. 19Fig. 18e Fig. 19_Radial and azimuthal resolution.