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I case study Fig. 20a–c_ Cone-beam effect. Fig. 20d_Cone-beam effect interpolation. _Reliability of STL surgical guides The study by Stumpel15 provides important in- formation on the accuracy of STL surgical guides. Theirreliabilityisascertainedviaateeth-bornesur- gical guide. After a stone model has been scanned and matched to the planning, the surgical guide is used like a jig and the correspondence between the STL model and the mouth is measured. An HU threshold appropriate for the bone algo- rithmisnecessaryinordertoavoidproducinganSTL model of inadequate size. The merging of planning andstonemodelscanningcanfurtherhelpimprove its accuracy. The dimensional tolerance of an STL modelisabout0.3%whenSLSorLSandstereolith- ography (either SL or SLA) are applied. These tech- niquesyieldtolerancesof+/-0.3%andaminimum of +/- 0.005. Since less resolution is needed to produce a sur- gical guide than to ascertain implant position, the softwarecanonlybeusedforplanningandSTLsur- gical guide production. It cannot, however, be used forverifyingtheimplantposition.Inordertoembed either smooth or thread-timed sleeves that can guide drills and implants while respecting the pt. anatomy, 0.1 mm is sufficient. _Moving on Superimposition cannot differentiate between inaccuratesleeveplacementandinaccuraciesofthe sleeve position and axis of the surgical guide or in- accuracy resulting from using a smooth sleeve. In- stead, these are confused, which leads to the con- clusionthatacomparisonofplanningandpost-op- erativescanswillnotleadtoanyconvincingresults, even if the superimposition was perfectly executed anddifferentkindsofsoftwarewereusedinunique clinical situations. At worst, the ALARA principle cannotbefollowedandpatientsaresubjectedtoan inordinate amount of radiation. Once we accept that errors are likely when su- perimposition is done, we can consider other tech- niques. These techniques should be designed to avoiderrorsderivedfromusingasmoothsleeve.An ideal system, for example, would allow for a pros- thesis, and the surgical guide would allow for iden- tical implant and analogue positions both in the model and in the mouth. Thus,fromnowon,wecanbeextremelyaccurate whenworkingwithathread-timeddeviceintheim- plantphase.Afterthesurgicalguidehasbeenmade, we must demonstrate the accuracy of the implant placement.Thesurgicalguidewithitsrepeatablere- sults allows us to work on an infinite number of master casts. Our nth master cast is the mouth, and its correctness can be evaluated by means of a jig. In 2007, Nobel engineered a threaded device for zygomatic implants, which was considered for use in other Nobel implants (patent number: WO 2007/129955 A1). Their threaded guiding sleeve functions with a threaded implant mounter. They claim that these devices lack any vertical fastening features and do not use any notches to index the hex. Consequently, they warn that there may be no hex correspondence. Therefore, additional rotation may be needed. Additional rotation amounts to missingdepth(itismathematics:ifyougoonscrew- ing, you deepen the screw itself); therefore, with a threaded sleeve, missing the depth because a sys- tem has not been adequately fastened means miss- ing the hex as well. Additional rotation is only ap- proximately adjusting a device that has lost the phaseandthesetwoparameters.Thesetwoparam- eterswillbemissed.Inordertoobtainthecorrectfi- nal hex position (and consequently also the depth), I invented a helical gear. _Conclusion Accuracy in implant placement appears to de- pend on the context of the respective case; for ex- ample, it appears less relevant when immediate loading is not the preferred option or if an impres- sion can be taken immediately after implant place- ment. However, accuracy in implant placement can 26 I implants1_2012 Fig. 20bFig. 20a Fig. 20dFig. 20c