CAD/CAM - international magazine of digital dentistry

I special _ guided implantology Fig. 20a–c_ Cone-beam effect. _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 software can only be used for planning and STL surgical guide production. It cannot, however, be used for verifying the implant position. In order to embed 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 inaccurate sleeve placement and inaccuracies of the sleeve position and axis of the surgical guide or inaccuracy resulting from using a smooth sleeve. Instead,theseareconfused,whichleadstothecon- clusionthatacomparisonofplanningandpost-op- erativescanswillnotleadtoanyconvincingresults, even if the superimposition was perfectly executed anddifferentkindsofsoftwarewereusedinunique clinical situations. At worst, the ALARA principle cannot be followed and patients are subjected to an 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 avoid errors derived from using a smooth sleeve. Anidealsystem,forexample,wouldallowforapros- 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. The surgical guide with its repeatable results 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. Con- sequently, they warn that there may be no hex correspondence. Therefore, additional rotation may be needed. Additional rotation amounts to missing depth (it is mathematics: if you go on screwing, you deepen the screw itself); therefore, with a threaded sleeve, missing the depth because a system has not been adequately fastened means missing the hex aswell.Additionalrotationisonlyapproximatelyad- justingadevicethathaslostthephaseandthesetwo parameters. These two parameters will be missed. In order to obtain the correct final hex position (and consequentlyalsothedepth),Iinventedahelicalgear. _Conclusion Accuracy in implant placement appears to de- pend on the context of the respective case; for example, 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 help prevent cortical vascular perfusion disorders (cortical plate perfusions) or arterial vessel damage. This appears to be especially important in areas in which hard- and soft-tissue stability is required for long-term results, for example for biomechanical concepts that require submillimetric precision. Fur- thermore, tissue stability should be considered in all areasofthemouthforaestheticandtrophicreasons. On the one hand, CT scans to date offer low-res- olution 3-D images of the bone. The software avail- able,ontheotherhand,deliversbothgoodplanning and safe sleeve positions and axes independently of the technique used to obtain a surgical stent. 10 I CAD/CAM 2_2012 Fig. 20a Fig. 20b