CAD/CAM - international magazine of digital dentistry

special _ guided implantology I a good means of evaluating the optical properties of CT scans. Spatial frequency has been widely studied and it is now generally accepted that line pairs (black and white) can be perceived up to a tenth of a millimetre (human visual acuity). The same is true for hearing (in hertz) and hand move- ments (we cannot control a movement beyond 0.1 mm). A passive device therefore appears necessary to ensure that the same implant position can be reproduced repeatedly and independently of the operator within the threshold defined above. This fulfils my definition of “passivity”. _The limitations of infra-red control systems This last point also means that infra-red control sys- tems are excluded by defini- tion, since their accuracy is operator dependent. Apart from spatial resolution lim- its, this kind of technology is affected by time- delay problems, partially due to the machine itself and partly due to the temporal resolution limits of the operator (eye, ear, hand). Therefore, infra-red control should not be considered passive. These systems are equipped with a virtual smooth sleeve andareoperatordependent.Furthermore,theycan be monitor or mouse guided, when the handpiece is transformed into a computer mouse. Ironically, wetendtoconsiderthesurgicaltutoringtoyapas- sive tutoring system only because it is provided with sensors along its holes (Figs. 16a & b), but not because of its functionality. Itismyopinionthereforethatanentirelypassive device, in which all necessary information is in- cluded, is superior to semi-active devices. Further- more, passive devices should be easy to handle and intuitive to use, and their design should not allow any freedom for the operator (the operator has already decided upon the location of the implant through planning and the surgical guide). _Accuracy verification Many studies on accuracy verification have been conducted. In these, scientists have sought to determine and measure accuracy by means of comparing the planning data and data acquired post-operatively. Their aim is to evaluate which of the marketed systems delivers the most accurate results. InPartI,Idiscussedsmooth- sleeve-relatedinaccuracyfrom a theoretical perspective. We must also realise that a reli- able evaluation of accuracy re- quires measuring device. Is CT a reliable technique? Is super- imposition a good means of evaluating accuracy in implant placement? Does it consider all the parameters that define the implant position (including the hex)? To prove validity and measurement accuracy, repeatability shouldbeconsideredasimportantasitsunderlying mathematics. Even if a perfect superimposition has been carried out, CT artefacts and the voxel size (which is 0.125 mm at the best) not being an issue, and considering the CT scan as a continuum, its results appear to be invalid information. Scanners, like any I 07CAD/CAM 2_2012 In this „little Surgeon“ toy is my hand guided by a passive tutoring system down the hollow to get the target? No! Only a red nose will notice me I’m touching the guardrail. Does it make sense? Is it transformed into a passive methos if I can do the same thing with a mouse-handpiece and looking into a screen? No! These are semi-active monitor-guided systems. Fig. 16b Fig. 17a Fig. 17b Fig. 16a Fig. 16a_The surgical tutoring toy. Fig. 16b_Operator-dependent super technological system. Fig. 17a–b_CT scan MTF limits.