implants0112

case study I 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 amillimetre(humanvisualacuity).Thesameistrue for hearing (in hertz) and hand movements (we cannot control a movement beyond 0.1 mm). A passive device therefore appears necessary to ensure that the same implant position can be re- produced repeatedly and independently of the op- erator within the threshold defined above. This ful- fils my definition of “passiv- ity”. _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 fromspatialresolutionlimits, 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 con- trol should not be considered passive. These sys- temsareequippedwithavirtualsmoothsleeveand are operator dependent. Furthermore, they can be monitor or mouse guided, when the handpiece is transformed into a computer mouse. Ironically, we tend to consider the surgical tutoring toy a passive tutoring system only because it is provided with sensors along its holes (Figs. 16a & b), but not be- cause 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 al- ready decided upon the location of the implant through planning and the surgical guide). _Accuracy verification Manystudiesonaccuracyverificationhavebeen conducted. In these, scientists have sought to de- termine and measure accuracy by means of com- paring the planning data and data acquired post- operatively. Their aim is to evaluate which of the marketed systems delivers the most accurate re- sults. In Part I, I discussed smooth- sleeve-related inaccuracy from a theoretical perspective. We must also realise that a reliable evalua- tionofaccuracyrequiresmeasur- ing device. Is CT a reliable tech- nique? Is superimposition a good means of evaluating accuracy in implant placement? Does it con- sider all the parameters that de- fine the implant position (including the hex)? To prove validity and measurement accuracy, repeata- bility should be considered as important as its un- derlying mathematics. Even if a perfect superimposition has been car- ried out, CT artefacts and the voxel size (which is 0.125 mm at the best) not being an issue, and con- sidering the CT scan as a continuum, its results ap- pear to be invalid information. Scanners, like any I 23implants1_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 guar- drail. 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.