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CAD/CAM - international magazine of digital dentistry

special _ guided implantology I I 13CAD/CAM 1_2012 Figs. 14b & c_Euler’s formula in applied mathematics. describe a sine wave (in our field, the sine wave period can be identified with the implant thread pitch).Withthisinmind,Idevelopedthedevicedis- cussed in this article, which controls the threading pattern. In mechanical engineering, this is called thread timing, and the hex position can be defined as hex timing. For both of them we can speak of phase control (i.e. we can speak of the phase of the implant, both for the thread and the hex). Along this spiral track, the implant can be theoretically and actually screwed and unscrewed as many times as we desire (back and forth), and it will al- ways be possible to know the hex position at the end of the spiral path (final analogue and implant position; Figs. 14a–c). As a spiral circular motion is transformed into a pure translation, a threaded device will respect also position and axis. The information needed to correctly (position and axis, anti-rotational feature and depth) place an implant is in its platform and inside its threads. By creating in the surgical guide a track along which the implant is screwed before its contact with the bone, it is logically possible to start and stop the implant with a final seating with all the parameters always reproduced. We can thus decidewhentostoptheimplantduringitsfallalong thisspiraltrack.Thefinalpositionwillalwaysbethe same,thatisrepeatable,andoperatorindependent. The device meets my earlier definition of a passive system. The maximum precision possible will be what manufacturers can effectively offer (a 1/100 mm is expected to be realistic), which corresponds to the actual implant placement. With a threaded system, there is no axial deviation. Therefore, there will only be a 1/100 mm position deviation (in the arch this will signify a possible 2/100 mm deviation), no axial deviation, depth and anti-rotational feature corre- spondence.Thisdiscrepancyiswithinthelimitsthat allowthecliniciantomakeapremadefinalprosthe- sis and allows for presumably optimal long-term tissue stability. Some of the systems available also consider hex orientationposition,butinordertoseattheimplant correctly with regard to the anti-rotational feature, an extra rotation may be needed. Speaking of “correctly”,atwhichangleresolution?Ifthefeature described is in the shape of two points (painted or alike) to be vertically aligned, what is the point dimension?Whatistheeyeresolution?Isitpossibly aparallaxerror?Extra-rotationisanimplicitadmis- sion of inaccuracy: the depth will not be respected as well, and the implant platform depth may be a littleaboveorbelowthedesiredposition(itdepends on the degree to which the operator is out of phase, more or less than 180°). It is easy to realise that, un- less all this has been calculated, all attempts to find the anti-rotational feature position and depth are onlyguesswork—awasteoftime!Threadtimingand implant phase have not been respected. Forget any notches on the implant mount and smooth sleeves, if anti-rotational feature orientation is the goal. Notches are history in digital guided implantology. Once we have set a threading pattern, it is possi- bletosetthestoppointsimplymakingahelicalgear (a helical gear is realised by contouring the thread along its 360° run; a vertical step will be present once we have gone 360° all round) both in the bottle-neck plug and in the embedded sleeve (the coordinating feature inside the surgical guide), so thataverticalstopisrealisedinthedevice.Whenthe two vertical parts match up, we can be certain that the hex is just where we have engineered it to be. The device pitch must have the same implant pitchbecausedifferenceswillleadtobonestripping. In fact, a difference in implant and mount insertion speed(i.e.thedistancecoveredindepthevery360°) and a different wave period (i.e. thread pitch), will lead to something different from an out of phase working device; it will lead to bone stripping. In Fig. 14c Fig. 14b