implants the journal of oral implantology United Kingdom Edition

28 I I clinical _ guided implantology implants2_2012 axis, the depth and the antirotational feature (classically, a hex) orientation. The mechani- cal engineering of a screw is quite different from that of a bullet (smooth sleeve) and was defined by Archimedes (applications of an endless screw are still in use today, like the meat mincer) and by Euler (Swiss mathema- tician, who died in St Petersburg more than two centuries ago). In particular, Euler point- ed out that the movement of a circle (in our field, the implant platform) can be described with mathematical formulas: a point along the circumference (in our field the perimetric projection of a part of the hex) can be pro- jected along a plane orthogonal to the di- rection of the circle movement itself (in our field, the progression of the platform while the implant is being screwed in multiplanar reconstructions). The projection will in ap- plied mathematics. describe a sine wave (in our field, the sine wave period can be identi- fied with the implant thread pitch). With this in mind, I developed the device discussed in this article, which controls the threading pat- tern. In mechanical engineering, this is called threadtiming,andthehexpositioncanbede- fined as hex timing. For both of them we can speakofphasecontrol(iewecanspeakofthe phase of the implant, both for the thread and the hex). Along this spiral track, the implant canbetheoreticallyandactuallyscrewedand unscrewed as many times as we desire (back and forth), and it will always be possible to know the hex position at the end of the spiral path (final analogue and implant position) As a spiral circular motion is transformed into a pure translation, a threaded device will respect also position and axis. The informa- tion 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 be- fore 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 decide when to stop the implant during its fall along this spi- ral track. The final position will always be the same, that is repeatable, and operator inde- pendent. The device meets my earlier defini- tionofapassivesystem.Themaximumpreci- sion possible will be what manufacturers can effectively offer (a 1/100mm 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 1/100mm position deviation (in the arch this will signify a possible 2/100mm deviation), no axial deviation, depth and anti- rotational feature correspondence. This dis- crepancy is within the limits that allow the clinician to make a premade final prosthesis and allows for presumably optimal long- term tissue stability. Some of the systems available also con- sider hex orientation position, but in order to seat the implant correctly with regard to the anti-rotational feature, an extra rotation may be needed. Speaking of “correctly”, at which angle resolution? If the feature described is in the shape of two points (painted or alike) to beverticallyaligned,whatisthepointdimen- sion?What istheeyeresolution?Isit possibly a parallax error? Extra-rotation is an implicit admission of inaccuracy: the depth will not be respected as well, and the implant plat- formdepthmaybealittleaboveorbelowthe desired position (it depends 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 only guesswork—a waste of time! Thread timing and implant phase have not been respected. Forget any notches on the implant mount and smooth sleeves, if anti-rotationalfeatureorientationisthegoal. Notches are history in digital guided implan- tology. Once we have set a threading pattern, itispossibletosetthestoppointsimplymak- ing a helical gear (a helical gear is realised by contouring the thread along its 360° run; a vertical step will be present once we have