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special _ guided implantology I density bone. It could imitate the implant, with sharp threads and narrow body, to be screwed to the bottle-plug, or a bottle-plug dedicated to the tapping step, with the tapping part integral to the bottle-plug itself. In both clinical cases, the device was assembled chairside to allow for minimal vertical clearance (Figs. 7a–d). A base-plate resin was then used to create jigs to check accuracy between the models and the mouth. _Results The case results were satisfactory. The device waseasytouse(Figs.8a&b)andjigcorrespondence between the abutments screwed on the analogue models and the clinical implant positions was ob- tained. For the STL case, four abutments were modelled on the STL model, the resin jig was created directly in the mouth, and then its correspondence to the same abutments was checked on the STL model (Figs. 9a–c). For the stone case, a transfer was screwed onto the analogue, the resin jig was cre- ated, and then its correspondence was clinically checked (Figs. 10a & b). _Discussion The present systems do not offer sufficient and reliable accuracy because they do not consider the conceptsofthreadtimingandimplantphase.Their weak point is the smooth sleeve (whether metal or virtual), which does not have any control over the mechanics of a screw, which an implant is. Shooting a bullet makes sense, but shooting a screw does NOT. Smoothsleeve-dependentinaccuracy Thefirstelementtobeconsideredisthegapbe- tween the implant mount and the sleeve. A twist- ing implant apex is the natural effect. When the implant is guided by a smooth sleeve, the position inthearchwillbecorrectonlyiftheimplantmount does not ever touch the sleeve during the process, but when the dentist is working there will always be contact, which will results in an error in B-L and M-V position. This is what I call the “position pa- radoxeffect”ofaguidingsmoothsleeve(similarto a guard-rail). Since the sleeve has a top and a bottom plane, this paradox effect is reproduced in both these two planes, and an axis deviation is a natural con- sequence (what I call the “axis paradox effect of asmoothsleeve”).Thegapaffectspositionandaxis: these parameters go hand in hand. Depending on the gap entity, it is possible to calculate the implant apex twisting entity, using simple proportionality (Fig. 11a). At a 20 mm depth from the top of the sleeve (approximately 13 mm below the ridge), the linear deviation will be 0.8 mm (1.6 mm on the diameter that is the possible implant apex twisting entity). Trigonometry is an easy way to calculate the deviation angle of the implant axis (sine/cosine and tan/cot rules). If the gap is 0.1 mm (0.2 on the diameter), the axis deviation will be a deviation of 2° 20' (Figs. 11b–d). Tapered implants can engage bone at an even greater angle, particularly if the driver is conical at its first part. Consequently, it will work only at the end of the implant placement phase. According to the previous considerations, I suggest that it does not work efficiently. This cone-shaped driver limits toolargeaninsertiontorquebecauseitmaybedam- aging; however, the larger the axis deviation, the Figs. 9a & b_Jig created in the mouth for the STL case. Fig. 9c_Jig verified against the model in the STL case. Fig. 10a_Jig created on the stone model in the stone case. Fig. 10b_Jig verified in the mouth in the stone case. I 09CAD/CAM 1_2012 Fig. 9cFig. 9bFig. 9a Fig. 10bFig. 10a