implants the journal of oral implantology United Kingdom Edition

assembledchairsidetoallowforminimalver- tical clearance. A base-plate resin was then usedtocreatejigstocheckaccuracybetween the models and the mouth. The case results were satisfactory. The device was easy to use and jig correspon- dence between the abutments screwed on the analogue models and the clinical im- plant positions was obtained. 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. For the stone case, a transfer was screwed onto the analogue, the resin jig was created, and then its correspondence was clinically checked. _Discussion The present systems do not offer suffi- cient and reliable accuracy because they do not consider the concepts of thread timing and implant phase. 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. Smooth sleeve-dependent inaccuracy. The first element to be consid- ered is the gap between the implant mount and the sleeve. A twisting implant apex is the natural effect. When the implant is guided by a smooth sleeve, the position in the arch will becorrectonlyiftheimplantmountdoesnot 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 “positionparadoxeffect”ofaguidingsmooth sleeve (similar to a guard-rail). Since the sleevehasa topanda bottomplane,thispar- adox 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 a smooth sleeve”). The gap affects position and axis: these parameters go hand in hand. Depending on the gap entity, it is possible to calculatetheimplantapextwistingentity,us- ing simple proportionality. At a 20mm depth from the top of the sleeve (approximately 13mm below the ridge), the linear deviation will be 0.8mm (1.6mm 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/co- sine and tan/cot rules). If the gap is 0.1mm (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 too large an insertion torque because it may be damag- I 25 clinical_ guided implantology I implants2_2012 Fig.8b Fig.9a Fig.9b Fig.9c Fig.10a Fig.10b Fig.11a Fig.11b Fig 8b_Surgical guide in the mouth, showing the helical gear in particular . Fig 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. Fig 11a_Mathematical proportion to calculate the linear radial apex deviation. Fig 11b_Calculation of the trigonometric angle deviation.