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implants - internationalmagazine of oral implantology

case study _ computer-guided implantology I _Results The case results were satisfactory. The device was easy to use (Figs. 8a & b) and jig correspondence be- tweentheabutmentsscrewedontheanaloguemodels andtheclinicalimplantpositionswasobtained. For the STL case, four abutments were modelled on the STL model, the resin jig was created directly in the mouth,andthenitscorrespondencetothesameabut- mentswascheckedontheSTLmodel(Figs.9a–c).Forthe stone case, a transfer was screwed onto the analogue, the resin jig was created, and then its correspondence wasclinicallychecked(Figs.10a&b). _Discussion Thepresentsystemsdonotoffersufficientandreli- able accuracy because they do not consider the con- cepts of thread timing and implant phase. Their weak point is the smooth sleeve (whether metal or virtual), whichdoesnothaveanycontroloverthemechanicsof a screw, which an implant is. Shooting a bullet makes sense,butshootingascrewdoesNOT. Smoothsleeve-dependentinaccuracy Thefirstelementtobeconsideredisthegapbetween the implant mount and the sleeve. A twisting implant apex is the natural effect. When the implant is guided byasmoothsleeve,thepositioninthearchwillbecor- rectonlyiftheimplantmountdoesnotevertouchthe sleeveduringtheprocess,butwhenthedentistiswork- ingtherewillalwaysbecontact,whichwillresultsinan errorinB-LandM-Vposition.ThisiswhatIcallthe“po- sitionparadoxeffect”ofaguidingsmoothsleeve(sim- ilartoaguard-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 consequence (what I callthe“axisparadoxeffectofasmoothsleeve”).Thegap affects position and axis: these parameters go hand in hand.Dependingonthegapentity,itispossibletocal- culate the implant apex twisting entity, using simple proportionality(Fig.11a).Ata20mmdepthfromthetop of the sleeve (approximately 13 mm below the ridge), the linear deviation will be 0.8 mm (1.6 mm on the di- ameterthatisthepossibleimplantapextwistingentity). Trigonometry is an easy way to calculate the deviation angleoftheimplantaxis(sine/cosineandtan/cotrules). Ifthegapis0.1mm(0.2onthediameter),theaxisdevi- ationwillbeadeviationof2°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 theimplantplacementphase.Accordingtotheprevious considerations, I suggest that it does not work effi- ciently. This cone-shaped driver limits too large an in- sertion torque because it may be damaging; however, thelargertheaxisdeviation,thegreaterthetorqueper- ceivedbytheoperator,whowillbegivenaninaccurate senseofimplantstability. Fig. 4_Analogues in the STL model. Fig. 5_Surgical guide created for stone surgery. Fig. 6_Surgical guide created for stone surgery. Fig. 7a_Assembly. Fig. 7b_Assembly. Fig. 7c_Assembly in the stone-based surgical guide. Fig. 7d_Assembly in the STL-based surgical guide. Fig. 8a_Surgical guide in the mouth. Fig. 8b_Surgical guide in the mouth, showing the helical gear in particular. I 29implants4_2011 Fig. 8bFig. 8a Fig. 5 Fig. 6Fig. 4 Fig. 7a Fig. 7b Fig. 7c Fig. 7d