implants - internationalmagazine of oral implantology

I case study _ computer-guided implantology Fig. 12a_Missed implant position parameters in the depth-control systems owing to congruent triangle considerations (implant axis deviation and endo-stop angle). Fig. 12b_Missed implant position parameters in the depth-control systems owing to congruent triangle considerations (implant axis deviation and endo-stop angle). platformandinsideitsthreads.Bycreatinginthesurgi- calguideatrackalongwhichtheimplantisscrewedbe- fore its contact with the bone, it is logically possible to start and stop the implant with a final seating with all theparametersalwaysreproduced.Wecanthusdecide whentostoptheimplantduringitsfallalongthisspiral track. The final position will always be the same, that is repeatable,andoperatorindependent.Thedevicemeets myearlierdefinitionofapassivesystem. The maximum precision possible will be what man- ufacturerscaneffectivelyoffer(a1/100mmisexpected toberealistic),whichcorrespondstotheactualimplant placement.Withathreadedsystem,thereisnoaxialde- viation. Therefore, there will only be a 1/100 mm posi- tion deviation (in the arch this will signify a possible 2/100mmdeviation),noaxialdeviation,depthandanti- rotational feature correspondence. This discrepancy is withinthelimitsthatallowthecliniciantomakeapre- made final prosthesis and allows for presumably opti- mallong-termtissuestability. Someofthesystemsavailablealsoconsiderhexori- entation position, but in order to seat the implant cor- rectlywithregardtotheanti-rotationalfeature,anex- tra rotation may be needed. Speaking of “correctly”, at whichangleresolution?Ifthefeaturedescribedisinthe shape of two points (painted or alike) to be vertically aligned, what is the point dimension? What is the eye resolution?Isitpossiblyaparallaxerror?Extra-rotation isanimplicitadmissionofinaccuracy:thedepthwillnot be respected as well, and the implant platform depth maybealittleaboveorbelowthedesiredposition(itde- pends on the degree to which the operator is out of phase, more or less than 180°). It is easy to realise that, unless all this has been calculated, all attempts to find theanti-rotationalfeaturepositionanddepthareonly guesswork—awasteoftime!Threadtimingandimplant phase have not been respected. Forget any notches on the implant mount and smooth sleeves, if anti-rota- tional feature orientation is the goal. Notches are his- toryindigitalguidedimplantology. Oncewehavesetathreadingpattern,itispossibleto setthestoppointsimplymakingahelicalgear(ahelical gearisrealisedbycontouringthethreadalongits360° 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 that a vertical stop is realised in the device.Whenthetwoverticalpartsmatchup,wecanbe certainthatthehexisjustwherewehaveengineeredit tobe. The device pitch must have the same implant pitch becausedifferenceswillleadtobonestripping.Infact,a differenceinimplantandmountinsertionspeed(i.e.the distance covered in depth every 360°) and a different wave period (i.e. thread pitch), will lead to something different from an out of phase working device; it will leadtobonestripping.Inparticular,alongermounting periodwillforcetheimplantdownwardsintothebone, with consequent vertical bone stripping, whereas a shortermountingperiodwillforcetheimplanttorotate horizontally, with consequent horizontal bone strip- ping. Self-tapping implants should show better torque control. Rigidity The device must be secured to the surgical guide to resist the rotational torque and vertical torque al- ways present during the implant rotation inside the bone. 32 I implants4_2011 Fig. 12a Fig. 12b Fig. 12c Fig. 12d Fig. 14a Fig. 13a Fig. 13b