implants - internationalmagazine of oral implantology

I case study _ computer-guided implantology Fig. 9a_Jig created in the mouth for the STL case. Fig. 9b_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. Thegoodresultsreportedinpublicationscouldhave beenaffectedbyright-handedoperatorsinisotropicD2 and D3 bone or by working in sites in which cortical platescandirectionallyaddressimplantplacement.Ex- cellent results reported could have been affected by working in low-density bone, where the marketed sys- temallowsforagoodaxisanddepth,butthedrillscre- atedatruncatedconevolumedevitalisedarea(depend- ing on the drill blades’ cutting power and operator’s handforce),becausethelow-densitytrabeculaewould bedrilled360°around.Thehexwouldbemissedanyway. The second matter to be considered is bone guidance. Depth and anti-rotational feature orientation depend onbonemorphologyanddensity. Whentheimplanthasstarteditsrotationinsidethe bone,itisnotpossibletochangethethreadingpattern: while screwing the implant, the platform will move in- creasingly deeper downwards to the bone. Since it is possible to index a hex to a peripheral point along the circumference and a point along the same circumfer- ence can be indexed to the implant thread, the need to change the platform depth and hex orientation and controlthethreadingpattern(implantphase)willbein- dicated. Any painted notch to index the hex and the sleeve is misleading information and naïve, as it is ap- proximate,thatis,noimplantphase,anddependenton notchsize,pointofview(parallax)andoperator’svisual acuity. Once the implant has started its rotation, it is not possible to correct the position by redirecting the im- plant, as the apex is inserted into the bone and will act as a fulcrum. Even if the operator redirects the implant axis,theimplantbodywillremaindisplacedinposition (B-LandM-D).Moreover,theredirectionwouldbedone by sight, which is dependent on the operator’s visual acuityandaparallaxerrorisapossibility. Theaxisdeviationintroducesanotherconcept:bone responseintermsofbonedensityandboneanisotropy. As a matter of fact, on the other side of the surgical guide, when the implant touches the bone, with a smooth sleeve it is impossible to predict when it starts beingscrewed.Themomenttheimplantstartsrotating dependsonthebonefriction,dependingonthedensity (HU),andtheprogressionoftheosteotomyandtheim- plant insertion will be dependent on the HU gradient (anisotropy), which describes how rapidly the density changesperunitoflengthalongthethreespatialcoor- dinates inside the bone. Unless we use a device able to force implants in a precise position (referred to as the surgical guide) along a path engineered according to a particular mechanics, the bone will determine the im- plantthreadingpattern(bonedensityforinitialscrew- ing, whether or not a crestal bone drill has been used) andbonedensitygradient,oranisotropyforthesubse- quentaxis. Accepting inaccuracy, manufacturers and re- searchers have created depth-control systems in the hopeofofferingcertaintyaboutthisparameteratleast, butthegapwillberesponsiblefornotonlypositionand axisdeviations,butalsodeptherrors.Infact,theimplant mount endo-stop will match up with the sleeve at an angle.Thefirstcontactwillbebeyondthedesireddepth, andkeepingonscrewingtheimplantwillcreateagreat torquewithsurgicalguidedeformationandtensionon thebone. The complete contact will correspond to a deeper implantpositionthandesired.Thecorrectdepthmaybe halfway (maybe operator dependent and determined using the naked eye). Depth error, axis deviation and translation in crestal position in the axial deviation di- rectionwillbetheresults(Figs.12a–e). The likelihood of ideally positioning two implants is oneoutofsevenbillionand500millionpossibilities(just afewmillionless,ifitisanycomforttous).Andthiseval- 30 I implants4_2011 Fig. 9b Fig. 9cFig. 9a Fig. 10bFig. 10a