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I special _ guided implantology Fig. 11a_Mathematical proportion to calculate the linear radial apex deviation. Fig. 11b_Calculation of the trigonometric angle deviation. Fig. 11c_Calculation of the trigonometric angle deviation (sine/cosine rule). Fig. 11d_Calculation of the trigonometric angle deviation (tan/cot rule). greater the torque perceived by the operator, who will be given an inaccurate sense of implant sta- bility. The good results reported in publications could have been affected by right-handed operators in isotropic D2 and D3 bone or by working in sites in which cortical plates can directionally address im- plant placement. Excellent results reported could havebeenaffectedbyworkinginlow-densitybone, where the marketed system allows for a good axis and depth, but the drills created a truncated cone volume devitalised area (depending on the drill blades’ cutting power and operator’s hand force), becausethelow-densitytrabeculaewouldbedrilled 360° around. The hex would be missed anyway. Thesecondmattertobeconsideredisboneguid- ance.Depthandanti-rotationalfeatureorientation depend on bone morphology and density. When the implant has started its rotation inside the bone, it is not possible to change the threading pattern: while screwing the implant, the platform will move increasingly deeper downwards to the bone. Since it is possible to index a hex to a periph- eralpointalongthecircumferenceandapointalong the same circumference can be indexed to the im- plantthread,theneedtochangetheplatformdepth and hex orientation and control the threading pat- tern (implant phase) will be indicated. Any painted notch to index the hex and the sleeve is misleading information and naïve, as it is approximate, that is, no implant phase, and dependent on notch size, point of view (parallax) and operator’s visual acuity. Oncetheimplanthasstarteditsrotation,itisnot possible to correct the position by redirecting the implant,astheapexisinsertedintotheboneandwill act as a fulcrum. Even if the operator redirects the implantaxis,theimplantbodywillremaindisplaced inposition(B-LandM-D).Moreover,theredirection would be done by sight, which is dependent on the operator’s visual acuity and a parallax error is a possibility. The axis deviation introduces another concept: bone response in terms of bone density and bone anisotropy. As a matter of fact, on the other side of the surgical guide, when the implant touches the bone,withasmoothsleeveitisimpossibletopredict when it starts being screwed. The moment the im- plant starts rotating depends on the bone friction, dependingonthedensity(HU),andtheprogression of the osteotomy and the implant insertion will be dependent on the HU gradient (anisotropy), which describes how rapidly the density changes per unit of length along the three spatial coordinates in- side 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 theimplantthreadingpattern(bonedensityforini- tial screwing, whether or not a crestal bone drill has 10 I CAD/CAM 1_2012 Fig. 11d Fig. 11a Fig. 11b Fig. 11c