implants the journal of oral implantology United Kingdom Edition

26 I I clinical _ guided implantology implants2_2012 ing; however, the larger the axis deviation , the greater the torque perceived by the op- erator, who will be given an inaccurate sense of implant stability. 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 cor- tical plates can directionally address implant placement. Excellent results reported could havebeenaffectedbyworkinginlow-density bone, 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), because the low- density trabeculae would be drilled 360° around. The hex would be missed anyway. The second matter to be considered is bone guidance. Depth and anti-rotational feature orientation 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 peripheral point along the circumference and a point along the same circumference can be indexed to the implant thread, the need to change the platform depth and hex orientation and con- trol the threading pattern (implant phase) will be indicated. Any painted notch to index the hex and the sleeve is misleading informa- tion 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. Once the implant has started its rotation, it is not possible to correct the posi- tion by redirecting the implant, as the apex is inserted into the bone and will act as a ful- crum. Even if the operator redirects the im- plant axis, the implant body will remain dis- placed in position (B-L and M-D). Moreover, theredirectionwouldbedonebysight,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, with a smooth sleeve it is impossible to predict when it starts being screwed. The moment the implant starts ro- tating depends on the bone friction, depend- ing on the density (HU), and the progression of the osteotomy and the implant insertion will be dependent on the HU gradient (an- isotropy), which describes how rapidly the density changes per unit of length along the threespatialcoordinatesinsidethebone.Un- less we use a device able to force implants in a precise position (referred to as the surgical guide)alongapathengineeredaccordingtoa particularmechanics,thebonewilldetermine Fig 11c_Calculation of the trigonometric angle deviation (sine/ cosine rule). Fig 11d_Calculation of the trigonometric angle deviation (tan/ cot rule) Fig. 12a-d_Missed implant position parameters in the depth-control systems owing to congruent triangle considerations (implant axis deviation and endo-stop angle). Fig.11c Fig.11d Fig.12a Fig.12b Fig.12c Fig.12d