Biomechanics and Specificities

  -E-   IMPLANT SPECIFICITIES

Impl elast

   Fig. E2: The visco-elastic periodontal ligament and the tapered shape of a natural root, move the center of rotation toward the Apex. Then the dissipation of transverse forces is shared along the root. It is different for a cylindrical implant, on which the transversal forces are concentrated at the neck of implant. Tapered implants, elude partially this phenomenon. In fact conical design improve the rigidity, the solidity and the distribution of the transversal forces in the peripheral bone. However if the conic shape is too accentuated with an oversized volume, related to the crest size, the peri implant bone at the neck will be insufficient in quantity and poorly irrigated, with an increased risk of resorption.

Compared to natural teeth, the lack of ligament around implants, reduces their clinical mobility. The implants, like the extended fixed restorations, have a clinical mobility nonexistent. And the motionlessness amplifies the consequences of occlusal traumas.

Fig. E3: A natural tooth has, a visco-elastic way of response to the mechanical load. In a first step, light forces easily provoke a reversible displacement limited to the natural mobility of the ligament, then in close contact with the bone, it’s only the elasticity of the bone that allow a very limited displacement. On an implant, it’s only the elastic way that works, with a linear and direct response to load, in association with the important elasticity of titanium, that can become critical for crest bone with small diameter implant.

The lack of  periodontal ligament mechanoreceptors, reduces interferences perception, and the capacity to install avoiding mechanisms. But some patient (Hammërle 1995) have a similar level of pressures perception, as on implants as on natural teeth. But the majority have a pressures perception very very reduced on implants.

Fig. E4: When functional overcontacts and/or overguidances are present on the implant prostheses, with a tactile sensitivity threshold very low, they are not detected, then not avoided, resulting in numerous micro-traumas, possibly responsible for bone loss.

Reduced mobility amplify malocclusions that are not properly detected.
Clinical mobility of adjacent teeth is a key factor. If their mobility is reduced, balancing is almost similar, to natural teeth. The contacts and guidances must be present but not dominant at the first balancing. On the other hand, if their mobility is large, there is a high risk to overload the implants, from which the necessity to do a specific balancing on built-in implants, or the obligation to realize an extended stabilization to limit the whole mobility.

Fig. E5: The contacts and guidances non dominant at the beginning, are generally found well balanced at the following recall (Le Gall et Le Gall 2016). Thank’s to the natural wearing of the neighbours and opposite teeth. Then the functional  equilibrium remains. In case of important mobility of the next teeth, an extended connection and the prosthetic geometry, as well as functional equilibrium, are the keys of the long lasting.

Tooth and cortical bone have the same modulus of  Young. The modulus of titanium is five to ten time higher (Lemons et Philips 1993). Titanium is more stiff, but more elastic than the tooth. A direct response to load, follows the lack of periodontal ligament (Sekine et col 1986).
Consequently: there is a wide stress area, around the neck, at bone crest level, that is amplified by occlusal trauma (Kilamura et col 2004).
At the end of the first year of loading: bone loss has the same shape that the maximum stress area (Zechner et col 2004). These results directly involve occlusion in bone loss.
The concept and design of the implants, the surface processing and the occlusal balancing concept, modify the level of bone loss. But other causes may be involved in bone loss, like infection, leakages at microgap (abutment or trans-screwed crowns), rebuilding of biologic space, number of surgeries (Misch et col. 2005), bio-corrosion and of course infection.
Our first objective is to choose an implant capable to eliminate or minimize infectious risk and to withstand the functional transverse forces, when they are optimally balanced and controlled.

Fig. E6: These considerations lead to prefer a slightly tapered implant, with a flared neck, capable to withstand and distribute well, lateral forces, in order to preserve neck bone. The abutment interface of this implant must be leakage-proof, stable,with a mere design, and remote from the bone level, to eliminate infectious risk. 

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