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88 rev port estomatol med dent cir maxilofac. 2017;58(2):79-90

Discussion

Analysing the tendencies of both models is possible to ob-
serve an absolute match of their biomechanical behaviours.
The stress pattern measured on the FEA was confrmed
with the failure location on the SCLT and the sequence in
which each part reached the yield strength was the same
as the observed on the interrupted SCLT. As Kim et al and
35
Eser et al have demonstrated, a coincidence of tendencies
36
and behaviours is the condition needed to validate a nu-
merical model, once all the simplifcations and assump-
tions made during the numerical modelling impair a quan-
Figure 28. Correspondence between the damage titative coincidence of the results. So, because the results
sequences on both studies: 64% of FEA (implant screws). show the necessary coincidence of behaviours, the model
was considered valid to study the biomechanical behaviour
of an implant -fxed oral rehabilitation with recourse to
short implants.
The calculation performed using the validated model,
simulating a clinical situation, indicated that the metal
parts of the model are in a very safe zone, away from the
plastic deformation. The interpretation of the results on the
bone part is, due to its orthotropy, more complex. As the
Von Mises equivalent stress express the six components of
stress in only one value, it is very difficult to be compared
with an anisotropic materials yield strength. The values of
182 MPa and 121 MPa for the yield strength, considering,
respectively, a compression load parallel or normal to the
bone major axis, were suggested by Natali et al. However,
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the Frosts mechanostatic theory, without referring to the
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orthotropic variations, indicated that, for a stress of 60 MPa
and a strain of 3000 microstrain, the bones yield point
Figure 29. Correspondence between the damage would be achieved resulting in woven bone rather than
sequences on both studies: final static test where the
implant screws remained on the elastic deformation zone. lamellar formation. Likewise, Carters hypothesis 13,53 sus-
Comparing the images 22, 24, 26 and 28 respectively with tain that a bone strain over 4000 microstrain can cause bone
the images 23, 25, 27 and 29, is possible to confirm the loss.
match between the stress pattern for each part and the
experimental model failures on the same part. It is possible to observe that the microstrain measured on
the X and Y axis, as well as the stress measured on the Z axis,
are on a dangerous zone with a very high possibility of bone
overloading and bone loss.
Table 4. The implants, prosthetic framework and These results highlight the fact that bone is the weakest
implant screws are situated in a safe zone, unlikely to part of such a rehabilitation, especially the buccal cortical
fracture with a physiologic mastication load. bone, around the implants platform. This finding is of extreme
importance once it may result in bone resorption, initiating a
Relation with
Model part Stress (MPa)
yield strength sequence of events that may result in implant failure. It is also
extremely important because bone, the only biological tissue
Implants 348.1 46.41%
considered in this experiment, is also the most difficult part
Prosthetic framework 332.5 50.45% to replace.
23
22
24
Tabrizi et al, Bhat et al and Himmlová et al, share the
Implant screws 341.1 42.90%
same concerns related with the cortical bone preservation
around short implants platform. On contrary, analysing un-
splinted short -implant rehabilitations, Nissan et al found
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Table 5. Stress and strain values obtained for the bone part. prosthetic failures for a crown -to -implant ratio of 1,75 -1.
To decrease the stress and strain observed at the cortical
Stress (MPa) Microstrain bone level, the intervention at the prosthetic framework ge-
X axis 67.25 3165 ometry could represent a valid alternative. It is worth re-
member that the presupposition of this study was to avoid
Y axis 61.6 4034
complex surgical techniques to augment the available bone
Z axis 79.2 1814 volume.