Biomedical Engineering Reference
In-Depth Information
In the natural tooth, the periodontum, which forms a shock-absorbing layer, is in
this position between the tooth and jaw bone [Skalak, 1983]. Natural teeth and
implants have different force transmission characteristics to bone.
Compressive strains were induced around natural teeth and implants as a
result of static axial loading, whereas combinations of compressive and tensile
strains were observed during lateral dynamic loading [Oshida, 2007a]. Magnitude
of strain around the natural tooth is signifi cantly lower than the opposing implant
and occluding implants in the contra-lateral side for most regions under all
loading conditions. It was reported that there was a general tendency for increased
strains around the implant opposing natural tooth under higher loads and par-
ticularly under lateral dynamic loads [Hekimoglu et al., 2004].
By means of fi nite element (FEM) analysis, stress-distribution in bone around
implants was calculated with and without stress-absorbing element [van Rossen
et al., 1990]. A freestanding implant and an implant connected with a natural
tooth were simulated. For the freestanding implant, it was concluded that the
variation in the modulus of elasticity of the stress-absorbing element had no
effect on the stresses in bone. Changing the shape of the stress-absorbing element
had little effect on the stresses in cortical bone. For the implant connected with
a natural tooth, it was concluded that a more uniform stress was obtained around
the implant with a low modulus of elasticity of the stress-absorbing element. It
was also concluded that the bone surrounding the natural tooth showed a decrease
in the height of the peak stresses.
The dental or orthopedic prostheses, particularly the surface zone thereof,
should respond to the loading transmitting function. The placed implant and
receiving tissues establish a unique stress-strain fi eld. Between them, there
should be an interfacial layer. During the loading, the strain-fi eld continuity
should be held, although the stress-fi eld is obviously in a discrete manner due
to different values of modulus of elasticity between host tissue and foreign
implant material. If the magnitude of the difference in modulus of elasticity is
large, then the interfacial stress, accordingly, could become so large that the
placed implant system will face a risky failure or detachment situation. Therefore,
materials for implant or surface zone of implants should be mechanically compat-
ible to mechanical properties of receiving tissues to minimize the interfacial
discrete stress. This is the second compatibility and is called as the mechanical
compatibility.
Figure 5.4 [Oshida, 2007a] compares yield strengths and modulus of elasticity
of various biomaterials in log-log plot, where P: polymeric materials, B: bone,
HSP: high strength polymers (such as Kevlar, Kapton, PEEK, etc.), D: dentin,
TCP: tricalcium phosphate, HAP: hydroxyapatite, E: enamel, TI: commercially
pure titanium (all unalloyed grades), TA: titanium alloys (e.g., Ti-6Al-4V), S:
steels (e.g., 304 and 316 stainless steels), A: alumina, PSZ: partially stabilized
zirconia, and CF: carbon fi ber, respectively.
From the fi gure, it is greatly surprising to note the large differences in both
yield strength and modulus of elasticity between TI as a foreign implant material
and B as a receiving hard tissue. Even depositing HAP onto TI surface would
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