Biomedical Engineering Reference
In-Depth Information
2.3.2
Interface Constitutive Model
In THA, the interface is the region immediately adjacent to the prosthesis, that
is, the region through which the forces are transmitted from the prosthesis to
the bone tissue (Figure 2.3). This consists of a thin layer of deformable material
with particular characteristics. Immediately after the implantation, the shaft is
partially stabilized because of its geometric characteristics, since it is inserted
into the medullary canal under a moderate pressure, generating a distribution
of stresses over the bone and the interface. Experimental studies show that the
osseointegration does not occur immediately after the implantation and, even over
time, only a small amount of the porous covering is osseointegrated. Some places
may also show formation of both the bone and the fibrous tissue, that is, a kind of
material mixture at the same place [45-47].
The real mechanisms responsible for the formation of the fibrous tissue, in the
interfacial region, are still not completely clear. One of the clearest causes is related
to the occurrence of excessive micromovements between the prosthesis and the
bone. Some authors state that the mechanical environment is more important
than the presence of particles originating from acetabular wear and, therefore, the
formation of interfacial tissue is directly related to the historical loadings to which
the prosthesis has been subjected [48].
In vivo
studies have shown that relative
micromovements between the prosthesis and the bone of the order of 20 µmdonot
influence the bone repair process, but for micromovements greater than 150
m
the formation of fibrous tissue around the prosthesis may reach a thickness of
1-2 mm , within six weeks after the implantation [49].
In the structure of the fibrous tissue, the collagen fibers are distributed randomly
in several overlapping layers. This tissue has a nonlinear behavior, low stiffness,
µ
Figure 2.3
Schematic of a hip replacement and interface layer.
