Biomedical Engineering Reference
In-Depth Information
and a moderate bone loss (reduced stress shielding effect) has a bigger probability
to perform efficiently.
10.3.1
Implant Stability
The fixation of a cementless hip stem is biological. After surgery, the stem is
press-fitted to the bone and fixation is achieved through an appropriate geometry
and the friction interaction. Good primary stability is achieved when the bone
starts to attach to the coating surface (osseointegration), establishing the necessary
biological fixation. To have a good initial stability, ''small'' relative tangential dis-
placements and ''small'' contact stresses at stem-bone interface are necessary [9].
On the other hand, thigh pain is related to excessive interface displacements and
contact stresses [10]. In summary, initial stability is an essential requirement for
the success of THA and can be quantified by the relative displacement and interface
stress.
With respect to displacements, ''large'' relative tangential displacements at
stem-bone interface lead to the formation of a soft fibrous tissue with reduced
fixation capacity. Indeed, high values of displacements can lead to absence of bone
ingrowth into porous surface. However, the threshold value of the displacement
to obtain bone ingrowth is not precisely known. For instance, Rancourt
et al
.
[27] suggested 28
m as the limit value for relative tangential displacement. For
Viceconti
et al
. [28], the limit is between 30 and 150
µ
µ
m, and displacements between
150 and 220
m lead to the formation of a fibrous tissue layer. Finally, to achieve a
very fast osseointegration the displacement value should be less than 30
µ
m [28].
Contact stresses can also avoid bone ingrowth. In fact, for human femur, cortical
and trabecular bone has an ultimate compressive strength of 170 MPa [29] and
7.89 MPa [30], respectively. Since proximal stem is in contact with trabecular bone
(see Figure 10.1), osseointegration in this region is very sensitive to contact stress.
On the contrary, the distal part of implant is in direct contact with cortical bone
or is surrounded by marrow. This way, interface stresses can be greater, but not
excessive to avoid thigh pain and stress shielding effect.
Besides the contact stress and relative displacement, the size of the interface gap
is also an important parameter to obtain bone ingrowth. However, this parameter
is not considered as a requirement in the design model described in this chapter
since its relevance is relative. In fact, even with a gap of 150
µ
µ
m between the bone
and stem, osseointegration can occur [31].
10.3.2
Stress Shielding Effect
Another important requirement for a bone implant is the minimization of stress
shielding effect.
Actually, bone is a living tissue in continuous adaptation, and its morphology
depends on applied loads. This relation between loads and bone remodeling was
