Page 277 - Biomedical Engineering and Design Handbook Volume 1, Fundamentals

254 BIOMECHANICS OF THE HUMAN BODY

anastomoses (joinings of the native and graft arteries). O’Brien et al. (2005) utilized CFD to evaluate
a new design for arterial grafts in the mid leg (near the knee) using a rigid wall, Newtonian pulsatile
model to evaluate wall shear and regions of recirculation.
Stents. In recent years, the insertion of stents has become popular to maintain patency in
stenosed coronary arteries after dilation. Stents are also used to bypass aneurysms in arteries with-
out having to resort to replacing the vessel segment with a graft. While the author will not enter
the debate as to their effectiveness, it should be noted that many CFD papers have been published
evaluating stents.
Strut design is critical to its proper function and has been the subject of many studies. The gen-
eral topic of stent design remains an area of study, as does the effect on local blood motion and wall
shear. Seo et al. (2005) evaluated stent wire size using a pulsatile, non-Newtonian fluid and rigid
walls. LaDisa Jr. et al. (2005) utilized a Newtonian pulsatile model to evaluate strut mesh geome-
tries. Papaioannou et al. (2007) utilized a rigid wall model with pulsatile Newtonian flow in their
evaluation of wall shear, and validated their model with a bench-top system. Frauenfelder et al.
(2007b) evaluated an abdominal aortic aneurysm model based on CT scans with Newtonian pulsatile
flow and deformable walls, validated the steady-flow model, and examined both the stent and the
surrounding walls. Chen et al. (2005) used biplane cine angiography to produce coronary models for
hemodynamic modeling of stents.

Skin. Due to its bidirectional mechanical properties and (obvious) importance, an increasing num-
ber of FEA studies are investigating skin biomechanics. Of particular interest is the issue of skin
deterioration of damage. These evaluations include both the skin surface and subdural tissue layers.
Some authors have investigated skin in general, while others evaluated specific regions.
Hendricks et al. (2003) used an isotropic nonlinear 2D model to evaluate the effect of suction on
skin. Kuroda and Akimoto (2005) used a 2D linear, elastic, isotropic model to evaluate the stresses
caused by various sizes of ulcerations to investigate the growth of ulcers.
Working from the top down, the following studies give a brief overview of the areas being inves-
tigated via FEA. To evaluate the effect of high and low frequency vibration, Wu et al. (2007) pro-
duced a model of a fingertip including the nail, bone, and skin. While the other structures were
assumed to be linear and elastic, the skin and subcutaneous tissue was modeled as a nonlinear
elastic and viscoelastic 2D material. Linder-Ganz et al. (2007) used MRI data to construct a FEA
model to evaluate stresses in subdermal tissue (just below the skin) while sitting, under weight-bearing
and nonweight-bearing conditions. Finally, to the foot, Erdemir et al. (2005) utilized a 2D model
taken during maximum pressure of stance to evaluate the effects of various shoe sole materials on
the reduction of local plantar pressures. This FEA model used a hyperelastic strain energy function
for the foot’s soft tissue.
Prosthetics. A subset of skin FEA evaluation is that of the anatomic stump remaining in limb
prostheses. When replacing a portion of a limb, especially the leg, great care must be taken so that
the local soft tissue does not deteriorate. Much of this work focuses on the local soft tissue stresses.
Portnoy et al. (2007) validated a FEA model by matching their studies with sensors in the sock nor-
mally placed between the limb and prothesis, along with devices monitoring indentation in to the
stump. Goh et al. (2005) produced a FEA model of a below-the-knee residual limb which included
bone and soft tissue, both modeled as linear, elastic, isotropic, homogeneous materials. They also
validated their model via sensors built into the prothesis. A third study in this brief review, by Jia
et al. (2005), investigated stresses during walking. This study utilized isotropic, linear material prop-
erties, and further simplified the model by assuming the knee motion to be hinge like. Actual knee
motion includes the revolving and sliding of the instantaneous center of rotation but, as noted before,
simplifications of this sort are very useful in early studies (and often are adequate for significant
advances in the field).

Dental. A final review area the author would like to present is that of dental FEA. The realm of
implants and tooth behavior is of particular interest considering the number and variety of implants
and procedures available today. The complexity of tooth interaction both with its neighbors and
underlying bones and ligaments is well suited for FEA. Many studies evaluate implanted teeth and