Biomedical Engineering Reference
In-Depth Information
82 CHAPTER 5. FUTUREDIRECTIONS
Friction tests are applied by sliding a probe across the cartilage surface and collecting data for the two
forces of interest. This can be done at the macroscale level of the joint [
657
] or the microscale [
77
]to
determine the frictional characteristics of the sample. An alternative approach is to measure friction
using a rheometer, which moves two surfaces rotationally. This device can also measure both the
simple and dynamic shear properties of cartilage. Simple shear testing can provide a measure of the
shear modulus,
G
:
τ
=
Gγ
where
τ
is the shear stress and
γ
is the shear strain. Oscillatory measurements are used to determine
the storage and loss moduli, as well as the complex shear modulus,
G*
, which is simply the sum of
the two.
5.2.2 DESIGN STANDARDS - FUNCTIONAL IMPROVEMENTVERSUS
REGENERATION
The determination of design standards for articular cartilage tissue engineering largely depends on
the type of restoration sought after. It appears obvious that long-term, fully functional restoration is
desirable, but whether this is to be achieved immediately or some amount of time post-implantation
will alter the design standards. Using non-terminally differentiated chondrocytes will also result in
additional considerations. Nonetheless, of prominence are (1) mechanical properties, (2) biochemical
properties, (3) integration, (4) construct size, (5) contour, and (6) ease of implantation.
The biomechanical properties of native tissue has long been well-characterized in different
anatomical locations, at various ages, diseased states, and under various hormonal and drug con-
centrations to provide ample direction for tissue engineered constructs to emulate. As it has been
identified that cartilages of different anatomical locations can have different properties [
50
-
52
],
should a cartilage product be tailored specifically for the talus and another for the knee, and at
what point does stiffness mismatch result in ill-borne stresses that can lead to articular damage?
If the engineered cartilage is softer than native tissue, can functional improvement nonetheless be
achieved, or will the construct break down like the mechanically inferior fibrocartilage? These are
all questions that should be considered.
There are other issues that complicate articular cartilage regeneration when using implanted
grafts or cells. Integration between the implanted construct and the surrounding tissue is of crucial
importance for mechanical function since the cartilage surface acts as one entity to distribute applied
loads. The interface between new and old tissues is often weak, especially on the surface, and failure is
probable unless sufficient healing takes place that helps to integrate the two tissues. Ideally, implanted
constructs should account for the tissue microstructure and create a replacement that has correctly
aligned collagen fibers and regional variations [
658
]. The alternative is complete remodeling of the
tissue
in vivo
, which may never occur in an environment that has only a limited repair capacity.
Various studies have been performed to examine the integration of cartilage to cartilage or tissue-
engineered constructs to cartilage, and the general consensus regarding the main factors that hinder
integration are the following:
