NANOINDENTATION OF SOFT TISSUES AND OTHER BIOLOGICAL MATERIALS - Nanoindentation with Biological Applications - page 277

Articular cartilage is a highly studied tissue for clinical reasons.
Articular cartilage is hyaline cartilage that covers the ends of long bones
and sesamoid bones within synovial joints. 10 The function of the cartilage
is to cushion the ends of the bones that articulate at the joint, provide
a low friction sliding interface, and provide lubrication to the joint.
Articular cartilage is of clinical importance because it is often the site
of osteoarthritis and other damage. If the cartilage on the ends of
the long bones deteriorates, joint replacement surgery is required. Hence,
there has been much research into understanding structure-property
relationships in healthy, diseased, and repair cartilage in an effort to
develop better diagnostic tools and treatments for cartilage degeneration.
To understand why cartilage is particularly suited for nanoindentation
testing, an understanding of cartilage structure and traditional mechanical
testing is necessary. The following sections will briefly describe cartilage
structure and the mechanical testing techniques commonly used to
characterize cartilage. Then several cartilage nanoindentation studies will
be reviewed, concluding with a discussion of the implications of the
results of cartilage studies for the future development of soft tissue
indentation techniques.
2.1 . Cartilage structure
The structure of articular cartilage makes it challenging to test using
traditional macroscale mechanical testing techniques, such as bulk
tension or compression tests. Articular cartilage is found as a thin layer
on the ends of long bones, with thicknesses ranging from micrometers to
millimeters depending on the joint and animal being studied. The thin
cartilage layer is made up of chondrocytes (cartilage cells) in a highly
organized matrix of collagen II fibers (to resist tensile and shear loads)
and hydrated proteoglycans (to resist compressive loads). 10,11
Cartilage tissue has a complex microstructure and composition that
varies with depth. The cartilage thickness is typically divided into three
zones: the superficial zone (10-20% of the cartilage thickness), the
middle zone (40-60% of the cartilage thickness), and the deep zone (30%
of the cartilage thickness). 10,11 The collagen orientation varies with depth