Properties of natural materials - Orthopaedic Biomaterials in Research and Practice - page 110

0.6

G u 6

Enzymatically
degraded

5

0.5

0.4

0.3

0.2

Normal

G R

0.1 1

20 40 60 80 100 20 40 60 80 100

Hexosamine (µg/mg dry wt) Hexosamine (µg/mg dry wt)

FIGUre 5.6 dependence of mechanical properties of articular carti-
lage on GaG content (as measured by hexosamine content). (adapted
from Parsons, J.r., The viscoelastic properties of rabbit articular carti-
lage . Phd thesis, university of Pennsylvania, Philadelphia, Pa, 1977.)

proteoglycans, fibrillation of the cartilage surface, wearing away of the
collagen fibrils, and eventual exposure of the underlying bone.
Extensive studies of articular cartilage in other modes of deformation
have been made, and there is a large resulting literature. It seems clear that
the properties of articular cartilage are matrix related and are not depen-
dent on the small number of indwelling cells (chondrocytes), despite their
relatively large size. Perhaps the most important finding from these studies
is the observation that articular cartilage is mildly subject to fatigue, with
failure lines propagating parallel to the superficial collagen fiber direction.
It is unknown whether this occurs in vivo and, if it does occur, whether it
represents a source of cumulative damage or it is repaired by remodeling.

Other soft tissues There have been extensive studies of mechanical behavior of other soft

tissues, including skin, fascia, peripheral nerve, and so on, and a wide
literature exists. However, in the vast majority of cases, these tissues
possess sufficiently low effective moduli at the strain rates that they
experience and sufficient ductility that they simply follow movements
of the major elements of the musculoskeletal system without playing a
major role in determining either kinetics or mechanics.

Bone

Material aspects Bone is the term used to describe both the primary mineralized tissue

in mammalian bodies and the structures that are composed of it. As a
material, it consists of a framework of collagen fibers, essentially all
type I; a mineral matrix, primarily calcium hydroxyapatite; and a small
quantity of mucopolysaccharides and protein polysaccharides, collec-
tively referred to as ground substance or cement.