Properties of natural materials - Orthopaedic Biomaterials in Research and Practice - page 102

In vitro versus in vivo properties

There is a general problem involved in understanding the mechani-
cal behavior of tissues. By and large, mechanical properties have been
determined in the laboratory, on “dead” tissue. A frequent observation is
that the stress-strain relationship observed on an initial test is not repro-
ducible on subsequent tests that are immediately repeated. However, the
subsequent tests tend to closely reproduce each other, so long as physical
conditions are not changed and tissue autolysis has not begun. This phe-
nomenon, of initial mechanical irreproducibility, is called the “precondi-
tioning effect,” and it is the practice, rather than the exception, in studies
of tissue mechanics to “cycle” or exercise the tissue specimens until
reproducibility is assured. This misdemeanor is compounded, again in
the vast majority of cases, by failure to report the initial results.
The origins of the preconditioning effect are unclear but they are
related to the differences between the stable in vivo situation and the
artificial and often poorly controlled in vitro test conditions. Release of
stresses, owing to failure to maintain original resting length of the tissue
as a starting point or to removal of tissue from a supporting structure,
is a major contributing factor, as is the inability to restore mechanically
expressed fluid owing to the loss of microcirculation. Furthermore, there
are continuing property changes in the immediate postexcision period,
up to 5-6 h after specimen preparation at room temperature, paralleling
the macroscopically observable events of rigor mortis. These occur in
all tissues, but are more prominent in soft tissues. Similarly, the storage
conditions of the tissue, for example, if they are stored in formalin or
some other solution, can also affect the properties.
The property values cited in this chapter are in vitro post precondi-
tioning values. In general, overlooking the obvious fact that tissues are
viscoelastic rather than elastic-plastic, in vitro (post preconditioning)
behavior is more elastic and brittle than that in vivo .*

Tissue constituents

Poverty of
structural
elements

The atomic composition of the human body is surprisingly simple.
Essentially all of the vast intricacy of the structure is composed of com-
binations of four elements: carbon, oxygen, hydrogen, and nitrogen. A
number of additional elements contribute to creation and preservation
of structure: calcium, sulfur, and phosphorus. Once past these seven ele-
ments, which together constitute more than 99% of body weight, those
remaining are present in amounts treated in general engineering as
“trace elements” or “impurities”: perhaps possessing chemical activity

* For a more complete discussion, the reader is referred to Black J: Tissue properties:
Relationship of in vitro studies to in vitro properties. pp. 5-26. In Hastings GW,
Ducheyne P (eds): Natural and Living Biomaterials . CRC Press, Boca Raton, FL,
1984. The reader is also referred to Part 1 of Black J, Hastings, G (eds): Handbook of
Biomaterial Properties , Chapman & Hall, London, 1998.