Biomedical Engineering Reference
In-Depth Information
CHaPTer ONe
Forces and equilibrium
The orthopaedic surgeon uses materials to multiple and various ends
in treating disabilities of the musculoskeletal system. There are many
requirements that these materials must meet if they are to form either a
temporary or a lasting union with the part of the body being treated, and
the recipe for each material is often unique and tailored to the particular
application. Considerations for the ideal material for orthopaedic appli-
cations include its material strength and stiffness, biologic response,
toxicity, and conductivity, just to name a few. All of these factors derive
from the structure and composition of materials, and their study consti-
tutes the materials science called
biomaterials.
Frequently, the first questions asked about a material are in relation
to its strength. Mechanical behavior holds a position of primacy among
these factors: the materials being used must have sufficient integrity to
sustain the forces placed upon them. Before it can be discussed whether
a material is strong enough, it is important to know exactly for what
it needs to be strong enough. In other words, what is the mechanical
environment to which the material will be subjected? Discussion of
mechanical behavior requires a language suitable first for dealing with
the position and motion of objects acted on by external forces and then
for examining the internal effects of these forces.
The external description of the position and motion of objects has
been a traditional problem for philosophers, physicists, and engineers. In
the modern era, we speak of the field of
statics
as dealing with the rela-
tionships between objects that are in
equilibrium—
that
is, objects that
are seen to be either at rest or moving with a constant velocity—whereas
the field of
dynamics
deals with objects undergoing a change in velocity,
called
acceleration.
Units
Both fields, which are analytic and quantitative, require the use of units
of measurement to permit exact comparison of physical quantities. We
can describe the weight of a patient by deciding which unit of weight
