& Biomaterials: Introduction
Orthopedic surgery is the branch of medicine concerned with restoring
and preserving the normal function of the musculoskeletal system.
As such, it focuses on bones, joints, tendons, ligaments, muscles,
and specialized tissues such as the intervertebral disk. Over the
last half century, surgeons and investigators in the field of orthopedics
have increasingly recognized the importance that engineering principles
play both in understanding the normal behavior of musculoskeletal
tissues and in designing implant systems to model the function of
these tissues. The goals of the first portion of this chapter are
to describe the biologic organization of the musculoskeletal tissues, examine
the mechanical properties of the tissues in light of their biologic
composition, and explore the material and design concepts required
to fabricate implant systems with mechanical and biologic properties
that will provide adequate function and longevity. The subject of
the second portion of the chapter is gait analysis.
Most biologic tissues are either porous
materials or composite materials. A
material such as bone has mechanical properties that are influenced
markedly by the degree of porosity, defined as the degree of the
material’s volume that consists of void. For instance,
the compressive strength of osteoporotic bone, which has increased
porosity, is markedly decreased in comparison with the compressive
strength of normal bone. Like composite materials, alloys consist of two or more different
metallic elements that are in solution. Although composite materials
can be physically or mechanically separated, alloyed materials cannot.
Generally, composites are made up of a matrix material, which
absorbs energy and protects fibers from brittle failure, and a fiber,
which strengthens and stiffens the matrix. The performance of the two
materials together is superior to that of either material alone
in terms of mechanical properties (eg, strength and elastic modulus)
and other properties (eg, corrosion resistance). The mechanical properties
of various types of composite materials differ, based on the percentage
of each substance in the material and on the principal orientation
of the fiber. The substances in combination, however, are always
stronger for their weight than is either substance alone. Microscopically, bone
is a composite material consisting of hydroxyapatite crystals and
an organic matrix that contains collagen (the fibers).
The mechanical characteristics of a material are commonly described
in terms of stress and strain. Stress is
the force that a material is subjected to per unit of original area,
and strain is the amount of deformation
the material experiences per unit of original length in response
to stress. These characteristics can be adequately described by
a stress–strain curve (Figure 1–1), which plots the effect
of a uniaxial stress on a simple test specimen made from a given
material. Changes in the geometric dimensions of the material (eg,
changes in the material’s area or length) have no effect
on the stress–strain curve for that material.
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