Chapter 1

### Biomechanics & 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.

### Basic Concepts & Definitions

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.

###### Figure 1–1.

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