Structural Biological Materials

Design and Structure-Property Relationships

M. Elices

• Publisher: Pergamon
• Year: 2000
• Print ISBN: 9780080434162
• eBook ISBN: 9780080541907
• Topic: Technology & Engineering
• Subtopic: Materials Science
• Index: Technology & Engineering

The ongoing process of bio-evolution has produced materials which are perfectly adapted to fulfil a specific functional role. The natural world provides us with a multitude of examples of materials with durability, strength, mechanisms of programmed self-assembly and biodegradability. The materials industry has sought to observe and appreciate the relationship between structure, properties and function of these biological materials. A multidisciplinary approach, building on recent advances at the forefront of physics, chemistry and molecular biology, has been successful in producing many synthetic structures with interesting and useful properties. Structural Biological Materials: Design and Structure-Property Relationships represents an invaluable reference in the field of biological materials science and provides an incisive view into this rapidly developing and increasingly important topic within materials science.This book focuses on the study of three sub-groups of structural biological materials:• Hard tissue engineering, focussing on cortical bone• Soft tissue engineering• Fibrous materials, particularly engineering with silk fibers.The fundamental relationship between structure and properties, and certain aspects of design and engineering, are explored in each of the sub-groups. The importance of these materials, both in their intrinsic properties and specific functions, are illustrated with relevant examples. These depict the successful integration of material properties, architecture and shape, providing a wide range of optimised designs, tailored to specific functions.Edited by Manuel Elices of the Universidad Politécnica de Madrid, Spain, this book is Volume 4 in the Pergamon Material Series.

Soft Tissue Engineering

Chapter 4

Structure – Properties of Soft Tissues Articular Cartilage

Dan Bader; David Lee

4.1 INTRODUCTION

Soft tissues are biological composite structures. In all cases they contain, in various amounts, the ubiquitous biological macromolecule, collagen, in fibrous form. The building block of collagen fibres is the tropocollagen molecule, 300 nm long and 1.5 nm wide, with a molecular weight of approximately 300 kDa. Its molecular form is ideally designed to support tensile loads, to which the structural composites are subjected. However, its precise role in the biological composite largely depends upon the nature of loading, which is present when performing the various functions of the individual soft tissues. Thus tendons and ligaments contain collagen as the main structural component arranged in the form of fibres approximately parallel to the long axis of the tissues. Thus their mechanical properties are largely determined by those of the collagen fibres. This design is ideal to support the tensile forces during normal physiological activities. Soft tissues such as spinal ligaments and skin are subjected to mixed loading modes, involving a combination of tension, torsion, shear and compression. In these tissues collagen is associated with other structural proteins such as elastin, which confer distinct mechanical properties such as an ability to permit large extensions even at relatively low applied tensile stresses. Other soft tissues, such as intervertebral disc and articular cartilage, contain collagen fibres closely associated with macromolecular gels. Their interaction provides mechanical integrity for theses tissues to support compression loading during the normal loading at synovial joints. This chapter will concentrate on the unique tissue, articular cartilage, which has traditionally attracted much research interest in the field of biomechanics and biomaterials.

4.1.1 General properties of articular cartilage

There are three types of cartilaginous soft tissues found in the body, namely elastic cartilage, fibrocartilage and hyaline cartilage. Articular cartilage, which is located in synovial joints, is a type of hyaline cartilage and is the focus of the present chapter. It is the soft tissue, which covers the articulating ends of the bones, which terminate at a synovial joint.

In this position cartilage is subjected to the forces which pass through the joint. Thus, the main functions of articular cartilage are to reduce the contact stresses to safe values, thereby protecting the subchondral bone from damage and to provide the joint with low-friction and low-wear bearing surfaces. Indeed, articular cartilage in conjunction with its lubricant, synovial fluid, produce a coefficient of friction in normal healthy joints, which is lower than can be achieved with any man-made engineering system.

The resultant forces, which are transmitted through the major load-bearing joints of the lower limb, are largely compressive and the cartilages, with their inherently low frictional properties, are subjected mainly to compression perpendicular to the articular surface. The magnitude of the resultant forces regularly attain five times body weight, equivalent to approximately 3500 N, during the period just after the heel-strike of the gait cycle. In some cases, involving vigorous sporting activities, joint forces can exceed 10,000 N. Taking into account the surface area of the supporting cartilage in major load bearing joints, contact stresses of the order of 5 MPa are common in normal walking.

The thickness of cartilage varies from joint to joint and with location on a joint surface. Cartilage thickness in major load bearing joints is generally between 1.5 and 3.5 mm, although cartilage located on the lateral facet of the patella can be 5 mm thick. The surface of articular cartilage demonstrates directional properties, when examined by a pinpricking technique to induce directional splits. The resulting split direction is pronounced and generally site specific.

Normal adult human articular cartilage does not contain any blood vessels and the mature subchondral bone/cartilage junction is generally believed to be impermeable to nutrients. Thus pathways must exist to supply its cells with the vital nutrients to maintain viability and remove waste products. In addition, articular cartilage is devoid of nerve endings. Therefore any perceived pain within the joint is probably a direct result of abnormal bone contact or other structural damage from within the joints.

4.2 STRUCTURE AND COMPOSITION

Articular cartilage may be considered to be a fibre reinforced polymer gel containing cells, known as chondrocytes. The approximate proportions of the major constituents are given in Table 4.1. The properties of these extracellular components and their interactions determine the physical and mechanical properties of the tissue.

Table 4.1

Relative proportion of non-cellular components in adult human articular cartilage.

	wet weight
Collagen	15-20%
Proteoglycan	3-15%
Water	65-80%
Non-collagenous proteins and glycoproteins^*	1%

^* e.g. cartilage oligomeric matrix protein (COMP), fibronectin, anchorin.

4.2.1 Chondrocytes

The chondrocytes are responsible for the synthesis and maintenance of the extracellular constituents of cartilage. Chondrocytes are reported to vary considerably in size with values between 7 to 30 μm in diameter and are contained within spaces called lacunae. The ratio of cell volume to tissue volume is lower than most tissues, accounting for between 1% and 10% of the tissue volume, with a reported mean chondrocyte density of 14,000 cells.mm^–3 (Maroudas et al. 1975). The chondrocytes are most numerous near the articular surface.

The characteristics of chondrocytes change with depth from the articular surface. In normal adult articular cartilage, four cellular zones can be identified under the light microscope (Figure 4.1) namely:

Fig. 4.1 Structural variation through the thickness of articular cartilage showing zonal arrangement of chondrocytes and collagen fibres.

- a superficial zone beneath the articular surface in which the cells are discoid and oriented parallel to the surface.

- a transition zone in which the cells are more ellipsoidal in morphology with their long axes at a range of orientations to the articular surface.

- a deep zone, which contains ellipsoidal or spherical cells in groups of four to eight arranged in columns perpendicular to the articular surface.

- a calcified zone beneath the uneven basophilic line, known as the tidemark, marking the transition to calcified cartilage, which contain hypertrophic chondrocytes

It is the cells in the transition and deep zone, with relatively large cytoplasmic volumes containing well-developed endoplasmic reticulum and golgi complexes, which produce the main synthetic activity in cartilage.

The chondrocytes in adult articular cartilage are supplied by nutrients in the synovial fluid, which are transported across the cartilage surfaces. Clearly the permeability of cartilage will affect cell nutrition via fluid transport and thus structural changes which occur with age and disease will affect these pathways.

The extracellular matrix (ECM) surrounding each chondrocyte can also be divided into three regions, known as the pericellular, territorial and interterritorial matrix. These regions differ in both their proximity to the chondrocytes and their structure and organisation. The interterritorial region, comprising the majority of the tissue volume and is the main contributor to the mechanical properties of the matrix.

The behaviour of chondrocytes in a mechanical environment will form the basis of section 4.4 of this chapter.

4.2.2 Collagen

A large proportion of the non-aqueous ECM of cartilage is composed of a network of collagen fibrils (see also Chapter 8). The building blocks of collagen, the tropocollagen molecule, consists of three polypeptide chains, designated α chains, coiled together in a right-handed helical structure. Each α chain contains approximately 1000 amino acid residues. With the exception of the short non-helical sequences at the end of each chain, one third of the amino acid residues are composed of the small molecule glycine. Of the remai...

Table of contents

• Cover image
• Title page
• Table of Contents
• Copyright page
• Series Preface
• Acknowledgements
• List of contributors
• Introduction
• General Concepts
• Hard Tissue Engineering
• Soft Tissue Engineering
• Engineering with Fibers
• Glossary
• Subject index