The Biomechanics of Cartilage-An Overview

Abstract

Articular cartilage (AC) sheathes joint surfaces and minimizes friction in diarthrosis. The resident cell population, chondrocytes, are surrounded by an extracellular matrix and a multitude of proteins, which bestow their unique characteristics. AC is characterized by a zonal composition (superficial (tangential) zone, middle (transitional) zone, deep zone, calcified zone) with different mechanical properties. An overview is given about different testing (load tests) methods as well as different modeling approaches. The widely accepted biomechanical test methods, e.g., the indentation analysis, are summarized and discussed. A description of the biphasic theory is also shown. This is required to understand how interstitial water contributes toward the viscoelastic behavior of AC. Furthermore, a short introduction to a more complex model is given.

Keywords: Articular cartilage; biomechanics; osteoarthritis; tissue engineering; viscoelasticity.

Figure 1

Schematic, cross-sectional diagram of healthy articular cartilage: cellular organization in the zones of articular cartilage (adapted from [24,25]).

Figure 2

A schematic representation of the collagen network interacting with the proteoglycan network in AC. They have complex macromolecules, also known as aggrecans an elongated protein core to which long glycosaminoglycan chains attach. These carry numerous negative charges and are via chondroitin-4-sulphate, chondroitin-6-sulphate, and keratan sulphate covalently with the aggrecan chain connected. The interstices of this porous solid matrix are filled with water and dissolved ions (adapted to [5,32,33]).

Figure 3

A constant stress σ₀ applied to a sample of AC (top left); creep response ε of the sample under the constant applied stress σ (top right). The boxes below the loading and creep curves (A to F) illustrate that creep is accompanied by fluid exudation from the tissue. At equilibrium (t → ∞), fluid flow ceases, and the load is borne entirely by the solid matrix (F). (taken and modified from [35]).

Figure 4

Drawing of an apparatus used to perform a simple compression test of AC. (taken and modified from [15]).

Figure 5

Representation of an apparatus used to perform an indentation test on articular cartilage (taken and modified from [15]).

Figure 6

Schematic representation of a device used to measure the permeability of cartilage. A slice of cartilage is supported on a porous plate in a fluid-filled chamber. High pressure applied to one side of the cartilage drives fluid flow. The average fluid velocity through the cartilage is proportional to the pressure gradient, and the constant of proportionality is called the permeability. (taken and modified from [15]).

Figure 7

Theoretical tensile loading curves at infinitely high and low strain rates (adapted from [37]), (σ → stress, ε → strain). From A to B the strain is constant (ε₀) and this means a stress relaxation (σ_A). From A to C the stress is constant (σ₀) and this means a strain retardation and creep (ε_A), respectively. An idealized behavior is shown.

Figure 8

Stress relaxation related to the behavior shown in Figure 7. (σ → stress, ε → strain; the strain is constant; t ≥ 0).

Figure 9

Creep related to the behavior shown in Figure 7. (σ → stress, ε → strain; the stress is constant; t ≥ 0).

Figure 10

Simple model consisting of a mixture of spring and damper for describing viscoelastic behavior (= Kelvin–Voigt model). (σ → stress, ε → strain, E → is a modulus of elasticity; η → is the viscosity).