Hyaline articular cartilage

BASIC SCIENCE

Hyaline articular cartilage

in 1743 when he described articular cartilage damage as a problem that will not heal.3 The cellular component of the hyaline cartilage represents 3–5% of the total cartilage mass. More than 95% of the cartilage volume consists of extracellular matrix (ECM), which is the functional element of this tissue.4

Ehab Kheir David Shaw

Cellular component Chondrocytes are sparse but essential in the production and maintenance of the ECM including collagen, glycosaminoglycans (GAGs) and proteoglycans.5 They are distributed either singularly or in clusters known as isogenous groups. The latter group represents newly divided cells. The cytoplasm of chondrocytes varies in appearance depending on their level of activity.6 The newly divided chondrocytes that are active in the production of matrix material display areas of cytoplasmic basophilia, where protein is synthesised, and clear areas, containing large Golgi apparatus. In older less active chondrocytes, the Golgi apparatus is reduced in size and clear areas of cytoplasm usually indicate extracted lipid droplets and glycogen stores. Each chondrocyte is surrounded by a thin, pericellular matrix that provides hydrodynamic protection for the chondrocyte; a complex known as the chondron.7 Each chondron is composed of a chondrocyte linked to a high pericellular concentration of proteoglycans, as well as hyaluronan, biglycans and matrix ­glycoproteins.8 Figure 1 illustrates hyaline cartilage cross sections, stained with haematoxylin and eosin. This figure is extracted from MSc research undertaken by the first author. The stained tissue sections demonstrated cartilage to have four zones:

Abstract Hyaline articular cartilage forms an important component of synovial joints, the integrity of which is crucial for normal function. Knowledge of its composition and dynamics is important for a better understanding of joint diseases pathophysiology and management. This article describes the basic characters of hyaline cartilage, its biochemical and biomechanical properties and reviews the current concepts of cartilage diseases.

Keywords articular cartilage; collagen; glycosaminglycans; ­ osteochondral defect; osteoarthritis

Introduction Hyaline articular cartilage is the most important structure of a synovial joint. Its main function is to protect the articular surface of bones from abrasion and to provide a smooth lubricated surface for joint movement distributing load evenly.1 It can withstand an astonishing amount of repetitive physical stress. The complex and highly specialized composition of normal hyaline articular cartilage makes it a formidable challenge to replace or repair once damaged or lost. The predominant repair tissue found in damaged areas is fibrocartilage, which is mechanically and biochemically inferior to hyaline cartilage.2 Thus, chondral lesions often result in progressive deterioration and eventually osteoarthritis.

Tangential layer Chondrocytes were rather small, flattened and parallel to the surface. Transitional zone The chondrocytes were slightly larger, round and arranged both singly and in isogenous groups. Radial zone Fairly large chondrocytes oriented perpendicular to the articulating surface.

Structure of hyaline cartilage Hyaline cartilage is a tough, semi-transparent, elastic, flexible tissue consisting of cartilage cells (chondrocytes and chondroblasts) scattered through a complex woven network of collagen fibres and proteoglycans distributed within interstitial water, the extra-cellular matrix. The articular surface of cartilage is covered by a dense fibrous membrane called the perichondrium.1 Cartilage is an avascular and aneural tissue, and when damaged, it does not heal readily. This concept was first described by Hunter

Calcified cartilage layer This is adjacent to the underlying cortex of the bone. The matrix of the calcified cartilage layer stained slightly darker (H&E) than the matrix of the other layers.

Ehab Kheir MRCS MRCS is a Registrar in Trauma and Orthopaedics at Bradford Royal Infirmary, UK.

Biochemical component of the hyaline cartilage matrix The ECM of hyaline cartilage is mostly water (60-80%). The remaining organic component predominantly is collagen molecules amounting to 15% of the ECM, of which type II is the most abundant. The rest of the ECM is formed of proteoglycans (aggrecan) (10%) and multiadhesive glycoproteins (5%)9 (Figure 2).

David Shaw MSc FRCS Ed FRCS (Orth) is a Consultant Orthopaedic Surgeon at Bradford Royal Infirmary and a Consultant in Trauma and Orthopaedics, Honorary Senior Lecturer Medical & Mechanical Engineering at Leeds University, UK.

Collagen molecules Collagen represents the major ECM protein. It constitutes the only fibrillar component of the cartilage. 80% of the hyaline articular cartilage is type II collagen.10 Each collagen type

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Hyaline cartilage stained with haematoxylin and eosin. a Original magnification ×40, b original magnification ×100, c the Arrow points to chondrocytes cluster, original magnification ×200, d the arrow indicates a single chondrocyte, original magnification ×400. Figure 1

Proteoglycans and glycosamoniglycans The ECM in hyaline cartilage contains three kinds of GAGs: hyaluronan, chondroitin sulphate, and keratan sulphate (Figure 4). The hyaluronan molecule is exceptionaly important in the structure of the ECM as it forms a linear aggregate and is interwoven with the network of collagen fibrils.7 A proteoglycan monomer consists of a core protein joined by GAGs. The most important proteoglycan monomer in the ECM is aggrecan, which is joined by chondroitin sulphate and keratan molecules.15 Proteoglycans are hydrophilic; this property results in the ability of hyaline cartilage to retain water which is essential to its proper function.

has a specific role in maintaining the structure of cartilage (Table 1). The arcade concept Alfred Benninghoff first described the arcade concept which explained the three-dimensional (3-D) organization of the collagen fibres within the ECM.13 The arcade concept proposed that the collagen fibrils anchored deeply in the calcified zone, run vertically towards the articular surface in the radial zone, turn obliquely in the transitional zone and become parallel to the articular surface in the tangential zone14 (Figure 3).

Figure 2 a Cartilage constituents b Structure of the solid component of the extracellular matrix in hyaline cartilage.

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e­ lectromagnetic fields can produce a sustained up-regulation of growth factors in articular cartilage.17

Function of collagen in hyaline cartilage Collagen type

Function

II

forms the network of fibrils within which the proteoglycans are contained found mainly at the periphery of the chondrocytes where it helps to attach these cells to the matrix framework10 facilitates fibril interaction with the matrix proteoglycan molecules11 organizes the collagen fibrils into a threedimensional hexagonal lattice12 regulation of fibrils size Function in articular cartilage unknown

VI

IX X XI III, XII & XIV

Biochemical properties of hyaline cartilage The solid phase of hyaline cartilage consists of proteoglycan aggregates entrapped within the collagen meshwork forming a strong porous-permeable composite. Both molecules bind to water by different ways.18 Within the solid phase proteoglycans contain repeating sulphate and carboxylate groups along their chains, which become negatively charged when placed in an aqueous solution. These negatively charged molecules repel each other exerting a large swelling pressure which is resisted by the tensile stress of the surrounding collagen network.18 The balance of the expanding swelling pressure exerted by the proteoglycans and the constraining tensile force within the collagen network determines the degree of hydration in the cartilage. Any disruption of this balance, by damage to either component of the solid phase, causes an increase in tissue hydration.19

Table 1

Function The principal role of articular cartilage is to efficiently transmit weight loads to the underlying bone. Keratin sulfate and chondroitin sulfate on the GAGs carry a negative charge. This negative charge creates a high affinity for water that helps cartilage resist compressive loads and causes the aggrecans to repel one another, resulting in maximal volume expansion. The flow of water through charged regions of the proteoglycan-rich matrix generates piezoelectric charges that further modulate the rate of water flow contributing to the viscoelastic behaviour of articular cartilage.16 In addition, there is evidence that electric and

Biomechanical behaviour of hyaline cartilage Hyaline cartilage consists of a fluid component within a solid porous-permeable matrix. This biphasic nature profoundly influences its mechanical behaviour when loaded.20 Loading results in the flow of water through the solid permeable matrix generating a frictional drag on the matrix.21 Hyaline cartilage is permeable and fluid may flow through it.22 Permeability is a measure of the ability of fluid to flow through

Figure 3 The three-dimensional organisation of collagen in articular cartilage.

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a Proteoglycan aggregate structure, b The proteoglycan monomer. Figure 4

hydrated porcine hyaline cartilage was subjected to compression (1 Newton) using a flat pin (3 mm in diameter). The test allowed generation of results in the format of time against voltage. Results were converted to time against deformation (mm) on application of the calibration factor. The y-axis represents displacement (mm) and x-axis represents time (minutes).

a porous-permeable material such as an ECM and is inversely proportional to the friction drag exerted by the fluid.22 Thus, as the cartilage is compressed under load, fluid extrudes from it and this reduces the pore size and permeability of the ECM. As the permeability of the ECM decreases the drag forces on fluid movement increases.22 The overall result of this is to provide cartilage with a selfprotective mechanical feedback mechanism which stiffens the cartilage by limiting rapid fluid flow in response to high and increasing load. This behaviour is exhibited as creep and stress relaxation.20,23 Creep occurs when cartilage undergoes constant loading. Typically cartilage responds to loading by an initial rapid deformation followed by a more slow deformation as time goes on. This timedependant deformation continues until equilibrium is reached. At the same time cartilage will respond with high initial stress which progressively diminishes with time. This time-dependant stress response is known as stress relaxation.20

Cartilage response to injury – the three stages theory The extent of articular cartilage damage is dependent on the severity of the injury. With increasing traumatic insult the 1 0.9

Deformation (mm)

0.8

Laboratory simulation of cartilage mechanical behaviour The mechanical behaviour of hyaline cartilage in response to load can be investigated in vitro by the indentation test. This compressive testing of hyaline cartilage was first described by Mow24 in 1989. Mow demonstrated that hyaline cartilage initially deforms rapidly and as compression is sustained the rate of deformation slows down gradually until it reaches a plateau phase. Figure 5 illustrates the results of the indentation test as part of a Master of Science undertaken by the first author. Fully

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0.7 0.6 0.5 0.4 0.3 0.2 0.1 0 0

10

20

30

40

50

60

70

Time (minutes)

Figure 5 Compressive (Indentation) testing of hyaline cartilage. Data are expressed as the mean (n = 9) ± 95% confidence limits. 453

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Water

Softening Water permeability Water content

2

3 Proteoglycans

Stiffness of subchondral bone load transmission to the partially damaged cartilage

1

Figure 6 Schematic illustrations of the three stages of cartilage damage following low energy trauma and development of osteoarthritis.

effect may range from fibrillation to development of an osteochondral defect.25 Nevertheless low energy trauma has been reported to injure the chondrocytes, compromise their metabolic capacity to repair, and lead to decreased proteoglycan concentration,26 increased hydration (Figure 6 [1]), and altered fibrillar organization of collagen.19 This has been reported to result in softening of the cartilage due to increased water content and permeability which will eventually lead to increased force transmission to the underlying subchondral bone, which will increase its stiffness (Figure 6 [2]) and, in turn, cause impact loads to be more readily transmitted to the partially damaged cartilage (Figure 6 [3]). This vicious cycle is thought to contribute to the progression of partial-thickness articular cartilage injuries to full-thickness defects and the development of osteoarthritis (OA).19 ◆

8 Poole CA, Flint MH, Beaumont BW. Chondrons in cartilage: ultrastructural analysis of the pericellular microenvironment in adult human articular cartilages. J Orthop Res 1987; 5(4): 509–522. 9 Poole AR, Pidoux I, Reiner A, Rosenberg L. An immunoelectron microscope study of the organization of proteoglycan monomer, link protein, and collagen in the matrix of articular cartilage. J Cell Biol 1982; 93(3): 921–937. 10 Eyre DR, Wu JJ, Woods PE. The cartilage collagens: structural and metabolic studies. J Rheumatol Suppl 1991; 27: 49–51. 11 Mendler M, Eich-Bender SG, Vaughan L, Winterhalter KH, Bruckner P. Cartilage contains mixed fibrils of collagen types II, IX, and XI. J Cell Biol 1989; 108(1): 191–197. 12 Schmid TM, Linsenmayer TF. Immunohistochemical localization of short chain cartilage collagen (type X) in avian tissues. J Cell Biol 1985; 100(2): 598–605. 13 Benninghoff A. Form und Bau der Gelenkknorpel in ihren Beziehungen zu Funktion. II Tell: der Aufbau des Gelenkknorpel in semen Beziehungen zu Funktion. Z Zellforsch 1925; 2: 783–862. 14 Huber M, Trattnig S, Lintner F. Anatomy, biochemistry, and physiology of articular cartilage. Invest Radiol 2000; 35(10): 573–580. 15 Hardingham TE, Muir H. Hyaluronic acid in cartilage and proteoglycan aggregation. Biochem J 1974; 139(3): 565–581. 16 Vidal Bde C, Vilarta R. Articular cartilage: collagen II-proteoglycans interactions. Availability of reactive groups. Variation in birefringence and differences as compared to collagen I. Acta Histochem 1988; 83(2): 189–205. 17 Aaron RK, Boyan BD, Ciombor DM, Schwartz Z, Simon BJ. Stimulation of growth factor synthesis by electric and electromagnetic fields. Clin Orthop Relat Res 2004(419): 30–37. 18 Jaffe FF, Mankin HJ, Weiss C, Zarins A. Water binding in the articular cartilage of rabbits. J Bone Joint Surg Am 1974; 56(5): 1031–1039. 19 Mankin HJ. The response of articular cartilage to mechanical injury. J Bone Joint Surg Am 1982; 64(3): 460–466. 20 Mow VC, Kuei SC, Lai WM, Armstrong CG. Biphasic creep and stress relaxation of articular cartilage in compression? Theory and experiments. J Biomech Eng 1980; 102(1): 73–84.

References 1 Buckwalter JA. Articular cartilage. Instr Course Lect 1983; 32: 349–370. 2 Furukawa T, Eyre DR, Koide S, Glimcher MJ. Biochemical studies on repair cartilage resurfacing experimental defects in the rabbit knee. J Bone Joint Surg Am 1980; 62(1): 79–89. 3 Hunter W. Of the structure and disease of articulating cartilages. 1743. Clin Orthop Relat Res 1995(317): 3–6. 4 Anderson CE, Ludowieg J, Harper HA, Engleman EP. The Composition of the organic component of human articular cartilage: relationship to age and degenerative joint disease. J Bone Joint Surg Am 1964; 46(6): 1176–1183. 5 Stockwell RA. The cell density of human articular and costal cartilage. J Anat 1967; 101(Pt 4): 753–763. 6 Stockwell RA. The interrelationship of cell density and cartilage thickness in mammalian articular cartilage. J Anat 1971; 109(Pt 3): 411–421. 7 Poole CA. Articular cartilage chondrons: form, function and failure. J Anat 1997; 191(Pt 1): 1–13.

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21 Mow VC, Holmes MH, Lai WM. Fluid transport and mechanical properties of articular cartilage: a review. J Biomech 1984; 17(5): 377–394. 22 Maroudas A, Bullough P, Swanson SA, Freeman MA. The permeability of articular cartilage. J Bone Joint Surg Br 1968; 50(1): 166–177. 23 Kempson GE, Freeman MA, Swanson SA. The determination of a creep modulus for articular cartilage from indentation tests of the human femoral head. J Biomech 1971; 4(4): 239–250. 24 Mow VC, Gibbs MC, Lai WM, Zhu WB, Athanasiou KA. Biphasic indentation of articular cartilage–II. A numerical algorithm and an experimental study. J Biomech 1989; 22(8–9): 853–861. 25 Alford JW, Cole BJ. Cartilage restoration, part 1: basic science, historical perspective, patient evaluation, and treatment options. Am J Sports Med 2005; 33(2): 295–306. 26 Lohmander LS, Dahlberg L, Ryd L, Heinegard D. Increased levels of proteoglycan fragments in knee joint fluid after injury. Arthritis Rheum 1989; 32(11): 1434–1442.

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Practice Points • Cartilage is an avascular and aneural tissue • When cartilage is damaged, it does not heal readily • The predominant repair tissue found in such defects is fibrocartilage, which is mechanically and chemically inferior to hyaline cartilage • Hyaline cartilage consists of a fluid component within a solid porous-permeable matrix; the biphasic nature of hyaline cartilage • The balance of the expanding swelling pressure exerted by the proteoglycans and the constraining tensile force within the collagen network determines the degree of hydration in the cartilage

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