Intervertebral Disc

Fig. 1

Concentric lamellae of collagen surrounding the nucleus with fibres running at 65° to the horizontal

../images/451372_1_En_14_Chapter/451372_1_En_14_Fig2_HTML.jpg — Fig. 2

Structure of the disc and the vertebral end-plate

The nucleus pulposus (NP) of young adults is a semiliquid mass of mucoid material with the consistency of toothpaste. NP is a remnant of the notochord and cartilage cells with a few irregular collagen fibres disseminated in a semiliquid medium. The fluidic nature of NP allows it to deform under pressure but its volume cannot be compressed, similar to a balloon filled with water. Subject to multidirectional pressures, the NP deforms and stretches the collagen walls of the IVD. In fact, histological studies show that the nucleus is almost indistinguishable from the annulus in a young IVD (Fig. 3). Subsequently, after bone maturation in the spine, the articular cleft is an acquired fissure.

../images/451372_1_En_14_Chapter/451372_1_En_14_Fig3_HTML.jpg — Fig. 3

Microtome section of a lumbar disc of a young subject; it is difficult to macroscopically distinguish nucleus and annulus

AF is composed of collagen fibres arranged in 10–20 lamellae (Latin lamella: small leaf) [2], organized in concentric rings surrounding the NP. The lamellae are thicker on the ventral and lateral slopes of the annulus and thinner on the dorsal side. The orientation of the fibres is constant—at 65° to the vertical with an alternating direction for each lamella [3–6].

The end-plates (EP) are formed of a layer of cartilage 0.6–1 mm thick. The two EPs of each disc cover each vertebra, except at the periphery of the disc at the level of the marginal or peripheral bone ridge in the adult. Hyaline cartilage is present in newborns and children. In adults, the EPs are fibrocartilaginous and formed by the insertion of AF collagen fibres into bone.

The collagen fibres of the lamellae penetrate the EP and turn parallel to the EP. The NP is thus surrounded by a spherical “capsule” of collagen; EPs are attached to the disc via the AF [4]. EPs are poorly attached to the vertebral bodies [5, 6] and can become detached in certain traumas in the growing child (scallop type detachment and rupture of the EP limbus) and in growth dystrophies at the marginal non-ossified apophyseal ring (see chapter “The Cranial and Pelvic “Vertebrae” Are They Real Vertebrae?”).

IVD Ultrastructure (Fig. 4)

The IVD is composed of glycosaminoglycans, collagen, and water.

../images/451372_1_En_14_Chapter/451372_1_En_14_Fig4_HTML.png — Fig. 4

Schematic view of the structure of the interstitial space of the disc

Glycosaminoglycans (GAG) [7–13]

They are present in the skin, bone , cartilage, tendons, synovial fluid, the aqueous humour of the eye, and especially at the level of NP in the IVD. They form chains of polysaccharides composed of 20 repeating units (disaccharides hexoses) of sugar–amine–sugar.

In the disc, the dominant GAG are chondroitin sulphate (CS) , keratan sulphate (KS), and hyaluronic acid .

Proteoglycans (PG) are 20–100 chains composed of GAG bound to hyaluronic acid by binding proteins. The hydrophilic capacity of PGs depends on the size and spiral shape to form 3D molecules akin to the entanglement of cotton with similar absorbent properties as a cotton ball. The hydrophilic strength of the PGs, however, depends on the sulphate and carboxylate radicals. CS has both sulphate and carboxylate radicals while KS has only sulphate. These GAG have fixed negative charges [9] that attract positive sodium and calcium ions. The sodium concentration in the disc is higher than in the plasma and adjacent tissues, the excess of ions is responsible for the significant osmotic pressure in the IVD matrix and its ability to withstand high loads. The hydrophilic capacity of a PG is proportional to the structural density of the ionic radicals.

The swelling pressure of the disc is equal to the osmotic pressure of the PGs minus the tensile force in the collagen fibres which is , in general, very low.

Collagen [14–16]

The basic unit of collagen is the tropocollagen molecule (15 Angstroms) consisting of three polypeptide chains helically wound and linked by hydrogen bridges. The joining of several chains of tropocollagen results in a large fibril (0.04 μ). The collagen fibre (0.1–0.5 μ) is formed by the association of several large tropocollagen fibrils. There are 11 types of collagen determined by the chemical nature of the polypeptide chains:

Type 1 of elastic nature is found in tissues subject to tension and compression (skin, bone, tendon, meniscus) and type 2 more elastic is found in tissues exposed to pressure (articular cartilage). These two types are present in the IVD. Type 1 is the predominant form in AF and Type 2 is dominant in NP.
Type 3 (dermal vessels, synovium) is in the NP and in trace elements in the AF.
Type 9 (cartilage) is found with type 2 (2% collagen type 2 concentration) in NP.
Type 6 (vessels, viscera, muscles) and type 10 (growth cartilage) are present in the IVD in very small amounts.

The chemical structure of the vertebral end-plate corresponds to that of the disc with PGs and collagen fibres with cartilage cells aligned along the collagen fibres.

Water and Chemical Composition of Human Discs [8]

The water and the chemical composition of the human disc represent 70–90% of the NP. PG represents 65% of the dry weight of the NP with only 25% of the PGs linked to the hyaluronic acid. The PGs of the NP are smaller than those of articular cartilage with 8–18 units of PG on a small chain of hyaluronic acid.

Water forms a gel with PGs. The concentration of PG is four times greater in the central part of the disc than in its periphery with twice the amount of hydration.

Collagen represents 15–20% of the NP, disseminates in the PG medium, and serves as a support for the PGs. Water represents 60–70% of the AF and collagen 50–60% of the dry weight of AF, a quantity twice as large at the peripheral level (Table 1) (at the end of the text). Between the collagen fibres, the spaces are filled by PGs forming a fine network between which very fine pores of 30–100 angstroms persist. The disc loses fluid under pressure only very slowly.

Table 1

Chemical composition of the various components of the intervertebral disc (Maroudas [8])

Parameter	External annulus	Internal annulus	Nucleus
Hydration (mg H₂O) mg of dry weight	1–2	2–3	3–4
Concentration of GAG (CS & KS) mg of dry weight	5–7	10–15	22–30
Collagen (% of dry weight)	50	25–30	15–25
Ratio KS/CS	0.2–0.3	1.0	1–2

The chondrocytes are located in the NP near the vertebral end-plates and provide PG synthesis and collagen type 2 for the NP.

Proteolytic Enzymes [17]

Proteolytic enzymes are the matrix metalloproteinases (MMP) with collagenase (MMP 1), gelatinase (MMP 2), and stromelysin (MMP 3).

Collagenase can cleave collagen type 2, and stromelysin is the most destructive for collagen type 2 and PG.

Under normal conditions, enzymes allow the addition of new components.

By removing the worn components of the disc matrix, the chondrocytes synthesize collagen and PGs forming the matrix and retain water. Enzymes are controlled by activators such as plasmin and inhibitors such as tissue inhibitors of MMPs. The state of the matrix is based on a delicate balance between synthesis and destruction activities.

The swelling pressure of the disc decreases with the level of PG. Water content decreases with age and degeneration. The hydration of the aged discs decreases as the discs are subjected to physiological pressures of 6–10 atmospheres in an active subject. Discs lose water when exposed to higher loads. The liquid flow is made more rapidly when the pores are larger and as the swelling pressure is lower. An aged or degenerate disc dehydrates faster during a day of work than that a young disc.

The cells in the centre of the disc are remote from blood vessels, thus do not receive significant glucose or other nutrients. The lactic acid concentration is ten times higher than plasma. The pH, decreasing to 6, can promote the action of proteolytic enzymes with progressive degradation of the disc matrix and in particular the PGs responsible for the hydrophilic power.

The equilibrium hydration under a load of 78 atmospheres (0.7–0.8 NM/m²) is 1.5–2.2 g of water/g of dry weight for an aged or degenerate disc as opposed to 3 g of water/g of dry weight for a young disc (Maroudas [8], Urban [9, 10]). A less hydrated and thinned disc is less able to perform its mechanical functions. The functional deterioration of IVD leads to an overload of other structures such as the facet joints whose degeneration follows that of IVD.

Vascularization and Innervation of the Disc

Classically, the IVD is non-vascularized and non-innervated. In fact, the periphery of the disc is a real “living area”.

During growth, the epiphyseal vessels at the marginal apophysis provide some peripheral vascularization of the disc.

In degenerate discs, neoangiogenesis allows penetration of the disc with an inflammatory membrane and disc cavitation clearly visible on endoscopy. According to Bogduk [5], the sinuvertebral nerve resulting from an anastomosis of a branch of the anterior branch of the spinal nerve and grey communicating branch of the paravertebral sympathetic lymph node chain gives a direct branch or Roofe nerve which is distributed to the dura mater, dorsal longitudinal ligament, and commonly at the annulus.

The outer 1/3 of the periphery of the disc thus has a radicular and sympathetic segmental innervation. Disc cracking is the trigger for neovascularization and neoinnervation of the disc. It is the characteristic of intradiscal rupture, which is an acquired traumatic condition rather than linked to degeneration.

The nerves seem to accompany the vessels that grow in the cracks.

These discs are symptomatic and painful enough on discography–manometry which gave rise to coblation with thermomodulation radiofrequency of the fissure zone.

Role of the Disc

The IVD is hydrophilic and a hydraulic damper.

Under pressure during the day, the water leaves the disc for the vertebra through the vertebral end-plate and the subject can lose up to 2 cm in height during the day (the height of the discs represents a quarter of the spine height).

With age, the hydrophilicity decreases; after 65 years, the disc contains only 65% of water versus 90% in children. The size of the subject is reduced by up to 5 cm.

Osteoporosis bone compression is associated with disc compression.

The NP and AF are involved in the load with transmission of load from one vertebra to another.

The mechanism of load transfers in the disc is as follows:

The compression increases the pressure in the NP which is exerted radially on the AF and increases the tension of the AF. The disc is a model of tensegrity (tensile integrity) where there is a balance of compressive and tensile stresses (Fig. 5).
The tension in the AF is applied to the NP, preventing it from widening radially, hence the importance of the AF tension forces. The NP pressure is then exerted on the vertebral end-plate.

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