Intervertebral disc

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Definition/Description

The intervertebral disc (IVD) is important in the normal functioning of the spine. It is a cushion of fibrocartilage and the principal joint between two vertebrae in the spinal column (Figure 1).

Clinically Relevant Anatomy

Morphology

The IVD consists of three distinct components (Figure 2):
• a central nucleus pulposus (NP);
• a peripheral annulus fibrosus (AF);
• two vertebral endplates (VEPs).

Figure 2: Detailed structure of the IVD (adapted from Bogduk 2005)

Nucleus Pulposus

The nucleus pulposus is a gel-like mass composed of water and proteoglycans held by randomly arranged fibres of collagen [1-3]. With it’s water-attracting properties, any attempt to deform the nucleus causes the applied pressure to be dispersed into various directions, similar to a person on a waterbed.

Annulus Fibrosus

The annulus fibrosus consists of “lamellae” or concentric layers of collagen fibres [4]. The fibre orientation of each layer of lamellae alternate and therefore allow effective resistance of multidirectional movements.

Vertebral endplate

The vertebral endplate is a plate of cartilage that acts as a barrier between the disc and the vertebral body. They cover the superior and inferior aspects of the annulus fibrosus and the nucleus pulposus.

Innervation

The disc is innervated in the outer few millimetres of the annulus fibrosus [5].

Vascular supply and nutrition

The IVD is largely avascular, with no major arterial branches to the disc [1]. The outer annular layers are supplied by small branches from metaphysial arteries [1]. Due to the avascular nature of the disc, the nutrition is dependent on metabolite diffusion [6-8].

Biomechanics

Weight bearing

The disc is subjected to various loads, including compressive, tensile and shear stresses [9, 10]. During compressive loading, hydrostatic pressure develops within the NP, which thereby disperses the forces towards the endplates as well as the AF [1, 11, 12]. This mechanism slows the rate applied loads are transmitted to the adjacent vertebra, giving the disc its shock absorbing abilities [1].

Movements

The disc is also involved in permitting movements between vertebral bodies, which include:
• axial compression / distraction;
• flexion / extension;
• axial rotation;
• lateral flexion.

Nuclear migration

Asymmetric compressive loading disc can cause the NP to migrate in a direction opposite to the compression [13-16]. For example, during forward bending (or flexion) of the lumbar spine, the NP migrates posteriorly or backwards (Figure 4). Conversely, during backwards bending (or extension), the nucleus is squeezed anteriorly or forwards. This concept is known as the dynamic disc model [17]. Although NP migration has been shown to behave predictably in asymptomatic discs, a variable pattern of migration occurs in people with symptomatic and/or degenerative IVDs [17].

Figure 4: Direction of nuclear migration within the IVD during spinal movements (adapted from McKenzie 1981)

Epidemiology /Etiology

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Characteristics/Clinical Presentation

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Differential Diagnosis

Refer to Lumbar discogenic pain and Thoracic disc syndrome.

Diagnostic Procedures

Refer to Lumbar discogenic pain and Thoracic disc syndrome.

Outcome Measures

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Examination

Refer to Lumbar discogenic pain.

Medical Management

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Physical Therapy Management

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Key Research

Bogduk, N., Clinical anatomy of the lumbar spine and sacrum. 4th ed. 2005, New York: Churchill Livingstone.

Resources

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Clinical Bottom Line

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Recent Related Research (from Pubmed)

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References

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