The intervertebral disc (IVD) plays an important role in the normal functioning of the spine. It is a cushion of fibrocartilage and the main joint between two adjacent vertebrae. There are 23 discs in the human spine: 6 in the cervical region, 12 in the thoracic region and 5 in the lumbar region.
IVDs allow the spine to be flexible without losing its strength. They also provide a shock-absorbing effect within the spine and prevent the vertebrae from rubbing against each other. IVDs consist of three main components: the internal, nucleus pulposus, the outer, fibrous ring and cartilaginous plates, through which the IVDs articulate with the vertebral bodies.
Clinically Relevant Anatomy
Thus, the MTD consists of three separate components:
- Central nucleus pulposus.
- Fibrous peripheral ring.
- Two end plates.
Nucleus pulposus
It is a gel-like structure that sits at the center of the intervertebral disc and provides much of the strength and flexibility of the spine. The nucleus pulposus is composed of 66-86% water, with the remainder consisting primarily of type II collagen (it may also contain types VI, IX and XI) and proteoglycans. Proteoglycans include the larger aggrecan and versican, which bind to hyaluronic acid, as well as several small leucine-rich proteoglycans. Aggrecan is largely responsible for retaining water within the nucleus pulposus. This structure also contains a low cell density. Being sparse, these cells generate extracellular matrix products (aggrecan, type II collagen, etc.) and maintain the integrity of the nucleus pulposus.
Fibrous ring
Consists of concentric layers of collagen fibers. The orientation of the fibers of each layer alternates and therefore provides effective resistance to multidirectional movements. The annulus fibrosus contains an inner and an outer part. They differ primarily in the composition of collagen. While both are primarily composed of collagen, the outer ring contains primarily type I collagen and the inner ring contains predominantly type II. The inner ring also contains more proteoglycans than the outer ring.
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Note:
- type I collagen: skin, tendons, blood vessels, organs, bones (the main component of the organic part of bone);
- collagen type II: cartilage (the main collagen component of cartilage).
End plate
The superior and inferior cartilaginous end plates (each about 0.6-1 mm thick) cover the superior and inferior surfaces of the disc. The end plate allows for diffusion and is the main source of nutrients for the disc. The hyaline endplate is also the last part of the disc to wear due to disc degeneration.
- Plates of cartilage that connect the disc to the corresponding vertebral bodies.
- Each end plate covers almost the entire surface of the adjacent vertebral body; only a narrow rim of bone, called the annular apophysis, around the perimeter of the vertebral body remains open.
- The part of the vertebral body to which the cartilaginous endplate is attached is called the vertebral endplate.
- The endplate completely covers the nucleus pulposus, but it does not cover the peripheral portion of the annulus fibrosus.
- The collagen fibers of the annulus fibrosus fuse at the endplate and fuse with it, resulting in all parts of the nucleus being enclosed in a fibrous capsule.
Innervation
Only the outer third of the fibrous ring has vascular supply and is innervated in a normal (non-pathological) state. With aging and inflammatory processes, both nerve growth and granulation tissue growth are stimulated. In addition, granulation tissue secretes inflammatory cytokines, which further increases sensitivity to pain.
Vascularization and nutrition
The IVD is largely avascular because it has no major arterial branches approaching it. The outer layers of the annulus fibrosus are supplied by small branches of nearby arteries. Blood vessels near the junction of the disc with the vertebral body, as well as those in the outer ring, supply the nucleus pulposus. Glucose, oxygen, and other nutrients reach avascular areas by diffusion. The evacuation of metabolic products is associated with the same process.
So, what hernias require surgical treatment?
• Cauda syndrome
This is the most serious complication of a disc herniation. With cauda syndrome, there is very strong compression of the nervous structures, and dysfunction of the pelvic organs occurs. Simply put, the patient loses control of his urination and bowel movements. The patient develops paralysis of a leg or both limbs. Here it is necessary to carry out surgical treatment as quickly as possible, otherwise the patient risks remaining disabled.
• Acute root compression
This is the case when the patient is ready to climb the wall in pain and is ready to jump onto the operating table himself. With acute compression, the patient experiences an antalgic posture - he bends down or stands on all fours. This position is protective, since the body’s task is to reduce the tension on the nerve as much as possible. Therefore, he flexes the lumbar spine or moves it away from the hernia. For acute compression, surgical treatment is also recommended.
In all other cases, you need to focus on such an indicator as quality of life. What is quality of life? It is an assessment of some set of conditions and characteristics of a person's life, usually based on his own degree of satisfaction with these conditions and characteristics.
Criteria for violation of quality of life:
- Taking painkillers daily
- Inability to move normally (lameness)
- Inability to do your job properly
- Pain when driving long distances, etc.
If you have these signs, you should consider surgical treatment. Hernias that slightly compress the root, and pain does not affect the quality of life, can be treated conservatively or undergo a minimally invasive procedure - intradiscal decompression (hydroplasty).
In our ]hydroplasty[/anchor] is an outpatient procedure. Two hours after the procedure, the patient is discharged.
Biomechanics
Weight load
The intervertebral disc is subjected to various loads, including compressive, tensile and shear stresses. During a compressive force, hydrostatic pressure increases within the nucleus pulposus, which thereby dissipates forces towards the end plates as well as the annulus fibrosus. This mechanism slows the rate at which applied load is transmitted to the adjacent vertebra, providing the disc with its shock-absorbing capabilities.
Movement
The disc is also involved in movements between adjacent vertebral bodies, which include:
- Axial compression/distraction.
- Flexion/extension.
- Axial rotation.
- Lateroflexion.
Disk Core Migration
Asymmetrical loading can lead to migration of the nucleus pulposus in the direction opposite to the compressive force. For example, when the lumbar spine is flexed, the nucleus pulposus migrates posteriorly. Conversely, during extension, the disc core is squeezed anteriorly. This concept is known as the dynamic disk model. Although nucleus pulposus migration has been shown to behave predictably in normal (asymptomatic) discs, individuals with symptomatic and/or degenerative IVDs may experience a variable migration pattern.
Physiological options
The thickness of the discs usually increases from top to bottom. Relative to the size of the vertebral bodies, it is highest in the cervical and lumbar regions. This reflects the increased range of motion observed in these regions.
In the cervical and lumbar regions, the intervertebral discs are thicker in the front. This creates a secondary curvature of the spine - cervical and lumbar lordosis.
Pathology
Neuropathic pain.
Disc herniation. Valentina Yudakova and Konstantin Sinelnikov There are several terms to describe IVD pathologies:
- Protrusion of the disc, i.e. the disc circumference extends beyond the vertebral bodies.
- A herniated disc involves the nucleus pulposus. This condition is unpleasant because it can compress the adjacent spinal nerve. IVD herniations affect the nerves associated with the lower vertebrae (for example, an L4/L5 herniation affects the L5 nerve root). The most common location for disc herniation is at the L5-S1 level, which may be due to thinning of the posterior longitudinal ligament towards its caudal end. There are three subtypes of hernias: Disc protrusion is characterized by the fact that the width of the base of the protrusion is greater than the diameter of the disc material that is the herniation.
- During disc extrusion, the annulus fibrosus is damaged, allowing the nucleus pulposus to be extruded beyond the confines of the disc. In this case, the hernial material forms a mushroom-shaped dome, which is wider than the neck, connecting it with the body of the nucleus pulposus. The herniation may extend above or below the level of the disc.
- During disc sequestration, the hernial material is torn away from the body of the nucleus pulposus.
Stages of protrusion
There are several stages in the development of protrusion:
- Stage 1 (mild) – consists of the formation of a barely noticeable protrusion of the intervertebral disc. Its dimensions do not exceed 0.1–1 mm, so not every specialist is able to detect protrusion at this stage even with MRI.
- Stage 2 (moderate) – the protrusion increases to 1–3 mm, which makes it possible to easily detect it on MRI images.
- Stage 3 (severe) - the protrusion already noticeably protrudes beyond the boundaries of the vertebral bodies, but there are no signs of rupture of the outer part of the fibrous membrane yet. Its dimensions are about 3–6 mm.
Thus, the 3rd stage borders on extrusion, i.e. the formation of a true intervertebral hernia. Therefore, at almost any moment, the thinned fibrous membrane can rupture, and the nucleus pulposus will begin to move outward. The part that falls outside the boundaries of the fibrous ring can separate over time, which is called sequestration. If ordinary hernias are considered dangerous from the point of view of the development of severe neurological symptoms and the risk of irreversible consequences, then sequestered hernias require emergency surgical care.
Interesting Facts
- There is no intervertebral disc between the C1 and C2 vertebrae, which is unique to the spine.
- Two major ligaments support the IVD: The anterior longitudinal ligament is a broad ligament that covers the anterolateral surface of the spine from the foramen magnum to the sacrum. This ligament allows the spine to avoid hyperextension and prevents the formation of intervertebral hernias that spread in the anterolateral direction.
- The posterior longitudinal ligament covers the posterior surface of the vertebral bodies, within the spinal canal, and serves primarily to prevent the formation of posterior herniations of the intervertebral discs, and is therefore responsible for the majority of herniations extending in the posterolateral direction.
Diagnostic methods
To diagnose the disease, the patient should consult a neurologist or vertebrologist. The doctor will conduct a survey and examination, during which the patient will be asked to undergo several special tests designed to determine his neurological status. An experienced diagnostician is often able to suspect the presence of protrusion based on the results of the initial examination.
But to confirm the diagnosis and differentiate the disease from other pathologies of the central nervous system, the following must be prescribed:
- MRI;
- x-ray;
- CT.
Magnetic resonance imaging is the most informative, since it is this method that allows one to obtain the largest amount of data on the condition of the intervertebral discs themselves, the spinal cord, the musculo-ligamentous apparatus and other soft tissues. With its help, you can detect protrusions up to 2 mm.
But the data obtained during a survey, examination of the patient and using X-ray methods are of great importance for diagnosing protrusion and selecting the most effective treatment tactics. They provide information about:
- the nature, severity, time of appearance and frequency of episodes of malaise;
- muscle strength, coordination, connection of pain with certain movements;
- the level of physical development of the patient before the onset of protrusion formation and its change at the time of examination;
- curvature of the spine;
- preservation of reflexes;
- quality of tactile sensitivity;
- the presence and size of osteophytes in the spine;
- presence of spondylolisthesis;
- the size of the intervertebral spaces and bone density;
- the nature of the patient’s professional activity and his lifestyle in general.