Anatomo-biomechanical features of the spine

The spinal column should be considered from the anatomical (biomechanical) and functional side.

Anatomically, the spine consists of 32, sometimes 33 individual vertebrae, connected to each other by intervertebral discs (art. intersomatica), which represent a synchondrosis, and joints (art. intervertebrales). The stability or firmness of the spine is ensured by a powerful ligamentous apparatus connecting the bodies of the vertebrae (lig. longitudinale anterius et posterius), and the capsule of the intervertebral joints, ligaments connecting the vertebral arches (lig. flava), ligaments connecting the spinous processes (lig. supraspinosum et intraspinosum).

From a biomechanical point of view, the spine is like a kinematic chain consisting of individual links. Each vertebra articulates with the neighboring one at three points:

At the two intervertebral joints at the back and by the bodies (through the intervertebral disc) at the front.

The connections between the articular processes constitute true joints.

Situated one above the other, the vertebrae form two columns - the anterior one, built from the bodies of the vertebrae, and the posterior one, formed from the arches and intervertebral joints.

The mobility of the spine, its elasticity and resilience, the ability to withstand significant loads are to a certain extent provided by the intervertebral discs, which are in close anatomical and functional connection with all the structures of the spine that form the spinal column.

The intervertebral disc plays a leading role in biomechanics, being the “soul of movement” of the spine (Franceschilli, 1947). Being a complex anatomical formation, the disc performs the following functions:

• fusion of vertebrae,
• ensuring mobility of the spinal column,
• protection of vertebral bodies from constant trauma (shock-absorbing role).

ATTENTION! Any pathological process that weakens the function of the disc disrupts the biomechanics of the spine. The functional capabilities of the spine are also disrupted.

The anatomical complex consisting of one intervertebral disc, two adjacent vertebrae with the corresponding joints and ligamentous apparatus at this level is called a vertebral motion segment (VMS).

The intervertebral disc consists of two hyaline plates that fit tightly against the endplates of the bodies of adjacent vertebrae, the nucleus pulposus, and the fibrous ring (annulus fibrosus).

The nucleus pulposus, being a remnant of the dorsal notochord, contains:

• interstitial substance chondrin;
• a small number of cartilage cells and intertwined collagen fibers that form a kind of capsule and give it elasticity.

ATTENTION! In the middle of the nucleus pulposus there is a cavity, the volume of which is normally 1-1.5 cm ³.

The fibrous ring of the intervertebral disc consists of dense connective tissue bundles intertwined in various directions.

The central bundles of the fibrous ring are loosely located and gradually pass into the capsule of the nucleus, while the peripheral bundles are closely adjacent to each other and are embedded in the bone marginal edge. The posterior semicircle of the ring is weaker than the anterior, especially in the lumbar and cervical spine. The lateral and anterior sections of the intervertebral disc slightly protrude beyond the bone tissue, since the disc is somewhat wider than the bodies of adjacent vertebrae.

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Spinal ligaments

The anterior longitudinal ligament, being the periosteum, is firmly fused with the bodies of the vertebrae and freely passes over the disc.

The posterior longitudinal ligament, which participates in the formation of the anterior wall of the spinal canal, on the contrary, is freely thrown over the surface of the vertebral bodies and is fused with the disk. This ligament is well represented in the cervical and thoracic spine; in the lumbar region it is reduced to a narrow band, along which gaps can often be observed. Unlike the anterior longitudinal ligament, it is very poorly developed in the lumbar region, where disc prolapses are most often observed.

Yellow ligaments (23 ligaments in total) are located segmentally, starting from the C vertebra to the S vertebra. These ligaments seem to protrude into the spinal canal and thereby reduce its diameter. Due to the fact that they are most developed in the lumbar region, in cases of their pathological hypertrophy, phenomena of compression of the equine tail can be observed.

The mechanical role of these ligaments is different and is especially important from the point of view of the statics and kinematics of the spinal column:

• they maintain cervical and lumbar lordosis, thus strengthening the action of the paravertebral muscles;
• determine the direction of movement of the vertebral bodies, the amplitude of which is controlled by the intervertebral discs;
• protect the spinal cord directly by closing the space between the plates and indirectly through their elastic structure, due to which, during extension of the trunk, these ligaments remain fully stretched (provided that if they contracted, their folds would compress the spinal cord);
• together with the paravertebral muscles, they help bring the trunk from ventral flexion to a vertical position;
• have an inhibitory effect on the nucleus pulposus, which, through interdiscal pressure, tend to move two adjacent vertebral bodies apart.

The connection of the arches and processes of adjacent vertebrae is carried out not only by the yellow, but also by the interspinous, supraspinous and intertransverse ligaments.

In addition to the discs and longitudinal ligaments, the vertebrae are connected by two intervertebral joints formed by articular processes that have features in different sections. These processes limit the intervertebral openings through which the nerve roots exit.

The innervation of the outer parts of the fibrous ring, posterior longitudinal ligament, periosteum, joint capsule, vessels and membranes of the spinal cord is carried out by the sinuvertebral nerve (n. sinuvertebralis), consisting of sympathetic and somatic fibers. Nutrition of the disc in an adult occurs by diffusion through the hyaline plates.

The listed anatomical features, as well as data from comparative anatomy, allowed us to consider the intervertebral disc as a semi-joint (Schmorl, 1932), while the nucleus pulposus, containing synovial fluid (Vinogradova T.P., 1951), is compared to the joint cavity; the endplates of the vertebrae, covered with hyaline cartilage, are likened to the articular ends, and the fibrous ring is considered as the joint capsule and ligamentous apparatus.

The intervertebral disc is a typical hydrostatic system. Since liquids are practically incompressible, any pressure acting on the nucleus is transformed uniformly in all directions. The fibrous ring, with the tension of its fibers, holds the nucleus and absorbs most of the energy. Due to the elastic properties of the disc, the shocks and concussions transmitted to the spine, spinal cord and brain are significantly softened when running, walking, jumping, etc.

The turgor of the core varies significantly: when the load decreases, it increases and vice versa. Significant pressure of the core can be judged by the fact that after being in a horizontal position for several hours, the straightening of the discs lengthens the spine by more than 2 cm. It is also known that the difference in human height during the day can reach 4 cm.

The vertebral bodies in different parts of the spine have their own distinctive anatomical and functional features.

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Cervical spine

In accordance with the functional tasks of support, the sizes of the vertebral bodies gradually increase from the cervical to the lumbar regions, reaching their greatest size in the S vertebrae;

• The cervical vertebrae, unlike those located below, have relatively low, ellipsoid-shaped bodies;
• the bodies of the cervical vertebrae are not separated from each other by a disk along their entire length. These elongated upper-lateral edges of the vertebral bodies, called the semilunar or hook-shaped processes (processus uncinatus), connecting with the lower-lateral angles of the bodies of the overlying vertebrae, form the so-called Luschka joint, or uncovertebral articulation, according to Troland's terminology. Between the processus uncinatus and the facet of the upper vertebra there is an uncovertebral gap of 2-4 mm;
• the uncovertebral articular surfaces are covered with articular cartilage, and the joint is surrounded on the outside by a capsule. In this region, the vertical fibers of the annulus fibrosus on the lateral surface of the disc diverge and run in bundles parallel to the opening; however, the disc does not directly adjoin this articulation, since, approaching the uncovertebral fissure, it gradually disappears;
• an anatomical feature of the cervical vertebrae is the presence of openings at the base of the transverse processes, through which the a. vertebralis passes;
• intervertebral openings C5 _, C6 _and C7 _have a triangular shape. The axis of the opening in the section passes in an oblique plane. Thus, conditions are created for narrowing of the opening and compression of the root with uncovertebral growths;
• the spinous processes of the cervical vertebrae (except C7 ₎ are split and lowered;
• the articular processes are relatively short, they are in an inclined position between the frontal and horizontal planes, which determines a significant volume of flexion-extension movements and somewhat limited lateral tilts;
• rotational movements are carried out mainly by the upper cervical vertebrae due to the cylindrical articulation of the odontoid process with the articular surface of the C1 vertebra;
• the spinous process of C ₇ protrudes maximally and is easily palpated;
• the cervical spine is characterized by all types of movements (flexion-extension, bending to the right and left, rotational) and in the greatest volume;
• the first and second cervical roots emerge behind the atlantooccipital and atlantoaxial joints, and there are no intervertebral discs in these areas;
• In the cervical region, the thickness of the intervertebral discs is 1/4 of the height of the corresponding vertebra.

The cervical spine is less powerful and more mobile than the lumbar spine, and is generally subject to less stress. However, the load on 1 cm2 ^of the cervical disc is no less, and even greater, than on 1 cm2 ^of the lumbar spine (Mathiash). As a result, degenerative lesions of the cervical vertebrae are as common as in the lumbar spine.

R. Galli et al. (1995) showed that the ligamentous apparatus provides very little mobility between the vertebral bodies: horizontal displacements of adjacent vertebrae never exceed 3-5 mm, and angular inclinations - 11°.

Instability of the PDS should be expected when there is a distance of more than 3-5 mm between the bodies of adjacent vertebrae and when the angle between the bodies of the vertebrae increases by more than 11°.

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Thoracic spine

In the thoracic region, where the range of motion of the spine is relatively small, the vertebrae are higher and thicker than the cervical ones. From Th ₅ to Th12 thoracic vertebrae, their transverse size gradually increases, approaching the size of the upper lumbar vertebrae; the intervertebral discs in the thoracic region are smaller than in the lumbar and cervical regions; the thickness of the intervertebral discs is 1/3 of the height of the corresponding vertebra; the intervertebral openings in the thoracic region are narrower than in the cervical region; the spinal canal is also narrower than in the lumbar region; the presence of a large number of sympathetic fibers in the thoracic roots not only causes a peculiar vegetative coloring of thoracic radiculopathies, but can also cause the development of visceral pain and dyskinesia; relatively massive, thickened at the ends transverse processes of the thoracic vertebrae are inclined somewhat posteriorly, and the spinous processes are sharply inclined downwards; The tubercle of the rib adjoins the anterior surface of the thickened free end of the transverse process, forming a true costotransverse joint; another articulation is formed between the head of the rib and the lateral surface of the body of the vertebra at the level of the disc.

These joints are strengthened by strong ligaments. When the spine rotates, the ribs and the lateral surfaces of the vertebral bodies with transverse processes follow the spine, turning around the vertical axis as a single unit.

The thoracic spine has two distinctive features:

• normal kyphotic curve as opposed to lordotic curve of the cervical and lumbar spine;
• articulation of each vertebra with a pair of ribs.

Stability and mobility of the thoracic spine

The main stabilizing elements are: a) the costal framework; b) intervertebral discs; c) fibrous rings; d) ligaments (anterior and posterior longitudinal ligaments, radial ligament, costotransverse ligament, intertransverse ligaments, yellow ligament, inter- and supraspinous ligaments).

The ribs with the ligamentous apparatus provide sufficient stability and at the same time limit mobility during movements (flexion - extension, lateral bending and rotation).

ATTENTION! When moving in the thoracic region, rotation is the least restricted.

The intervertebral discs, together with the fibrous ring, in addition to cushioning, perform a stabilizing function: in this section, the discs are smaller than in the cervical and lumbar sections, which minimizes mobility between the vertebral bodies.

The condition of the ligamentous apparatus determines the stability of the thoracic spine.

A number of authors (Heldsworth, Denis, Jcham, Taylor, etc.) have substantiated the theory of three-point stability.

The key role is given to the posterior complex: its integrity is an essential condition for stability, and damage to the posterior and middle supporting structures is manifested by clinical instability.

An important stabilizing element is the joint capsule, and the anatomy of the joints also ensures the integrity of the structures.

The joints are oriented in the frontal plane, which limits flexion-extension and lateral bending; therefore, subluxations and dislocations of joints are extremely rare in the thoracic region.

ATTENTION! The most unstable area is the Th10-L1 zone due to the relatively stable thoracic and more mobile lumbar regions.

Lumbosacral spine

In the lumbar spine, which supports the weight of the overlying section:

• the bodies of the vertebrae are the widest, the transverse and articular processes are massive;
• the anterior surface of the lumbar vertebral bodies is slightly concave in the sagittal direction; the body of the L vertebra is slightly higher in front than behind, which anatomically determines the formation of lumbar lordosis. Under lordosis, the load axis shifts backwards. This facilitates rotational movements around the vertical axis of the body;
• the transverse processes of the lumbar vertebrae are normally located frontally; the ventral parts of the transverse processes of the lumbar vertebrae are underdeveloped remnants of the corresponding lumbar ribs, which is why they are called costal processes (processus costarii vertebrae lumbalis). At the base of the costal processes are smaller accessory processes (processus accessorius);
• the articular processes of the lumbar vertebrae protrude noticeably, and their articular surfaces are located at an angle to the sagittal plane;
• the spinous processes are thickened and directed backwards almost horizontally; on the posterolateral edge of each superior articular process on the right and left there is a small conical mammillary process (processus mamillaris);
• the intervertebral openings in the lumbar region are quite wide. However, in conditions of spinal deformation, degenerative processes, and static disorders, radicular pain syndrome most often appears in this region;
• the lumbar discs, in accordance with the greatest load performed, have the greatest height - 1/3 of the body height;
• the most frequent localization of disc protrusions and prolapses corresponds to the most overloaded sections: the space between L4 _and Ls _and, somewhat less frequently, between C and S1;
• The nucleus pulposus is located on the border of the posterior and middle third of the disc. The fibrous ring in this area is significantly thicker in front, where it is supported by a dense anterior longitudinal ligament, most powerfully developed in the lumbar region. Behind, the fibrous ring is thinner and is separated from the spinal canal by a thin and less developed posterior longitudinal ligament, connected to the intervertebral discs more firmly than to the vertebral bodies. This ligament is connected to the latter by loose connective tissue, in which the venous plexus is embedded, which creates additional conditions for the formation of protrusions and prolapses into the lumen of the spinal canal.

One of the characteristic features of the spinal column is the presence of four so-called physiological curvatures located in the sagittal plane:

• cervical lordosis, formed by all cervical and upper thoracic vertebrae; the greatest convexity is at the level of _C5 and _C6;
• thoracic kyphosis; the maximum concavity is at the level of Th ₆ - Th ₇;
• lumbar lordosis, formed by the last thoracic and all lumbar vertebrae. The greatest curvature is located at the level of the body L ₄;
• sacrococcygeal kyphosis.

The main types of functional disorders in the spine develop either by the type of smoothing of physiological curves, or by the type of their increase (kyphosis). The spine is a single axial organ, its division into different anatomical sections is conditional, therefore there cannot be hyperlordosis, for example, in the cervical spine with smoothing of lordosis in the lumbar, and vice versa.

Currently, the main types of functional disorders in smoothed and hyperlordotic variants of changes in the spine have been systematized.

1. When the physiological curves of the spine are smoothed out, a flexion type of functional disorders develops, characterized by a forced position of the patient (in a flexion position) and including:

• limited mobility in the motor segments of the cervical spine, including in the area of the head joints;
• inferior oblique capitis syndrome;
• lesions of the deep flexor muscles of the neck and sternocleidomastoid muscle;
• anterior scalene syndrome;
• scapular region syndrome (levator scapulae syndrome);
• anterior chest wall syndrome;
• in some cases - scapulohumeral periarthritis syndrome;
• in some cases - lateral elbow epicondylosis syndrome;
• limited mobility of the 1st rib, in some cases - the I-IV ribs, clavicle joints;
• lumbar lordosis flattening syndrome;
• paravertebral muscle syndrome.

Limitation of mobility in the motor segments of the lumbar and lower thoracic spine: in the lumbar - flexion and lower thoracic - extension:

• limited mobility in the sacroiliac joint;
• adductor syndrome;
• iliopsoas syndrome.

2. With an increase in physiological curves in the spine, an extension type of functional disorders develops, characterized by a straightened "proud" gait of the patient and limited extension in the lumbar and cervical spine during the manifestation of clinical manifestations of the disease. It includes:

• limited mobility in the motor segments of the mid-cervical and cervicothoracic spine;
• cervicalgia of the neck extensor muscles;
• in some cases - internal elbow epicondylosis syndrome;
• limited mobility in the motor segments of the thoracic spine.
• lumbar hyperlordosis syndrome;
• limitation of extension in the motor segments of the lumbar spine: L1-L2 and L2 _- L3 _, in some cases - _L3 - _L4;
• hamstring syndrome;
• hip abductor syndrome;
• piriformis syndrome;
• coccydynia syndrome.

Thus, when the symmetry of active efforts is disturbed even under normal physiological conditions, a change in the configuration of the spine occurs. Due to physiological curves, the spinal column can withstand an axial load 18 times greater than a concrete column of the same thickness. This is possible due to the fact that in the presence of curves, the load force is distributed evenly throughout the spine.

The spine also includes its fixed section - the sacrum and the slightly mobile coccyx.

The sacrum and the fifth lumbar vertebra are the basis of the entire spine, providing support for all its overlying sections and experiencing the greatest load.

The formation of the spine and the development of its physiological and pathological curves is significantly influenced by the position of the IV and V lumbar vertebrae and the sacrum, i.e. the relationship between the sacral and overlying parts of the spine.

Normally, the sacrum is at an angle of 30° to the vertical axis of the body. A pronounced tilt of the pelvis causes lumbar lordosis to maintain balance.

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