Anatomical and biomechanical features of the spine

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

Anatomically, the spine consists of 32, sometimes 33 separate vertebrae, interconnected by intervertebral discs (art. Intersomatica), which represent synchondrosis, and joints (art. Intervertebrales). The stability or stability of the spine is provided by a powerful ligamentous apparatus that connects the vertebral bodies (lig. Longitudinale anterius et posterius), and the capsule of the intervertebral joints, the ligaments connecting the vertebral arches (lig. Flava), the ligaments connecting the spinous processes (lig. Supraspinosum et intraspinusinusinusinusinusinusinusinusinusinusinusinus).

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

In two intervertebral joints in the back and bodies (through the intervertebral disc) in front.

The joints between the articular processes are the true joints.

Located one above the other, the vertebrae form two pillars - the anterior, built at the expense of the vertebral bodies, and the posterior, which is formed from the arches and intervertebral joints.

The mobility of the spine, its elasticity and elasticity, the ability to withstand significant loads to a certain extent are 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:

• connecting vertebrae
• ensuring the mobility of the spine,
• protection of the vertebral bodies from permanent traumatization (depreciation role).

ATTENTION! Any pathological process, weakening the function of the disk, violates the biomechanics of the spine. Functional capabilities of the spine are also impaired.

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

The intervertebral disc consists of two hyaline plates, closely adjacent to the endplate plates of the bodies of adjacent vertebrae, the pulpal nucleus (nucleus pulposus) and the fibrous ring (annulus fibrosus).

Pulpous nucleus, being a remnant of the spinal chord, contains:

• interstitial substance chondrin;
• a small number of cartilage cells and intertwining collagen fibers, forming a kind of capsule and giving it elasticity.

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

The fibrous ring of an intervertebral disc consists of dense connective tissue bundles intertwining in different directions.

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

[1], [2], [3], [4], [5],

Spinal ligaments

The anterior longitudinal ligament, being the periosteum, is firmly adhered to the vertebral bodies and freely spreads over the disk.

The posterior longitudinal ligament, which participates in the formation of the anterior wall of the spinal canal, on the contrary, freely spreads over the surface of the vertebral bodies and is spliced with the disk. This ligament is well represented in the cervical and thoracic spine; in the lumbar part, it is reduced to a narrow ribbon, during which even gaps can often be observed. Unlike the anterior longitudinal ligament, it is very poorly developed in the lumbar region, in which disc prolapse is most often noted.

The yellow ligaments (a total of 23 ligaments) are segmented, ranging from vertebra C to S vertebra. These ligaments act as if 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 horsetail compression can be observed.

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

• they preserve the 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 intervertebral discs;
• protect the spinal cord directly by closing the space between the plates and indirectly through their elastic structure, thanks to which during the extension of the body these ligaments remain fully stretched (provided that if they were reduced, their folds would squeeze the spinal cord);
• together with the paravertebral muscles contribute to bringing the body from the ventral flexion into an upright position;
• They have an inhibitory effect on the pulpal nuclei, which, by means of interdisk pressure, tend to distance two adjacent vertebral bodies.

The connection of the handles and processes of the adjacent vertebrae is carried out not only yellow, but also between the interostases, the hypostases, and the intertransverse ligaments.

In addition to the disks and the longitudinal ligaments, the vertebrae are connected by two intervertebral joints, formed by articular processes with features in different parts. These processes limit the intervertebral foramen through which the nerve roots exit.

The innervation of the outer divisions of the fibrous ring, posterior longitudinal ligament, periosteum, capsule of the joints, vessels and membranes of the spinal cord is performed by the sinus-vertebral nerve (n. Sinuvertebralis), consisting of sympathetic and somatic fibers. The nutrition of the disc in an adult occurs by diffusion through hyaline plates.

The listed anatomical features, as well as comparative anatomy data, allowed to consider the intervertebral disk as a half-joint (Schmorl, 1932), while the pulpal nucleus containing synovial fluid (Vinogradova TP, 1951) is compared with the joint cavity; the vertebral endplate, covered with hyaline cartilage, is likened to the articular ends, and the fibrous ring is considered as a joint capsule and ligamentous apparatus.

The intervertebral disc is a typical hydrostatic system. Due to the fact that liquids are practically incompressible, any pressure acting on the core is transformed uniformly in all directions. The fibrous ring, by energizing its fibers, holds the core and absorbs most of the energy. Due to the elastic properties of the disc, tremors and tremors transmitted to the spine, spinal cord, and brain are considerably softened when running, walking, jumping, etc.

The core turgor is variable within considerable limits: with decreasing load it rises and vice versa. A significant pressure of the nucleus can be judged by the fact that after being in a horizontal position for several hours, straightening the discs lengthens the spine by more than 2 cm. It is also known that the difference in the height of a person during a day can reach 4 cm.

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

[6], [7], [8], [9],

Cervical spine

According to the functional tasks of the support, the sizes of the vertebral bodies gradually increase from the cervical to the lumbar, reaching the maximum size in S vertebrae;

• cervical vertebrae, in contrast to those located below, have relatively low ellipsoidal bodies;
• the bodies of the cervical vertebrae are separated from each other by a disk not all over. These elongated upper side edges of the vertebral bodies, called the semilunar or hooked processes (processus uncinatus), connecting with the lower lateral corners of the bodies of the overlying vertebrae, form the so-called Lyushka joint, or uncovertebral articulation, according to Troland terminology. Between the processus uncinatus and the facet of the upper vertebra there is an uncovertebral fissure 2-4 mm;
• uncovertebral articulated surfaces are covered with articular cartilage, and outside the joint is surrounded by a capsule. In this area, the vertical fibers of the annulus fibrosus on the lateral surface of the disk diverge and run in bunches parallel to the hole; at the same time, the disk does not directly adjoin this joint, since, as it approaches the uncovertebral fissure, it gradually disappears;
• Anatomical feature of the cervical vertebrae is the presence of holes at the base of the transverse processes, in which a. Vertebralis;
• intervertebral holes C ₅, C ₆ and C ₇ have a triangular shape. The axis of the hole in the section passes in an oblique plane. Thus, conditions are created for narrowing the aperture and compressing the spine during uncovertebral growths;
• the spinous processes of the cervical vertebrae (except C ₇ ) are split and lowered down;
• the articular processes are relatively short, they are in an inclined position between the frontal and horizontal planes, which determines a significant amount of flexion-extension movements and somewhat limited lateral inclinations;
• rotational movements are carried out mainly by the upper cervical vertebrae due to the cylindrical joint of the tooth-like process with the articular surface of vertebra C1;
• spinous process C ₇ appears maximally and easily palpated;
• all types of movements (flexion-extension, tilts to the right and left, rotational) and to the greatest extent are characteristic of the cervical spine;
• the first and second cervical roots extend behind the atlanto-occipital and atlanto-axial joints, and there are no intervertebral discs in these areas;
• in the cervical spine, the thickness of the intervertebral discs is 1/4 the height of the corresponding vertebra.

The cervical spine is less powerful and more mobile than the lumbar, and generally subject to less stress. However, the load on 1 cm ² of the cervical disc is not less, but even greater than 1 cm ^{2 of the} lumbar (Mathiash). As a result, degenerative lesions of the cervical vertebrae occur as often as in the lumbar region.

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

The instability of the PDS should be expected if there is a distance of more than 3-5 mm between the interstices of adjacent vertebrae and with an increase in the angle between the vertebral bodies more than 11 °. 

[10], [11], [12], [13]

Thoracic spine

In the thoracic region, where the volume of spinal movements is relatively small, the vertebrae are higher and thicker than the cervical ones. From Th ₅ to TH12 of the thoracic vertebra, their transverse size gradually increases, approaching the size of the upper lumbar vertebrae; intervertebral discs in the thoracic region have a lower height than in the lumbar and cervical regions; intervertebral disc thickness is 1/3 of the height of the corresponding vertebra; intervertebral holes in the thoracic region narrower than in the cervical; the spinal canal is also narrower than in the lumbar region; the presence in the chest roots of a large number of sympathetic fibers not only causes a peculiar vegetative color of thoracic radiculopathy, but may also cause the development of visceral pain and dyskinesias; relatively massive, thickened at the ends, the transverse processes of the thoracic vertebrae are inclined somewhat posteriorly, and the spinous processes are sharply inclined downward; the hillock of the rib adjoins the front surface of the thickened free end of the transverse process, forming a true costal-transverse joint; another joint is formed between the head of the rib and the lateral surface of the vertebral body at the level of the disk.

These joints are reinforced with strong ligaments. When the spine rotates, the ribs and side surfaces of the vertebral bodies with transverse processes follow the spine, turning around the vertical axis as a whole.

The thoracic spine is distinguished by two features:

• normal kyphotic bend in contrast to the lordal bend of the cervical and lumbar regions;
• articulation of each vertebra with a pair of ribs.

Stability and mobility of the thoracic spine

The main stabilizing elements are: a) rib cage; b) intervertebral discs; c) fibrous rings; d) ligaments (anterior and posterior longitudinal ligaments, radiant ligament, costal-transverse ligament, inter-transverse ligaments, yellow ligament, inter- and supraspinous ligaments).

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

ATTENTION! During movements in the thoracic region, rotation is least limited.

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

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

A number of authors (Heldsworth, Denis, Jcham, Taylor, and others) substantiated the theory of three-support stability.

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

An important stabilizing element are articular bags, 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 tilts; therefore, in the thoracic region, subluxations and dislocations of the joints are extremely rare.

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 severity of the overlying department:

• vertebral bodies wide, transverse and articular processes massive;
• the anterior surface of the bodies of the lumbar vertebrae is slightly concave in the sagittal direction; the body of the L vertebra in the front is slightly higher than in the back, which determines the anatomically formation of lumbar lordosis. In lordosis conditions, the load axis is shifted backwards. This facilitates rotational movements around the vertical axis of the body;
• transverse processes of the lumbar vertebrae are normally located frontally; ventral parts of the transverse processes of the lumbar vertebrae are the underdeveloped remnants of the corresponding lumbar ribs, therefore they are called the rib processes (processus costarii vertebrae lumbalis). At the base of the rib processes, there are smaller incremental processes (processus accessorius);
• the articular processes of the lumbar vertebrae prominently protrude, and their articular surfaces are angled to the sagittal plane;
• spinous processes are thickened and posteriorly almost horizontal; there is a small conical mastoid process (processus mamillaris) on the posterior-lateral margin of each superior articular process on the right and on the left;
• intervertebral holes in the lumbar spine are quite wide. However, in conditions of spinal deformity, degenerative processes, static disorders in this section pain radicular syndrome most often appears;
• lumbar discs, respectively, performed the greatest load have the greatest height - 1/3 of the height of the body;
• the most frequent localization of protrusions and prolapses of the disk corresponds to the most overloaded sections: the gap between L ₄ and L _s and somewhat less frequently between C and S1;
• pulpal nucleus is located on the border of the back and middle third of the disk. The fibrous ring in this area is much thicker in the front, where it is supported by a dense anterior longitudinal ligament, which is most strongly developed in the lumbar region. The back of the fibrous ring is thinner and is separated from the spinal canal by a thin and more poorly developed posterior longitudinal ligament, which is more firmly connected to the intervertebral discs than to the vertebral bodies. With the latter, this ligament is connected by a loose connective tissue, in which a venous plexus is laid, which creates additional conditions for the formation of protrusions and prolapses in 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 bulge is at level C ₅ and C ₆;
• thoracic kyphosis; maximum concavity is at the level of Th ₆ - Th ₇;
• lumbar lordosis, which is 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 according to the type of smoothness of physiological curves, or according to the type of their increase (kyphosis). The spine is a single axial organ, dividing it into different anatomical divisions conditionally, therefore there can be no hyperlordosis, for example, in the cervical spine with the smoothness of lordosis in the lumbar, and vice versa.

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

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

• restriction of mobility in the motor segments of the cervical spine, including in the area of the head joints;
• syndrome of the inferior oblique muscle of the head;
• lesions of the deep flexors of the muscles of the neck and sternocleidomastoid muscle;
• anterior scalene muscle syndrome;
• syndrome of the upper fissure region (syndrome of the muscle that raises the scapula);
• anterior chest wall syndrome;
• in some cases - the syndrome of humeroscapular periarthritis;
• in some cases, external ulnar epicondylosis syndrome;
• restriction of mobility of the 1st rib, in some cases - I-IV ribs, clavicle joints;
• lumbar lordosis smoothness syndrome;
• paravertebral muscle syndrome.

Restriction 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;
• adrenal muscle syndrome;
• ileo-lumbar muscle syndrome.

2. With an increase in physiological bends in the spine, a flexing type of functional impairment develops, characterized by a straightened “proud” gait of the patient and limitation of extension in the lumbar and cervical spine during the manifestation of clinical manifestations of the disease. It includes:

• restriction of mobility in the motor segments of the mid-cervical and cervical ovaries of the spine;
• cervicalgia of the muscles - neck extensors;
• in some cases, the syndrome of internal ulnar epicondylosis;
• restriction of 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 L ₂ -L ₃, in some cases - L ₃ - L ₄;
• femoral back muscle group syndrome;
• femoral muscle discharge syndrome;
• piriformis syndrome;
• coccygodynia syndrome.

Thus, when the symmetry of active forces is disturbed even under normal physiological conditions, a spinal configuration changes. 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 bends, the load force is distributed evenly throughout the spine.

The spine also includes its fixed division, the sacrum and the slow-moving tailbone.

The sacrum and the fifth lumbar vertebra are the basis of the entire spinal column, which provide support for all its overlying departments and experience the greatest strain.

The formation of the spine and the formation of its physiological and pathological bends are greatly influenced by the position of the IV and V lumbar vertebrae and the sacrum, i.e. The ratio between the sacral and the overlying part of the spine.

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

[14], [15], [16], [17]