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The spine forms a strong and at the same time flexible base of the human body, and therefore it is sometimes compared to the scaffolding on which everything rests. In addition to the support function, shaping the figure and enabling the vertical body posture, the key function of the spine is to protect the spinal cord. The spine seen from the side resembles a slightly stretched letter S, which is formed by natural curves: cervical lordosis, thoracic kyphosis, lumbar lordosis and sacral kyphosis. The support function and the associated heavy loads on the spine make it vulnerable to degeneration and numerous injuries.

Human spine consists of7 cervical vertebrae(C),12 thoracic vertebrae(Th),5 lumbar vertebrae(L),5 sacral vertebrae(S) (they merge into a single sacrum during fetal development) and4 to 5 coccyx(What). In total,human has from 33 to 34 vertebrae , including24 vertebraein the strict sense of the word ( unconnected,mobilne ) and two bones made of fused vertebrae - sacral and coccyx.

The spine is thus formed by 26 bones (24 vertebrae, sacrum and coccyx) connected byintervertebral discs(23, because between the head and the first cervical vertebra, as well as between the first and second vertebrae There are no intervertebral discs in the cervical vertebrae; similarly between the sacrum and coccyx) and the inter-process joints, which are additionally stabilized by ligaments and articular capsules.

But the anatomical-functional complex commonly known as the spine also includes elements of the nervous system (spineand nerve roots ) that benefit from the protection of the bone spine (the spinal canal is spine), as well as numerous muscle groups responsible for both stabilization and mobility of the spine.

Spine - structure and functions

As the name suggests - the spine isa pole made of circles . Indeed, when viewed from the front it forms a straight line. In the sagittal plane, however,four curves :lordozy(forward bend)cervicaland lumbarandkyphosis(backward bend)thoracicandcross . Inaccuratethere is also the second part of the name, because there areintervertebral discsbetween the vertebrae, whichadd up to about a quarter of the length of the entire spine . They are also slightly wider than the vertebral bodies, and therefore extend beyond their outline, giving the spine a characteristic bamboo shape.

As a whole, the spine performs several important functions for the entire body:

  • issupporting axis , which allows you to achieve a vertical silhouette and keep it in balance;
  • amortisesallloadsin the long axis of the body;
  • protects the sensitive nerve tissue of the spinal cordthat runs inside the spinal canal;
  • representsscaffolding for the vegetative nervous system ;
  • representsscaffolding for internal organschest and abdomen;
  • is a stable element in relation to mobile limbs, as well asmuscle attachment sitelimbs;
  • enables neck and torso movementsin all three planes;

The primary tasks of the spine arestabilization, protection and maintenance of the skeleton structure , and only then are its functions related to mobility. Jerzy Stodolny justifies it using examples: “It is known from practice that one can function perfectly well with a stiff spine. Examples include children with implanted metal distractors after some scoliosis surgeries or patients with ankylosing spondylitis. On the other hand, with an unstable spine, e.g. after injuries, sprains, fractures, not to mention muscular paralysis, unfortunately it is impossible to live actively. Hence a very important conclusion: the activity of the spine is a compromise between the greatest possible stabilization and the desired mobility. "

A number of mechanisms can be identified that enable effectivelimiting and controlling the mobility of the spine :

  • systemmuscleswhichkeep it upright ;
  • fourcurves(cervical and lumbar lordosis as well as thoracic and sacral kyphosis), which increase the resistance ofspineto loads from above up to 17- times compared to the same, but completely simple construction;
  • ribs , whichrestrict mobilityof the thoracic sectionof the spine ;
  • ligament systemand fibrous ringsintervertebral discsthat control passive and active movements between the vertebrae;
  • nuclei of intervertebral discs ,which allow the vertebral bodies to change the orientation of the planes in different directions, but within a limited angular range;
  • different depending on the level of the spinestructure of articular processes and intervertebral jointswith articular surfaces that allow movement only in certain directions and the correct range;
  • structure of the sacrumand its connection with the pelvis by wide sacroiliac joints, thanks to which this structure is the strongest shock absorber of the human body, reducing loads in the vertical axis.

The motor functions of the individual sections of the spine are varied in terms of the range and directions of movement.The greatest range of motion occurs in the cervical section, then in the lumbar section, and the smallest in the sacral section . The so-calledkey points of the spine, i.e. interstitial connections :

  • cervical-cervical (Co-C1), where there are flexion, extension, lateral flexion, rotation and combinations of the directions mentioned;
  • cervico-thoracic, with the same movements as in the cervical-cervical junction, but with a greater range;
  • thoracolumbar with flexion, extension, lateral flexion and rotation movements;
  • lumbosacral with flexion, hyperextension and lateral flexion movements.

The increased mobility and range of motion of the above-mentioned places is influenced by the angles of the articular surfaces of the inter-process joints and the height of the intervertebral discs at individual points. The mentioned places of increased mobility are, under additional loads, especiallysubject to overload damage(including discopathy).

Due to the fact thatthe spine has two opposite functions at the same time-stabilizing and motor- its structures must be appropriately different in terms of morphological and functional. To understand the biomechanics of the entire system, it is necessary to get to know the structure of its individual parts.

Constructing a circle

Between vertebraethere are different sections of the spinedifferences due to different functionstypical for different sections of the spine.

Each circle consists of two basic parts: a massive cylindrical body and a thinner and more varied arc.Each arch has 7 processes : spiky from the back, transverse on the sides, and even upper and lower articular processes from above and below. A free triangular space remains between the wings of the arch and the back of the shaft - the spinal canal. This is wherespinal cord .

Flatcoils' shafts are designed to support a large mass of loads . In addition to the spongy substance, their part may also include cartilage boundary plates, which protect it from pressure and mediate the exchange of fluids between the shafts and intervertebral discs. On the other hand,arches are mechanical protection for the spinal cord.andhave articular surfaces that connect individual vertebrae with each other and allow them to move relative to each other . In addition, the transverse processes are sites for attachments of the intervertebral ligaments and the muscles of the spine.

Construction of the intervertebral disc

The intervertebral discs connect the vertebral bodies with each other(they are part of the interbody joints),carry body weight , anddetermine the shape of the spine(in the lordotic cervical and lumbar sections the discs are higher in front) andallow it to move . Intervertebral discs are the highest in relation to the vertebral bodies in the most mobile (approx. 4 mm) and lumbar (approx. 10 mm) sections. From above and below, the intervertebral discs adjoin the border plates that separate them from the spongy substance of the vertebral bodies.

Apart from the cartilage plates, the intervertebral disc consists of two elements that are characteristic and crucial due to the functions of the entire system:fibrous ringsandnucleus. The rings are made of overlapping plates. The fibers of the individual layers of the fibrous ring run obliquely in alternating directions, which gives the so-called rope effect. There is loose fibrous tissue between the plates. In the lumbar region, the fibrous ring consists of 10-12 plates - thick on the sides and thinner at the front and back. Eachfiber ring is firmly attached from the front and sides to the vertebral body . Additionally,connects from the front to the anterior longitudinal ligament .The rear trailers are a bit weaker . The fibers of the inner layers connect through an intercellular scaffold with the nucleus pulposus, soit is difficult to define a strict boundary between the ring and the nucleus .

From a functional point of view, the fibrous ring is the most important element that stabilizes the spine . It connects the individual vertebrae into a functional whole andplays the role of a safety brake in the event of an attempt to move the spine in too large a range . However, it does not completely limit the mobility of the so-called scaffolding the body, because thanks to its (ring) stretch it allows you to make certain movements. InThe fibrous ring surrounds, and thus protects, the nucleus pulposus smaller than the functional whole of the scale - the main shock absorber of the spine.

The nucleus inside the fibrous ringtransversally occupies about 50-60 percent of the volume of the intervertebral disc . It is white, shiny and has a semi-gelatinous texture. It is not an ideal center of the intervertebral disc, as it lies closer to the posterior part of the vertebral body (hence the tendency towards the posterolateral direction of the disc). In a newborn baby, it consists mainly of mucus interspersed with cartilaginous-fibrous threads. Over the years, the ratio of the components of the testicle changes as the mucus is replaced by cartilage-fibrous tissue. This process is lifelong. As the years go by, the differences between the nucleus and the fibrous ring become more and more blurred.

The main functions of the pulmonary nucleusare:

  • mediating fluid exchangebetween the disc and the vertebral bodies;
  • supporting the vertebralocated above the intervertebral disc;
  • amortizing tensions and pressures , transferring them evenly to all sides: to the fibrous ring and cartilage plates of the vertebral bodies;

The effectiveness of the cushioning of the nucleus depends on the properties of thegel it contains, and more specifically on the possible strength of water absorption by it. During pressure, water is squeezed out of the nucleus pulposus, and when the pressure is reduced, it is soaked up again.As the body ages, the absorption and reabsorption abilities of the testes decrease, so that the older spine is able to withstand less pressure. The shock-absorbing function is also severely impaired by damage to the fibrous ring , which may interfere with the even load transmission by the nucleus pulposus.

Biomechanics of the spine on the example of the lumbar spine

The lumbar spine consists of 5 vertebrae with massive bodies. The last of them, connected to the opposite sacrum and has a wedge-shaped shape (it is clearly higher than the front). The vertebral arches are the strongest in this segment, and the vertebral openings are the largest along the entire spine. The articular processes are almost vertical (at an angle of 45 degrees to the frontal plane), so the articular surfaces on them run in the sagittal plane. As a result, they limit rotation and extend the range of flexion and extension (at a young age it is around 80 degrees, in an old age it is usually around 25 degrees).

There are two functional units of the spinevertebrae connected by an intervertebral disc . The front part (with the shafts and disc) carries the weight of the body, the back part (with the appendages and articular surfaces) is responsible for the movements. A strong ligament system also plays an important role as it counteracts the expanding force of the discs. The effect of these opposing forces is best evidenced by the fact thatdaily differences in the length of the spine reach 12 mm in women and 18 mm in men(it is mainly related to changes in the hydration of the nucleus pulposus gel). The effect of these forces is the stabilization of the spine (additionally supported by the muscles of the spine and torso).

In the horizontal cross-section, the spine resembles the letter T, so a system known for its high strength. It is additionally increased (17 times in relation to the same, but without curvature structure) natural curvature of the spine, which is subjected to numerous pressures and loads acting in different planes. They mostly concern the lumbar section, which also has the greatest endurance. Its lifting force is on average about 400 kg (for comparison: the average lifting force of the cervical - 113 kg; the thoracic - 210 kg). Of course, the standard loads on the spine in an upright position are much lower and amount to about 125 kg in the lumbar spine, 75 kg in the thoracic and 50 kg in the cervical.

The strength of the spine is inversely proportional to the duration of the load . In the case of prolonged pressure, the stiffness of the tissues increases and the resistance to damage decreases. Vibration, which makes it difficult for the spine's structures to adapt to the increased pressure, has a similar negative impact on the resistance to injuries.

The axis of the individual spine movements is in the center of the nucleus pulposus and moves with the nucleus - forward during hyperextension and backward during flexion. Therefore, a disorder of the supportive function of the testicle is always the beginning of the pathology of the spine. In addition to excessive loads, they are also caused by the decreasing hydration of the testicle with the passage of years (after birth it contains about 88% of water, at the age of 18 it is 80%, and in old age it is less than 70%). The pressure transferred by the nucleus on the fibers of the intervertebral disc is often distributed asymmetrically, which is related to the shape of the disc. The tension in the posterolateral part (this is where the testicle is pushed back most often) may exceed the existing pressure even 5 times.

The mechanism that protects the nucleus from damage is that it can absorb water despite the pressure exerted on it. However, this only happens with intermittent pressure. With constant overpressure, the testicle losesthis property and is destroyed.The position in which the intervertebral disc is subjected to the greatest pressure (often constant) is sitting with the so-called slouch(rounded back, abdominal extensor shortened, spine extensor in elongation, its tension is reduced). Bending the lumbar spine while standing on straight legs in the kneesis also a harmful position for the disc. As Artur Dziak writes: "It is assumed that in a person weighing 70 kg - the 3rd lumbar vertebra is exerted by a force of 142 kg in a sitting position, 99 kg in a standing position, and 20 kg in a lying position (when sleeping). The pressure of the intra-lumbar intervertebral disc is 1-1.5 MPa (145-210 psi) in the sitting position (resting position giving the greatest pressure). When standing, the pressure drops by about 30 percent, while lying on the side by 50 percent. In the lying position on the back, the pressure inside the intervertebral disc is only 10-20 percent. what while sitting and 25 percent. what in a standing position. So it is obvious that patients with discopathy should, above all, limit their sitting position. "

But the scale of stresses and loads on the spine depends not only on the position, but also to a large extent on all activities that stimulate the work of the muscles. Each movement is accompanied by a change in muscle tension and an elongation of the lever of forces falling on the lumbar spine. The very long lever arm for the lumbar spine forms the torso and upper limbs. Balance is ensured by a short lever arm from the nucleus pulposus to the dorsal extensors. The ratio of these levers is 15: 1, which means that a 40 kg lift must be counterbalanced by a contraction of the back muscles of about 600 kg.

The place where the last lumbar vertebra joins the sacrumis crucial for transferring the load on the spine. It has the shape of a wedge bent forward, made of 5 vertebrae fused together (the final fusion usually occurs between the age of 20 and 25). Together with the two iliac bones, the sacrum forms the pelvis - the base and scaffolding of the spine - to which all loads are transferred (running through the lumbar spine and then the sacrum).The optimal angle between the lumbar spine and the sacral spine is 30 degrees . The transfer of loads takes place in a specific place, because lumbar lordosis transforms into sacral kyphosis. Changing the position of the spine and the wedge-shaped, very high anterior intervertebral disc provide this place with a large range of motion. At the same time, the loads transmitted through the entire spine accumulate here. Intherefore, changes in position, activities and extension of the lever arm are most clearly felt in this area. An example is Artur Dziak: “in the standing upright position of a competitor weighing 105 kg, height 195 cm, holding a barbell of 150 kg on his chest, the load on the L5-S1 section is 220 kg. In the case of a forward tilt of the torso at an angle of 30 degrees, these pressures reach a value of as much as 990 kg, due to the shift of the center of gravity and the extension of the force arm. It should be added that this force is divided into two components, one of which acts parallel to the articular surface of the sacrum and the other perpendicular; for example, with a 52 degree inclination of the base of the sacrum, these components are 348 and 509 kg. It follows that the lumbosacral connection is subjected to very significant pressures, with one force tending to slide the 5th lumbar vertebra forward off the sacrum. "

In order to develop overload changes in the musculoskeletal system, we do not need loads that exceed the strength of the tissues. Frequent overloads are sufficient. If they are strong and frequent enough, without exceeding the threshold of tissue strength, they can even break the fibrous ring - a pathology that cannot be stopped or undone.

So how is it that during some work or sports that involve lifting heavy loads or lengthening the lever arm, the spine does not (or at least not immediately) be damaged? It turns out thatprotects, relieves and stabilizes the muscles (especially postural muscles) and the pressure generated in the body cavities (abdomen, chest) . Not only that, as Artur Dziak writes: “Muscle activity and pressure in the body cavities increases with the weight of the lifted load. By taking over some of the pressure on the spine, the thoracic section can be relieved by as much as 50 percent and the lumbar spine by 30 percent (measured at the L5-S2 intervertebral disc level) ”. This example clearly shows how important a compensatory role is played bypostural muscles , which - adequately strong and fit -not only protect the spine from damage, but also partially take over its functions .

Bibliography :

  • Bochenek A., Reicher M., Human Anatomy, vol. I, Państwowy Zakład Wydawnictw Lekarskich, Warsaw 1997 and Sobotta J., Atlas of human anatomy, vol. 1: General anatomical concepts. Organs of movement. Ed. Urban & Partner, Poznań 2012.
  • Dziak A., Spine pains and dysfunctions, Medicina Sportiva,Krakow 2007.
  • Sobotta J., Atlas of Human Anatomy, Volume 1: General anatomical concepts. Organs of movement. Ed. Urban & Partner, Poznań 2012.
  • Stodolny J., Spinal overload disease, ZL-Natura, Kielce, 2000 and Dziak A., Pains and spine dysfunctions, Medicina Sportiva, Kraków 2007.

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Anatomy