Basic knowledge of anatomy, biophysics and biomechanics of the locomotor apparatus
January 6, 2017

Basic knowledge of anatomy and biomechanics of the spine

A complex segment, of highly functional importance, the spine consists of 33 or 34 bone segments (vertebrae), 344 articulation surfaces, 23 intervertebral disks and 365 ligaments with 730 insertion points. On the spine act no less than 730 direct-acting muscles. To all this, it must be added nervous formations (somatic and vegetative), vascular formations, etc.

Bone segments that make up the spine are called vertebrae. Vertebrae have an anterior part called body and a posterior part called arc. These two parts close together the spinal canal.

The vertebral body is the most voluminous and is shaped like a short cylinder which has two sides (top and bottom) and a circumference. The vertebral arch has an irregular shape. On the posterior and median side, it presents a spinous process, on the lateral, two transverse processes, and above and beneath, two articular processes (in total, four articular processes disposed vertically). Between the spinous process and the articular processes lay the vertebral laminae. The portions which connect the vertebral arc to the vertebral body are called pedicles.

The spine is divided into four regions, each of which consists, normally, of a fixed number of vertebrae:

  • The cervical region: 7 vertebrae;
  • The thoracic region: 12 vertebrae;
  • The lumbar region: 5 vertebrae;
  • The sacrum and coccyx: 9 – 10 vertebrae;

The vertebrae of each region have morpho-functional characteristics related to fulfilling the two important functions of the human spine: the function of supporting the weight of the head, trunk and upper limb and the function of ensuring an adequate amount of mobility.

The bearing surfaces of the vertebral bodies grow from one vertebra to another, their shape being determined by the dynamic solicitations; in the cervical and lumbar regions, the transverse diameter of vertebral bodies is proportionally higher than the antero-posterior one, which explains the greater possibilities of these regions to make flexion and extension movements.

Each vertebra presents morpho-functional differentiation, resulting from the mechanisms which helped to adapt the body to the demands of bipedal statics and dynamics.

The joints

Between the vertebrae are made a series of joint lines, which are classified as:

Joints of the vertebral bodies: Their joint surfaces are given by the lower and upper faces (slightly concave). Between these bone surfaces are found the intervertebral disks. They are fibrocartilaginous formations, consisting of a fibrous peripheral portion – annulos fibrosus – and a central portion – nucleus pulposus.  The nucleus puplosus does not have a fixed position, being mobilized during movement. Its movements are possible because it is deformable, elastic and expansive, these qualities being related to its water content. The nucleus is thus under constant pressure and it is easily understandable why any lesion of the annulos fibrosus which surrounds it, permits its herniation. The lower limit of the disks is composed of cartilage strips, which protect the nucleus pulposus of excessive pressure. Regarding vascularization, the presence of blood vessels appears only in pathological cases for adult individuals. The nutrition of the cartilage is made through passive diffusion, through the terminal laminae of the vertebral articular surfaces. Therefore, it is easy to understand why, when intervertebral joints are permanently under pressure, the disk does not have the possibility to feed properly, facilitating thus its early degeneration (through the diminishment or even the loss of its physical & chemical properties). When the disc is loaded, it diminishes in size and widens. It should be noted that the nucleus pulposus has a great capacity of diffusion, increasing its volume while latent, being able to produce the extension of the spine (by summing up the enlargement of all disks volume) with 1 to 3 cm in young, healthy subjects. The nucleus pulposus is not innervated. The annulus fibrosus is innervated by nerve branches from the spinal nerves which also innervate the posterior common vertebral ligament (this also explains the nature of the intervertebral disc herniation/compaction pain).

The role of the intervertebral discs is multiple:

  1. They contribute, through their resistance, to maintaining spinal curvatures;
  2. They favor, by their elasticity, the return to an equilibrium state after movement;
  3. They transmit the body weight in all directions of the various segments of the spine;
  4. They attenuate the shocks or pressures being put on each segment of the spine in particular during movement and effort;

Consequently, sparing the intervertebral discs when it comes to inherent solicitations is an obligation to the body itself, and it must be taken into consideration even in the most common situations, such as sitting on a chair. When the back rest is tilted back, the normal lordosis disappears, the coxofemural joint extends and the intervertebral disc solicitation is more important. Correctly, one must sit so that the lumbar lordosis is maintained, implying a better balancing of the spine.

The execution of a physical exercise is more correct if the biomechanical laws of protection against the solicitation of intervertebral discs are respected.

Joints of the vertebral laminae: Between the vertebral laminae there are not actual joints. However, they are joined by special ligaments, called yellow ligaments, composed of elastic fiber bundles, which through their structure allow the vertebral laminae to come closer or to drown from one another.

Joints of the spinous processes: Much like the vertebral laminae, the spinous processes are joined together through two kinds of ligaments: the interspinous and supraspinous ligaments. The first are located between two spinous processes and the last is a string that stretches along the entire spine. In the cervical region, the supraspinous ligament is very well developed and through its proximal extremity, it attaches itself to the external occipital protuberance; it is called the posterior cervical ligament and its purpose is to maintain the head and neck passive in order not to flex forward.

Joints of the transverse processes: the transverse processes are joined together through transverse ligaments.

The articular processes joints are flat and allow only the mere glide of the articular surfaces on one another.

The Occipitoatlantal Joint is a bicondylar diarthrosis. The articular surfaces of the occipital are represented by the two occipital condyles which “look” up, forward and outside and have a convex form in every way.

The articular surfaces of the Atlas are represented by the two glenoid cavities looking up, forward and inside that have a concave shape in every sense. All these four articular surfaces are covered in a hyaline cartilage. The articular surfaces are linked together by a thin capsule, strengthened by two ligaments – anterior and posterior. The main ligament apparatus of the spine is formed of two ligaments (anterior longitudinal ligament and posterior longitudinal ligament), which form two strips that stretch from one end of the spine to another.

The motor segment. The mobility of the spine is given by the motor segment, which is composed of the intervertebral disc and its ligaments, the conjugation holes, the articular processes joints and the spinous processes with its ligaments.

The motor segment can be divided in an anterior column and a posterior one. The anterior column is less mobile, harder; it presents relatively few muscular insertions and represents the main element of passive mechanical support of the spine. The posterior column presents various muscular insertions and represents the main motor element of the spine.

The spine vascularization

The spinal cord irrigation is realized in a very distinctive way and its understanding is a must when it comes to the knowledge of ischemic syndromes (shortage or lack of blood circulation), with their major neurological deficiencies that can appear as a result of different affections. The cervical cord is irrigated by several important arteries that have their origins in the anterior and posterior spinal arteries, (branches of the vertebral artery and of the posterior cerebral artery). The lumbar cord is irrigated by arteries that originate in the lateral sacral artery, while the dorsal cord is irrigated by the cervical and lumbar arteries. The most critical vascular zone is found at the D4 level, at the border of the two vascular territories.

The muscles of the spine

The spine movements are produced by a large number of muscles, which insert themselves whether on the spine, whether at a certain distance from it, like some neck or abdominal muscles.

The neck muscles: the sternocleidomastoid; the scalene muscles; the rectus capitis anterior; the rectus capitis lateralis; the longus colli; the splenius capitis, splenius cevicis, etc.

The abdominal muscles: the rectus abdominis; the internal and external obliques; the transverses abdominis; the pyramidalis; the quadratus lumborum.

The posterior muscles of the trunk: the trapezius; the latissimus dorsi; the rhomboid; the angular; the erctor spinae: iliocostalis, longissimus, spinalis; the transversospinales; etc.

The statics of the spine

The spine can be compared to a mast whose correct position depends on the extension of the bowlines. A bowline deficiency can constitute a cause for mast deviation or breaking.

THE SPINE CURVATURES

When in orthostatism and in repause, the spine has a vertical direction and a little sinuous form especially in sagittal plane. The curvatures attenuate the shocks and favour the maintaining of the spine equilibrium on the pelvis, rendering easier the spine muscular belt efforts.

This attitude and form are maintained because of the muscular tonicity, the ligament and disc elasticity and also because of the anatomic junction of the 24 bone segments from which the spine is made, segments that adapt each other’s different articular surfaces. The attitude of the spine depends on age, sex, profession, fatigue, mental state, health, etc.

In order to maintain the biped equilibrium, at the beginning of the second year of life, the lumbar curvature (with the bump forward) appears (compensatory lordosis).

The intrinsic equilibrium. In adults, when in vertical position, the gravitational line goes through tragus, that is before the occipitoatlantal joint, through the anterior part of the shoulder, a bit posterior related to a line which would unite the two femoral heads, through the middle of the exterior part of the great trochanter, anterior to the transversal axis of the knee joint and somewhat posterior to the tibio-tarsal one.

Because of the curvatures of the spine, the center of gravity projections of different segments is not found on the projection line of man’s general center of gravity. Therefore, the gravity’s action determines from a vertebra to another, rotational solicitations that tend to accentuate the curvatures and need to be neutralized, otherwise the spine would collapse.

The elements that fight against the rotational solicitations are the ligaments. In the dorsal spine, the projection of the center of gravity goes in front of the spine. This would collapse forward if the force of the posterior common vertebral ligament, interspinous ligaments and yellow ligaments would not intervene. The situation is reverse when it comes to the lumbar and cervical spine; the center of gravity projection is situated posterior to the spine, and the forces that fight against the collapsing are represented by the resistance of the anterior common vertebral ligament. The vertebral ligaments have, therefore, the role to absorb much of the solicitations.

Other elements known to absorb some of the solicitations are the intervertebral discs. They do not stand in tension like the ligaments, but under pressure. Between these two categories of anatomic elements, found under the influence of opposite forces, a certain balance state is established, called intrinsic equilibrium.

Except for the intrinsic equilibrium, the spine possesses (as mentioned above) a great number of muscular groups, that through their tonicity confers it with an extrinsic equilibrium, the muscular corset.

THE TYPES OF POSTURE

The balance of the spine is not realized the same in all normal individuals. This makes the spine posture different from an individual to another and it must correlate with the accentuation or diminishment of the spine curvatures from the antero-posterior plane, as a result of the inclination degree of the pelvis.

There are, therefore, five general posture types:

  • The normal back – it is the type of posture in which the vertebral curvatures present a normal arching. It is the military posture, in which the pelvis inclination is normal.
  • The round back – it is a very common posture type. The dorsal bump/convexity goes down and covers the lumbar vertebrae, and the lumbar region concavity decreases both as depth and length. The pelvis is slightly inclined down and forward.
  • The flat back – it is a posture type less common. The dorsal convexity and lumbar concavity disappear but the pelvis inclination remains small. The scapulae appear drawn back. This back type produces the worst types of scoliosis.
  • The concave-flat back – it is even rarer; the lumbar concavity accentuates severely through the powerful forward inclination of the pelvis, while the dorsal convexity disappears.
  • The round-concave back – it is the rarest type of posture. Both the lumbar concavity and dorsal convexity accentuate.

The spine must be considered as a functional unit and every type of posture must be considered as a spontaneous adaptation to certain special statics and dynamics conditions.

Also, it must be taken into account the important role of the pelvis in the determining of the spine posture. The pelvis constitutes itself in the functional support of the spine, actively participating at its statics and dynamics. Any disorder at pelvis level (affection, pain, functional or static asymmetry) has severe consequences on the biomechanical-functional ensemble of the spine.

The pelvis together with the lower limbs represents the support of the spine biomechanics.

The biomechanics of the axial organ (the spine)

The spine’s movements, regardless of their amplitude, are complex movements that comprise several vertebral segments. They are realized through the accumulation of the slight displacing of the vertebral bodies, which take place at the intervertebral discs as well as the other joints level. These movements are limited by the ligaments resistance, intervertebral joints form and fibrocartilaginous tissue (from which the disc is made) compression degree.

The small intervertebral displacements are possible only thanks to the nucleus pulposus presence, which must have normal consistency, form and location. The vertebral movements are executed on the nucleus pulposus as if it were an axis, the nucleus playing the role of a real mechanical ball (roller bearing). It can be understood that on such a base, all movements are possible; still, these will be limited or guided by the various articular processes conformations and positions, spine ligaments and its muscular corset.

Thanks to the tension of the liquid found between its components, the nucleus pulposus has the property of being elastic. By virtue of this property, the spine movements are possible and the damaging effects of the excessive pressures or shocks of the rachis are eliminated. When in forced flexion, the vertebral bodies come together – on the anterior side – through the partial compression of the disc in its anterior half and through the slight posterior push of the nucleus pulposus; when in extension, things happen the other way around. If the nucleus pulposus must be considered the bearing on which the spine movements are made, the annulus fibrosus remains the most important element of the intervertebral disc, which resists to the compression and decompression forces.

From the point of view of normal goniometry, the spine presents complex movements resulted from the micro-movements of all the intervertebral joints: flexion – extension, lateral inclination, rotation, and as a resultant of all these – circumduction.

The ventral flexion movement of the trunk on the lower limbs is realized through the participation not only of the spine, but also of the hips.

The sacrum being fixed, the rest of the spine can fully execute a flexion movement, but not all its segments participate in the same way. The greatest flexion amplitude is registered in the cervical and lumbar regions.  The flexion movement has the biggest amplitude in the last two dorsal vertebrae and lumbar vertebrae.

The spine in its whole forms an anterior concavity arc, which is not a circle arc, but a curve line, made of three segments: one with a smaller radius formed by the cervical spine, one with a bigger radius formed by the dorsal spine and one with a small radius formed by the lumbar region.

When in flexion, the anterior portion of the intervertebral discs is compressed while the posterior common vertebral ligament, yellow ligaments, interspinous ligaments, superspinous ligament and back muscles are under tension.

When in orthostatic position, the muscles which initiate the flexion movement are those from the abdominal wall, especially the rectus abdominis and the two obliques, iliopsoas, as well as the infrahyoid and sternocleidomastoid. Once initiated the movement, the antagonist group of extensors come into action and graduate the flexion of the trunk, overcoming the gravitational forces.

To what concerns the extension movement in orthostatic position, things occur exactly the other way around. The extensor muscles are those that initiate the movement which is later controlled by the anterior muscular group. If, on the other hand, the extension movement is realized in ventral position, the extensors will keep holding the movement.

While in extension, the posterior parts of the intervertebral discs are compressed, while the anterior common vertebral ligament is put under tension. The extension is blocked in the final phase by the articular processes and ultimately by the spinous processes.

The lateral inclination movement reaches its maximal amplitude in the dorsal segment. When the spine coils a little, then the trunk bends even more to the lateral.

The rotation movement is maximal in the cervical region. The dorsal spine rotates a little and only if it bends laterally, the coiling movement in the lumbar spine is executed when the spine is in extension, especially in the dorso-lumbar segment. When the spine is flexed, the coiling movement in the lumbar segment is impossible because the vertebrae condyles are vertically positioned in the joints and stop the movement; this is also the reason why when in flexion the lumbar segment cannot bend laterally.

The biomechanics of the axial-atlantal joint

The movements realized between Atlas and Axis present some particularities because between these two vertebrae there is no joint to link the vertebral bodies, the Atlas not having such a thing. Also it neither presents inferior articular processes, the existing ones being reduced to mere articular surfaces, found on the inferior facets of its lateral masses. Together, the superior processes of the Axis realize the lateral axial-atlantal joints, flat joints similar to those between the articular processes of the other vertebrae.

There is, however, a median atlantal-axial joint realized by the axis odontoid process and the atlas osteo-fibrous annulus in which the odontoid process enters.

In the axial-atlantal joint only the rotation movement of the head is possible. During this movement, the odontoid process stays still, like a bolt, while the atlas ring rotates around it. For the atlas movement to be possible, this slides on the lateral axial particular facets. The rotation movement permitted by the axial-atlantal articular complex is only of 30º on both sides (left, right). Large amplitude rotations are realized through the participation of the subjacent vertebral joints.

The biomechanics of the occipitoatlantal joint

This joint permits flexion, extension, lateral inclination movements of the head, but not rotation movements.

The flexion-extension movement is realized around a transversal axis which passes through the superior area of atlas’ glenoid cavities, the head acting on the spine like a 1st degree lever, where the force is given by the neck muscles, the resistance by the weight of the head which tends to fall forward, and the fulcrum is on the spine.

The head flexion amplitude permitted by the occipitoatlantal joint is about 20º, while the extension one is of about 30º. This amplitude is only enough for the head moves which allow us to confirm something. The enhancement of the flexion-extension amplitude is possible only with the participation of the subjacent vertebrae.

The lateral inclination movement is limited to only 15º in the occipitoatlantal joint and it is realized around a sagittal axis that traverses every occipital condyle.