The spine, or rachis, is a stack of articulated bones called vertebrae. It supports the back of vertebrates, especially mammals. The ribs are attached to the spine. It houses the spinal cord.
In humans, it supports the head and transmits the body weight to the hip joints.
It is composed of 24 vertebrae (or 33 if we count the fused sacro-coccygeal vertebrae): seven cervical, twelve thoracic and five lumbar (plus five sacral and four coccygeal).
The spine is curved in the median sagittal plane, in a frontal plane. It has two primary curves (concave forward), also called kyphoses, in the thoracic and sacral spines, and two secondary curves (concave backward) called lordoses in the cervical and lumbar spines.
The cervical spine
It is composed of seven cervical vertebrae, named by the letter C: from C1 to C7. The first two cervical vertebrae are very particular and together with the occipital bone, the atlas and the axis form the craniocervicum, which is very mobile.
Upper cervical spine or “craniocervicum.
The occipito-cervical joints:The atlas
First cervical vertebra (C1), and consequently, first vertebra of the rachis.
Carries the head (by analogy to the Greek titan Atlas who carries the world).
Composed of :
- Of an anterior arch bearing anteriorly the anterior tubercle and posteriorly the fovea dentis for the axis tooth;
- A posterior arch ;
- Two unituberculated transverse processes with a foramen: the transverse foramen which allows the vertebral artery to pass;
- Two lateral masses where the articular surfaces for the condyles of the occipital bone are located on the upper surface, and the articular surfaces for the axis (2nd cervical vertebra) on the lower surface. Behind the articular surfaces of the upper surface, there are the two grooves of the vertebral artery (one groove on each side for each artery).
It has no body or spinous process.
Its vertebral foramen is large and roughly a rounded square.
It has two tubercles in its anterior portion where the transverse ligament of the atlas is inserted. This ligament delimits two lodges (ventral and dorsal).
In front, a compartment for the axis tooth and behind a compartment for the spinal cord.
The axis : Second cervical vertebra.
Defines an axis of rotation for the atlas with its odontoid process (or axis tooth).
Composed of :
- A body to which is attached, on its upper surface, the odontoid process, and on its lower surface an articular surface for C3 ;
- Two lateral masses bearing the articular surfaces with the atlas on its upper surface and C3 on its lower surface;
- Two unituberculated transverse processes, perforated by the transverse foramen which allows the vertebral artery to pass;
- A bituberculated spinous process connected to the transverse process by two blades.
The odontoid process (or axis tooth)
Vertical bony projection located on the upper surface of the body of the axis
On its posterior-superior surface, a dorsal articular facet articulating with the transverse ligament of the atlas
On its anterosuperior surface a ventral articular facet articulates with the fovea dentis of the atlas.
Serves as a pivot for the atlas and helps hold this vertebra in place to protect the spinal cord.
The vertebral foramen of the axis is roughly round, slightly smaller than that of the atlas, and does not have any special features.
Embryological and postnatal development of the craniocervicum
Vascularization of the axis tooth
No intervertebral disc between occiput and C1 and between C1 and C2
Particular vascular anatomy
Lower cervical spine
The C3-C4-C5-C6-C7 vertebrae are the continuation of the transition made by the axis.
They are composed of :
- An articular body on its superior and inferior surface with the sub- and overlying vertebrae. It has two uncus on its superior-lateral edges stabilizing the articulation with the overlying vertebra;
- Two bituberculated transverse processes (an anterior and a posterior tubercle), perforated by the transverse foramen which allows the vertebral artery to pass;
- Two lateral articular processes for the overlying and underlying vertebrae, separated from the body by the two pedicles;
- A bituberculated spinous process connected to the articular processes by two blades.
The vertebral foramen is triangular (the final shape of the vertebral foramen for the other vertebrae of the spine) and allows the spinal cord to pass through.
On the upper side of the transverse processes, there is a groove that starts from the body, passes through the pedicle, and ends in the foramen of conjugation. The nerve roots of the spinal nerves pass through this groove.
C6 and C7 are standard cervical vertebrae, with some differences.
C6 vertebra
The anterior tubercle of its transverse process is larger than those of the other cervical vertebrae. It is called the carotid tubercle.
C7 vertebra
C7 is a transitional vertebra between cervical and thoracic spine.
Its spinous process is unituberculated, very long and very inclined backwards and downwards. It represents the posteroinferior limit of the neck.
It is easily palpable under the skin: going down the nape of the neck, it is the first large projection under the skin.
It is at this height that the buffalo hump, which is a form of cellulitis, forms.
The dorsal or thoracic spine
It is composed of twelve dorsal or thoracic vertebrae, designated by the letters T or D: from T1 to T12, or from D1 to D12.
It follows the cervical spine and precedes the lumbar spine.
The dorsal spine forms a physiological convex posterior curve called
dorsal cyphosis in case of excess or
dorsal or thoracic lordosis in case of inversion, or even “flat back”.
Lumbar spine, sacrum and coccyx
The lumbar spine is composed of five lumbar vertebrae, designated by the letter L: L1 to L5.
The lumbar spine forms an anterior curvature called lordosis.
It follows the dorsal spine and precedes the sacral spine.
The sacral spine is also called the sacral spine. The five sacral vertebrae are fused together in adulthood, forming a single bony block called the sacrum.
It is composed of five sacral vertebrae, named by the letter S: from S1 to S5.
It follows the lumbar spine and precedes the coccygeal spine.
It is inclined about 45 degrees backwards.
It forms the posterior part of the pelvis and thus ensures its solidity.
The transverse processes, because of this welding, are no longer distinguished and form a bony blade on both sides, these are the wings of the sacrum.
The same is true for the spinous processes, which now form only small bumps on the posterior surface of the sacrum.
The upper articular surface of the first sacral vertebra S1 forms the head of the sacrum, which articulates with the last lumbar vertebra, L5.
On the edges of the sacrum, in the upper part, there is an articular surface with the iliac bone (coxal bone), this is the auricular articular surface (because it is shaped like an ear). It faces backward and is connected to its iliac bone counterpart to form the sacroiliac joint (a synovial joint, reinforced by anterior, posterior and interosseous sacroiliac ligaments). This joint allows very little movement and transmits the weight of the upper body to the hip joints when the person is standing.
On the anterior and posterior surfaces, four pairs of sacral foramina allow the ventral and dorsal branches of the spinal nerves to pass through.
At the proximal (lower) part of the sacrum is the sacrococcygeal joint, which articulates it with the coccyx. It is a cartilaginous joint that allows almost no movement.
Coccyx
Bone remnant: tail of mammals.
Four or five coccygeal vertebrae fused together.
Follows the sacrum and constitutes the lower end of the rachis, thrown forward.
It is often the site of pain during a shock to the posterior or even a fracture-luxation.
Intervertebral joints
Three joints
Each vertebra articulates with the vertebra above and below it (except for the atlas which articulates with the occipital condyles and of course the coccygeal vertebrae).
The vertebrae are joined by three joints:
the intervertebral disc uniting the plates of two vertebrae.
the two posterior inter-apophyseal joints, where the upper articular processes of a vertebra are articulated with the lower articular processes of the overlying vertebra.
Strength is provided by:
the common anterior and posterior vertebral ligaments,
the interspinous ligaments,
the yellow ligaments,
the interapophyseal ligaments,
the spinal muscles antagonists of the movements,
the capsules of the posterior apophyseal joints.
Mobility
The movements of the spine are possible thanks to the existence of a complex articular system, the spinal articular segment, a functional unit constituted by :
the disco-corporeal or disco-somatic complex,
the posterior inter-apophyseal joints,
the intervertebral ligaments and the vertebral muscles.
This articular segment allows movements in a plane, the amplitude of which is very variable according to the vertebral stage considered:
sagittal (flexion extension) ;
frontal (lateralities) ;
transverse (rotation in the axis of the spine).
The differences in amplitude are due to vertebral anatomical differences, notably :
sagittal inclination of the articular apophyses ;
the sagittal inclination of the spinous processes;
the difference in thickness of the intervertebral disc;
the articulation with other elements (head, thoracic cage);
the type of convexity (front or back) of the floor.
Anatomical relationships
The spine is obviously not isolated in the organism, it is even the pillar of it and presents as such many relationships with the surrounding structures.
Anterior
Blood vessels
In addition to the vascularization of the spine itself, there are vessels that pass through or near the spine.
The vertebral artery
The vertebral artery is a branch of the subclavian artery and ascends into the canal formed by the superposition of the transverse foramina (transverse canal) at the level of the cervical spine. It usually enters this canal at the level of C6, but may enter at C7 or much higher (sometimes it crosses only three or four vertebrae). At the level of the atlas, it bends at a right angle and enters the brain through the foramen magnum. There, it joins the other vertebral artery, forming the basilar trunk and participating in the arterial circle of the brain (the polygon of Willis).
Inferior vena cava, renal veins and thoracic aorta (then abdominal aorta)
It is the continuity of the aorta and descends along the thoracic spine at its anterior wall. It is offset to the left in relation to the axis of the spine and thus allows, for example, orientation on a medical image. At the level of the lumbar spine, it gives the two common iliac arteries.
Anterior radicular artery of Adamkiewicz
Sympathetic nerve plexus (erection)
The aerodigestive axis
This is the set formed by the esophagus and the larynx and then the trachea. It begins opposite C4 and continues to T4 (where the trachea divides into two bronchi). It is medial, i.e. it runs just in front of the vertebral bodies.
Posterior
The muscles
The rachis is an important zone of muscular insertions. Indeed, it serves as an anchor for the posture muscles and, at the neck level, it receives a good part of the cephalogy muscles, which move the head. Main muscles with insertions on the spine :
Cervical spine
Posterior rectus muscle of the head
Posterior rectus muscle of the head
Anterior rectus abdominis muscle of the head
Anterior rectus muscle of the head
Superior oblique muscle of the head
Lower oblique muscle of the head
Longissimus muscle of the head (petit complexus)
Longissimus muscle of the neck
Semi-spinous muscle of the head (grand complexus)
Splenius
Trapezius muscle
Long muscle of the head
Long neck muscle
Scalene muscles
Thoracic spine
Dorsalis major muscle
Trapezius muscle and all the
interspinous muscles (between the spinous processes), then all the other muscles
muscles of the posture.
Embryology of the spine
In an embryo at the initial stage, we will see at the dorsal level that the outline of the neural system is open and is made of the neural gutter at the origin of the nervous system.
In front of the neural gutter there is the chord that will partially regress. On both sides, there are cellular clusters called somites at the origin of two types of structures:
The myotomes, at the origin of the paravertebral musculature (which surround the vertebrae)
The sclerotomes which will approach the neural tube and will be at the origin of the cartilaginous formation which will give the vertebra by ossifying
The neural gutter will close on the median line and is of ectodermal origin (embryonic sheet giving the skin and the nervous system). This will give the neural tube then the central nervous system. The tube is dorsal to the sclerotomes. These will divide into two. A vertebra is the union of two lower half sclerotomes and two upper half sclerotomes. We thus have the formation of a cartilaginous core, meeting of four halves of sclerotome.
The following sclerotome will give rise to a structure called the intervertebral disc. This disc will allow the chord to persist in the form of a residue called nucleus pulposus in the center of the disc with a fibrous ring in the periphery. Dorsally, we have the neural tube which will be protected by a posterior arch and will give the spinal cord. At the level of the rachis, the vertebral bodies emit extensions which will close and contain the neural tube.
Biomechanics of spinal instrumentation
The pathology of the spine has a double impact on :
the statics: deformations
the content of the spinal canal: spinal cord above L1, roots of the cauda equina below, with the constitution of neurological disorders.
The instrumentation takes into account this double impact, if necessary.
Trauma to the spine
real traumas (accidents)
Cervical spinal trauma
upper cervical
low cervical
thoraco-lumbar
in case of metabolic disease: fractures due to osteoporosis or osteomalacia anomalies
Disc trauma and “degenerative”
Herniated discs
Degenerative conditions: osteoarthritis and its derivatives
Tumors and infections
Primary or secondary tumors (secondary bone cancer)
Infections of the epidural space (without bone involvement!), of the vertebral bodies (spondylitis, rare), of the discs (discitis) or of both (spondylodiscitis, frequent)
At the level of the cranium, there is a connection called cordoma, a tumor of the cord residue (see the part “embryology of the spine”). It is benign and extremely recurrent.
Rheumatism
Inflammatory diseases (inflammatory rheumatism), mainly
ankylosing spondylitis or
rheumatic arthritis
Spinal deviations
Congenital” malformations such as, for example, a somitic nucleus has not ossified, so we have a vertebral body where 1/4 of the structure is missing, so we have a deviation of a part of the spine. Sometimes, a hemivertebra is missing.
Scoliosis: Deviation of a curvature in a coronal plane such as the thoracic spine with a right lateral concavity. Associated with this curvature, there is a vertebral rotation.
Neuromuscular: poliomyelitis
Idiopathic: that which has no determined etiology in the current state of knowledge but among them, idiopathic scoliosis, the most frequent
