Vertebral Column and Back

What you will be able to do

One column, two contradictory jobs

Stiffness and mobility rarely come in the same structure. The column solves that by dividing the work between many small units. No single joint between two vertebrae moves far, but add two dozen together and you get a trunk that bends in every direction.

There are 33 vertebrae: seven cervical, twelve thoracic, five lumbar, five fused into the sacrum and three to five fused into the coccyx. Only the top twenty-four move independently. Everything below has traded movement for carrying your weight into the pelvis.

Figure 1 · The five regions

Seen from the side the column is not straight. At birth it forms one long curve concave forwards, and two parts of it stay for life — the thoracic and the sacral. These are the primary curves, convex backwards, present from the beginning.

The other two are earned. When an infant lifts its head against gravity the neck develops a curve convex forwards; when the child stands and walks, the low back does the same. These secondary curves, cervical and lumbar, exist because of what the child learned to do.

A typical vertebra

Every vertebra follows one plan, and once you have it the regional differences become small edits rather than new bones. In front is the body. Behind it is the vertebral arch, built from two pedicles running backwards and two laminae closing the ring. Between the two is a hole. Seven processes grow off the arch: one spinous pointing backwards, two transverse pointing sideways, and four articular — two up, two down.

Figure 2 · A typical vertebra, how two of them meet, how the regions differ, and the top two that break the plan

Illustration to be added

Four panels on white, bone warm ivory with navy outlines, cartilage pale blue. Panel one: a typical vertebra from above, filling the panel, every part reachable by a leader line - body in front, two pedicles running backwards, two laminae closing the arch, the vertebral foramen shaded pale grey-blue, one spinous process, two transverse processes, and the superior articular processes with facet surfaces tinted. Panel two: two vertebrae articulated, from the left side, disc between the bodies in a distinct tint and the intervertebral foramen a clear opening - shade the inferior notch of the upper pedicle and the superior notch of the lower pedicle in two different tints so the reader sees the opening is built from both, with the spinal nerve in gold leaving through the upper part of it, not the middle. Panel three: cervical, thoracic and lumbar vertebrae from above, side by side to the same scale - cervical small and broad with a foramen transversarium pierced in each transverse process and a red vertebral artery through it, thoracic heart-shaped with a costal facet on the body and another on the transverse process, lumbar massive and kidney-shaped with long thin transverse processes. Panel four: atlas and axis drawn separately then assembled, from above, showing the atlas as a ring with no body and no spinous process, the axis carrying the dens upwards, and the transverse ligament in navy holding the dens against the anterior arch with the cord space behind. No text inside the artwork; leader lines out to labels set in the margin, panel by panel.

Hold that last row. The intervertebral foramen is bounded in front by the disc and bodies, and behind by the facet joint. A bulging disc narrows it from the front, an arthritic facet from behind, and the nerve is the structure with least room to give. In the neck a third structure crowds the same door: the small uncovertebral joints along the upper edges of the bodies, whose bony spurs are a common cause of a narrowed cervical foramen.

What makes each region different

The hole in each cervical transverse process is the foramen transversarium, and it exists to protect a vessel. The vertebral artery ascends through these foramina, usually entering at the sixth vertebra and climbing to the skull. Bony change in the neck can therefore matter to more than the nerves.

The top two vertebrae abandon the plan altogether. The atlas has no body and no spinous process: it is a ring of two arches and two lateral masses, a washer between skull and column. The axis carries the body the atlas gave up, projecting upwards as the dens. The atlas turns around that peg.

The joints between two vertebrae

At every ordinary level below the axis there are three joints, of two completely different kinds. The top two levels are the exception, and are dealt with a little further on.

In front, the bodies are joined by the intervertebral disc. This is a secondary cartilaginous joint, a symphysis, with no cavity and no synovial membrane. It is built to carry load and to permit a small movement in every direction.

Behind, the articular processes meet as a pair of zygapophyseal joints, usually called facet joints. These are small plane synovial joints, each with a capsule and a little fluid. Their real job is to guide movement and to prevent the movements the region should not make.

From the third to the seventh cervical vertebra there are two more. Small lipped joints, the uncovertebral joints, sit where the raised side edge of one body meets the one above, making five articulations at those levels rather than three.

Figure 3 · Why each region moves the way it does

The facet joints block unwanted movement by geometry alone. A surface lying almost flat lets one bone slide across another in any direction. A surface standing upright and facing sideways becomes a wall. In the lumbar spine the facets face each other across the midline like two hands pressed together, and rotation runs straight into bone.

The top of the column: nodding and turning

Neither of these joints has a disc, so the pattern of three is not the pattern here. The joint with the skull is a pair of synovial joints and nothing else. Below it, the atlas meets the axis at three synovial joints: one in the midline on the dens, and one on each side.

The dens is held against the front arch of the atlas by the transverse ligament of the atlas. That band is the main restraint keeping the peg forward; behind it lie only the tectorial membrane and the dura before the cord is reached.

Anything that loosens the ligament, or a fracture of the dens itself, leaves the peg free to be driven backwards into the upper cord. That is why a suspected dens fracture or a lax transverse ligament means no neck movement and no manual technique until it has been imaged.

The intervertebral disc

Twenty-three discs run from between the second and third cervical vertebrae down to the joint with the sacrum. Each has three parts.

Figure 4 · The intervertebral disc

Illustration to be added

Three panels on white. Panel one: a disc from above, front towards the top, drawn as a large flattened oval. Show the anulus fibrosus as concentric rings and cut a wedge from the near quadrant so four or five successive rings are exposed in depth - in each exposed ring draw the collagen fibres as fine parallel diagonal lines, each ring sloping the opposite way to the one beside it so the alternating criss-cross is unmistakable. Place the nucleus pulposus centrally as a soft pale blue gel, sitting slightly behind the middle, and make the anulus visibly thinner at the back than the front. Panel two: a sagittal cut through two bodies and the disc between, cartilaginous endplates as thin pale blue sheets on the bone surfaces, and small gold arrows passing from the marrow through the endplate into the disc to show nutrition arriving by diffusion - no blood vessel of any kind may enter the disc itself. Beside it repeat the cut with the disc compressed and shorter, arrows pointing outward, for fluid lost under load through the day. Panel three: a transverse view at the fourth-to-fifth lumbar level, the body kidney-shaped - anulus torn at the back corner, nucleus material pushed through, the posterior longitudinal ligament drawn narrow in the midline in navy. Draw two roots in gold: the fourth lumbar root already leaving high in its foramen, out to the side and clear of the bulge, and the fifth lumbar root still descending in the canal with the extruded material pressed against it. Distinguish the two by a tint, not by text. No text inside the image.

That last point governs the rest. The adult disc has no blood supply. Nutrients diffuse in through the endplate and waste leaves the same way, and load drives the exchange: squeezing pushes fluid out, releasing draws it back. A spine held in one position all day is a spine whose discs are fed badly, and a torn disc has almost no capacity to repair itself.

You can measure the consequence on yourself. Through the day the discs lose water and you get shorter, commonly by a centimetre or two, and lying down overnight returns it.

The ligaments

Two long ligaments run the length of the column and short ones tie neighbouring arches together. Each resists the movement that would pull its two ends apart, so you can work out its job from where it lies.

The elasticity of the ligamentum flavum is worth a second look. Being elastic rather than merely tough, it lengthens smoothly in flexion and shortens again without folding into the canal. With age it thickens and loses that quality, and a thickened ligament buckling inwards is one of the things that narrows a canal.

The muscles of the back, in layers

Not every muscle on your back is a back muscle. The large superficial sheets are limb muscles that migrated backwards in development and took their nerves with them. Trapezius and latissimus dorsi lie on the back and belong to the upper limb.

Before the rule that sorts them, one piece of wiring. Every spinal nerve splits as it leaves the column. A small branch turns backwards to the muscles and skin of the back, and that is the posterior ramus. A much larger one runs forwards to the trunk wall and the limbs, and that is the anterior ramus.

Deep to the thoracolumbar fascia are the muscles that actually move the column. These are the intrinsic back muscles, and almost all of them are supplied by posterior rami. A large muscle lying on the back but fed by an anterior ramus is a visitor.

The exceptions are small and worth knowing rather than worth fearing. Among the short segmental muscles, the ones running in front of the neck's transverse processes and out to the side of the lumbar ones take anterior rami instead.

Figure 5 · The muscles of the back, layer by layer

Illustration to be added

One tall posterior view of the trunk and neck, dissected in stages from the reader's left to the reader's right, so a single figure carries four depths at once. Far left: skin removed only, showing trapezius above and latissimus dorsi below in brick red with their fibre directions drawn. Next: those removed to show levator scapulae, the rhomboids and the two serratus posterior sheets. Next: those removed to expose the thoracolumbar fascia as a broad pale cream sheet over the lumbar region, with a window cut in it revealing the erector spinae as three distinct vertical columns - iliocostalis laterally, longissimus centrally, spinalis medially - and splenius capitis and cervicis in the neck above. Far right: erector spinae removed to show the transversospinalis group running upwards and inwards from transverse to spinous processes, semispinalis spanning the most segments in the neck, multifidus filling the groove beside the lumbar spines, and short rotatores deepest. Add a small inset at the foot: a transverse cut through the lumbar region showing the three layers of thoracolumbar fascia, with erector spinae in one compartment and quadratus lumborum in another. Muscle brick red, fascia pale cream, bone warm ivory, nerves gold. No text inside the image; label to the margin.

The thoracolumbar fascia makes the arrangement work. In the lumbar region it has three layers, wrapping the erector spinae in one compartment and quadratus lumborum in another. Latissimus dorsi, transversus abdominis and internal oblique all attach to it, so a pull on the abdominal wall reaches the back directly.

Figure 6 · What keeps a spine stable

What is inside the canal

The canal runs the whole length of the column; the spinal cord does not. In the adult the cord ends at about the first or second lumbar vertebra. Below that the canal holds a bundle of roots hanging down like a horse's tail, which is what cauda equina means.

The reason is growth. In the newborn the cord reaches to around the third lumbar vertebra. The bony column then grows faster and further than the cord, so each root is drawn downwards to reach the foramen it started opposite. The lower the root, the longer it travels inside the canal before leaving.

That decides where a needle goes. Below the second lumbar vertebra there is fluid and a loose bundle of roots that a blunt needle pushes aside, and no cord to injure. The line joining the highest points of the iliac crests finds the level, crossing near the fourth lumbar spine. In the midline the needle passes skin, fat, the supraspinous and interspinous ligaments and the ligamentum flavum. It then crosses the epidural space, with its fat and veins, and finally pierces dura and arachnoid — the second give — to reach the fluid.

What goes wrong here

Where students get this wrong

Expecting 33 discs to go with 33 vertebrae

There are 23. None between the skull and atlas, none between atlas and axis, and none inside the fused sacrum or coccyx. The first is between the second and third cervical vertebrae.

Saying that a disc slips

It cannot. The disc is bound firmly to the endplates above and below. The anulus tears and material from the centre is pushed through. The wording matters, because patients hear "slipped" and stop moving.

Expecting a disc to bulge straight backwards

The posterior longitudinal ligament defends the midline. The corner between behind and to the side is defended by nothing, which is where the bulge goes and where the roots are.

Naming the root from the upper bone of the pair

In the lumbar spine the root leaves high in its foramen, so the usual bulge misses it and catches the root travelling past. In the neck the roots leave above their own vertebra. Work it out from the anatomy each time.

Thinking the cord fills the canal down to the sacrum

It ends at about the first or second lumbar vertebra. Below that are loose roots in fluid, which is precisely why a needle can be placed there safely.

Treating everything on the back as a back muscle

Trapezius and latissimus dorsi are limb muscles that ended up there. The intrinsic muscles lie deeper and take posterior rami — with the short intertransversarii of the neck and loin as the recognised exceptions, which is worth saying rather than pretending the rule is absolute.

Check yourself

Ten questions on this chapter. Tap one to see the answer and the reasoning.

Quick review

Further reading

Reviewed by the Physiotherapist India Team. · Human Anatomy contents