The vertebral column

Why it matters

The vertebral column is a flexible column that supports the head, neck and body and allows them to move ^[1]. It also protects the spinal cord, which passes down the back through openings in the vertebrae ^[1].

What it is

The column develops as a series of 33 vertebrae, but this number is reduced to 24 vertebrae plus the sacrum and coccyx ^[1]. It has five regions, and the vertebrae in each are named for the region and numbered from the top down ^[1]:

• seven cervical vertebrae, C1 to C7, in the neck ^[1];
• twelve thoracic vertebrae, T1 to T12 ^[1];
• five lumbar vertebrae, L1 to L5, in the lower back ^[1];
• the sacrum, formed by the fusion of five sacral vertebrae ^[1];
• the coccyx, formed by the fusion of four small coccygeal vertebrae ^[1].

How it works

A typical vertebra has a body, a vertebral arch and seven processes ^[1]. The body is the front part and supports the body weight; the arch forms the back part, with a pedicle on each side and laminae forming its roof ^[1].

The large opening between the arch and the body is the vertebral foramen, which contains the spinal cord; in the intact column the foramina line up to form the vertebral canal ^[1]. Between neighbouring vertebrae, notches in the pedicles form the intervertebral foramina, through which the spinal nerves leave ^[1].

Why it is built this way

The adult column is not straight: it has four curves, which increase its strength, flexibility and ability to absorb shock ^[1].

• The thoracic and sacrococcygeal curves are primary curves, kept from the single forward-concave curve of the fetus ^[1].
• The cervical and lumbar curves are secondary curves, concave towards the back; the cervical curve develops as an infant holds its head up, and the lumbar curve as the child begins to stand and walk ^[1].

When the load on the spine increases, for example with a heavy backpack, the curves deepen to accommodate the extra weight ^[1].

When it does not behave

Excessive or abnormal curves include kyphosis, an excessive backward curve of the thoracic region; lordosis, an excessive forward curve of the lumbar region; and scoliosis, a sideways curve with twisting of the column ^[1]. Compensatory curves may develop elsewhere in the column to keep the head over the feet ^[1].

How we see and measure it

The size of a sideways curve is measured as the Cobb angle, which equals the sum of the tilts of the upper and lower end vertebrae ^[2]:

Cobb angle from the end-vertebra tilts^[2]

$$\angle\mathrm{Cobb} = \alpha + \beta$$

The Cobb angle of a curve in the coronal plane equals the tilt of the upper end vertebra plus the tilt of the lower end vertebra, each measured from the horizontal. Wang and colleagues derived this from plane geometry and used it to measure the angle without drawing the classical perpendicular lines.

Variables of Cobb angle from the end-vertebra tilts

Symbol	Meaning	Unit
$\alpha$	upper end vertebra tilt	°
$\beta$	lower end vertebra tilt	°
$\angle\mathrm{Cobb}$	Cobb angle	°

Valid when

• Tilts measured on the same coronal image, on opposite sides of the horizontal
• Which vertebrae are the end vertebrae is a judgement; the angle depends on it

Worked example^[2]

The worked case in Wang et al. (their figure): tilts of 41° and 30° give a Cobb angle of 71°.

alpha = 41 °, beta = 30 ° → cobb = 71 °

upper end vertebra tilt (°)lower end vertebra tilt (°)

Cobb angle: 71 °

Common misconceptions

Check yourself

Which two curves are present before birth?

The thoracic and sacrococcygeal curves, the primary curves kept from the fetal curvature ^[1].

What forms the vertebral canal?

The vertebral foramina of all the vertebrae, lined up in the intact column ^[1].

Read next

• The intervertebral disc.
• Sagittal alignment.
• Alignment and load lab.