Vertebra

A vertebra is one of the bones stacked on top of each other to form the spine. Humans have 33 vertebrae: 7 cervical in the neck, 12 thoracic in the upper and middle back, 5 lumbar in the lower back, 5 sacral that fuse into the sacrum and 4 coccygeal that form the tailbone. Together with the skull, ribs and breastbone they make up the axial skeleton (DeSai et al., 2023).

What is a vertebra made of?

A typical vertebra has two main parts (Waxenbaum et al., 2026):

• The vertebral body, the thick cylinder at the front, carries most of the load passing down the spine. Inside is spongy (trabecular) bone containing red marrow, wrapped in a thin shell of compact bone.
• The vertebral arch at the back forms, with the body, the vertebral canal: the tunnel that holds the spinal cord. Seven processes project from it. The spinous process is the bump you can feel down the middle of your back, two transverse processes point sideways, and four articular processes meet the vertebrae above and below at the facet joints.

Between neighbouring bodies sit the intervertebral discs. An adult has 23 of them, with none between the top two neck vertebrae, and together they make up roughly a quarter to a third of the spine’s length (Waxenbaum et al., 2026). The spinal nerves leave the canal through the gaps formed by the discs, arches and facet joints of neighbouring vertebrae.

How vertebrae change from the neck to the lower back

The vertebral bodies get larger from top to bottom (Waxenbaum et al., 2026), so the smallest are in the cervical spine, where the topmost vertebra, the atlas, carries the skull.

In the thoracic spine, each vertebra has small facets where the ribs attach. The ribs and the long, downward-sloping spinous processes limit bending forwards and backwards here, while T5 to T8 allow the most rotation (Waxenbaum et al., 2026).

The five vertebrae of the lumbar spine have the biggest bodies. L5 is the largest of all, and its front is taller than its back, which helps create the angle where the lower back meets the sacrum (Waxenbaum et al., 2026).

Does sitting squash your vertebrae?

A common worry is that hours at a desk crush the spine. Pressure inside a lumbar disc was measured directly in one 45-year-old volunteer. Relaxed sitting gave 0.3 MPa, sitting upright without a backrest 0.46 MPa and relaxed standing 0.5 MPa, while sitting slumped as far forward as possible rose to 0.83 MPa (Wilke et al., 1999).

These numbers are only a guide. The authors cautiously concluded that sitting may load the disc less than standing, and that changing position often helps the flow of fluid that nourishes the disc. That supports regular microbreaks more than a hunt for one perfect posture.

When bone strength matters

The body of a vertebra is mostly spongy bone. In osteoporosis it can lose enough strength to squash down on itself, a vertebral fracture. These are the most common complication of osteoporosis (Delmas et al., 2005), and they don’t always cause the pain you would expect from a broken bone (Royal Osteoporosis Society).

What scans of the vertebrae can and can’t tell you

Signs of wear are very common on the scans of people with no pain. In a review of 3,110 people without symptoms, disc degeneration was found in 37% of 20-year-olds and 96% of 80-year-olds (Brinjikji et al., 2015). Much of this is normal ageing, which is why imaging is used to answer specific questions, not as a routine check (see X-ray). For everyday aches from desk work, lower back pain from sitting covers the common causes.

When to see a doctor

See a GP if back pain doesn’t improve after a few weeks, if you’ve lost weight without trying, if your back has changed shape, or if the pain is worse at night or comes from between the shoulder blades. Call 999 or go to A&E if back pain comes with numbness or weakness in both legs, loss of feeling around the genitals or anus, changes in bladder or bowel control, or chest pain, or if it started after a serious accident (NHS).