The Lower Back - Bones

What are vertebrae?

The spine is made up of bones called vertebrae.

There are 33 vertebrae in the human spine (7 cervical, 12 thoracic, 5 lumbar, 5 sacral, and 4 coccygeal).

The bones are shaped in a way that allows them to bear weight, and resist shear forces, as well as move in various directions.

While vertebrae have unique regional features, every vertebra has three functional parts:

1. A drum-shaped body designed to bear weight and withstand compression (blue)

2. An arch-shaped bone that protects the spinal cord (green)

3. Star-shaped processes designed as outriggers for muscle attachment (nude)

There are parts of the vertebrae which have different functions:

• Vertebral body

  The main part, which bears weight

• Pedicles

  Make up the lateral portion of the spinal canal, where the spinal cord passes through

• Transverse processes

  Attach to muscles and allow rotary movement of the spine

• Laminae

  Make up the posterior part of the spinal canal

• Spinous processes

  Muscles attach to allow spinal extension

• Articular facets

  Connect each bone to each other, and allow the spine to resist shearing forces

The lumbar vertebrae are the largest.

Compared to other parts of the spine, the lumbar region is able to flex and extend as well as rotate to a high degree.

• The bones are numbered 1-5 from top to bottom.

• When viewed from the side, an adult spine has a natural S-shaped curve. 

• The neck (cervical) and low back (lumbar) regions have a slight concave curve, and the thoracic and sacral regions have a gentle convex curve. 

• The curves work like a coiled spring to absorb shock, maintain balance, and allow range of motion throughout the spinal column.

What connects the spine?

There are two types of joint in the lumbar spine. Both of these articulations are not unique to the lumbar vertebrae, and are present throughout the vertebral column.

• Between vertebral bodies – adjacent vertebral bodies are joined by intervertebral discs, made of fibrocartilage. This is a type of cartilaginous joint, known as a symphysis.

• Between vertebral arches – formed by the articulation of superior and inferior articular processes from adjacent vertebrae. It is a synovial type joint.

Where are the discs?

Between the bones, there are discs which help to absorb compressive load.

The disc is soft and made up of two main parts:

1. Nucleus pulposus

2. Annular fibrosis

The majority of the disc itself is not innervated by nerves, meaning it is not sensitive to pain.

Discs function like coiled springs.

• The crisscrossing fibers of the annulus pull the vertebral bones together against the elastic resistance of the gel-filled nucleus. 

• The nucleus acts like a ball bearing when you move, allowing the vertebral bodies to roll over the incompressible gel. 

• The gel-filled nucleus contains mostly fluid. This fluid is absorbed during the night as you lie down and is pushed out during the day as you move upright.