Facet joint orientation in the lumbar and thoracic spine
Facet joints (small paired joints at the back of each vertebra) lie close to the sagittal plane in the lumbar spine and the coronal plane in the thoracic spine.
Remember
Lumbar facets lie close to the sagittal plane, so the lumbar spine mainly bends forwards and backwards and restricts twisting, whereas thoracic facets lie close to the coronal plane and, with the rib cage, favour rotation over flexion and extension.
Memory aid
The thorax is a twister in a cage
The ribs stiffen the thoracic spine like a cage, but it can still twist.
Facet joints set the directions in which a spinal segment can move. The sagittal plane is the vertical front-to-back plane that divides left from right. The coronal plane is the vertical plane that divides the body into front and back halves.
In the lumbar spine the facets lie close to the sagittal plane, at about 90 degrees. Joints set this way suit flexion (bending forwards) and extension (bending backwards), while they restrict axial rotation, the twisting of one vertebra on the next around the vertical axis. They also help to carry load in the lower back and are a common source of low back pain.
The thoracic spine is the contrast. Its facets lie close to the coronal plane. Facets set this way resist the forward and backward gliding that bending needs, so flexion and extension are limited, yet they still allow some rotation. The ribs add further stiffness. Thoracic vertebrae are joined to ribs at the costovertebral joints (where the head of a rib meets the vertebral body), and the rib cage acts like a splint. Long spinous processes (bony spikes that can be felt down the back) that slope downwards also limit flexion and extension.
The cervical facets lie at about 45 degrees and the cervical spine is a very mobile region. The change from stiff thoracic to mobile lumbar facets is at the thoracolumbar junction (where the thoracic spine meets the lumbar spine), and the orientation changes there from coronal to sagittal.
Key facts
- Lumbar facets
- they lie close to the sagittal plane, at about 90 degrees, so flexion and extension predominate and axial rotation is restricted.
- Thoracic facets
- they lie close to the coronal plane, which favours rotation over flexion and extension.
- Cervical contrast
- the cervical facets lie at about 45 degrees, and the cervical spine is a very mobile region.
- Rib cage
- the rib head joints help to stabilise the thoracic spine, which is a relatively immobile region.
- Thoracic discs
- they are on average the thinnest in the spine, so disc thickness does not explain thoracic stiffness.
- Change of orientation
- the facets change from a coronal to a sagittal orientation at the thoracolumbar junction.
Exam traps
- A coronal facet orientation is not the lumbar pattern, because the facet joints change from a coronal to a sagittal orientation at the thoracolumbar junction, so a coronal orientation belongs to the region above.
- Lumbar facet joints do not leave twisting unrestricted: flexion and extension do predominate in the lumbar spine, but the facet joints restrict axial rotation.
- Greater thickness of the thoracic intervertebral discs does not explain the stiffness, because thoracic discs are on average the thinnest in the spine, and a thicker disc would increase movement rather than reduce it.
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