Facet joint tropism

Facet joint tropism refers to a situation where there is a difference in the orientation/angle of facet joints (i.e. between the left and right sides) with respect to each other in the sagittal plane. This can lead to unequal biomechanical forces on the facet joints and intervertebral disc during rotation and other physiological movements.

Epidemiology

The reported incidence in the lumbar spine has been variable at around 40-70%.

In the lumbar spine, the L4-L5 level is considered the most commonly affected level ¹. 

Radiographic features

Angle is calculated in the axial planes, with angle assessed between lines along the posterior border of the vertebral body and another line bisecting the superior and inferior tips of the facet joint process. The angle calculated was considered the facet joint angle and if the difference in the angulation of either facet joint at the same level, was 8° or more, it was considered as facet tropism⁷.

Associations

In the lumbar spine, it has been reported in some studies ²  but not in others ⁶ to have a higher association with:

• disc degeneration / lumbar disc herniation
• facet joint degeneration / lumbar degenerative spondylolisthesis