Spinal Motion
The spinal motion segment is also known as the functional spinal unit and represents a segmental model of spinal function. It consists of two adjacent vertebrae and the three joints (two posterior facet joints and the intervertebral disc) and associated ligaments in between [Figure 1 ]. Your physician or physical therapist may discuss parts of the spinal motion segment to explain spinal motion, strength, and flexibility and how the spine works. Any two vertebrae (except the fused sacral and coccygeal vertebrae) can represent a motion segment of that spinal region.
Figure 1: Anatomy of a spinal motion segment: two vertebrae, one intervertebral disc, and ligaments
Artificial functional spinal units have been used in research and tested for mobility and strength under various conditions of fatigue and weight to understand better the stresses the human spine sustains under normal and diseased states. Your doctor can explain why more pressure is placed on the nucleus of the spinal disc when sitting as compared to standing or lying down. Or why support to the lumbar region when sitting reduces pressure on the disc and may be a good habit to apply.
Figure 2: spondylolisthesis
Each individual motion segment has a normal range of motion, the limits
of which help contribute to the stability of the spine. When one or more
parts of a motion segment are affected by disease, trauma, or degeneration,
the tissues do not interact normally. The result may be pain, limited motion,
or instability. Motion segment laxity results in an increase in vertebral
column motion (instability) when the spine is under stress. The quality
of spinal motion becomes diminished and you may feel unstable when bending
or pushing and lifting an object. Because many tissues and factors contribute
to motion segment stability, a balance among factors exists that, if attended
to and managed by therapeutic exercises, limit abnormal motion segment
motion and relieve pain. Early treatment may slow or prevent more advanced
disease in many cases. Degenerative
spondylolisthesis can result from severe laxity of a spinal motion
segment due to degenerative changes in the facet
joints. These joints are no longer able to keep the vertebra to which
they belong from slipping forward and past the vertebra below.
[Figure
2