Anatomically, quadratus lumborum originates from iliac crest and inserted into the transverse processes of lumbar vertebrae and twelfth rib. From the point of biomechanics, it has been presented as side-flexor (lateral flexor) of spine. In this article, we shall analyze the possibilities of lateral flexion of lumbar spine taking their facet orientation seriously into account. There has been a wide range of studies related to the plane of orientation of lumbar facets. But when compared to facets of thoracic spine, lumbar facets are more saggitally oriented. This sagittal plane orientation makes the lumbar spine more specialized for flexion and extension. (It should be noted at this point that the coronal plane orientation of facets of thoracic spine makes them more specialized for lateral flexion and transverse plane rotation as well).

If the entire vertebral column is personified as a plant, lumbarest then its root is sacrum which is deeply rooted between two innominate bones. The whole world of biomechanics will agree that the sacrum (root of spine) is sacrificed for its stability not for mobility. Thus, any vigorous movement of trunk should not disturb the root of spine, so the saggital plane orientation of lumbar facets can effectively prevent root disturbing forces as they can act as a barrier for lateral flexion and transverse plane rotations of lumbar spine.

Hypothetically, we consider two factors that are responsible for lateral stability of lumbar spine or lumbar lateral stability index;

1. Facet joint space (distance between articulating facets)

2. Facet height/ transverse length of lumbar vertebral body ratio (Lumbar Lateral stability index?)

We shall understand the concept of lumbar lateral stability index through a simple geometrical method.

Draw a line whose length is 6 cm. Let the terminal points of this line be A and G. Plot 5 points on the line AG at 1 cm interval and name them as B, C, D, E and F, from point A towards point G. Draw a line running downwards from point B to about 1 cm and let this 1 cm line be perpendicular to the line AG. Let the tip of this 1 cm line be H. Draw another 1 cm line (IJ) parallel to line GH ensuring 0.3 cm interval between them.

Regard;

1. Line AG as transverse length of body of a lumbar vertebra (say, L-3)

2. Line GH as the height of right inferior facet of a lumbar vertebra (say, L-3)

3. Line IJ as the height of right superior facet of a lumbar vertebra located below (say, L-4)

4. 0.3 cm interval between line GH and IJ as facet joint space

5. Parallel arrangement of GH and IJ as saggital plane orientation of lumbar facets.

5. Point A as left lateral edge of L-3 and point G as right lateral edge of L-4.

6. This entire arrangement as the posterior view of L-3 and L-4.

Now, take let us draw arcs from various points on the line AG to see whether point H collides on the line IJ. This arc study is to identify the scopes and magnitude of lateral flexion of lumbar spine. In this example, we shall study the possibilities and the extent of left lateral flexion of L-3 on L-4. For example, after selecting a point (say, F), keep the compass on point F and align the pencil to be on point H, and then draw an arc superiorly. An arc drawn from point F like this will intersect the line IJ. That point of intersection can be called as “Limiting point” which is the point that restricts lateral flexion. Likewise, try on all points from G to A. You will find the arcs drawn from D, C, B and A are not intersecting, so no limiting points. (Arc drawn from D will go very close (tangential?) to IJ but no intersection on IJ takes place. You also can find the arc drawn from a point between D and E does not intersect IJ. Even the arc drawn from point E intersects IJ on a point close to I). According to this example, the distance between point G and D is 3 cm.

Hypothetically, we shall also consider Lumbar lateral stability index = Facet height / Transverse length of lumbar vertebral body. If lumbar lateral stability index equals 1 or greater than 1, then lateral flexion of lumbar spine is very much restricted. If lumbar lateral stability index is less than 1, then lateral flexion of lumbar spine is unrestricted. Not only that, if lumbar lateral stability index is much less than 1, then the lumbar spinal column must require very strong collateral musculo-ligamentous support to prevent lateral-ward dislocations.

Example: 1

If the transverse length of lumbar vertebral body (in this experiment, FG) is equal to the height of the facet (GH), then;

Lumbar lateral stability index = Facet height/ Transverse length of lumbar verterbral body

= 1 cm / 1 cm

= 1

Example: 2

If the transverse length of lumbar vertebral body (in this experiment, DG) is greater than the height of the facet (GH), then;

Lumbar lateral stability index = Facet height/ Transverse length of lumbar verterbral body

= 1 cm / 3 cm

= 0.33

Obviously, if the transverse length of all lumbar vertebral body is greater than the height of the facets; then lateral flexion of lumbar spine is also feasible. But we have taken the facet joint space comfortably as 0.3 cm but in reality facet joint space plays crucial role in deciding lumbar lateral stability index. Smaller the facet joint space (distance between GH and IJ), greater will be the lumbar lateral stability index.

In case, if the lumbar facet joint space in vivo is too narrow, then the role of quadratus lumborum is questionable. I mean, quadratus lumborum may have a very limited function as side-flexor of lumbar spine.