Lumbosacral Angle

How Does It Affect Your Spinal Curves?

Model of the lumbosacral spine from the back.
Model of the lumbosacral spine from the back. Images

The spine in human posture (to visualize, think about when you stand erect) has 4 main curves. They are categorized by regions like this:

  1. Cervical, or neck
  2. Thoracic, or upper and mid-back area
  3. Lumbar, which is your low back and
  4. Your sacral curve, located at the base of your spine

The directions of these curves alternate – one right after another.  This construction feature helps provide both support and balance to your body as you go through your day. Common positions and movements like sitting, standing, walking, bending reaching, twisting, lifting and much more benefit greatly from these alternating spinal curves.

The entirety of your spine from the neck down through the lowest lumbar vertebra (called L-5) rests onto the top of your sacrum.  This bottom joint (L5 – sacrum) is called the lumbosacral joint. 

The sacrum is a triangular bone that is wedged in between the 2 hip bones in back to help stabilize, as well as to ease, the load of your spine as it transfers down to your lower body.  It does this by distributing the weight of your spine throughout the pelvis and lower extremity (ie your thighs, knees, lower legs, ankles and feet.)

And the lumbosacral spine, according to Renee Calliet, MD and author, is comprised of the 5 lumbar segments, including that lowest L5-sacrum joint.  A "segment" is basically an intervertebral joint; in other words, it consists of an upper spinal bone and a lower spinal bone with a disc in between the two. (Each region of the spine which corresponds to the curves in the list above has a specific number of segments.)

The Lumbosacral Angle Defined

Because your whole spine sits on top of that sacrum bone, the angle of the sacrum determines the degree of each of the spinal curves..

As you can imagine, the weight of your upper body transfers from the spine, through the L5 vertebra to the sacrum.  The angle of the top of the sacrum bone (called the sacral base) can be, relatively speaking, steep or flat, and places in between. 

As a base of support for the spine, then, this sacral angle determines the amount of the lumbar, thoracic and cervical curves.  In other words, starting at the foundation (the top of the sacrum) and going up the spine, one angle determines another.  In fact, the sacral part of the lumbosacral joint is more technically known as the sacral base.

The Lumbosacral Angle and Spondylolisthesis

One common spinal problem that occurs at the L5-sacrum joint is called spondylolisthesis, or forward slippage of the top bone, relative to the bottom.  This condition affects young and old alike, though in different forms.  In youth, it tends to start as injury to a small area at the back of the spine known as the pars in athletes whose sports require repetitive spinal movements.  Over time, the pars injury can develop into spondylolysis and finally spondylolisthesis. In older people, spondylolysis and spondylolisthesis are caused by degenerative changes in the spine. 

Hosoe and Ohmori, in their study entitled “Possible Factors which Predispose the Fifth Lumbar Vertebra to Slip,” which was published in March 2008 issue of The Journal of the Bone and Joint Surgery, reported that among other things, a greater “incline” of the sacral table, as they call it (which refers to the angle of the top of the sacrum bone where the spine sits) was associated with a higher incidence of spondylolisthesis.  

View Article Sources
  • Sources:
  • Calliet, R., M.D., The Illustrated Guide to Functional Anatomy of the Musculoskeletal System. American Medical Association Press. USA. 2004.
  • H. Hosoe, Hl, Ohmori, K. Possible Factors which Predispose the Fifth Lumbar Vertebra to Slip. The Journal of the Bone and Joint Surgery. March 2008