Lower Back Anatomy

The unique shape, curvature, and upright orientation of the lower back, gives us the ability to walk on two legs, and many of the physical abilities that have allowed us our nearly limited physical and mental capabilities. With the unique abilities that the lower back gives us, it also poses some unique challenges for our bodies. Because of the upright nature of the spine, the energy of gravity is not dispersed throughout the spine as we sit upright and walk, but is concentrated towards the bottom of the spine, and at the junction (joint) where the spine connects with the pelvis. These increased pressures on the lower back structures may result in an increased rate of wear and tear and resulting lower back pain in adulthood. Because of this vertical orientation of the spine, the amount of energy the spine takes on is concentrated closer to its base, and particularly the last few intervertebral discs. Here will take a closer look at lower back anatomy, and how it's unique shape and structures affect our bodies.

The Lumbar Discs and Bony Joints: The lumbar spine includes the last five true or mobile vertebrae of the vertebral spine. True or movable vertebra includes all 24 vertebrae that are present at birth and remain un-fused. The lumbar spine also includes the lumbar discs, bony joints, and all the other associated structures that move and protect this part of our backs The lumbar spine is bordered superiorly (from above) by the thoracic spine (T1-T12) and inferiorly (below) by the sacrum/coccyx.

The parts of the lumbar spine are labeled as follows:
  • There are five lumbar vertebrae, labeled L1-L5, with L1 being the uppermost vertebra and L5 being the bottom vertebra.
  • The intervertebral discs are labeled according to the vertebra that the discs are located between. For example, the disc located between the last thoracic vertebra and the first lumbar vertebra (L1) is labeled T12-L1. The disc located between the last lumbar vertebra (L5) and the first sacral body (S1) is labeled as L5-S1.
  • The spinal nerve roots are not labeled consistently throughout the spine. At the top of the spine (Cervical spine or C-Spine), the nerve roots are labeled according to the lower spinal segment located beneath it. In the lumbar spine (aka L-Spine), the nerve root is labeled according to the upper segment that the nerve runs between. For example, the nerve root between the 2nd and 3rd lumbar vertebra is called the L3 nerve root. At each level of the spine, there are pair of nerve roots that exit the spine between the vertebral notches of the adjacent vertebra. The opening created by the adjacent vertebral notches is known as the intervertebral foramina.
  • The vertebral segments together make up the second lordotic curvature. The lordotic curvature is convex anteriorly and concave posteriorly. This curvature is normal and beneficial to the body, as opposed to the medical condition known as lordosis, which involves and excessive, unhealthy curvature beyond and angle of 60 degrees. An angle of curvature in the lumbar spine of 40-60 degrees is considered normal. The normal lumbar curvature is also known as lumbar lordosis.

The lumbar spine is a beautifully engineered structure with two primary functions: movement and protection of the spinal cord. The lower back anatomy of the lumbar spine includes the structures mentioned above, as well as ligaments, muscles, nerves, and blood vessels. The lumbar spine supports the torso, arms, head and neck, and mainly allows for the body movements of forward and backward motion (flexion and extension). Below the lumbar spine is supported from below by the sacrum and coccyx. The sacrum consists of 5 sacral segments that are separated at birth and become fused in childhood. Below the sacrum is the coccyx, which consists of 3-5 vertebral segments that are individual at birth and also become fused during childhood. The sacrum/coccyx is connected to the lumbar spine at the L5-S1 intervertebral disc. The sacrum is suspended between the bones of the pelvis (ilia) by the sacroiliac joints, and at the auricular surfaces of the sacrum and ilium.