All About T12-L1 (Thoracolumbar Junction)

The T12-L1 spinal motion segment, also called the thoracolumbar junction, connects the thoracic spine (upper back) with the lumbar spine (lower back). It is located in the mid-back area and aligns with the level of the last rib.

Understanding the T12-L1 Spinal Segment

The distinguishing features of this spinal segment are:

• It acts as a fulcrum between the less flexible upper back and a relatively mobile lower back.
• The outward curvature (kyphosis) of the thoracic spine reverses into an inward curvature (lordosis) of the lumbar spine.

This anatomical setup creates an area of biomechanical instability in this segment, making it susceptible to injury and trauma.

The majority of traumatic spinal fractures occur at the thoracolumbar junction, and almost 25% to 50% of these traumatic injuries are known to cause neurologic deficits.¹Fernández-de Thomas RJ, De Jesus O. Thoracolumbar Spine Fracture. [Updated 2023 Aug 23]. In: StatPearls [Internet]. Treasure Island (FL): StatPearls Publishing; 2024 Jan-. Available from: https://www.ncbi.nlm.nih.gov/books/NBK562204/^,2Waxenbaum JA, Reddy V, Futterman B. Anatomy, Back, Thoracic Vertebrae. [Updated 2023 Aug 1]. In: StatPearls [Internet]. Treasure Island (FL): StatPearls Publishing; 2024 Jan-. Available from: https://www.ncbi.nlm.nih.gov/books/NBK459153/ Additionally, up to one in four people with a thoracolumbar fracture have another spinal fracture elsewhere, usually in the neck (cervical spine).³Wood KB, Li W, Lebl DR, Ploumis A. Management of thoracolumbar spine fractures [published correction appears in Spine J. 2014 Aug 1;14(8):A18. Lebl, Darren S [corrected to Lebl, Darren R]]. Spine J. 2014;14(1):145-164. doi:10.1016/j.spinee.2012.10.041

Anatomy of the T12-L1 Spinal Motion Segment

This motion segment typically includes the following structures:

T12 and L1 vertebrae

Each vertebra consists of a vertebral body in front and an arch in the back that has 3 bony protrusions: a prominent spinous process in the middle and two transverse processes on the sides. These protrusions serve as attachment points for ligaments.

The essentials of these vertebrae are:

• T12 is the last vertebra of the thoracic spine and is considered an atypical thoracic vertebra because its shape differs from most of the upper thoracic vertebrae and more closely resembles a lumbar vertebra.
  • The spinous process is long and projects more downward compared to the other thoracic vertebrae.
  • It has 2 costal facets which connect the vertebra with the last rib on each side
  • It has special bony protrusions for muscle attachments, unlike the other thoracic vertebrae.
• L1 is the first vertebra of the lumbar spine.
  • This vertebra has a larger vertebral body (region of solid bone in the front of the vertebra) compared to T12.
  • It is a shorter and thicker spinous process compared to T12, which projects perpendicularly from the vertebral body.

T12 and L1 are connected through facet joints lined with articular cartilage. These vertebrae are held together with ligaments at multiple attachment points.

T12-L1 intervertebral disc

A disc made of a gel-like material (nucleus pulposus) surrounded by a thick fibrous ring (annulus fibrosus) is situated between the vertebral bodies of T12 and L1. It provides cushioning and shock-absorbing functions to protect the vertebrae during spinal movements.

T12 and L1 vertebral endplates

A thin vertebral endplate forms the transition between the vertebral bodies of T12 and L1 and the intervertebral disc. It is made up of two layers:

• A bony layer that attaches to the vertebral body
• A cartilaginous layer that attaches to the intervertebral disc

The endplate facilitates the transport of nutrients to the discs and protects the vertebrae and discs from compressive loads on the spine.

T12 spinal nerve

The T12 spinal nerve roots exit the spinal cord through small bony openings (intervertebral foramina) on the left and right sides of the spinal canal.

• The T12 dermatome is an area of skin that receives sensations through the T12 spinal nerve and includes parts of the abdomen and flank.
• The T12 myotome is a group of muscles controlled by the T12 spinal nerve and includes various muscles in the front and side of the abdomen.

The T12-L1 motion segment provides a bony enclosure for the spinal cord and other delicate structures.

Common Problems at T12-L1

Spinal conditions affecting the thoracolumbar junction occur from sudden trauma due to accidents or sports, and gradual wear-and-tear of the spinal tissues.

Common conditions affecting the T12-L1 motion segment are discussed below.

Fracture

Almost 7% of spinal vertebral fractures occur at the thoracolumbar junction.⁴Katsuura Y, Osborn JM, Cason GW. The epidemiology of thoracolumbar trauma: A meta-analysis. J Orthop. 2016;13(4):383-388. Published 2016 Jul 21. doi:10.1016/j.jor.2016.06.019 These fractures are of 2 types:

• Compression fractures: the vertebra is crushed in the front of the spine, causing it to collapse into a wedge-shape
• Burst fractures: the vertebra is crushed in all directions, affecting most of the vertebral body

The vertebrae may also get dislocated where they are completely separated or move out of alignment due to torn or overstretched ligaments.

Compression fractures are the most common type of fracture at T12-L1.¹Fernández-de Thomas RJ, De Jesus O. Thoracolumbar Spine Fracture. [Updated 2023 Aug 23]. In: StatPearls [Internet]. Treasure Island (FL): StatPearls Publishing; 2024 Jan-. Available from: https://www.ncbi.nlm.nih.gov/books/NBK562204/ A common cause of vertebral compression fractures is osteoporosis, a condition where the bones become thin, brittle, and weak, leading to fractures even with minor trauma to the spine.

Disc herniation

A herniated disc occurs when the outer portion of a spinal disc breaks down and the inner portion leaks out. The extruding inner portion can then irritate or compress nearby nerves, causing inflammation and pain.

Less than 5% of spinal disc herniations occur at the thoracolumbar junction.⁵Wang J, Zhou Y, Zhang ZF, Li CQ, Zheng WJ, Huang B. Disc herniation in the thoracolumbar junction treated by minimally invasive transforaminal interbody fusion surgery. J Clin Neurosci. 2014;21(3):431-435. doi:10.1016/j.jocn.2013.04.029

The naturally unstable biomechanics and increased vulnerability of this motion segment make it prone to sudden disc herniation from acute trauma.

Watch Thoracic Herniated Disc Video

Disc degeneration

The T12-L1 disc may degenerate over time due to increased loads at the thoracolumbar junction, leading to a condition called degenerative disc disease.  Obesity and excessive or reduced spinal curvatures are common risk factors that contribute to disc degeneration at the T12-L1 segment.

Spinal stenosis

Narrowing (stenosis) of bony openings for nerves due to the presence of bone spurs (abnormal bone growth) or other degenerative changes may cause compression of the nerve roots or spinal cord in the area of spinal stenosis.

Degenerative spondylolisthesis

Degenerative changes in the thoracolumbar junction, such as disc and facet joint degeneration, may cause instability in the motion segment resulting in a forward movement of the T12 vertebral body over the L1 vertebra. This condition is called degenerative spondylolisthesis.

In rare cases, tumors and infections may affect the T12-L1 vertebrae and spinal segment.

Other potential problems that can affect the T12-L1 segment include ankylosing spondylitis, Scheuermann’s disease, scoliosis, and facet joint arthritis.

Common Symptoms and Signs Stemming from T12-L1

Vertebral, disc, or muscle pain from T12-L1 may occur suddenly following an injury or develop over time. The symptoms from this motion segment are felt in the front of the abdomen and along the flank (the mid-back region and side of the abdomen – the area between the lower ribs and pelvis).

Inflammation or compression of the T12 spinal nerve root may cause radiculopathy symptoms, characterized by:

• Sharp, shooting, or searing pain in the abdomen, mid-back, and/or side of the body
• Numbness and/or weakness in the abdomen, mid-back, or side of the body
• Inability to bend backward, bend sideways, or sit for long periods of time

These symptoms typically affect one side of the body, but, sometimes, both sides may be affected together.

Serious Symptoms and Signs Stemming from T12-L1

Compression of the spinal cord at the T12 spinal level results in serious symptoms, including:

• Severe, unrelenting pain in the abdomen mid-back, or side of the body
• Loss of sensation in the legs, causing an inability to bear weight or walk
• Loss of bowel and bladder function (inability to hold or to pass urine or stool)

These issues must be treated on an urgent basis to preserve nerve function and restore bowel and/or bladder function.

See When Back Pain May Be a Medical Emergency