The thecal sac is a protective, membranous sheath that safeguards the delicate structures of the central nervous system within the spine. This structure is essentially a fluid-filled tube running inside the bony vertebral column. It serves as the immediate environment for the spinal cord and the critical nerves extending from it, and is often referenced in medical imaging reports concerning back pain or neurological symptoms.
Structure and Composition
The thecal sac is not a single layer but is formed by the two outermost protective membranes, called meninges, that surround the central nervous system. The most substantial layer forming the sac is the dura mater, a thick, tough, fibrous outer covering often called the “dural sac.” Lying immediately inside the dura mater is the arachnoid mater, a thinner, web-like membrane that completes the sac’s boundaries.
The space contained within the sac, known as the subarachnoid space, is filled with cerebrospinal fluid (CSF). This clear fluid is continuously produced and circulated, providing a cushion that suspends the nervous tissue and protects it from mechanical shock. The CSF also supplies nutrients to the spinal cord and maintains appropriate pressure and fluid balance around the nerves.
Floating within this cerebrospinal fluid in the lower spine are the nerve roots known as the cauda equina, or “horse’s tail.” These nerve roots, responsible for sensation and movement in the lower body, are completely enclosed within the sac in the lumbar region. The sac is anchored to the surrounding bone and ligaments by connective tissue formations, which stabilize it within the spinal canal.
Precise Anatomical Placement
The thecal sac is located entirely within the vertebral canal, the hollow passage created by the stacked bony rings of the vertebrae. It begins high up, adhering to the bone at the base of the skull (the foramen magnum), and extends downward, traversing the entire length of the cervical, thoracic, and lumbar spine.
The sac’s lower boundary is a defining anatomical feature, typically tapering and closing off at the level of the second sacral vertebra (S2). This termination point is considerably lower than the spinal cord itself, which usually ends much higher, around the first or second lumbar vertebra (L1 or L2). This lower portion, extending below the spinal cord, is called the lumbar cistern and contains only CSF and the cauda equina nerve roots.
The bony vertebral column completely encases the sac, providing protection against external forces. The sac is separated from the inner bone surfaces and ligaments of the spinal canal by the epidural space, a layer of tissue and fat. This arrangement means the thecal sac does not contact the bone directly, but is cushioned and suspended within the bony passage, making it vulnerable to pressure from surrounding structures like bone spurs or herniated disc material.
Involvement in Medical Procedures and Conditions
The presence of cerebrospinal fluid within the thecal sac makes it a target for specific medical procedures and a factor in several spinal pathologies. One common procedure involving this structure is the lumbar puncture, or spinal tap. During this diagnostic test, a needle is carefully inserted into the lumbar cistern, the lower part of the thecal sac below the end of the spinal cord.
This technique allows clinicians to safely draw a sample of CSF for laboratory analysis, helping diagnose conditions affecting the nervous system. The procedure is typically performed in the lower back, between the L3-L4 or L4-L5 vertebrae, to avoid injury to the spinal cord, which is absent at these levels. The sac is also a consideration in pain management, as anesthetic agents can be injected into the epidural space (just outside the sac) to diffuse and block the nerve roots exiting the sac.
Clinically, the thecal sac is frequently discussed as “thecal sac compression” or “indentation,” a finding commonly noted on magnetic resonance imaging (MRI) scans. This compression occurs when a surrounding structure pushes into the sac, often resulting from a herniated intervertebral disc. When the soft inner material of a disc bulges or ruptures, it can encroach upon the spinal canal and press against the dural sac.
Another frequent cause of this pressure is spinal stenosis, a condition where the bony spinal canal narrows due to age-related changes like thickened ligaments or bone spur formation. When these changes or a herniated disc compress the sac, the pressure affects the nerve roots contained inside, leading to symptoms like radiating pain, numbness, or weakness in the limbs. The severity of the compression determines whether a patient experiences neurological symptoms requiring intervention.