Causes of Thecal Sac Indentation

Thecal sac indentation is a medical condition that occurs when the spinal canal narrows or experiences pressure, causing compression on the thecal sac—a protective membrane surrounding the spinal cord and cerebrospinal fluid. This condition can lead to various symptoms, ranging from mild discomfort to severe neurological issues. In order to effectively address thecal sac indentation, it is crucial to understand its underlying causes.

Any condition that narrows the spinal canal can cause thecal sac indentation. We will review some of the more common conditions below.

Herniated Discs

Herniated discs, also known as slipped or ruptured discs, are another significant contributor to thecal sac indentation. When the soft, gel-like center of an intervertebral disc protrudes through the outer layer, it can impinge on the thecal sac. This pressure can lead to discomfort, pain, and neurological symptoms.

Degenerative Changes

Degenerative changes refer to wear and tear on the spine usually due to age and overuse. Degenerative changes can lead to thickening and overgrowth of bone and ligaments, which can lead to narrowing of the canals. Ligamentum flavum hypertrophy and osteophytes are two degenerative conditions that can cause thecal sac indentation.

Ligamentum Flavum Hypertrophy

Ligamentum flavum hypertrophy refers to the thickening and enlargement of the ligamentum flavum, a connective tissue structure within the spinal column. This ligament runs along the posterior aspect of the vertebral bodies and helps to maintain stability in the spine. When it undergoes hypertrophy, it can lead to narrowing of the spinal canal and thecal sac compression.


Osteophytes in the spine, commonly known as bone spurs, are small bony outgrowths that form along the edges of vertebrae.Often associated with aging or degenerative conditions, osteophytes can be a result of the body’s attempt to stabilize areas of the spine where structural integrity has been compromised.


Spondylolisthesis is a spinal condition characterized by the displacement or slippage of one vertebra over an adjacent one. This displacement can occur forwards (anterolisthesis) or backwards (retrolisthesis), and it commonly arises in the lower lumbar region of the spine. It can be caused by various factors, including congenital abnormalities, stress fractures, or degenerative changes in the spine.

Trauma and Injuries

Acute injuries to the spine, such as fractures or dislocations, can cause displacement of bone fragments or other structures, leading to thecal sac indentation. Additionally, conditions like whiplash or severe spinal contusions can result in swelling or bleeding that puts pressure on the thecal sac.

Tumors and Abnormal Growths

Tumors and abnormal growths within the spine or surrounding tissues can directly exert pressure on the thecal sac. These growths may be benign or malignant, and they can originate from various structures including the spine itself, adjacent soft tissues, or metastasized from other parts of the body.


Thecal sac indentation is a multifaceted condition with various potential causes. Understanding the underlying factors contributing to this condition is vital for accurate diagnosis and effective treatment. It is essential for individuals experiencing symptoms of thecal sac indentation, such as back pain, numbness, or weakness, to seek medical attention promptly. A thorough evaluation by a healthcare professional, often including imaging studies like MRI or CT scans, is crucial in determining the cause and developing an appropriate treatment plan tailored to the individual’s specific needs.


Lee BH, Moon SH, Suk KS, Kim HS, Yang JH, Lee HM. Lumbar Spinal Stenosis: Pathophysiology and Treatment Principle: A Narrative Review. Asian Spine J. 2020;14(5):682-693. 

Raja A, Hoang S, Patel P, et al. Spinal Stenosis. [Updated 2023 Jun 12]. In: StatPearls [Internet]. Treasure Island (FL): StatPearls Publishing; 2023 Jan-. Available from:

Katz JN, Zimmerman ZE, Mass H, Makhni MC. Diagnosis and Management of Lumbar Spinal Stenosis: A Review. JAMA. 2022;327(17):1688-1699. 

About the Author

Dave Harrison, MD

Dr. Harrison is a board certified Emergency Physician with a part time appointment at San Francisco General Medical Center and is an Assistant Clinical Professor-Volunteer at the UCSF School of Medicine. Dr. Harrison attended medical school at Tufts University and completed his Emergency Medicine residency at the University of Southern California. Dr. Harrison manages the editorial process for