What Do L4-L5 and L5-S1 Mean? Letters and Numbers on Your MRI Report - Herniated Disc in 14 Questions Day 2

What Do L4-L5 and L5-S1 Mean? Letters and Numbers on Your MRI Report

Many clients who pick up their MRI report pause at the same spot: "L4-L5, L5-S1... What does this mean?" Although the letters and numbers on the report may look complicated at first glance, they are simply an anatomical address.

On the second day of our "Herniated Disc in 14 Questions" series, we answer this question. You can swipe through the 6-page post above (in Turkish) and also take a look at our Instagram post. For the first day of the series, see our article What Is a Herniated Disc? Is Every Low Back Pain a Hernia?.

What Do the Letters and Numbers Mean?

1. L1-L5: The five lumbar vertebrae in the lower back.
2. S1: The uppermost segment of the sacrum (the triangular bone at the base of the spine).
3. The logic: The letter indicates the spinal region, and the number indicates its position in order.

Where Is L4-L5?

L4-L5 is the level between the 4th and 5th lumbar vertebrae. Between these two vertebrae sits an intervertebral disc. When an MRI report mentions "L4-L5," it means the note is describing how the disc and surrounding tissues look at that specific level.

Where Is L5-S1?

L5-S1 is the level between the last lumbar vertebra (L5) and the sacrum (S1). It is the lowest disc level in the lower back. Seeing "L5-S1" on your report simply indicates that the observation belongs to this segment.

Level Does Not Mean Severity

1. The level indicates location: Writing L4-L5 or L5-S1 describes where a finding is located.
2. It does not show severity: The name of the level alone does not tell how significant or serious a finding is.
3. It does not automatically mean a herniated disc: Having a spinal level listed on a report does not mean there is a herniation there.

As we emphasized on Day 1, MRI findings can also appear in people who have no pain at all. Therefore, level notation on a report is not a standalone diagnosis or a treatment decision.

Is the MRI Report Enough on Its Own?

The report is an important part of the evaluation, but it is not the only part.

Important: MRI findings + symptoms + physical examination are assessed together. Care is never planned based on the spinal level alone.

From a kinetic chain perspective, the lower back does not work in isolation; the hips, upper back and posture habits are also part of the picture. During an assessment, the history of the complaint, its impact on daily life, and—when needed—nervous system findings (muscle strength, sensation, reflexes) are considered together. For a broader overview of low back pain, see our Low Back Pain Physiotherapy article, and for the approach specific to disc herniation, see Herniated Disc Physiotherapy. You can also visit our Manual Therapy and Orthopedic Rehabilitation pages for related clinical applications.

🚩 When You Should See a Physician

• Progressive weakness or numbness in the leg.
• Changes in bladder or bowel control, or numbness in the saddle/groin area.
• Severe low back pain following trauma.
• Fever, unexplained weight loss, or nighttime pain that does not ease with rest.

With these symptoms, a physician's evaluation comes first; seek emergency medical care if necessary.

Key Takeaways for Today

1. L1-L5 refer to the lumbar vertebrae, and S1 refers to the top segment of the sacrum.
2. L4-L5 and L5-S1 are the intervertebral disc levels between those bones.
3. The level indicates location, not severity; management is never decided by the level alone.

Frequently Asked Questions (FAQ)

1. What do L4-L5 and L5-S1 mean on an MRI report? L4-L5 is the level between the 4th and 5th lumbar vertebrae; L5-S1 is the level between the last lumbar vertebra and the sacrum. Both levels contain an intervertebral disc.

2. Does seeing L4-L5 or L5-S1 on a report mean I have a herniated disc? Not on its own. The level specifies the location of a finding, not its severity.

3. Is an MRI report enough to decide on a treatment plan? Not on its own. Imaging findings are evaluated together with your symptoms and physical examination.

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📚 Scientific Sources

• NCBI Bookshelf (StatPearls). Lumbar Vertebrae; Intervertebral Discs. ncbi.nlm.nih.gov/books
• AANS (American Association of Neurological Surgeons). Herniated Disc. aans.org

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Low Back Pain Assessment in Antalya Konyaaltı

If you have questions about the terms on your MRI report, with your physician's approval we can evaluate your symptoms and movement patterns together at our clinic in Konyaaltı, Antalya.

📍 Ali Öksüz | Fizyoterapist | Özel Sağlık Meslek Hizmet Birimi Toros Mah. 801. Sok. No:4/4 Konyaaltı / Antalya

For information regarding the physiotherapy and rehabilitation consultation process, you can visit our contact and information page.

Legal Disclaimer: This page is for general information; it does not replace a physician's diagnosis, an individual assessment or a definitive cure promise. It has been prepared in accordance with the Health Advertising Regulation.