Why Do Older People Tend to Have Low Back Pain?

Why are Spinal Discs so Important?

Humans have 23 spinal discs that sit between bone segments, or vertebrae, which together make up the spine (Figure 1). While vertebrae support body weight and protect the spinal cord, spinal discs allow for flexibility and protect the vertebrae [1]. Have you ever fallen when you were wearing knee pads? Chances are, even if it was a big fall, your knees were fine because the knee pads absorbed the force. Similarly, spinal discs absorb forces on the spine to prevent vertebrae from breaking. Furthermore, discs also allow the spine to bend and rotate easily because of their unique structure.

Figure 1- Overview of the spine, and an enlarged section showing two vertebrae and a spinal disc between them.

How are Spinal Discs Structured?

Spinal discs are made of three key components. A central nucleus pulposus and a surrounding annulus fibrosus make up most of the spinal disc (Figure 2A). The third component, endplates, cover the top and bottom of each disc.

Figure 2 - (A) Top view of a spinal disc. (B) Alternating 30-degree fiber layers in the annulus absorb forces from all directions. (C) A water balloon before and after a compressive force. The water is pushed to the sides of the balloon, stretching it. (D) Side view of the spinal disc before and after a compressive force. The nucleus pushes back against the force and expands outward to press onto the annulus fibers. The fibers stretch and absorb the force. The green regions on the top and bottom of the disc are the endplates.

The nucleus pulposus is a jelly-like ball that sits in the center of the spinal disc and is made mostly of water. The high water content of the nucleus allows it to maintain its shape, even under pressure [1]. When force is applied, the nucleus may smush a little, but it will not completely flatten. Instead, the nucleus pushes back against the force and expands outwards onto the surrounding annulus fibrosus.

You can think of the spinal disc as a water balloon. When you push down on a water balloon, the water moves and presses on the sides of the balloon, similar to how the nucleus pushes onto the annulus. In reality, the annulus is much larger, stronger, and stiffer than a balloon, so the disc does not bulge as much under pressure.

The annulus fibrosus consists of many layers of collagen fibers that surround the nucleus like an onion. These fibers are arranged in alternating layers, angled at roughly 30-degrees (Figure 2B). This layout makes the annulus strong against forces coming from all directions [1]. Think of the water balloon again. As the water pushes on the sides of the balloon, the balloon stretches (Figure 2C). Likewise, the annulus’ many layers stretch outward to absorb some of the force when the nucleus pushes on them (Figure 2D). Since the nucleus and annulus can share this force, less pressure is put onto the vertebrae.

Connecting the discs to the adjacent vertebrae are thin, but strong, layers called endplates (Figure 2D). Endplates sandwich each disc on the top and bottom and play an important role in keeping discs healthy.

How do Discs Stay Healthy?

Like us, discs need nutrients and oxygen to survive. Most body parts get their nutrients from blood vessels, but spinal discs lack them. As a result, nutrients must travel from the closest blood vessels in the surrounding vertebrae to get to the disc [2]. This is where the endplates come in. They contain small holes, or pores, that let nutrients flow into the nucleus and annulus. Think of pores like a sponge. They expand when wet and shrink when dry. Water helps to keep pores large so that nutrients can easily pass through [3]. This is another reason why it is important to stay hydrated.

Activities like walking and running can help larger nutrients flow into spinal discs. The forces from these actions squish the nucleus and expel water through the pores of the adjacent endplates. During a given day, 3%-10% of the fluid in a person’s discs leaves. Then, when a person sleeps, the pressure on the discs releases and invites water back into the disc. The water carries fresh nutrients with it to feed the disc. You can mimic this process by squeezing a wet sponge and then letting go so it can refill with water. Stationary activities, such as sitting, also provide compressive forces on the spine. However, excessive inactivity can actually shrink the pores and decrease nutrient flow. Sitting for too long is like squeezing a sponge and never letting go. The sponge will not be able to absorb water, and the same goes for your discs. Thus, it is best to stay as active as possible to keep your spinal discs healthy.

Why are Discs Destined to Break Down?

Sometimes the spinal discs start to break down, or degenerate, and struggle to absorb forces as efficiently as they once did. This is called degenerative disc disease. While exercise and hydration can help to prevent degenerative disc disease, the breakdown of spinal discs is practically bound to happen. Many factors can lead to disc degeneration, but aging is the most common one [4]. A typical change in disc structure seen with aging is a loss of disc hydration [3]. A healthy spinal disc has a hydrated nucleus and an intact annulus (Figure 3A). There is a special protein called aggrecan in discs, which helps the nucleus absorb water. With age, there is less aggrecan in the nucleus, so discs shrink because they cannot hold as much water. The shrunken nucleus does not absorb forces as well, similar to a deflated tire. As a result, more of the force that should have been absorbed by the nucleus passes to the annulus [4]. The increased force on the annulus causes the fiber layers to rub against one another. This rubbing between annulus layers leads to structural damage such as tears. If the body senses damage, it sends help to fix these cracks. In doing so, nerves grow through the tears toward the nucleus (Figure 3B).

Figure 3 - Top view of three different spinal discs. (A) A healthy spinal disc. (B) A spinal disc with significant nerve growth into the annulus. (C) A herniated spinal disc that pushes on the spinal cord, causing pain.

Nerves make up a signaling network that sends messages between the brain and body. They let you feel a bug land on your arm or tell your brain to remove your hand from a hot stove. Needless to say, nerves are extremely sensitive. When they grow into disc tears, they become subject to the forces the disc usually encounters. Once they become compressed, they send signals to the brain that cause pain. [3]. Since nerves are sensitive enough to tell when a small bug lands on you, imagine how they feel when they are crushed under the weight of your upper body.

While nerves can grow into the tears of the annulus, this does not always happen. Instead, a hernia may form. The jelly-like nucleus can expand into tears and push through layers of the annulus [1]. If the nucleus completely breaks through all layers of the annulus, it becomes a herniated disc (Figure 3C). Herniated discs can cause a lot of pain if they press against surrounding nerve roots or the spinal cord.

What Can be Done?

While there are treatments for back pain, there is still no cure for disc degeneration. The first line of defense for back pain is typically pain medication and physical therapy. If that does not work, patients may require surgeries to remove problematic parts of the disc or to fuse parts of the spine together. Unfortunately, these treatments do not always work because they focus on relieving pain rather than preventing degeneration [5]. Scientists are actively researching spinal disc structures to create therapies for degeneration. However, there is still a lot to uncover about discs to make these new methods effective. In the meantime, it is important to be preventive. We know how dehydration and being inactive can reduce the nutrient flow to the discs—so keep drinking water and make sure to exercise. While most people eventually get this disease, it is not one to be afraid of. Instead, it is something for you to be aware of and continue learning about. Who knows? Maybe one day you will help find a cure!

Glossary

Spinal Discs: ↑ Cushioning structures between vertebrae that absorb forces on the spine.

Spinal Cord: ↑ A collection of nerves running from the brain to the tailbone that is protected by the vertebrae.

Collagen Fibers: ↑ Small strands made up of a protein called collagen. They provide strength when pulled on.

Degenerate: ↑ To physically decline or break down.

Degenerative Disc Disease: ↑ The age-related breakdown of spinal discs, which can cause pain.

Aggrecan: ↑ A special protein found in the spinal disc. Aggrecan can attract water because the two substances have opposite charges.

Nerve: ↑ A bundle of fibers that sends impulses of sensation to the brain.

Hernia: ↑ Tissue that bulges through a weak point in the surrounding material.

Conflict of Interest

The author(s) declared that this work was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

Acknowledgments

This project was done in collaboration with the Department of Orthopedics at the University of Virginia. We want to thank the Department of Mechanical and Aerospace Engineering at the University of Virginia for funding support. We also want to thank Dr. William Landis from the Department of Preventative and Restorative Dental Sciences at the University of California in San Francisco for providing valuable feedback. Figures 1, 2A, 2D and 3 in this article contain content which is adapted from BioRender.

Figure 1 was created in BioRender. Tang, T. (2026) https://BioRender.com/mnd5bti.

Figures 2A, D were created in BioRender. Tang, T. (2026) https://BioRender.com/ymm152p.

Figure 3 was created in BioRender. Tang, T. (2026) https://BioRender.com/acoexmu.

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[1] ↑ Broom, N. D., and Thambyah, A., eds. 2018. “The intervertebral disc–endplate system”, in The Soft–Hard Tissue Junction: Structure, Mechanics and Function, eds N. D. Broom and A. Thambyah (Cambridge: Cambridge University Press), 155–282. Available online at: https://www.cambridge.org/core/product/6A43F28D82CB310D0695867AEEB6F786 (Accessed June 24, 2025).

[2] ↑ Crump, K. B., Alminnawi, A., Bermudez-Lekerika, P., Compte, R., Gualdi, F., McSweeney, T., et al. 2023. Cartilaginous endplates: a comprehensive review on a neglected structure in intervertebral disc research. JOR Spine. 6:e1294. doi: 10.1002/jsp2.1294

[3] ↑ De Geer, C. M. 2018. Intervertebral disk nutrients and transport mechanisms in relation to disk degeneration: a narrative literature review. J. Chiropr. Med. 17:97–105. doi: 10.1016/j.jcm.2017.11.006

[4] ↑ Kirnaz, S., Capadona, C., Lintz, M., Kim, B., Yerden, R., Goldberg, J. L., et al. 2021. Pathomechanism and biomechanics of degenerative disc disease: features of healthy and degenerated discs. Int. J. Spine Surg. 15:10–25. doi: 10.14444/8052

[5] ↑ Guterl, C., See, E., Blanquer, S., Pandit, A., Ferguson, S., Benneker, L., et al. 2013. Challenges and strategies in the repair of ruptured annulus fibrosus. Eur. Cell. Mater. 25:1–21. doi: 10.22203/eCM.v025a01