Intervertebral discs are the primary cushions between your vertebrae. These rubbery pads sit between each pair of bones in your spine, absorbing shock and allowing your back to bend and twist. But discs aren’t the only structures doing this work. Several supporting players, including cartilage-lined joints, thin cartilage plates, and elastic ligaments, all help protect your vertebrae from grinding against each other and absorbing the forces of daily life.
How Intervertebral Discs Work
Each disc has two distinct parts that work together. The center is a gel-like core called the nucleus pulposus, which contains around 70% water along with proteins that attract and hold moisture. This jelly-like interior is what actually absorbs compressive force when you walk, jump, or simply stand upright. When a load pushes down on the spine, pressure inside this core increases, and that pressure gets redirected outward rather than crushing the bones above and below.
Surrounding that gel core is a tough outer ring of layered cartilage fibers called the annulus fibrosus. Think of it like a radial tire wrapped around the softer center. Its job is to keep the gel contained and convert downward pressure into outward tension across its fibers, which the ring is built to handle. The layers run in alternating directions, giving the ring strength no matter which way you bend or rotate.
Your Discs Change Height Every Day
Here’s something most people don’t realize: you are measurably taller in the morning than at night. Throughout a normal day, the weight of your body gradually squeezes water out of your discs. By evening, this compression can shrink your spinal height by almost 2 centimeters, roughly 1% of your total height. While you sleep and the load comes off, the discs reabsorb water and plump back up.
This daily cycle of compression and rehydration is normal and healthy. It happens because standing and sitting create more hydrostatic pressure than the disc’s internal osmotic pull can resist, so fluid slowly migrates out. Lying down reverses the equation. The nucleus pulposus, with its high water and proteoglycan content, is especially affected by this cycle. As discs age or degenerate, they lose the ability to fully rehydrate overnight, which is one reason people gradually lose a bit of height over the decades.
Cartilage Endplates: The Disc’s Supply Line
Sandwiched between each disc and the vertebra above or below it is a thin layer of cartilage called the cartilage endplate, only about half a millimeter to a millimeter and a half thick. These plates serve two roles. First, they act as a buffer between hard bone and the softer disc material, distributing force more evenly. Second, and perhaps more importantly, they are the disc’s lifeline for nutrients.
Intervertebral discs have no blood supply of their own. Instead, nutrients like oxygen and glucose slowly diffuse from the vertebral bone through the endplate and into the disc. Waste products travel the opposite direction. Healthy endplates carefully regulate the speed of this exchange, maintaining a stable internal environment. When endplates calcify or deteriorate with age, nutrient delivery drops off, starving the disc from the inside and accelerating breakdown.
Facet Joints and Synovial Fluid
Discs handle most of the spine’s compressive load, but they aren’t the only cushioning system. At the back of each vertebra, small paired joints called facet joints connect one vertebra to the next. These are synovial joints, the same type found in your knees and knuckles, enclosed in a fibrous capsule and lined with smooth articular cartilage.
Inside each facet joint capsule is synovial fluid, a slippery liquid that lubricates the joint surfaces and reduces friction during movement. A key ingredient in this fluid is hyaluronic acid, which helps the joint glide smoothly. Facet joints guide and limit spinal motion while sharing some of the load, particularly during extension (leaning backward) and rotation. When these joints wear down, the cartilage thins and the bones can grind together, contributing to the stiffness and pain often associated with spinal arthritis.
Ligaments That Protect the Spinal Canal
Running along the back wall of the spinal canal is a yellow, elastic band called the ligamentum flavum. This ligament is roughly 80% elastic fibers, making it one of the most elastic tissues in the human body. Its primary job is to limit excessive forward bending and maintain a baseline tension that keeps it from folding inward and pressing on the spinal cord when you straighten up. Other spinal ligaments run along the front and back of the vertebral bodies, adding further stability, but the ligamentum flavum is unique in how much it relies on elasticity rather than rigid strength.
What Happens When Cushioning Fails
Disc problems generally take two forms. A bulging disc occurs when the tough outer ring weakens and expands outward, usually affecting a quarter to half of the disc’s circumference. The gel center stays contained, but the outer wall pushes beyond its normal boundary. This is extremely common with aging as discs lose flexibility, and it often causes no symptoms at all.
A herniated disc is different. In this case, an actual crack develops in the outer ring, and some of the softer inner gel pushes through that crack. Only the small area around the crack is affected, not the whole disc, but the protruding material is more likely to press on nearby nerve roots and cause pain, numbness, or weakness in an arm or leg.
Age-Related Disc Degeneration
Disc wear is one of the most predictable changes in the human body. In a study of 238 patients across age groups, degeneration scores on MRI increased steadily at every spinal level. At the L3/4 segment, for example, average degeneration grades roughly doubled from the youngest group (ages 20 to 39) to the oldest (ages 80 to 99). The correlation between age and disc degeneration was strong, with an r value of 0.703, meaning age alone explains a large share of the variation.
This degeneration isn’t just about the discs themselves. The muscles that support the spine deteriorate in parallel. In the oldest age group, fat infiltration of the deep spinal muscles reached 35 to 52%, meaning a substantial portion of muscle tissue had been replaced by fat. Weaker muscles shift more mechanical load onto the discs and facet joints, creating a cycle where declining support accelerates wear on the cushioning structures. Staying physically active and maintaining back muscle strength is one of the most effective ways to slow this process and keep your spine’s cushioning system functioning well into older age.