What Is Retrolisthesis of L5 on S1?

Retrolisthesis of L5 on S1 is a backward slippage of the fifth lumbar vertebra (L5) relative to the first sacral vertebra (S1) at the very bottom of your lumbar spine. Unlike the more commonly discussed forward slippage known as spondylolisthesis, retrolisthesis involves the upper vertebra shifting posteriorly, toward your back. Researchers have defined a clinically meaningful retrolisthesis as a posterior displacement of roughly 8% or more of the vertebral body’s width on imaging.¹ The L5-S1 junction is especially vulnerable because it sits at the transition between the mobile lumbar spine and the rigid, fused sacrum, making it a natural stress point during everyday movement.

Why L5-S1 Is a Vulnerable Spot

Your lumbar spine curves gently forward (a shape called lordosis), and L5-S1 sits right at the base of that curve where it meets the relatively immovable sacrum. Every time you bend, twist, or absorb shock from walking and running, L5-S1 bears a disproportionate share of the load. The angle between L5 and S1, the height of the disc between them, and the tilt of your pelvis all interact to determine how forces travel through that joint. Research using full-body standing X-rays has shown that the L5-S1 disc angle, vertebral translation, and disc height are closely linked to overall spinal alignment from the neck down to the pelvis.² In other words, what happens at L5-S1 doesn’t stay at L5-S1; it ripples up and down the entire spine.

The disc at L5-S1 is also the thickest in the spine, which allows a wide range of motion but also means there’s more material that can degenerate, bulge, or lose height over time. When that disc thins, the vertebra above it can shift in ways it normally wouldn’t, and backward slippage is one of those possibilities.

What Causes Retrolisthesis at This Level

Most cases of retrolisthesis at L5-S1 develop gradually through disc degeneration. As the disc loses water content and height with age, the ligaments and facet joints that normally keep L5 locked in place become less effective at restraining movement. The vertebra can then drift backward, especially when the facet joints themselves are oriented in a way that doesn’t resist posterior translation well. A case report of a 38-year-old woman with degenerative changes at L5-S1 suggested that an unusually sagittal (front-to-back) orientation of her facet joints on both sides contributed to the slippage, essentially giving L5 less bony resistance against sliding backward.³

Less commonly, trauma can cause an acute retrolisthesis. A published case described a 22-year-old who developed L5-S1 retrolisthesis along with a large disc extrusion after falling from a height of about eight meters.⁴ That kind of high-energy injury can disrupt ligaments and the disc all at once, allowing immediate displacement. But traumatic retrolisthesis in young, otherwise healthy people is rare; the vast majority of cases are degenerative and develop over years.

Other contributing factors include weakened core and paraspinal muscles, chronic poor posture, osteoporosis reducing bone density around the facet joints, and prior surgery at adjacent spinal levels that shifts mechanical load onto L5-S1. Obesity adds compressive force across the lower spine, accelerating disc wear. And smoking impairs blood flow to the discs, which rely on diffusion for nutrients, speeding up degeneration.

How It Differs From Forward Slippage

When people hear “slipped vertebra,” they usually think of anterolisthesis, where L5 slides forward over S1. That is the classic spondylolisthesis pattern and is far more common. Retrolisthesis is the mirror image: L5 slides backward. The distinction matters because the structures compressed differ. In forward slippage, the spinal canal itself narrows from front to back, which can squeeze the cauda equina (the bundle of nerve roots below the spinal cord). In retrolisthesis, the backward shift tends to narrow the neural foramen, the small tunnels on each side where individual nerve roots exit. That’s why retrolisthesis more often produces one-sided leg symptoms rather than the bilateral leg and bladder symptoms associated with severe forward slippage.

There’s also a less intuitive difference in how the two conditions behave with posture. Forward slippage often worsens with standing and extension (arching your back) because gravity pulls L5 further forward on the sloped sacral surface. Retrolisthesis, by contrast, can become more pronounced in slouched sitting postures. Research tracking spinal alignment between standing and slumped sitting positions found that the L5-S1 segment loses lordosis and can translate posteriorly when you slump, which is essentially the posture that loads the disc in a way that encourages backward slippage.²

Symptoms You Might Experience

Many people with a mild retrolisthesis at L5-S1 have no symptoms at all. The slippage shows up incidentally on an X-ray or MRI ordered for something else, and the person had no idea anything was out of alignment. When symptoms do appear, they tend to develop slowly alongside disc degeneration.

The most common complaint is low back pain, usually centered around the beltline and often worsened by prolonged sitting or standing. Because the backward shift narrows the space where nerve roots exit the spine, you may also feel pain, tingling, or numbness radiating into one leg, typically following the path of the sciatic nerve down the buttock, thigh, and sometimes into the calf or foot. Weakness in the foot or ankle is possible when the L5 or S1 nerve root is significantly compressed. In the traumatic case described earlier, the young patient presented with lower-extremity weakness severe enough to require surgery.⁴

Stiffness in the lower back is also typical, especially in the morning or after sitting for a long time. Some people describe a sensation of instability, as though their lower back might “give out,” particularly during transitions like standing up from a chair or bending to pick something up. These symptoms overlap considerably with plain disc degeneration or a herniated disc, which is part of why retrolisthesis often goes unrecognized without imaging.

How Retrolisthesis Is Diagnosed

A standing lateral X-ray of the lumbar spine is the primary tool. The radiologist measures how far the back edge of L5 has shifted behind the back edge of S1, typically expressed as a percentage of the vertebral body’s depth. A posterior displacement of about 8% or more is generally considered a retrolisthesis.¹ Standing films are important because lying-down images can understate or miss the slippage entirely; gravity and your body weight need to be acting on the spine for the displacement to show itself.

An MRI is usually added when nerve-related symptoms are present. It shows the discs, nerve roots, and soft tissues that X-rays can’t visualize, helping distinguish whether your symptoms come from the slippage itself, a herniated disc, or both. Flexion-extension X-rays, where you bend fully forward and then arch backward while being imaged, can reveal whether the slippage changes with movement, which tells clinicians whether the segment is unstable or simply displaced in a fixed position.

Grading retrolisthesis is less standardized than grading forward spondylolisthesis. There is no universally accepted four-grade system the way there is for anterolisthesis. Most clinicians simply report the percentage of slippage and describe whether it is mild (under roughly 25% of the vertebral body depth), moderate, or severe. The clinical significance depends less on the exact millimeters of slippage and more on whether the displacement is compressing neural structures and causing symptoms.

Non-Surgical Treatment

For most people with retrolisthesis at L5-S1, treatment starts conservatively. Physical therapy focused on core stabilization is the backbone of management. Strengthening the deep abdominal muscles, the multifidus muscles along the spine, and the gluteal muscles helps create a muscular “corset” that limits abnormal movement at L5-S1. Flexibility work targeting the hip flexors and hamstrings is also standard because tightness in these muscle groups alters pelvic tilt and increases mechanical stress at the lumbosacral junction.

Pain management often includes nonsteroidal anti-inflammatory drugs for flare-ups, along with activity modification. Because slumped sitting worsens the posterior loading at L5-S1, many clinicians recommend ergonomic adjustments: a lumbar support cushion, a sit-to-stand desk, and frequent movement breaks. Some people benefit from a short course of physical modalities like heat, ultrasound, or manual therapy to manage acute pain while building up exercise tolerance.

Epidural steroid injections are sometimes offered, but the evidence for their effectiveness in spondylolisthesis (including retrolisthesis) is not encouraging. A large study following patients with degenerative spondylolisthesis found that those who received epidural steroid injections fared no better in surgical rates over four years than those who did not. Among patients managed without surgery, those who skipped the injections actually showed greater improvements in bodily pain and physical function scores at multiple follow-up points compared with those who received them.⁵ That doesn’t mean injections never help an individual patient, but the group-level data suggest they are not a reliable long-term solution for this condition.

When Surgery Enters the Conversation

Surgery is typically reserved for people who have tried conservative treatment for several months without meaningful improvement, or who have progressive neurological deficits like worsening leg weakness or loss of bladder and bowel control. The most common surgical approach is decompression followed by spinal fusion, where the surgeon removes whatever is pressing on the nerve roots (disc material, thickened ligament, bone spur) and then stabilizes L5-S1 with screws, rods, and a bone graft or cage to prevent further motion at that segment.

In the traumatic retrolisthesis case mentioned earlier, the patient underwent surgical decompression and reduction with instrumentation, and achieved complete recovery of the neurological deficit.⁴ Acute traumatic cases with nerve compression tend to have the clearest indications for surgery and, when treated promptly, some of the best outcomes.

For degenerative retrolisthesis, the picture is more nuanced. A study that followed patients with retrolisthesis and lumbar disc herniation after surgery found that those with retrolisthesis did significantly worse in bodily pain and physical function over four years compared with patients without retrolisthesis. However, the retrolisthesis did not increase operative time, blood loss, length of hospital stay, complication rates, or the likelihood of needing additional spine surgeries or experiencing recurrent disc herniations.⁶ The takeaway is that having retrolisthesis alongside a disc herniation doesn’t make the surgery riskier, but it does seem to temper the degree of pain relief and functional improvement you can expect afterward.

What the Prognosis Actually Looks Like

Mild retrolisthesis discovered incidentally often requires no treatment at all and may never progress to the point of causing trouble. Many people live their entire lives with a small backward displacement at L5-S1 and never know it. When symptoms do develop, the majority of patients improve with conservative measures over weeks to months, particularly when they commit to a structured exercise program.

For those who need surgery, improvement in leg symptoms (pain, numbness, weakness) tends to be more reliable than improvement in back pain. The research on postoperative outcomes suggests that retrolisthesis patients start from a worse functional baseline and, while they do improve with surgery, they may not reach the same endpoint as patients without retrolisthesis.⁶ Disc degeneration and other degenerative changes visible on MRI, interestingly, did not independently affect those outcomes, suggesting the slippage itself is the variable that makes the difference.

One thing worth managing expectations around: fusion at L5-S1 eliminates motion at that segment, which means the disc and facet joints at L4-L5 (the level just above) take on extra mechanical demand. Over years, this increased stress can accelerate degeneration at L4-L5, a phenomenon called adjacent segment disease. It doesn’t happen to everyone, and it’s not a reason to avoid surgery when surgery is genuinely needed, but it’s part of the long-term calculus your surgeon should discuss with you.

Common Misconceptions

One widespread belief is that any amount of vertebral slippage is dangerous and will inevitably get worse. In reality, many retrolistheses are small, stable, and asymptomatic. The mere presence of slippage on an image does not mean you need treatment, let alone surgery. Overreacting to incidental imaging findings is one of the most common pitfalls in spine care.

Another misconception is that you should avoid all physical activity to prevent the slippage from progressing. The opposite is closer to the truth. Controlled exercise, especially movements that build core strength and maintain spinal mobility, is protective. Prolonged bed rest and inactivity weaken the very muscles that stabilize L5-S1 and can make symptoms worse.

People also sometimes confuse retrolisthesis with a herniated disc, assuming they’re the same problem. They can coexist, and the backward slippage may even contribute to disc herniation by altering the mechanical stresses on the disc, but they are distinct conditions with different imaging findings and sometimes different treatment strategies.

The Evolutionary Angle on Lumbar Vulnerability

It’s worth stepping back to ask why this part of the human spine is so prone to problems in the first place. The answer, in part, is evolutionary. The human lumbar curve is a relatively recent adaptation to upright walking, and it comes with inherent trade-offs. Research on lumbar lordosis variability has found that while the forward curve of the lumbar spine improves mobility and shock absorption during walking and running, it also creates higher shearing forces between vertebrae and reduces the spine’s resistance to bending deformations, making it less stable and more injury-prone.⁷ This trade-off between mobility and stability is baked into our anatomy.

Individual variation in pelvic shape adds another layer. A measure called pelvic incidence, which describes the geometric relationship between your sacrum and your hip joints, varies significantly from person to person and appears to be partly heritable. Studies have found that people with a lower pelvic incidence tend to have a more vertical pelvis and flatter lumbar curve, a configuration that resembles the spinal alignment seen in earlier hominin species and even in modern great apes. About 8% of healthy modern adults exhibit this straighter alignment.⁸ Researchers have proposed that this variation in pelvic and spinal geometry may predispose certain individuals to disc degeneration and vertebral displacement, not because something went wrong in their lifetime, but because their skeletal blueprint carries trade-offs rooted deep in human evolutionary history. The same research group has suggested that genetics may influence disc degeneration partly by determining pelvic shape, which then dictates how forces distribute across the lower spine.

None of this changes your treatment plan if you already have retrolisthesis, but it does reframe the condition. Rather than thinking of a slipped vertebra purely as damage or failure, it helps to understand that the human lumbosacral junction is an inherently imperfect engineering solution to the problem of balancing a flexible trunk on top of a rigid pelvis while walking upright. Some of us inherited a version of that solution that handles the stresses slightly less gracefully than others.