A 9mm anterolisthesis almost always falls into Grade I on the Meyerding classification, the standard system doctors use to rate how far one vertebra has slipped forward over the one below it. The catch is that the Meyerding system does not use millimeters directly. It uses the percentage of the lower vertebral body’s width that the upper vertebra has shifted, and 9mm translates to a different percentage depending on how large your vertebrae are. For most adults, though, 9mm works out to somewhere between 18% and 30% of the vertebral body width, placing it squarely in Grade I or right at the border of Grade I and Grade II.
How the Meyerding Grading System Works
The Meyerding classification divides the top surface of the lower vertebra into four equal quarters and measures how far the upper vertebra has slid forward across those quarters. Grade I means the slip covers up to 25% of the lower vertebra’s width, Grade II covers 26% to 50%, Grade III is 51% to 75%, Grade IV is 76% to 100%, and Grade V (called spondyloptosis) means the upper vertebra has slipped entirely off the front edge of the lower one.1Europe PMC / Clinical Orthopaedics and Related Research. Classification in Brief: The Meyerding Classification System of Spondylolisthesis Because the system is built on percentages rather than raw distance, the same millimeter measurement can technically land in different grades for different people.
In the lumbar spine, where anterolisthesis is most common, the vertebral bodies are roughly 35 to 50mm wide in adults, depending on the spinal level and the person’s frame. A 9mm slip on a 45mm-wide vertebra works out to about 20%, which is solidly Grade I. The same 9mm on a smaller 30mm vertebra would be 30%, which crosses the threshold into Grade II. So the honest answer to “what grade is 9mm?” is almost always Grade I, but if you have a smaller frame or the slip is at a narrower spinal level, it could technically nudge into early Grade II.
Why Your Imaging Report Might Not State the Grade
Radiology reports frequently describe the slip in millimeters without converting to a Meyerding grade. This is partly because the millimeter measurement is what the radiologist reads directly off the image, while computing the percentage requires an additional calculation that not all reports bother with. If your report says “9mm anterolisthesis at L4-L5” but does not assign a grade, your doctor will typically do the percentage math themselves or simply recognize that 9mm at a lumbar level is low-grade.
Some surgeons and spine specialists also prefer to work with the raw millimeter figure because it gives them a more precise picture of how much bone has shifted. The grade is useful shorthand for broad treatment planning, but a 5mm Grade I slip and a 24% Grade I slip can mean quite different things for a patient’s symptoms and surgical risk. In practice, knowing the absolute distance and the percentage together gives the most complete picture.
Standing Up Versus Lying Down Changes the Number
One of the less intuitive things about spondylolisthesis measurement is that the slip can change size depending on your body position when the image is taken. The vertebra slides forward a little more under the load of gravity when you are standing, and it can partially reduce when you lie flat for an MRI. Research comparing standing X-rays to supine MRI found that the average slip measured about 10mm on standing films but only about 7mm on supine MRI.2PubMed. Determination of dynamic instability in lumbar spondylolisthesis using flexion and extension standing radiographs versus neutral standing radiograph and supine MRI That gap matters. A slip that measures 9mm on a supine MRI might actually be 12 or 13mm when you are upright, which could push the percentage closer to Grade II territory.
A separate study comparing standing neutral X-rays to supine films confirmed this pattern, with standing images showing a mean slip about 1.7mm greater than supine ones.3PubMed Central. How Do Standing Neutral, Supine Lateral, Standing Flexion, and Standing Extension Radiographs Compare in Detecting the Presence and Magnitude of Stable and Dynamic Spondylolisthesis? This is why spine specialists sometimes order weight-bearing imaging: the position that reproduces your actual load-bearing posture gives a more clinically relevant measurement. If your 9mm figure came from a supine MRI, the functional slip when you are on your feet is likely a bit larger. If it came from a standing X-ray, it is closer to the real-world number.
Beyond just standing versus lying, bending forward (flexion) and backward (extension) can shift the vertebra further in either direction. Researchers have found that no single pair of imaging positions catches every case of dynamic instability, meaning a slip that moves with posture changes.3PubMed Central. How Do Standing Neutral, Supine Lateral, Standing Flexion, and Standing Extension Radiographs Compare in Detecting the Presence and Magnitude of Stable and Dynamic Spondylolisthesis? A 9mm anterolisthesis that stays about 9mm in every position is considered stable. One that jumps to 13 or 14mm when you bend forward may warrant closer monitoring even though the resting grade is low.
Does the Grade Predict How Much It Hurts?
Not reliably. Spondylolisthesis is often asymptomatic, discovered incidentally on imaging done for another reason.4PubMed Central. Spondylolisthesis When symptoms do appear, they tend to involve low back pain and sometimes leg pain or cramping with walking, which gets worse with standing, arching the back, or prolonged activity. But people with a 15mm slip can feel fine, while someone with a 5mm slip at the same level can be miserable. The grade tells you how far the bone has moved, not how compressed the nerves are or how inflamed the surrounding tissues have become.
What tends to matter more for symptoms is whether the slip is narrowing the spinal canal or the nerve exit tunnels (foramina). Lumbar spondylolisthesis is a well-known cause of spinal canal and foraminal narrowing, and that narrowing tends to get worse when a person stands upright, adding a positional component to the symptoms.5PubMed. Weight-bearing MRI of the Lumbar Spine: Spinal Stenosis and Spondylolisthesis A Grade I slip with significant foraminal narrowing on one side can produce burning leg pain, numbness, or weakness in a specific nerve distribution. A higher-grade slip with a roomy spinal canal might cause only mild stiffness. So the grade on your report is one piece of the puzzle, but the MRI findings showing how much space is left around the nerves generally predict symptoms more accurately.
What Causes a 9mm Slip in the First Place
In adults over about 50, the most common cause is degenerative anterolisthesis. Years of wear on the disc and facet joints at a given spinal level gradually loosen the segment’s restraints, letting the upper vertebra creep forward. This type tends to happen at L4-L5 more than anywhere else and is more common in women.
In younger people, particularly athletes, the cause is more often isthmic spondylolisthesis, where a stress fracture in a small bridge of bone called the pars interarticularis disconnects the front of the vertebra from its rear anchoring structures.6PubMed Central. Spondylolysis and spondylolisthesis: A review of the literature These fractures can develop during adolescence, especially in sports involving repetitive back extension and rotation like gymnastics, football, and cricket.7PubMed. Lumbar isthmic defects in teenagers resulting from stress fractures A person might sustain the pars fracture as a teenager, then notice the slip progressing to 9mm or so only years later as the disc at that level gradually loses height.
The distinction between degenerative and isthmic types matters for treatment planning even when the millimeter measurement is identical. Degenerative slips are generally more stable because the facet joints, though arthritic, are still intact and act as a partial brake. Isthmic slips have lost a bony connection, so they can behave differently under load. Your doctor will usually be able to tell which type you have based on the imaging and your age at onset.
Treatment for Low-Grade Anterolisthesis
For a 9mm (Grade I) anterolisthesis, the starting point is almost always nonsurgical. First-line management for low-grade spondylolisthesis includes physical therapy, activity modification, anti-inflammatory medications, and sometimes epidural steroid injections for flare-ups. Most people get meaningful symptom relief with this approach.8PubMed Central. Surgical and non-surgical management of spondylolisthesis: a comprehensive review Nonoperative care tends to work best in patients who do not have significant leg symptoms like nerve-related cramping or weakness, and whose slip is stable rather than dynamic.9Neurosurgery Clinics of North America. Management of Degenerative Spondylolisthesis
Physical therapy for spondylolisthesis typically emphasizes core stabilization, strengthening the deep abdominal and back muscles that act as an internal brace around the unstable segment. A meta-analysis of randomized trials found that lumbar segmental stabilization exercises showed a trend toward improving disability, and the benefit became statistically significant when these exercises were used as the primary standalone treatment rather than layered with other interventions.10Spine. Effectiveness of Lumbar Segmental Stabilization Exercises in Managing Disability and Pain Intensity Among Patients With Lumbar Spondylolysis and Spondylolisthesis Interestingly, the effect on disability was clearer than the effect on pain itself, suggesting these exercises help people function better in daily life even if their pain does not disappear entirely.
Activities that load the spine in extension, such as heavy overhead pressing, back walkovers in gymnastics, or sustained arching, tend to aggravate spondylolisthesis symptoms. Most therapists will steer you toward exercises done in a neutral or slightly flexed spinal position: think planks, bird-dogs, and modified deadlifts rather than sit-ups and roman chair extensions. Swimming and walking are commonly tolerated well. The goal is to stay active while avoiding postures that increase the shear force pushing the vertebra further forward.
When Surgery Becomes Part of the Conversation
Surgery enters the picture when conservative treatment has been given a fair trial (usually three to six months) and failed, or when there are progressive neurological problems like worsening leg weakness, numbness, or bowel and bladder changes. Lumbar degenerative spondylolisthesis can cause radiculopathy and neurogenic claudication, and when these symptoms do not improve with therapy and injections, decompression surgery to relieve pressure on the nerves is the standard next step.11Journal of the American Academy of Orthopaedic Surgeons. Surgical Management of Lumbar Degenerative Spondylolisthesis
A long-running debate in spine surgery is whether decompression alone is enough or whether fusion (bolting the two vertebrae together) should be added. A large randomized trial published in the New England Journal of Medicine found that at two years, patients who had decompression alone did essentially as well as those who had decompression plus fusion. About 71% of the decompression-only group and 73% of the fusion group achieved at least a 30% improvement in disability scores, with no meaningful difference between the two.12PubMed. Decompression with or without Fusion in Degenerative Lumbar Spondylolisthesis A systematic review pooling multiple studies backed this up, finding high-quality evidence of no difference in function, leg pain, or back pain between the two approaches at two years. Decompression alone did involve less blood loss and shorter hospital stays.13PubMed. Decompression alone versus decompression with fusion in patients with lumbar spinal stenosis with degenerative spondylolisthesis: a systematic review and meta-analysis
That said, fusion is still preferred in certain situations: when the slip is dynamic and worsens significantly with position changes, when there is substantial disc collapse and instability at the level, or when the slip is higher grade. For a stable 9mm slip that is not moving around, fusion may add surgical risk without clear additional benefit. Your surgeon’s recommendation will depend heavily on how the slip behaves on dynamic imaging and whether there is significant instability.
Anterolisthesis Versus Retrolisthesis
Anterolisthesis means forward slippage and retrolisthesis means backward slippage. People sometimes see both terms on the same MRI report at different spinal levels. The two types are not just mirror images of each other. A study of 269 patients with degenerative spondylolisthesis found that about half had pure anterolisthesis, about 40% had pure retrolisthesis, and a smaller group had both at different levels.14PubMed. Degenerative retrolisthesis: is it a compensatory mechanism for sagittal imbalance? The researchers investigated whether retrolisthesis might function partly as the spine’s attempt to compensate for overall sagittal imbalance, a different biomechanical story than anterolisthesis.
Research on the cervical (neck) spine has shown that anterolisthesis and retrolisthesis can have different implications for nerve function as well. In patients with cervical spondylotic myelopathy (spinal cord compression in the neck), anterolisthesis was a significant risk factor for poor neurological outcomes after surgery, while retrolisthesis did not affect outcomes.15Spine. Cervical Anterolisthesis: A Predictor of Poor Neurological Outcomes in Cervical Spondylotic Myelopathy Patients After Cervical Laminoplasty That finding is specific to the cervical spine and a particular surgery, but it illustrates a broader point: the direction of the slip is not just a geometric curiosity. Forward and backward slippage stress different structures and can create different patterns of nerve compression.
How Pelvic Alignment Fits Into the Picture
Spine specialists increasingly evaluate spondylolisthesis in the context of the whole spine-pelvis chain rather than just the single slipped segment. Parameters like pelvic incidence (the angle of your pelvis relative to the top of the sacrum) and pelvic tilt influence how much stress lands on any given lumbar level. Research on patients with low-grade degenerative spondylolisthesis found that pelvic incidence correlated with several other alignment measures and with functional balance scores, suggesting that the pelvis’s shape and tilt play a role in how much load the slipped segment bears.16Journal of Rehabilitation Medicine. Spino-pelvic alignment, balance, and functional disability in patients with low-grade degenerative lumbar spondylolisthesis
In practical terms, this means two people with the same 9mm L4-L5 anterolisthesis can have very different functional outcomes partly because of differences in how their pelvis is shaped and tilted. A person with high pelvic incidence naturally has more lumbar curve (lordosis), which increases the forward shear force at the lower lumbar levels. That extra force can make a low-grade slip more symptomatic or more prone to progression. It also explains why exercises that improve pelvic control and hamstring flexibility are often part of the rehabilitation plan: they help redistribute load away from the vulnerable segment.
Reliability of the Measurements Themselves
It is worth knowing that measuring spondylolisthesis is not perfectly reproducible even among experienced clinicians. Different radiologists can look at the same image and come up with slightly different millimeter values depending on where they place their measurement lines. Research into the reliability of spondylolisthesis measurements has found that intra-rater reliability (the same person measuring twice) and inter-rater reliability (two different people measuring the same image) are generally good when using standardized techniques.17PubMed. Reliability of functional imaging in assessing instability of lumbar degenerative spondylolisthesis But “good” does not mean identical. A measurement of 9mm on one reading might come back as 8 or 10mm on another, which is usually clinically irrelevant but can feel alarming if you are comparing two reports side by side.
If you have had imaging done at different facilities and noticed the numbers differ by a millimeter or two, measurement variability is the most likely explanation rather than actual progression of the slip. Genuine progression of a degenerative slip is typically slow, on the order of a millimeter or two over several years for most people with low-grade disease. A jump from 9mm to 11mm over a six-month period is more likely to reflect different body positions during the scan, different measurement techniques, or inter-reader variability than a suddenly worsening condition. Comparing images taken the same way (same position, same facility) over time gives the most reliable picture of whether a slip is actually progressing.