The spine is a column of bones, or vertebrae, separated by cushioning intervertebral discs. This structure provides flexibility and protects the spinal cord. When alignment is disrupted, spondylolisthesis occurs, where one vertebra shifts out of its normal position relative to the one below it. Anterolisthesis is the specific term used when the upper vertebral body slips forward (anteriorly) over the lower one. This movement is most commonly seen in the lower back (lumbar spine), particularly at the L5-S1 junction.
Defining Spinal Slippage
Spinal slippage is measured clinically using two metrics: an absolute distance in millimeters (mm) and a relative percentage. The millimeter measurement represents the absolute distance the upper vertebra has translated forward from the vertebra directly beneath it. This measurement is typically obtained from lateral X-ray images of the spine.
The absolute millimeter measurement alone does not determine the severity grade because vertebral body size varies significantly among individuals. To standardize severity, this distance must be converted into a percentage relative to the width of the vertebral body below the slip. This percentage calculation allows clinicians to compare slippage across different patients and forms the basis for classifying anterolisthesis severity.
Grading Severity Using the Meyerding Scale
The standard clinical tool for classifying anterolisthesis severity is the Meyerding Grading System, which relies entirely on the calculated percentage of slippage. This system divides the top surface of the lower vertebral body into four equal quarters. Grade I, the lowest level of displacement, corresponds to a forward slip of up to 25% of the lower vertebral body’s width.
Grade II covers a range from 26% to 50% of the vertebral body width. Grade III represents significant instability, occurring when displacement is between 51% and 75%. Grade IV is the most severe partial slip, encompassing a translation from 76% up to 100% of the lower vertebral body’s width.
The most extreme classification is Grade V, or spondyloptosis, which occurs when slippage is greater than 100%. Grades I and II are considered low-grade slips. Grades III, IV, and V are referred to as high-grade slips, which often carry greater clinical concerns.
Clinical Significance of a 9mm Measurement
Determining the grade of a 9mm anterolisthesis requires knowing the patient’s specific vertebral size. The fifth lumbar vertebra (L5), the most common site, typically has an anteroposterior width ranging from 30mm to 40mm in adults. Using an average width of 35mm for the lower vertebral body, a 9mm slip translates to 25.7%.
A 25.7% slippage falls just over the Grade I threshold and is classified as Grade II according to the Meyerding system. If the patient has a smaller vertebral width (e.g., 30mm), a 9mm slip calculates to 30%, remaining Grade II. Conversely, a larger vertebral width (e.g., 40mm) results in a 22.5% slip, classifying it as Grade I.
A 9mm measurement most frequently lands the condition at the high end of Grade I or, more commonly, within the Grade II range. High-grade slips (Grade III or higher) are more likely to be associated with nerve root compression or spinal canal narrowing, leading to neurological symptoms. A 9mm slip, especially if Grade II, indicates a moderate slip that may cause low back pain, stiffness, or mild nerve irritation.
Management and Next Steps
The treatment pathway for anterolisthesis is guided by the Meyerding grade, the patient’s symptoms, and any neurological compromise. For slips typically falling into Grade I or Grade II, conservative management is the initial approach. This non-surgical treatment focuses on stabilizing the spine by strengthening the core and back muscles through physical therapy.
Other conservative measures include bracing, anti-inflammatory medications, and epidural steroid injections to manage pain. Surgery often involves spinal decompression and fusion to stabilize the segment. Surgery is generally reserved for cases that fail to improve after conservative care, higher-grade slips, or if the patient exhibits progressive neurological deficits.