Anterolisthesis is a medical condition involving the spine where one vertebral body slips forward relative to the vertebra directly beneath it. This forward slippage is a specific type of spondylolisthesis, which is the general term for any vertebral displacement. The severity of this spinal instability is not determined by a simple millimeter measurement alone, but is instead assessed using a standardized, percentage-based grading system. This classification is the determining factor used by healthcare professionals to guide decisions about the most appropriate course of treatment.
Understanding Anterolisthesis
Anterolisthesis describes a forward displacement of a spinal segment. The lumbar spine, or lower back, is the most common region affected because it bears the majority of the upper body’s weight and handles significant biomechanical stress. The segments L4-L5 and L5-S1 are particularly susceptible to this forward shift.
The mechanism involves structural changes that compromise stability. When a vertebra shifts forward, the spinal canal can narrow, potentially compressing neural structures, such as the spinal cord or nerve roots. This displacement creates instability that can lead to pain, muscle tightness, or neurological symptoms in the lower back and legs.
The Meyerding Grading System
The severity of anterolisthesis is universally measured using the Meyerding classification system, which categorizes the vertebral slip based on a percentage of the width of the vertebra below it. This system uses a lateral X-ray image to calculate how far the superior vertebral body has translated across the top surface of the inferior vertebral body. The classification ranges from Grade I, the mildest, to Grade V, the most severe. This standardized percentage-based approach ensures a consistent method for evaluating the condition across different patients and clinics.
The grading is calculated by dividing the distance of the forward slip by the total anteroposterior width of the underlying vertebra, then multiplying the result by 100 to get a percentage.
- Grade I: 0 to 25% displacement.
- Grade II: 26% to 50% displacement.
- Grade III: 51% to 75% displacement.
- Grade IV (76% to 100%) and Grade V (over 100%, known as spondyloptosis) indicate severe instability.
The grade of a 5 mm anterolisthesis cannot be answered with a single, definitive grade without knowing the patient’s specific vertebral body dimensions. For instance, the average anteroposterior width of an adult L5 vertebral body is often between 30 and 40 millimeters. If a vertebra measuring 40 mm wide slips forward by 5 mm, the calculation results in a 12.5% slip, classifying it as Grade I. If the underlying vertebral body is narrower, such as 30 mm, a 5 mm slip results in a 16.7% slip. Therefore, a 5 mm slip is nearly always a Grade I anterolisthesis, but the final grade always depends on the percentage relative to the underlying bone structure.
Distinguishing the Causes of Vertebral Slip
The underlying reason for the vertebral slip provides context for the diagnosis. Physicians categorize anterolisthesis based on its etiology, with the two most common types being isthmic and degenerative, which affect different patient populations and stem from distinct structural failures.
Isthmic Anterolisthesis
Isthmic spondylolisthesis results from a defect in the pars interarticularis, a small segment of bone that connects the facet joints. This defect, often a fatigue or stress fracture known as spondylolysis, is commonly seen in younger, physically active individuals or athletes who participate in activities involving repetitive spinal hyperextension. The fractured pars interarticularis allows the vertebral body to shift forward, most frequently at the L5-S1 junction. This mechanical instability is caused by a break in the bony ring that normally secures the vertebra.
Degenerative Anterolisthesis
Degenerative spondylolisthesis is caused by the cumulative effects of aging and wear-and-tear on the spinal structures, not a fracture. This type is more prevalent in older adults, particularly women over the age of 50, and most often occurs at the L4-L5 level. Disc degeneration and chronic arthritis in the facet joints cause supporting ligaments to weaken and loosen over time. This gradual breakdown allows the upper vertebra to slowly slide forward, creating instability without a break in the pars interarticularis.
Treatment Pathways Correlated to Grade
The Meyerding grade plays a significant role in determining the appropriate management strategy for anterolisthesis.
Conservative Management
Low-grade slips, specifically Grade I and low Grade II, are typically managed with a conservative approach focused on alleviating symptoms and improving spinal stability. This initial management often includes a period of rest or modified activity to reduce stress on the spine. Physical therapy is a component of conservative treatment, focusing on strengthening the core and back muscles to provide better muscular support for the unstable segment. Nonsteroidal anti-inflammatory drugs (NSAIDs) or localized steroid injections may be used to manage pain and reduce inflammation related to nerve root irritation. This non-surgical pathway is successful for a majority of patients with low-grade slips who do not exhibit severe neurological deficits.
Surgical Intervention
Surgery is generally reserved for high-grade slips (Grade III, IV, or V) or for any lower-grade slip that continues to cause debilitating symptoms after a trial of conservative treatment. Surgery aims to decompress pinched nerves and stabilize the unstable segment to prevent further slippage. Decompression procedures remove bone or disc material to relieve pressure on the nerve roots or spinal cord. Spinal fusion is the most common procedure performed, which permanently joins the slipped vertebra to the one below it using bone grafts and metal hardware. This process eliminates motion at the unstable segment, which reduces pain and prevents the slip from progressing.