What is Spondylolisthesis
Spondylolisthesis is the forward displacement of one vertebra over the vertebra below it, most commonly occurring at L5-S1 or L4-L5, resulting from either a structural defect in the pars interarticularis (isthmic type) or degenerative changes in the facet joints (degenerative type). 1
Anatomical Definition and Mechanism
Spondylolisthesis occurs when the normal anatomical structures that resist anterior vertebral displacement fail 1. These protective structures include:
- The bony block of the posterior facets
- The intact neural arch and pedicle
- Normal bone plasticity that prevents pedicle stretch
- The intervertebral disc bonding vertebral bodies together 1
Classification by Type
Isthmic Spondylolisthesis
- Results from a defect in the pars interarticularis of L5 with resultant slippage of L5 on S1 1
- Most commonly seen clinically 1
- Complete discontinuity of the pars is typically necessary to allow for higher-grade anterior vertebral translation 2
Degenerative Spondylolisthesis
- Arises from remodeling of the facet joints, eliminating bony opposition to anterior displacement 1
- Most commonly occurs at L4-L5 level in women over age 40 1
- Defined as displacement of one vertebra over the subjacent vertebra associated with degenerative changes, without an associated disruption or defect in the vertebral ring 3
- One of the most common causes of spinal canal stenosis 4
Grading System
The degree of vertebral body anterior displacement is rated from Grade 1-4 depending upon the severity of the slip 1. Higher-grade slips can affect global sagittal balance and often result in altered gait patterns and compensatory posturing 2.
Clinical Presentation
Patients with spondylolisthesis associated with stenosis typically present with 5:
- Neurogenic claudication
- Radiculopathy
- Low-back pain 6, 3
- In severe cases: intermittent claudication or vesicorectal disorder 4
Associated Instability
The inherent instability associated with spondylolisthesis may progress following decompression and lead to further misalignment that results in pain or recurrence of neurological complaints. 5 This progressive instability develops from degeneration of the disc, facet joints, and supporting ligaments over years 7.
Common Pitfalls
Do not confuse radiographic findings with clinical significance—correlation between imaging findings and clinical symptoms is necessary for determining treatment 8. The presence of spondylolisthesis on imaging does not automatically indicate it is the source of symptoms 1.