Isthmic Spondylolisthesis¶

Definition¶

Isthmic spondylolisthesis is anterior translation of a vertebra due to a defect (spondylolysis) in the pars interarticularis — the bony bridge connecting the superior and inferior articular processes. This defect disconnects the posterior elements from the vertebral body, allowing the body to slide forward. It is most common at L5–S1 in young adults and athletes.

Spondylolysis (Pars Defect)¶

A stress fracture of the pars interarticularis from repetitive hyperextension and rotation
Most common at L5 (~85% of cases)
Bilateral pars defects are required for listhesis to occur (unilateral defects cause pain but not translation)
Common in young athletes: gymnastics, football linemen, cricket fast bowlers, weight lifters

Imaging Findings¶

Radiography¶

Lateral: anterior slip of L5 on S1; the degree is graded by Meyerding classification
Oblique: the pars defect appears as a break in the "neck of the Scottie dog" — the classic radiographic finding
Flexion-extension views assess for dynamic instability

CT — Best for Pars Evaluation¶

Sagittal reformats through the pars: directly demonstrates the defect as a lucent line or gap
Sclerotic margins of the defect suggest a chronic, non-healing fracture
Absence of sclerosis with adjacent marrow edema suggests an acute/active stress fracture

MRI¶

Pars defect: low-signal discontinuity on sagittal T1 through the pars
STIR/T2: marrow edema adjacent to the pars defect suggests an active (potentially healable) stress reaction
Evaluate for disc degeneration at the slip level, nerve root compression, and canal stenosis

Nuclear Medicine (SPECT)¶

More sensitive than CT or MRI for detecting active pars stress reactions
Focal uptake at the pars on SPECT indicates metabolically active bone turnover
Useful in young athletes with back pain and negative or equivocal CT/MRI

Clinical Pearl

In young athletes with low back pain and suspected spondylolysis, the distinction between an active stress reaction (potentially healable with rest and bracing) and a chronic established defect (non-healing) guides management. MRI with STIR showing marrow edema at the pars, or SPECT showing focal pars uptake, indicates an active process that may heal with conservative treatment (typically 3–6 months in a brace with activity restriction). A chronic defect with sclerotic margins on CT is unlikely to heal and is managed symptomatically.

Key Points¶

Isthmic spondylolisthesis results from bilateral pars interarticularis defects (spondylolysis)
Most common at L5–S1 in young athletes from repetitive hyperextension
CT sagittal through the pars is the best imaging for confirming the defect
STIR edema (MRI) or SPECT uptake indicates an active, potentially healable stress reaction
Chronic sclerotic defects are unlikely to heal — managed conservatively or with fusion if unstable
The Scottie dog "broken neck" on oblique radiographs is the classic finding

References¶

Gaillard F, et al. Wiltse classification of spondylolisthesis. Radiopaedia.org. https://radiopaedia.org/articles/wiltse-classification-of-spondylolisthesis
Gaillard F, et al. Spondylolysis. Radiopaedia.org. https://radiopaedia.org/articles/spondylolysis
Koslosky E, Gendelberg D. Classification in Brief: The Meyerding Classification System of Spondylolisthesis. Clin Orthop Relat Res. 2020;478(5):1125-1130. https://pmc.ncbi.nlm.nih.gov/articles/PMC7170696/
Margetis K, Gillis CC. Spondylolisthesis. StatPearls. NCBI Bookshelf. https://www.ncbi.nlm.nih.gov/books/NBK430767/
Syrmou E, Tsitsopoulos PP, Marinopoulos D, et al. Spondylolysis: a review and reappraisal. Hippokratia. 2010;14(1):17-21. https://pmc.ncbi.nlm.nih.gov/articles/PMC2843565/
Expósito Jiménez D, Álvarez de Sierra Garcia B. Magnetic resonance imaging (MRI) vs. computed tomography (CT) in the diagnosis and classification of spondylolysis and spondylolisthesis—a narrative review. Quant Imaging Med Surg. 2024;14(11):7891-7907. https://pmc.ncbi.nlm.nih.gov/articles/PMC11558484/