Diagnostic Efficacy of Posterior Epidural Fat Interposition on Magnetic Resonance T1-Weighted Sequence in the Diagnosis of Spondylolysis.

Objective: Supportive radiologic signs may be needed to diagnose spondylolysis (SL) via lumbar magnetic resonance imaging (MRI). In SL, the slight displacement of the corpus forward and lamina posteriorly can cause the interposition of posterior epidural fat (EFI), which is normally segmental. This study aimed to determine the diagnostic value of EFI, an indirect sign of SL, on lumbar mid-sagittal T1-weighted MRI.

Methods: The lumbar MRI of 115 randomly selected patients with SL and degenerative disc disease (DDD) was randomized and assessed for the presence or absence of EFI by 2 masked observers. These observers were not permitted to examine the pars region. Interobserver agreement was tested using Cohen's kappa coefficient.

Results: EFI was positive in 98 (85%) of 115 patients with SL, 14 (12%) in the DDD group, and 6 (5%) with an upper vertebral level adjacent to the SL. The difference was statistically significant (P < 0.01). In patients with SL, the EFI positivity rate was highest at lumbar 5 vertebrae (94%) and lowest at lumbar 3 vertebrae (33%). The specificity, sensitivity, positive predictive value, negative predictive value, and accuracy of EFI in diagnosing SL were mean 64%, 97%, 80%, 97%, and 95%, respectively. The highest diagnostic value of EFI was at the lumbar 5 vertebrae level, where intraobserver and interobserver reliability were nearly perfect.

Conclusions: EFI is an indirect radiological finding with high reliability in diagnosing SL with mid-sagittal T1-weighted images in lumbar MRI.