The "Mustache sign:" An ancillary radiological sign for detecting L5/S1 spondylolisthesis.

Introduction and aims: Grade 1 spondylolisthesis can be challenging to detect on magnetic resonance imaging (MRI), particularly for spinal surgeons and radiologists with limited experience interpreting spinal MRIs. This study aims to describe a unique sign described as a "mustache sign," which may assist in detecting subtle Grade I spondylolisthesis on sagittal sequences on MRI of the spine.

Patients and methods: A retrospective review of 50 lumbar spine MRI scans of patients with Grade I spondylolisthesis of L5/S1 performed over 3 years was conducted at a tertiary orthopedic spinal center in the United Kingdom. The scans were assessed for the presence of the "mustache sign" and findings were independently recorded by one musculoskeletal radiology registrar and one fellowship trained musculoskeletal radiologist with over 10 years of experience.

Results: There were 35 females (70%) and 15 males (30%). The patient's mean age was 54.3 years (13-82). The "mustache sign" was present in 13 (26%) of these patients. Twelve of 13 scans (92%) positive for the sign also demonstrated pars interarticularis defects (P < 0.001, Fisher's Exact test), compared to those without the sign. There was excellent interobserver reliability with a kappa of 1.

Conclusion: The "mustache sign" on MRI spine correlates well with the presence of Grade I spondylolisthesis. This ancillary sign can complement other previously described radiological findings on sagittal MRI sequences to confirm Grade I spondylolisthesis.