Accuracy of the kissing sign on lumbar spine MRI in cases of axillary disc herniation and the surgical correlation: an Indian multi-center study()

OBJECTIVES: Magnetic resonance imaging has proven to be a valuable tool in the assessment of disc abnormalities. Two types of disc extrusion can be described according to the direction of herniated disc material: shoulder type and axillary type. Axillary disc herniation is described when the extruded disc fragment lies in the recess between the lateral border of cauda equina and medial to the nerve roots, while in the shoulder type the disc lies lateral to the nerve roots. It is very important to describe the type of disc herniation, as the surgical approach differs in each type. To the best of the authors’ knowledge, no definite signs have been described in literature to date. This study aimed to address the accuracy of the kissing sign on MRI for he diagnosis of axillary disc herniation. METHODS: The MRIs of 72 patients undergoing spinal surgery were prospectively evaluated for axillary disc herniation by a senior radiologist and experienced spinal surgeon using the kissing sign on MRI. The kissing sign was considered positive when the herniated disc material was in direct contact with the lamina and/or ligamentum flavum on axial images. Subsequently, all surgeries were performed by two independent surgeons and the actual type of disc herniation was documented. The accuracy of the results was statistically assessed. RESULTS: The kissing sign on MRI was found to be 66.66% sensitive, 92.59% specific, and 76.38% accurate in detecting axillary disc herniation with significant correlation with the surgical findings. CONCLUSION: The type of disc herniation is an important parameter for patient selection in different surgical approaches. The kissing sign on MRI can be considered as an important tool for diagnosing axillary disc herniation due to its high specificity and accuracy.