In ankylosing spondylitis, what is the dagger sign? In this post we look at how to recognise it and he Anatomy and Pathology that helps explain what the Dagger sign is.
In Ankylosing spondylitis the dagger sign refers to a solid central line of bone along in AP spine view.
Linear density in the midline connecting the spinous processes (Pink arrows).
Image Above: Dagger sign (Pink arrows). Also SIJ fusion (Blue arrow) and Bamboo spine (Orange arrow).
It's due to ossification of the interspinous and supraspinous ligaments that extend between the spinous processes.
- INTERSPINOUS LIGAMENT (Pink shading)
- Attaches to the cranial and caudal spinous process at each level.
- But also has attachment to the lamina and ligamentum flavum.
- SUPRASPINOUS LIGAMENT (Blue shading).
- Attaches between the tips of the spinous processes (Thoracic and lumbar).
- Above C7 it is free of the spinous processes and attaches to the occiput as the ligamentum nuchae.
SP Spinous Process, SSL Supraspinous Ligament FJC Facet Joint Capsule.
Image from 2019 Asian Journal of Neurosurgery Anatomical and Biomechanical Study of the Lumbar Interspinous Ligament .
Caused by enthesitis of the ligamentous attachments to the spinous process followed eventually by ossification.
The dagger sign is visualized on frontal x-ray and coronal CT.
This post has been made in conjunction with Dr Joe Thomas, a Senior Consultant Rheumatologist at Aster Hospital in Kochi India, who has a vast amount of clinical experience. He also has a very strong interest in Imaging of Arthropathies and has joined us to bring a clinical perspective to the imaging and to advise on what rheumatologists want when we report their referrals.
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