What causes the blade of grass sign on radiography of a long bone?
Let me know in the comments.
Click to reveal the answer
Answer:
The blade of grass sign, also called the flame sign or candle flame sign, is a well-defined V-shaped or wedge-shaped lucent front advancing along the shaft of a long bone. It marks the leading edge of the osteolytic (first) phase of Paget disease, and the front may persist into the mixed phase. It characteristically begins at a subchondral or metaphyseal end of the bone and advances towards the diaphysis, most often in the femur or tibia, and it typically progresses at roughly 1 cm per year. The equivalent process in the skull is not called blade of grass but osteoporosis circumscripta, a large, well-defined geographic lucency that crosses sutures and favours the frontal and occipital bones. Recognising the sign matters because a purely lytic pagetic lesion can be mistaken for a permeative malignant process; the sharply defined V-shaped margin, cortical thickening behind the front and preserved bone contour argue for Paget disease.
Why is it called so?
The tapering, pointed lytic front resembles a blade of grass or the tip of a candle flame.
Pathophysiology
Paget disease begins with a wave of intense, disorganised osteoclastic resorption. The osteoclasts advance along the medullary cavity and endosteal surface as a discrete front rather than diffusely, so the resorbed bone behind them is lucent and the unaffected bone ahead of them is normal, and the interface between the two is sharp. Because resorption proceeds fastest centrally in the medulla and more slowly at the cortex, the front is not flat but tapers to a point, producing the V. Behind the advancing edge the reparative phase lays down coarse, thickened trabeculae and an expanded cortex, which is why the lucent front is bounded proximally by dense, disorganised pagetic bone.
Alternative names: Flame sign, candle flame sign, V-shaped lytic front
Other associated named signs: Osteoporosis circumscripta, cotton wool appearance, picture frame vertebra, banana fracture, and the Tam O’Shanter skull
References
Access all radiology signs posted so far: https://radiogyan.com/radiology-signs/
Improve Content And Help Your Colleagues!
Found an error in the post? Please let us know using our contact page and we will update it with due credits!
If you wish to contribute radiological images for this case (or any other article on the website), you can submit them here and you will be duly credited: Submit radiology case