Pedicle sign | Radiology Signs

Answer:

The pedicle sign is absence or erosion of the normally sharp cortical ring of a vertebral pedicle on the frontal radiograph, so that one vertebra appears to have only a single eye. It is frequently the earliest visible radiographic sign of a lytic vertebral metastasis, because the pedicle is a compact cortical structure viewed end-on against air-filled or soft tissue background, whereas destruction confined to the cancellous vertebral body must remove roughly 30 to 50 per cent of trabecular bone before any lucency becomes visible. The usual primaries are breast, lung, prostate, renal and thyroid carcinoma. Classical teaching contrasts metastasis with multiple myeloma, in which the pedicle is said to be relatively spared early because myeloma involves the red-marrow-rich vertebral body first; this is a tendency and not a rule, since advanced myeloma does destroy posterior elements. Important non-neoplastic causes are congenital absence or hypoplasia of a pedicle, which is distinguished by a sclerotic, hypertrophied contralateral pedicle and an intact adjacent cortex, previous surgery, infection, and pedicle erosion by a slow-growing intraspinal lesion such as a nerve sheath tumour, which widens the interpediculate distance and scallops the posterior vertebral body. Because radiography is insensitive, a suspected pedicle lesion should be assessed with MRI, which shows marrow replacement long before cortical destruction, or with CT or bone-specific PET.

Why is it called so?

The sign is named for the vertebral pedicle whose cortical outline disappears; the resulting asymmetric appearance of the vertebra is also called the winking owl sign.

Pathophysiology

The pedicle is a short tube of dense cortical bone connecting the vertebral body to the posterior elements. Batson’s valveless paravertebral venous plexus delivers tumour emboli preferentially to the posterior vertebral body and the pedicle, and tumour cells stimulate osteoclastic resorption through RANKL and parathyroid hormone-related peptide. Because the projected image of the pedicle is a thin, high-contrast cortical ring superimposed on cancellous bone, loss of even part of that cortex is conspicuous, while equivalent destruction within the vertebral body is hidden by the surrounding trabeculae. Pedicle destruction also signals loss of posterior column integrity, which is relevant to spinal instability scoring.

Alternative names: Winking owl sign; one-eyed vertebra

Other associated named signs: Ivory vertebra of sclerotic metastasis or lymphoma, vertebra plana, and the scalloped vertebra of a slow-growing intraspinal mass