Winking Owl Sign & Beyond

An absent pedicle in the AP view of the spine is a sign of a metastatic tumor to the spine that affects the pedicle. This is usually called the “winking owl” sign. It means that the pedicle is absent or destroyed due to the metastatic tumor. The tumor extends from the vertebral body into the pedicle and destroys its boundaries, the cortical rim, which creates its oval ring as seen on the AP view x-ray.

The pedicle is the strongest part of the spine and looking at the pedicle is important. In Achondroplasia, there can be congenital lumbar stenosis. When you have a widened interpedicular distance, this can be a sign of a burst fracture. You can also differentiate a tumor from an infection. The tumor can affect the vertebral body and can extend into the pedicle. If there is pedicle involvement, then it is probably a tumor and not osteoporosis fracture of infection. Infection will start at the disc space, so when there is disc space involvement, then this is an infection of the spine or osteomyelitis. The pedicle is part of the posterior element of the spine, and it can be involved in osteoid osteoma, osteoblastoma, and it can be involved in aneurysmal bone cysts.

In a sagittal section example of L4, when you place the pedicle screws, you want to make sure that you do not go inferior to the pedicle, or you will injure the nerve root of the same numbered vertebra. If you are at L4 vertebra, this will not affect the L4 nerve root. If you go medical (outside the pedicle) into the spinal canal, the result usually depends upon the amount of violation. If you just penetrated or violated the medical border of the pedicle, then you could be  putting pressure on the nerve.

Nabil Ebraheim, MD

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