I was searching the Internet for any information I could find on these new X-stop devices used for spinal stenosis. My surgeon is strongly suggesting I consider having one of these put in at the L4-L5 level. I found reference to the sandwich phenomenon as new complication of this operation. What is this?

You may have found what is the first mention of a rare complication with X-stop devices. X-stops are spacers placed between two vertebrae to hold them apart. They are used to manage various degenerative conditions of the lumbar spine such as spinal stenosis (narrowing of the spinal canal) and disc degeneration.

The surgeon implants the X-stop between the spinous processes — not between the main bodies of the vertebrae. The device is called an X-stop because it stops the movement of spinal extension at that level. The spinous process is the bony projection off the vertebra that you feel as you run your hand down along your spine.
The device is called an X-stop because it stops the movement of spinal extension at that level.

Neurosurgeons from Italy recently published a paper reporting a fracture of the spinous process in three patients who had double-level X-stop devices. Double-level means there were two X-stops: one between the spinous processes of L3 and L4 and another between L4 and L5. The surgeons named this new type of fracture the sandwich phenomenon because the broken spinous process (L4) is sandwiched between two X-stop distractors.

Although X-stop is a fairly new treatment technique, two-level implants of this type have been used in many patients around the world without problems. The atraumatic fractures described in this report affected three men of different ages. It is considered a rare postoperative complication.

What causes a spontaneous fracture like this? That presents an unsolved puzzle. There could be some anatomic difference in these three men contributing to the fracture. But what exactly that difference is remains a mystery. There was no sign of osteoporosis (decreased bone density leading to brittle bones) that could account for the fracture. The double-level procedure was the most likely key feature.

The authors of this report suggest a possible piston effect on the spinous process. The X-stop above and below the process applied pressure to the bone with every spinal movement until it finally snapped. They concluded that despite this rare sandwich phenomenon, X-stop implants are still safe and effective.