Why Do Spinous Process Fractures Occur in the Spine After X-Spacers?In this study, surgeons at Tufts University School of Medicine discover an important reason why half of their patients receiving an X-STOPÂ® device end up with a fracture of the spinous process.
The X-STOPÂ® is a metal implant made of titanium. It is a minimally invasive procedure for the treatment of mild lumbar spinal stenosis (LSS). The implant is inserted through a small incision in the skin of the low back. It is designed to fit between the spinous processes of the vertebrae in the lower back. It stays in place permanently without attaching to the bone or ligaments in the back.
Spinal stenosis describes a clinical syndrome of buttock or leg pain. These symptoms may occur with or without back pain. It is a condition in which the nerves in the spinal canal are closed in, or compressed. The spinous process is the piece of bone that sticks out behind the vertebra. It is the bump you feel down the back of your spine.
The spinal canal is the hollow tube formed by the bones of the spinal column. Anything that causes this bony tube to shrink can squeeze the nerves inside. As a result of many years of wear and tear on the parts of the spine, the tissues nearest the spinal canal sometimes press against the nerves. This helps explain why lumbar spinal stenosis (LSS) is a common cause of back problems in adults over 55 years old.
The X-STOPÂ® procedure has been very successful for carefully selected patients with LSS. And now, thanks to this study, surgeons may be able to screen patients for a specific risk factor that is associated with spinous process fractures after X-STOPÂ® surgery. The single factor predictive they discovered for spinous process fracture was the presence of lumbar spondylolisthesis.
Spondylolisthesis describes a condition in which one lumbar vertebra has slipped forward over the vertebra below. This slippage causes a narrowing of the spinal canal and traction (pulling) on the nerve tissue. Although their study was small (only 39 patients), the high rate (52 per cent) of spinous process fractures after X-STOPÂ® implantation was significant.
The surgeons aren't exactly sure why spondylolisthesis would increase the risk of fracture. They suspect that the change in alignment with the vertebra shifted forward may have something to do with it. The contact points (where the implant rests against the spinous process) may be further back in patients with spondylolisthesis. This placement is against a weaker part of the spinous process contributing to fracture.
Other factors explored for potential cause of fractures included patient age, gender (male or female), body mass index (BMI), and bone density. The researchers did not find any difference in these risk factors between patients with fractures and those without. Surgical level and number of levels treated were also evaluated for possible links to fracture risk. There weren't enough patients in the study to make any firm conclusions about these two possible risk factors.
In summary, stenosis and spondylolisthesis in the same patient may be a reason NOT to use the X-STOPÂ® device. The high rate of spinous process fractures as a complication of the X-STOPÂ® procedure may be a reason to try a different surgical approach. In fact, the presence of spondylolisthesis in anyone with lumbar spinal stenosis may be considered a contraindication for this device. Contraindication means a reason not to use these implants.
The authors suggest future studies address the following concerns: 1) this was a small study; results may be different with a larger group, 2) the fracture rate was consistent no matter which type of X-STOPÂ® was used. So it's not likely to be caused by the implant itself but may be the way it is inserted; this should be further investigated, and 3) perhaps there are other biomechanical or patient factors responsible for these results. More study is needed to explore these concerns.
One final note of importance: fractures of the spinous process did not show up on X-rays for any of the patients who did, in fact, have a spinous process fracture. CT imaging was needed to truly see fractures of this type.
David H. Kim, MD, et al. Association Between Degenerative Spondylolisthesis and Spinous Process Fracture After Interspinous Process Spacer Surgery. In The Spine Journal. June 2012. Vol. 12. No. 6. Pp. 466-472.
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