Artificial Disc Replacement Holds Up Well After Spine Trauma

Artificial disc replacements are gaining in popularity as studies show how well they are working. They are still used primarily for patients with degenerative disc disease, but the number and types of patients with this diagnosis who have benefited continues to expand. For example, younger patients (less than 65 years old) and younger adults with early disc degeneration from trauma or work-related repetitive motions are now getting artificial implants of this type.

Artificial disc replacements do have their own problems. Sometimes they break or migrate (move). In some cases, malposition of the implant results in uneven wear and eventual hardware failure. Bone growth around the implant is expected and helps hold the implant in place. But in some patients, ossification occurs -- so much bone growth that the implant is buried and nonfunctional.

But these complications are fairly rare and short-to-medium term studies report good-to-excellent results with patient satisfaction described by a majority of patients. What we don't know is how well would an artificial disc replacement hold up under significant trauma? This idea can be tested in the laboratory using cadavers (spinal segments preserved from humans after death). But without the dynamic effects of muscles, ligaments, and other connective or soft tissues, it's impossible to know how a traumatic force might affect the implant.

That's why this case report is so important. It details what happened when a 31-year-old manual laborer with an L4-5 Charité artificial disc fell off a roof and fractured his spine at the L3 level (third vertebral bone in the lumbar spine). The compressive load through the spine was powerful enough to cause the L3 vertebra to burst into tiny pieces. This injury is called a burst fracture. The fracture was unstable meaning that pieces of the fractured bone shifted, pushing into the spinal canal and pressing on the nerves. The accident occurred 10 months after the implant was put in the spine and while the patient was back to work full-time.

Fortunately, there was no evidence of damage to the artificial disc between the fourth and fifth lumbar vertebrae. The L4 and L5 vertebrae were also undamaged. In fact, the authors think that maybe the artificial disc actually protected the vertebrae above and below it. The young age of the patient (31 years old) might have contributed in a positive way as well, but there's no way to know that from a single case study -- just something to consider.

Of course, there are very few cases like this, so no clinical practice guidelines have been published to guide the surgeon in knowing the best way to treat it. In this case, surgery was done to fuse the spine from L2 to L4. Motion at the L4-L5 level was saved. The last lumbar vertebra (L5) was already fused to the sacrum in a previous L5-S1 fusion procedure. Now the patient only has motion at the first lumbar segment and at L4-5 where the artificial disc is located.

This is a unique case because now the spine is fused above and below the level of the still functional disc replacement. That potentially puts a lot of stress and strain on the artificial disc, which in turn affects the facet (spinal) joints on either side. Anytime the disc is compressed, the facet joints are also compressed. The opposing joint surfaces get squeezed together and rub instead of sliding and gliding against each other. The result over time can be degeneration of the spinal joints. The authors hope to follow this patient long-term to see how well the implant holds up and what effects the fusion on either side might have on motion, biomechanics, and spinal stability.

For now, cases like this (and other already reported in the literature) show that artificial disc replacements can be used in patients who have had other levels fused in previous surgeries. This case confirms the successful use of an artificial disc with fusion above and below the implant. It also proves that artificial disc replacements can sustain major trauma without breaking loose or being fractured along with the bone fracture.

Typically, artificial disc replacements don't have good shock absorption. The authors point out that one of the main reasons this implant held up so well was because of its optimal placement. Other studies have shown that when placed with good alignment, these implants do indeed hold up better and last longer with fewer problems over time. The makers of artificial disc replacements are considering ways to improve the design to help absorb compressive loading and vibration. The hope is that the implant could actually help protect the segments above and below it, rather than transfer the load to the vertebrae above and below.

What happened to this particular patient? Well, after surgery, his symptoms of back and leg pain, numbness, and motor weakness gradually improved until he was able to return to work full-time at his same job. Two years later, he reported minimal low back pain but continued decreased sensation in his right leg and a little bit of weakness in his right foot. X-rays confirmed a good solid lumbar fusion with preserved motion at the L4-L5 level where the artificial disc continued to work properly.

Oren N. Gottfried, MD, and Darrel S. Brodke, MD. Traumatic Burst Fracture in a Patient with a Lumbar Artificial Disc. In Journal of Neurosurgery: Spine. September 2009. Vol. 11. No. 3. Pp. 353-357.

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