Research news and notes

1. Injection of acrylic cement increased the strength of an interspinous process device 

Placement of an interspinous process device, such as X-stop, is an option for the treatment of lumbar spinal stenosis with neurogenic claudication. The device keeps the spinous processes spread apart, and thus, depends on the strength of the bone. Indeed, X-stop is not currently used in patients with severe osteoporosis—a common problem in the patient population that can benefit most from an interspinous device. In the Februay 2008 issue of Spine, Idler et al [3] reported an experiment where they used polymethylmethacrylate (PMMA) to strengthen spinout processes before placement of X-stop devices. The authors used cadaveric vertebrae with osteoporosis; in half of them, the spinous processes were injected with about 2 mL of PMMA. All received 2 X-stop implants and were tested by compressing the spinout process between the X-stops to failure. On average, PMMA injections doubled the strength of the spinous process compared to controls. Cement was found in the laminae and sometimes in the facets but never broke into the central canal. This is an interesting study and a simple but potentially useful idea.

2. Interspinous process device tends to fail in patients with spondylolisthesis 

The interspinous distraction device X-stop is an option for the treatment of neurogenic claudication caused by lumbar spinal stenosis. In some of the patients, spondylolisthesis is part of the degenerative process and contributes to the symptoms. A recent study by Verhoof et al [8] tried X-stop in 12 patients with degenerative spondylolisthesis and symptomatic lumbar spinal stenosis. Initially, 8 of 12 patients had complete symptom relief. There were no changes in the spondylolisthesis (percentage of slip or spinal dimensions) after X-stop placement. However, the improvement was short lived. After 24 months of follow-up, 3 patients had worsening of symptoms. In total, 7 patients had decompression with posterolateral fusion within 24 months of the X-stop procedure. The authors do not recommend X-stop for the treatment of lumbar spinal stenosis complicating spondylolisthesis. The results can also be interpreted more charitably—in this group of patients—as 8 of 12 had a temporary improvement, and at least some patients had a long-term improvement, even in a situation that is mechanically unfavorable for interspinous distraction. I agree with the authors that the better surgical treatment in this case is decompression and fusion, but X-stop can be tried in carefully selected cases where a larger procedure is very dangerous or patient's expected longevity and overall prognosis call for shorter term symptomatic relief.

3. Additional insight into the value of fusion in the treatment of lumbar canal stenosis with degenerative spondylolisthesis 

Over the past decade, spinal fusion, as an adjunct to laminectomy for the treatment of lumbar canal stensosis with degenerative spondylolisthesis, has become increasingly common. In 1991, Herkowitz and Kurz [2] published a randomized prospective study of 50 patients, comparing the efficacy of lumbar laminectomy and posterolateral fusion (PLF) to decompressive lumbar laminectomy alone. After a mean follow-up of 3 years, patients who underwent PLF demonstrated a statistically significant improvement in clinical outcome. Later studies corroborated these results [1], [6], [9]. However, although Herkowitz and Kurz postulated that even the attempt at arthrodesis decreased subsequent motion, thus improving long-term outcome regardless of fusion status, the clinical consequences to patients who went on to a pseudoarthrosis remained speculative.

A recent article by Tsutsumimoto et al [7] reported a retrospective study of 42 patients who underwent laminectomy with PLF for lumbar canal stensosis associated with degenerative spondylolisthesis. The authors assessed long-term outcome in relation to fusion status and attempted to identify demographic and clinical patient factors predictive of surgical outcome. Patients were assessed using the Japanese Orthopaedic Association's (JOA) score for the assessment of low back pain before surgery and at 1, 3 and 5 years after surgery and at the final follow-up. Average patient follow-up was 9.5 years. The JOA assigns 9 points for subjective symptoms, including low back pain and leg pain, 6 points for clinical signs, and 14 points for the restriction of activity of daily living; the total score is thus 29 points [4]. Clinical results were then measured as a percent recovery of the JOA score indicating the degree of normalization after surgery, which was calculated as follows: percent recovery (%) = [(postoperative JOA score − preoperative JOA score)/(29 − preoperative JOA score)] × 100.

At the final follow-up, the percent recovery was greater than 50% in 69.0% (29 of 42) of the patients. Nonunion developed in 26.2% (11 of 42) of the patients. At 1- and 3-year follow-up, there was no significant difference in the overall percent recovery between the union and nonunion groups, however, at 5-year and final follow-up, the union group demonstrated better outcomes. These results are consistent with a study published in 2004 by Kornblum et al [5] describing the long-term outcomes (mean, 7.7 years) of 47 patients treated with uninstrumented PLF in the studies by Herkowitz and Kurz [2]. There, the long-term clinical outcome was excellent or good in 86% of patients with solid arthrodesis, but only in 56% of patients who developed a pseudarthrosis (P < .01). The precise cause of deterioration in patients with a pseudoarthrosis is debatable. One possible explanation is that instability at the incompletely fused segment causes greater laminar regrowth and hypertrophy of the facet joints. These changes might lead to the recurrence of the spinal stenosis, resulting in less favorable outcomes over time.