Research news and notes

1. Advancement in robotic-assisted stereotactic neurosurgery 

Since its inception 100 years ago, stereotaxy has been widely used in animal neuroscience laboratories. However, the study of modern stereotactic procedures, such as deep brain stimulation in small animal models, requires probe placement that is beyond the precision of manually operated stereotactic frames. For example, the subthalamic nucleus in the rat brain is 100 × 100 × 100 μm. In a recent article, Ramrath et al [7] introduce the first stereotactic robot for neurosurgery on small animals, SASSU. The system features a mean mechanical positioning accuracy of 32 μm and a mean positioning repeatability of 11 μm. Although the maximum positioning error of 110 μm eventually leads to missing targets, such as the subthalamic nucleus, 90% of all positioning errors were less than 50 μm. This represents a substantial improvement in positioning performance compared to existing animal stereotactic devices. The SASSU also outperformed the mechanical accuracy of human stereotactic frames, for which the mechanical accuracy has been reported to be in the submillimeter range [1]. Therefore, this technology creates the potential to minimize collateral morbidity associated with probe placement during deep brain stimulation. The next step is to analyze the application accuracy of the system, which will differ from the mechanical accuracy due to effects during probe insertion and morphological variation among animals.

2. Charite artificial disk: 5-year follow-up study fails to allay concerns 

Despite the prevalence of degenerative disk disease, surgical options are developing quite slowly. Current attempts at surgical treatment, such as diskectomy and interbody fusion, produce abnormal biomechanics and may result in adjacent level degeneration. Consequently, the concept of artificial disk replacement offers the enticing possibility for true reconstructive surgery and restoration of normal intervertebral segment motion. In 2004, the Charite artificial disk became the first total disk arthroplasty approved by the US Food and Drug Administration. Composed of 2 chromium alloy end plates, housing a sliding polyethylene core, this artificial disk promised to restore disk space height, preserve motion segment flexibility, prevent disk failure at adjacent levels, and improve patient functional activity. Approval was based on the 2-year prospective Investigational Device Exemption (IDE) clinical trial, which demonstrated noninferiority of the total disk replacement (TDR) with the Charite artificial disk compared to the Bagby and Kuslich (BAK) interbody fusion [2], [6].

Guyer et al [4] recently reported the 5-year follow-up results of the IDE clinical trial. The original study consisted of 375 patients at 14 sites. A total of 304 subjects were randomized in a 2:1 ratio, with 205 in the investigational group (Charite) and 99 in the control group (BAK). A total of 71 TDR training cases were performed before randomization beginning at each site, and these data were not included in the study. For the 5-year follow-up study, 6 of the original sites declined participation. In addition, there were 4 deaths, 4 device removals and 117 patients lost to follow-up, of whom 11 declined continued participation in the study and 10 had early discontinuation. This resulted in a follow-up rate of 57% of eligible randomized patients and 44% of the total IDE patient cohort.

Overall, clinical success and patient satisfaction were mostly not statistically different between the 2 groups. One notable difference, long-term disability, was recorded for 8.0% of Charite patients and 20.9% of BAK patients (P = .0441); however, range of motion at index and adjacent levels, segmental translation and disk height was equivalent between both groups.

Seven device failures occurred in each group. Of the 7 BAK failures, one case required a bilateral hemilaminectomy with anterior diskectomy and posterior fusion due to a nonunion. The remaining 6 cases were treated with supplemental segmental instrumentation for posterior fixation due to pseudoarthroses (4 cases), facet joint arthrodesis, and undefined persistent back pain. Of the 7 Charite failures, one case required right-sided hemilaminotomy, foraminotomy and partial diskectomy. The remaining 6 cases were treated with supplemental segmental instrumentation for posterior fixation. Indications for supplemental fixation included symptomatic spondylolisthesis, low back pain secondary to device subsidence, facet degeneration (2 cases) and back pain secondary to early postoperative implant displacement.

Since being introduced on to the market, the Charite artificial disk has generated vigorous debate. In contrast to spinal fusion, artificial disks are placed through a higher risk abdominal approach. Development of scar tissue makes replacement of failed disks even more dangerous. Therefore, to justify these procedures, TDR with the Charite artificial disk must demonstrate superior efficacy in the context of improved spinal mobility, as well as adequate durability. Although the IDE clinical trial showed lower disability rates, spinal mobility was not improved. Therefore, the present study failed to validate the key mechanistic argument in favor of artificial disks.

In addition, there are several concerns regarding the statistical analysis. The largest drop in eligible patient population between the 2- and 5-year studies was due to withdrawals in participation of 6 of the original 14 sites. No additional information is given regarding the 4 device removals; therefore, it is unknown whether these patients experienced complications related to their artificial disks. These patients were not included as “device failures” in the statistical analysis. Additional information for the 21 patients who discontinued participation is also not provided. Statistical analysis by the authors showed no significant differences between the 5-year completers and patients lost to follow-up at 2 years postsurgery; however, when sites withdraw from a study in such numbers, it is wrong to assume they all did well or even had the same complication rate as those who stayed with the study. The statistical analysis was also potentially compromised by the exclusion of 71 TDR training cases. These patients represent 19% of the original cohort, a significant portion of the sample size, and any complications related to these procedures were not reported. When the procedure is widely introduced, many of the surgeons will have limited experience; a long training period or a difficult learning curve will tend to worsen the outcomes.

Comparison of the Charite artificial disk to BAK interbody fusion is less relevant to current practice. The study demonstrates noninferiority of the Charite artificial disk; however, due to high complication rates, BAK interbody fusion is now less popular. Also, the Charite artificial disk is not designed to improve spinal instability. However, use of spinal fusion in patients without spinal instability, for back pain alone, is itself controversial. Therefore, this comparison leaves the indications for TDR unclear.

Finally, like with any moving joint replacement, durability is a key issue with the Charite artificial disk. Revision surgery to remove the implant is potentially risky. Therefore, to be truly attractive, these devices should last a lifetime, and the average candidate for TDR is in their mid-40s. Johnson & Johnson, the manufacturer of Charite, contend the device can withstand 80 years of wear and tear. However, in 2007, van Ooij et al [8] published a clinical case series of 4 patients who underwent anterior lumbar revision due to failure of the artificial disks. Wear analysis of the retrieved prostheses demonstrated the clinical significance of polyethylene wear debris and the potential for osteolysis. The 5-year follow-up period of the present study is not yet sufficient to definitively demonstrate long-term durability. Total disk arthroplasty remains an exciting concept, but much further research is needed to clarify the indications, risks and benefits in a clinically relevant manner.

3. Laminotomy and foraminotomy vs total laminectomy for treatment of lumbar canal stenosis 

Acquired spinal stenosis is the most common indication for spine surgery in the geriatric population. Laminectomy is a popular surgical decompression technique, but the instability caused by extensive removal of the posterior elements may cause long-term back pain and necessitate additional fusion procedures [5]. Many spine surgeons choose a more limited decompression technique as follows: laminotomy and foraminotomy. This procedure decompresses the spinal and nerve root canals but preserves the spinous processes, interspinous ligaments, facet capsules and part of the laminae to preserve spinal integrity.

In the March 2008 issue of Spine, Fu et al [3] state that the laminotomy and foraminotomy is an efficacious, cost-effective alternative for the treatment of lumbar spinal stenosis. They call it a “windows” technique—a new name for an old approach. The study includes 152 patients who underwent either the limited laminotomy and foraminotomy or a traditional decompressive laminectomy. No instrumentation or fusion was performed in either group. After a mean follow-up of 40 months, 89% of patients who underwent the laminoforaminotomy reported pain relief and return to usual activities, compared to 63% in the laminectomy group. In addition, the laminoforaminotomy group underwent no repeat operations, whereas in the laminectomy group, 4 patients underwent repeat instrumented fusions for recurrent stenosis and degenerative instability. These results support the notion that limited laminotomy and foraminotomy can provide adequate decompression of the lumbar spine and may be superior to a total laminectomy in the long run.