Safety and efficacy of a new percutaneously implantable interspinous process device

Conventional decompression surgery has demonstrated good results and can be regarded as gold standard in the treatment of lumbar spinal stenosis [1, 2, 7, 10, 18, 19, 29]. Short- and long-term results have been reported to be excellent, with more than 92.2% of patients still improved after 5.6 years [19]. However, the necessary removal of bone and soft tissue can have destabilizing effects on the operated segments, and the technique has therefore been combined with posterior lumbar fusion [11]. In order to provide additional stability, while avoiding longer operating times and potential risks of instrumentation with pedicle screws, rods, and intervertebral cages, interspinous process implants have been developed. Wallis (Abbot spine, Austin, TX), Coflex (formerly Intraspinous U; Paradigm spine, New York, NY) [24], and Diam (Medtronic, Tolochenanz, Switzerland) were primarily designed to provide additional stability following conventional decompression surgery [13]. Combination of IDP with endoscopic decompression was described as a less invasive alternative [17].

In order to further minimize surgical trauma, IPD devices have been proposed as stand-alone technique, avoiding the need for conventional decompression surgery by indirect decompression through the distraction effect of the implant itself. The distraction of the posterior column of the spine (at level of the spinous processes and interspinous ligaments) leads to widening of foraminal diameter and reduces soft tissue compression of the spinal canal by stretching of the ligamentum flavum [14, 16]. The X-stop (Medtronic, Tolochenanz, Switzerland) is so far the best studied device developed for this task [8, 23]. Recent developments like the Aperius PercLID (Kyphon, Sunnyvale, CA) and the In-space (applied in this study) are designed for percutaneous implantation, further reducing surgical trauma.

In our cohort the In-space device demonstrated short implanting times (average of only 16.4 min per segment in our cohort), virtually no blood loss and low complication rate (only 1.1% misplacements and 2.2% failed implantation because of severely calcified interspinous ligament).

Average implantation times for other IPD devices are not regularly reported in literature. Yano and colleagues [30] report of average operation times of 44.7 min per segment with a ceramic device, Kuchta and colleagues report that operation time for implantation of the X-stop was “much shorter when compared with the open decompression group” but give no exact values. The short implantation time of the In-space can be attributed to the percutaneous implantation.

The complication rates of other implants for IPD are likewise comparatively low. Barbagallo and colleagues [4] report 11.6% complications for the X-stop device (four device dislocations and four spinous process fractures in 69 patients), while Kuchta and colleagues [14] observed no intraoperative complications, using the same implant in 175 patients. Korovessis and colleagues [13] observed one involuntary dura leakage in 25 patients treated with the Wallis implant in combination with conventional decompression surgery.

Unfortunately, we lack standardized outcome assessment (in the form of well-established questionnaires, etc.). However, like Verhoof and colleagues [27], we regarded pain reduction, neurological function an improvement of quality of life as the primary goals of IPD techniques, and defined reoperation as the end point of failure. The outcome in our cohort is comparable with those presented in the literature for other interspinous process implants such as the X-stop. Brussee and colleagues [5] report in their study of 65 patients who received an X-stop implant for lumbar spinal stenosis and spinal claudication without spondylolisthesis of 9.2% of reoperations within a follow-up period of 1.0 ± 0.75 years. Only 31.1% of the patients reported to be satisfied with their outcome. Likewise, Verhoof and colleagues [27] found good initial reduction of symptoms in 8 of 12 patients (66.7%) treated for lumbar spinal stenosis in the presence of spondylolisthesis. Within 24 months (mean follow-up, 30.3 months), 3 (25%) of these had recurrent symptoms and finally 7 patients (58%) required reoperation (microsurgical decompression and posterolateral fusion). Our results should be expected to be somewhere in between these, because we included patients with and without presence of spondylolisthesis in the group. In our study, however, mild spondylolisthesis was not associated with a worse outcome compared with the entire collective.

Apart from this finding the similarity of the general outcome in our study compared with those cited above, using different implants suggests that the failure rate is an expression of the limits of the principle of IPD itself, rather than of individual design of the implants. The good initial response might be related to a surgical placebo effect, which was also recently observed in two prospective studies comparing vertebroplasty with sham operation [6, 12].

However, other authors report of far better results using the X-stop implant. Kuchta and colleagues [14] report of only 8 reoperations in 175 patients (4.6%). The authors of this study suggest that this might partly be attributed to careful patient selection and application of functional (upright) magnetic resonance imaging for diagnosis. Likewise, other authors argue that implantation of IPD devices should primarily be considered in cases that present with typical spinal claudication with good reduction of pain under flexion of the lumbar spine only [15]. As most of these data are low-level evidence (like our own study), the last word concerning IPD cannot yet be spoken. However, presently there are 10 clinical trials with IPD devices going on [26], and it can be hoped that the remaining questions can be solved when these are completed.

Paralleling these findings, in our study of 42.2% patients with lasting relief from claudication and radicular symptoms, 13.3% suffered of lasting or new-onset lumbago. Whether the four cases of new-onset lumbago are a complication of In-space implantation or whether this belongs to the natural history of these patients cannot be identified by this retrospective analysis. These four patients, however, were the only ones who reported of this possible complication at last follow-up; none of the patients who did not experience lasting relief of claudication complained about new-onset or worsened low back pain at last follow-up. In the low back pain group in contrast, 26.7% of patients reported of lasting relief also for this symptom after In-space implantation. Thus, concerning the treatment of low back pain, no evident suggestion can be derived from this study.

The authors therefore suggest that in the presence of only this low-level evidence and the high recurrence rate, the device should primarily be used in clinical trials in order to get more information concerning the optimal indication. Patients who are anyway scheduled for implantation of an In-space should be carefully informed about the above-discussed limitation before surgery.

However, a large controlled, prospective trial comparing the In-space device with the X-stop is under way and will supply a more detailed view of short- and long-term results of the In-space device [26].