Design Rationale

Early criticisms of IPD devices stem from the apparent kyphosing nature of the implant. It is indeed counterintuitive to apply posterior distraction to the lumbar spine. Yet one need only look at the clinical presentation of a patient with lumbar spinal stenosis to appreciate the design rationale. A patient with lumbar stenosis typically walks with a forward stooped gate. Additionally, patients with spinal stenosis obtain symptom relief upon sitting down. The common feature in both of these postures is the relative flexion of the lumbar spine or avoidance of extension. Human beings are unable to segmentally kyphos their lumbar spine. Muscle insertions allow global motions of flexion and extension. However, spinal stenosis is often a focal phenomena presenting with its worst clinical level at one or two lumbar segments. The rationale of the device, therefore, is to implant the IPD device at the one or two levels where the stenosis is most severe. The implant then segmentally kyphoses the lumbar spine at the level of most severe stenosis and allows the rest of the lumbar spine to fall in its natural posture of extension, having relieved the local stenosis.

An additional concept in the treatment of patients with lumbar spinal stenosis is how much nerve compression is clinically significant? Although several studies have tried to elucidate this measurement, it does appear to be an elusive number. Any clinician who has been involved in the care of the stenosis patient can appreciate the fact that for every octogenarian who presents with new-onset stenotic symptoms and who has a spinal canal that is extraordinarily narrow, there is another patient who presents with similar symptoms in their 50s with magnetic resonance imaging (MRI) findings that are not nearly as impressive. With this variation in mind, it can be appreciated that spinal stenosis is a threshold disease. That is to say, the degree of tightness that elicits symptoms in any particular patient may in part be somewhat unique for that patient. By extension, therefore, a device that can create additional room for the neural elements may only need to create enough room so as to get that patient to the other side of their threshold for symptoms. As surgeons, we may be a bit uncomfortable with this rationale, preferring to directly decompress the neural elements in their entirety and confirm this by direct visualization. This may, however, represent overtreatment.

If we accept the concept of focal spinal stenosis causing symptoms at a particular threshold, then it must be shown the interspinous process decompressive devices can enact an effect on the canal diameter with acceptable, otherwise minimal, alterations to spine biomechanics. The majority of the current literature on this topic relates to the X-Stop interspinous process device. Whether these data can be extrapolated to other products in this category is up to the reader’s discretion, yet these studies are presented as a design rationale for IPD devices.

The first reasonable question to pose in evaluating the design rationale of an IPD device is its net effect on implantation of the dimensions of the spinal canal. Richards and colleagues³ attempted to address this question in studying eight cadaver specimens from L2-L5 that underwent an MRI before and after implantation of an X-Stop device at the L3-4 level. Canal and foraminal dimensions were measured. The specimens were positioned, and parameters were measured in both 15 degrees of flexion and 15 degrees of extension. In extension, the canal area was increased by 18% when compared with the noninstrumented spine. Similarly the subarticular diameter was increased by 50%, the canal diameter by 10%, the foraminal area by 25%, and the foraminal width by 41%.³ In a subsequent in vivo study, Siddiqui and colleagues^4,5 presented results on 12 patients with 17 instrumented levels in which positional MRIs were obtained before and after surgery in the sitting flexed, extended, neutral, and standing positions. The area for the dural sac increased from 77.8 to 93.4 mm at 6 months after surgery in the standing position. There was a similar increase in the foramina. Importantly, no change in overall lumbar lordosis was noted.^4,5

These studies demonstrate the passive decompression obtained in placing an IPD device. The question remains as to whether this degree of passive decompression is enough to be clinically relevant.

The other area of study as it relates to implantation of the IPD device is the net effect on the kinematics and load sharing within the lumbar spine at both the instrumented level and the adjacent levels.⁶ Swanson and colleagues presented data on eight human cadaveric lumbar spines in which they tested intradiscal pressure, before and after implantation. The spines were positioned in flexion, neutral, and extension with intradiscal pressure transducers placed in the anterior and posterior aspect of the nucleus pulposus. The implants were placed at L3-L4, and the measurements of intradiscal pressure were taken at L2-L3, L3-L4, and L4-L5. The device proved to be load sharing in both extension and neutral positions. At L3-L4, which was the instrumented level, the authors measured a 63% decrease in pressure at the posterior annulus and a 41% decrease in pressure in the nucleus pulposus. In neutral position the decrease in pressure was 38% in the posterior annulus and 20% in the nucleus pulposus. The adjacent levels did not show any significant change in intradiscal pressure.⁷

Wiseman and colleagues presented a similar study as it relates to facet loading. Pressure film was placed in the facets at the instrument level, which was L3-L4, as well as the facets at L2-L3 and L4-L5. The film could then be measured for contact area, mean force, mean pressure, and peak pressure. At the implanted level the contact area decreased by 47%, mean force decreased by 68%, mean pressure by 39%, and peak pressure by 55%. No changes of facet pressure were seen at adjacent levels.⁸ These mechanical studies provide the basis for the assumption that IPD devices may be helpful in the clinical treatment of patients suffering from facet arthropathy or discogenic or degenerative disc disease-induced back pain. Yet these are only biomechanical studies. The efficacy of IPD devices has not been shown clinically in these conditions.

The effect of the X-Stop IPD device on spinal kinematics was further measured by Lindsey and colleages.⁹ Seven cadaveric specimens from L2-L5 were loaded in flexion, extension, axial rotation, and lateral bending to 7.5 Nm. There was a superimposed axial load of 700 Newtons. Rigid markers were placed in each vertebral body, as well as the supporting frame to measure the relative motion. Measurements were taken both before and after implantation of an L3-L4 IPD device. There was no change in range of motion as measured in axial rotation or lateral bending. The intervertebral angle was changed by 1.9 degrees. An average of 7.6 degrees of extension at L3-L4 was reduced to 3.1 degrees after implantation of the device. Notably, the adjacent levels were not affected in flexion or extension with a device in place.⁹

Two studies are presented to assess the question of kyphosis of the lumbar spine. Siddiqui and colleagues, as referenced earlier, studied 12 patients with 17 implanted levels. Comparing his postoperative with preoperative MRIs, the mean intervertebral angle changed .83 degrees in extension. The over mean lumbar lordosis changed .08 degrees in extension. Therefore the change in overall lumbar lordosis was not statistically significant.^4–6 The mean intervertebral angle and mean lumbar lordosis were also measured in the pivotal study trial for FDA submission. This included 41 patients with data available preoperatively and postoperatively. In those patients undergoing a single-level implant, of which there were 23, the mean lumbar lordosis changed .1 degrees in extension. In 18 patients who underwent double-level implants the mean lumbar lordosis changed 1.2 degrees in extension.¹⁰