Malcolm Nicol

The Dynesys lumbar spinal stabilization system: a preliminary report on positional magnetic resonance imaging findings.

Study Design. We present the positional magnetic resonance imaging findings of a prospective case series of patients undergoing surgery with the Dynesys spinal stabilization device (Zimmer, Inc., Warsaw, IN). Objective. To explore the biomechanical impact of the Dynesys device in vivo. Summary of Background Data. Spinal fusion surgery is widely used for painful degenerative conditions of the lumbar spine that have not responded to conservative measures. It often produces good outcomes but can be associated with adjacent segment hypermobility that may lead to further degeneration and pain. Previous cadaveric biomechanical studies claim that the Dynesys Dynamic Spinal Stabilization System allows some lumbar movement, behaving similar to a normal spine in extension but similar to rigid fixation in flexion. Methods. Twenty-four patients with dominant low back pain, with or without leg pain, were treated with the Dynesys. All patients underwent positional magnetic resonance imaging before surgery and 9 months after surgery. Measurements were made to assess the differences at the operated level, adjacent level, and whole lumbar spine. Results. There was a statistically significant reduction in flexion-extension range of movement of both the whole lumbar spine by 13.37° (P = 0.002) and at the instrumented segments by 4.08° (P < 0.001) following surgery. There was an insignificant reduction in range of movement at the level above instrumentation (P = 0.807). Mean anterior disc height at the instrumented level reduced by 0.7 mm following insertion of the Dynesys (P < 0.027). Mean posterior disc height reduced by 0.3 mm (P = 0.453). In a neutral posture, the Dynesys had no significant impact on lordosis or inclination of operated or adjacent levels. Contrary to cadaveric study findings, the Dynesys appears to restrict extension more than flexion with respect to a neutral posture. Conclusions. In vivo, the Dynesys Stabilization System allows movement at the instrumented level, albeit reduced, with no significant increased mobility at the adjacent segments. There was reduction of the anterior disc height without a significant increase of the posterior disc height.

The Positional Magnetic Resonance Imaging Changes in the Lumbar Spine Following Insertion of a Novel Interspinous Process Distraction Device

Study Design. Patients with symptomatic lumbar spinal stenosis underwent magnetic resonance imaging to study changes in the lumbar spine in various postures before and after implantation of the X STOP Interspinous Process Distraction Device (St. Francis Medical Technologies, Concord, CA). Objective. To visualize the effect of the device in vivo. Summary of Background Data. Previous studies have shown vertebral canal and exit foraminal area to reduce from flexion to extension. Recently, reports on improved kinematics in vitro at the implantation level of the X STOP device have also been published. Methods. Using positional magnetic resonance imaging, patients were scanned before and 6 months after surgery. Images were taken with the patient in sitting flexed, extended, neutral, and standing positions. The total range of motion of the lumbar spine and individual segments were measured, along with changes in disc height, areas of the exit foramens, and dural sac. Results. In 12 patients with 17 distracted levels, the area of the dural sac at these levels increased from 77.8 to 93.4 mm2 after surgery in the standing position (P = 0.006), with increase in the exit foramens, but no change in lumbar posture. Conclusions. This study shows that the X STOP device increases the cross-sectional area of the dural sac and exit foramens without causing changes in posture.

Disc changes in the bridged and adjacent segments after Dynesys dynamic stabilization system after two years.

Study Design. Prospective case series. Objective. To study the radiologic changes in the intervertebral disc after Dynesys dynamic stabilization. Summary of Background Data. Adjacent segment disc degeneration is one of the potential complications of fusion surgery. It has been proposed that nonfusion motion preservation surgery may prevent accelerated adjacent segment degeneration because of the protective effect of persisting segmental motion. Methods. Thirty-two patients who underwent Dynesys procedure between November 2002 and June 2004 and have completed 2-year follow-up MRI scans were included in this study. Preoperative and 2 year postoperative lumbar MRI scans were evaluated by 2 independent observers. T2-weighted mid-sagittal images were used and disc degeneration classified according to the Woodend classification of disc degeneration. Anterior and posterior intervertebral disc heights were also measured. Results. Of the 32 patients, 20 patients underwent Dynesys procedure alone and 12 underwent additional fusion at 1 or more levels. A total of 70 levels were operated on, of which 13 levels were fused. There was a statistically significant increase in the mean Woodend score at the operated levels in the Dynesys alone group, a change from 1.95 before surgery to 2.52 after surgery (P < 0.001). The mean Woodend scores changed from 1.27 preoperative to 1.55 postoperative (P = 0.066) at the proximal adjacent levels, and from 1.37 to 1.62 at the distal levels (P = 0.157). There was good interobserver agreement (weighted &kgr; score of 0.819). The anterior intervertebral disc height reduced by 2 mm from 9.25 to 7.17 (P < 0.001). The posterior disc height increased by 0.14 mm but this change was not significant. Conclusion. Disc degeneration at the bridged and adjacent segment seems to continue despite Dynesys dynamic stabilization. This continuing degeneration could be due to natural disease progression.

Influence of X Stop on Neural Foramina and Spinal Canal Area in Spinal Stenosis

Study Design. Measurements of cross sections of exit foramen and spinal canal were performed before and after placement of X Stop in physiologic postures using positional MR scanner at the stenosed level in patients with lumbar spinal stenosis. Objective. To quantify the effect of the implant in vivo on the lumbar spine at the instrumented levels in various postures. Summary of Background Data. Dimensions of the spinal canal and neural foramen decrease from flexion to extension. Symptoms of spinal stenosis occur typically in standing or extension. The X Stop device is designed to distract the posterior elements of the stenotic segment and place it in flexion to treat neurogenic claudication. We think that the device will improve the dimension of the canal in standing and extension. Methods. Twenty-six patients with lumbar spine stenosis underwent a one- or two-level X Stop procedure. All had preoperative and postoperative positional MRI in standing, supine, and sitting flexion and extension. Measurements were carried out on the images acquired. Results. Significant increase in the dimensions of the neural foramen and canal area were demonstrated after surgery. Conclusions. The X Stop device improves the degree of central and foraminal stenosis in vivo.

Effects of X-STOP device on sagittal lumbar spine kinematics in spinal stenosis.

The X-Stop device is designed to distract the posterior elements of the stenotic segment and place it in flexion to treat neurogenic claudication. Previous biomechanical studies on X Stop have been done in vitro on cadavers looking at disc pressures and segmental range of movements. The objective of this study is to understand the sagittal kinematics in vivo of the lumbar spine at the instrumented and adjacent levels. Twenty-six patients with lumbar spine stenosis underwent 1 or 2 level X-Stop procedure. All had pre- and postoperative positional magnetic resonance imaging (MRI) in standing, supine, and sitting in flexion and extension. Measurements of disc heights, endplate angles, segmental and lumbar range of movement were performed after placement of X Stop at the stenosed level in patients with lumbar spinal stenosis. No significant changes were seen in disc heights, segmental and total lumbar spine movements postoperatively. The X-Stop device does not affect the sagittal kinematics of the lumbar spine in vivo.