Dynamic Spinal Stabilization Research Articles

Gradations of Posterolateral Fusion in the Lumbar Spine: The Rationale for Dynamic Spinal Stabilization

Gradations of Posterolateral Fusion in the Lumbar Spine: The Rationale for Dynamic Spinal Stabilization

Preclinical evaluation of the Dynesys posterior spinal stabilization system: a nonhuman primate model

Background context Posterior dynamic spinal stabilization systems are intended to restore near-normal biomechanical function of the spine without inducing unnatural stresses to the spinal elements or eliciting a histopathological response. These devices must resist loosening within the challenging biomechanical environment of the lumbar spine. Purpose To determine the biomechanical effects of the Dynesys dynamic stabilization system (Zimmer, Inc., Warsaw, IN, USA) in the acute postoperative period and after 6 and 12 months in vivo; to examine the facet joints at the same postoperative intervals for signs of degeneration; and to measure the incidence of screw loosening after in vivo loading. Study design/setting This was an in vitro and in vivo animal survival study. Methods Fourteen baboons were used. Eight animals underwent survival surgery to implant a posterior dynamic stabilization system spanning two lumbar levels. Six animals were sacrificed acutely, and their spines were biomechanically tested in the intact condition and with instrumentation implanted as described above. Six animals in the survival group were sacrificed at 6 months postoperatively and two animals at 12 months postoperatively. Their spines were biomechanically tested with instrumentation in situ and explanted. The facets were then processed using undecalcified technique. Microradiographs of the facets were examined for signs of arthrosis, inflammation, and degenerative changes. Results The range of flexion-extension motion for the acute group of instrumented spines was 27% of the intact condition. After 6 months with instrumentation in situ, flexion-extension was 56% of the intact condition. After 12 months with instrumentation in situ, flexion-extension was 70% of the intact condition. With instrumentation explanted, flexion-extension at 6 and 12 months was not different from the intact condition (p>.05). Similar results were observed for lateral bending. There were no significant differences in axial rotation between any groups at any time point (p>.05). The facet joints at the operative and adjacent levels exhibited normal articular cartilage at both the 6- and 12-month postoperative time points. There was no evidence of facet arthrosis in any animal. At 6 months postoperatively, 0 of 36 screws exhibited radiolucency at the bone-metal interface. At 12 months postoperatively, 3 of 12 screws exhibited radiolucency. Conclusions After 12 months in vivo, spinal motions were stabilized by the dynamic instrumentation system. No facet arthrosis was observed at 6 and 12 months postoperatively. Explantation of the instrumentation restored motion to intact levels. A 25% rate of screw loosening (3 of 12 screws) was observed at the 12-month postoperative time point.

Development and Use of Dynamic Spine Stabilization Devices

To understand the evolution of dynamic spine stabilization devices, it is helpful to look into the history of spine treatment concepts. Dynamic spine stabilization devices evolved from prior spine stabilization implants involving surgical fusion.

Influence of Trunk Curl-Up Speed on Muscular Recruitment

Although exercise speed is an acute variable to prescribe abdominal strengthening programs, current literature lacks studies analyzing the influence of speed on muscular activation in abdominal exercises. The aim of this work was to determine the influence of trunk curl-up speed on the amplitude of muscular activation and the way in which the trunk muscles were coactivated. Twenty recreationally trained volunteers (16 women and 4 men; age, 23.7 +/- 4.3 years; height, 166.2 +/- 6.3 cm; mass, 61.0 +/- 8.2 kg) participated in this study. Surface electromyographic data were collected from the rectus abdominis, external oblique, internal oblique, and erector spinae during 4 different curl-up cadences [1 repetition per 4 seconds (C4), 1 repetition per 2 seconds (C2), 1 repetition per 1.5 seconds (C1.5), 1 repetition per 1 second (C1)], and during maximum speed curl-ups (Cmax). The electromyographic amplitude was averaged and normalized using maximum voluntary isometric contractions (MVICs). Statistical analyses were performed using repeated-analyses of variance. Normalized electromyographic mean amplitudes of trunk muscles increased with curl-up speed. Although the rectus abdominis (ranged from 23.3% of MVICs at C4 to 49.6% of MVICs at Cmax) and internal oblique (ranged from 19.2% of MVICs at C4 to 48.5% of MVICs at Cmax) were the most active analyzed muscles at each speed, contribution of the external oblique increased appreciably with velocity (ranged from 5.3% of MVICs at C4 to 33.3% of MVICs at Cmax). Increasing trunk curl-up speed supposed greater trunk muscular coactivation, probably required for a faster performance and to ensure dynamic spine stability. On the basis of our findings, curl-up speed had an important effect on trunk muscular recruitment and must be taken into account when prescribing exercise programs for abdominal conditioning.

The Dynesys Lumbar Spinal Stabilization System

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). To explore the biomechanical impact of the Dynesys device in vivo. 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. 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. There was a statistically significant reduction in flexion-extension range of movement of both the whole lumbar spine by 13.37 degrees (P = 0.002) and at the instrumented segments by 4.08 degrees (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. 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.

P32. Dynamic Spinal Stabilization: An Alternative to Spinal Fusion? A Report on Positional MRI Findings at 2 Years

BACKGROUND CONTEXT: The Dynamic Neutralization System (Dynesys) is a semirigid spinal stabilization device that uses transpedicular screws linked by cords and spacers.

Spinal stabilization exercises for the injured worker.

Returning the injured worker to his previous occupation can be a difficult and complex rehabilitation process. The goal of standard stabilization exercises (SSE) is to facilitate the active system for improved dynamic spinal stability. Improving dynamic spinal stability can decrease the forces placed on the intervertebral joints, which minimizes the chance of reinjury. When used in conjunction with industrial rehabilitation, SSEs offer the optimal environment for an injured worker to practice his occupational demands while using appropriate spinal stabilization.

Treatment of chronic low back pain incorporating active patient participation and chiropractic: a retrospective case report

Objective To present a retrospective case report about a patient who suffered from chronic lower back pain for sixteen years, finding little relief from numerous medical and traditional chiropractic interventions, until active patient participation was incorporated in the chiropractic treatment process. Clinical Features A 43-year-old female experienced severe right lumbar, right sacrum, and right acetabular pain and muscle spasms occurring after playing a vigorous tennis match 16 years earlier. Pain intensity was rated as 8 out of 10 on a variable analog scale. Prior treatments included prolonged bed rest, cortisone injection, chiropractic manipulation, stretching, acupuncture, physical therapy, and other rehabilitation interventions. By the time of presentation she also experienced right arm and right upper back pain. A lumbar MRI scan showed an L4/5 disc bulge. Patrick's, Yeoman's and Kemp's tests were positive on her right side. She had an asymmetrical gait pattern with a right hip hike, lateral shift and rotation of the pelvis. Weakness of the left gluteus maxi-mus, gluteus medius, and right erector spinae muscles was present. Motion palpation revealed several fixations. There was tenderness to palpation of the right psoas muscle and a trigger point in the right illiacus muscle. Intervention and Outcome Home-based rehabilitation including low-tech muscle-specific patient controlled dynamic spinal stabilization exercises, cardiovascular training and various stretching techniques was incorporated with chiropractic manipulation. Significant improvement was noted within a 40 week treatment course. Conclusion Incorporation of active patient participation seemed to be a significant factor in the resolution of the patient's low back pain. Active patient participation improved the quality of life for this patient.

Trunk muscle responses to demands of an exercise progression to improve dynamic spinal stability

Clarke Davidson KL, Hubley-Kozey CL. Trunk muscle responses to demands of an exercise progression to improve dynamic spinal stability.. Objectives To compare relative activation amplitudes among abdominal and trunk extensor muscle sites of healthy people performing a leg-loading exercise protocol aimed at progressively challenging spinal stability. Design A prospective, comparative, repeated-measures design. Setting Motion analysis research laboratory. Participants Eighteen healthy male and female volunteers (mean age ± standard deviation, 23.8±3.4y). Interventions Subjects performed 5 progression levels in random order, while surface electromyograms were recorded from 5 abdominal and 2 back extensor muscle sites. Levels 2 through 5 were of interest because they included a leg-extension phase. The root mean square (RMS) amplitude during the leg-extension phase was calculated and normalized to maximal voluntary isometric contractions (MVICs) for each muscle. A 2-factor repeated-measures analysis of variance tested the muscle-by-level interactions and the 2 main effects for the abdominal and trunk extensor sites separately. Bonferroni adjustments were performed on significant results. Main outcome measure Normalized RMS (NRMS) amplitude for each muscle during the leg-extension phase. Results There were statistically significant muscle-by-level interactions ( P<.05) for both abdominal and trunk extensor sites. The relative amplitudes increased for the abdominal muscles and erector spinae muscles among levels, except between levels 3 and 4. Significant differences were found among the abdominal muscle sites for the lower-level exercises (levels 2–4), but not for level 5. The highest NRMS amplitude was at level 5, that is, 40% of MVIC. Conclusions The patterns of activation amplitudes differed among levels; therefore, this was not a simple progressive loading protocol because muscles responded in a nonuniform manner to the demands associated with the various levels of progression. Based on the results, the protocol met some of the objectives of dynamic stability protocols.

Basic scientific considerations in total disc arthroplasty

Background context Total disc arthroplasty serves as the next frontier in the surgical management of intervertebral discogenic pathology. Purpose As we move from an era of interbody spinal arthrodesis to one in which segmental motion is preserved, this promising new technology offers increasing clinical and research challenges in the areas of spinal kinematics, histologic osseointegration at the prosthetic–bone interface and the effects of particulate wear debris. Study design The primary focus of this paper is to provide a methodologic basis to investigate the spinal kinematics, histologic osseointegration and particulate wear debris after total disc arthroplasty by using in vitro and in vivo models. Methods Part I: Using an in vitro cadaveric model, multidirectional flexibility testing evaluated the functional unit kinematics under the following L4–L5 reconstruction conditions: 1) intact spine, 2) Charité disc prosthesis, 3) BAK cages, 4) BAK cages+ISOLA pedicle screw or rod fixation (anteroposterior). Part II: A total of 27 mature baboons (n=27, Papio cynocephalus) underwent L5–L6 total disk replacement procedures to investigate the biomechanical, histochemical and biologic ingrowth characteristics of two different lumbar disc prostheses (AcroFlex and Charité) for total disc arthroplasty. Functional spinal unit fusion status was assessed by using radiographic analysis, biomechanical testing, undecalcified histopathologic and histomorphometric analyses. Part III: Using a total of 50 New Zealand white rabbits, this investigation served to quantify the neural and systemic tissue histopathologic response, after epidural application of four different types of spinal instrumentation particulate wear debris: 1) sham (control) (n=10), 2) stainless steel 316LVM (n=10), 3) titanium alloy Ti-6AL-4V (n=10), 4) cobalt chrome alloy (n=10) and 5) ultrahigh molecular weight polyethylene (UHMWPE) (n=10). Results In vitro multidirectional flexibility testing demonstrates the operative and adjacent level motion-preserving properties of total disc arthroplasty versus interbody arthrodesis cages and pedicle screw spinal instrumentation. To this end, disc replacement preserves the normal centrode or locus of intervertebral rotation at the operative and adjacent intervertebral spinal levels compared with conventional stabilization implants. On the basis of nonhuman primate modeling in the current studies, porous titanium interface surfaces afforded the greatest percentage of trabecular ingrowth at the prosthesis–end plate interface. In vivo segmental motion under multidirectional testing was preserved with the Charité device and slightly diminished with the AcroFlex implants. The porous ingrowth coverage at the bone–metal interface was more favorable for total disk replacement (range, 40% to 50%) compared with that reported for cementless total joint components in the appendicular skeleton (range, 10% to 30%). Direct epidural application of spinal instrumentation particulate wear debris elicits a chronic histiocytic reaction localized primarily within the epidural fibrous layers. Moreover, particles have the capacity to diffuse intrathecally, eliciting a macrophage and cytokine response within the epidural tissues, cerebrospinal fluid and spinal cord itself. Overall, on the basis of the postoperative time periods evaluated, no evidence was observed of an acute neural or systemic histopathologic response to the materials included in the current project. Conclusion The implementation of dynamic spinal stabilization systems for fusionless correction of spinal deformity, dynamic posterior stabilization and total disc arthroplasty necessitates improved understanding with regard to spinal kinematics, patterns and mechanisms of histologic osseointegration and the neurohistopathologic response to particulate wear debris. Collectively, the current studies provide a methodologic basis to comprehensively evaluate these three areas.

Electromyographic activity of selected trunk muscles during dynamic spine stabilization exercises

Souza GM, Baker LL, Powers CM. Electromyographic activity of selected trunk muscles during spine stabilization exercises. Arch Phys Med Rehabil 2001;82:1551-7. Objectives: To compare the electromyographic activity of the trunk flexors and extensors during 2 spine stabilization exercises and to evaluate the changes in muscle activity with increasing levels of exercise difficulty. Design: Descriptive study. Setting: Research laboratory. Participants: Twelve healthy subjects without history of lower back pain. Intervention: Subjects were instructed how to perform the Dying Bug and the Quadruped exercises. Electromyographic and motion data were recorded from each muscle group during maximum voluntary isometric contraction (MVIC). Main Outcome Measures: Surface electromyographic recordings of the erector spinae, rectus abdominus, abdominal oblique, and gluteus maximus muscles. Results: During the Dying Bug exercise, the trunk flexors (rectus abdominus, abdominal oblique) were equally active and demonstrated proportional increases in electromyographic activity with increasing level of exercise difficulty. In the Quadruped exercise, significantly greater electromyographic activity was observed in the abdominal oblique compared with the rectus abdominus; however, abdominal oblique activity did not change with increasing level of difficulty. The erector spinae and gluteus maximus demonstrated a significant level effect and were most active during elevation of the ipsilateral leg. At no point did activity of any of the muscles studied exceed 41% of the MVIC. Conclusions: Results indicated the Dying Bug exercise predominantly recruited the abdominal musculature, while greater activity was observed in the trunk and hip extensors during the Quadruped exercise. The relatively low levels of electromyographic activity observed in both exercises suggests that the intensity of muscle recruitment is not likely sufficient to provide a strengthening effect in healthy subjects. © 2001 by the American Congress of Rehabilitation Medicine and the American Academy of Physical Medicine and Rehabilitation