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Fred H. Geisler, Scott L. Blumenthal, Richard D. Guyer, Paul C. McAfee, John J. Regan, J. Patrick Johnson and Bradford Mullin

of adjacent-level disc disease would follow. Rationale of Lumbar Artificial Disc Design Numerous artificial discs have been designed during the past 35 years, but most have never being produced. 33 There are four types of dynamic stabilization systems derived from artificial disc technology. First, nucleus pulposus replacements with a hygroscopic gel or fluid-filled cylindrical sacs are for use after standard discectomy in which the anulus maintains normal disc space height. 24, 53, 54 Second, posterior dynamic stabilization systems increase posterior

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Sylvain Palmer, Andrew Mahar and Richard Oka

Object

Biomechanical testing and fluoroscopic imaging were used to study an extension-limiting device that has been developed to support and cushion the facet complex. It is a titanium screw–based system with a polycarbonate-urethane bumper that lies against the inferior articular process and is anchored into the pedicle by the screw for posterior dynamic stabilization (PDS).

Methods

Six human cadaveric spines were dissected from L-2 to L-5, leaving all ligamentous structures intact. The intact spines were first tested in flexion and extension, lateral bending, and axial rotation at ±7.5 Nm. The PDS devices were inserted at L3–4 and testing was repeated. Fluoroscopic analysis of posterior disc height and foraminal area of the intact and instrumented spines while loaded was performed. All test data were compared using a one-way analysis of variance (statistical significance was set at p < 0.05).

Instrumented spines had 62% less motion during flexion and 49% less motion during extension compared with the intact spines. Neuroimaging analysis showed 84% less compression of the posterior disc of the instrumented spines during extension, and no difference during flexion compared with intact spines. After instrumentation was affixed, the foraminal area was 36% larger than in intact spines during extension and 9% larger during flexion. During axial loading, compression of the posterior disc was decreased by 70%, and analysis showed 10% decompression prior to loading just from implanting the devices.

Conclusions

The PDS system has the benefit of being a completely percutaneous one, which can be used at all levels of the lumbar spine, including S-1. The PDS system limits spinal motion, enlarges the foramina, and achieves discal decompression.

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Paul Khoueir, K. Anthony Kim and Michael Y. Wang

✓Numerous new posterior dynamic stabilization (PDS) devices have been developed for the treatment of disorders of the lumbar spine. In this report the authors provide a classification scheme for these devices and describe several clinical situations in which the instrumentation may be expected to play a role. By using this classification, the PDSs that are now available and those developed in the future can be uniformly categorized.

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Jason M. Highsmith, Luis M. Tumialán and Gerald E. Rodts Jr.

✓The widespread use of instrumentation in the lumbar spine has led to high rates of fusion. This has been accompanied by a marked rise in adjacent-segment disease, which is considered to be an increasingly common and significant consequence of lumbar or lumbosacral fusion. Numerous biomechanical studies have demonstrated that segments fused with rigid metallic fixation lead to significant amounts of supraphysiological stress on adjacent discs and facets. The resultant disc degeneration and/or stenosis may require further surgical intervention and extension of the fusion to address symptomatic adjacent-segment disease.

Recently, dynamic stabilization implants and disc arthroplasty have been introduced as an alternative to rigid fixation. The scope of spinal disease that can be treated with this novel technology, however, remains limited, and these treatments may not apply to patients who still require rigid stabilization and arthrodesis.

In the spectrum between rigid metallic fixation and motion-preserving arthroplasty is a semirigid type of stabilization in which a construct is used that more closely mirrors the modulus of elasticity of natural bone. After either inter-body or posterolateral arthrodesis is achieved, the fused segments will not generate the same adjacent-level forces believed to be the cause of adjacent-segment disease. Although this form of arthrodesis does not completely prevent adjacent-segment disease, the dynamic component of this stabilization technique may minimize its occurrence.

The authors report their initial experience with the use of posterior dynamic stabilization in which polyetheretherketone rods were used for a posterior construct. The biomechanics of dynamic stabilization are discussed, clinical indications are reviewed, and case studies for its application are presented.

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.2007.22.1.1 Use of computed tomography–single-photon emission computed tomography fusion for diagnosing painful facet arthropathy Matthew McDonald Robert Cooper Michael Y. Wang 1 2007 22 1 1 4 10.3171/foc.2007.22.1.2 022000e2 Classification of posterior dynamic stabilization devices Paul Khoueir K. Anthony Kim Michael Y. Wang 1 2007 22 1 1 8 10.3171/foc.2007.22.1.3 022000e3 Biomechanical and radiographic analysis of a novel, minimally invasive, extension-limiting device for the lumbar spine Sylvain Palmer Andrew Mahar Richard Oka 1 2007 22 1 1 6 10

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Young-Soo Kim, Ho-Yeol Zhang, Byung-Jin Moon, Kyung-Woo Park, Kyu-Yeul Ji, Won-Chang Lee, Kyu-Sung Oh, Gwon-Ui Ryu and Daniel H. Kim

Object

The purpose of this study was to analyze the usefulness of the BioFlex, a Nitinol spring rod dynamic stabilization system, and the Nitinol shape memory loop (KIMPF-DI Fixing System) as a posterior dynamic stabilization system in surgery for low-back pain.

Methods

The 103 patients who underwent treatment with the BioFlex system were divided into two groups: Group 1, dynamic stabilization with or without posterior lumbar interbody fusion (PLIF); and Group 2, rigid fixation (PLIF + BioFlex system only). A total of 66 segments were treated with only the BioFlex system; in these the preoperative range of motion (ROM) was 10.0 ± 4.3°, which changed to 4.1 ± 1.9° after surgery. Adjacent-segment ROM changed from 8.4 ± 3.4° to 10.7 ± 3.2° in Group 1 and from 6.5 ± 3.2° to 10.5 ± 4.6° in Group 2 postoperatively. A total of 110 segments received both BioFlex and PLIF, with a fusion rate of 90.0%. The visual analog scale score for back pain improved from 7.3 ± 3.1 to 1.4 ± 1.8 in Group 1 and from 7.4 ± 2.4 to 2.1 ± 2.3 in Group 2. The Oswestry Disability Index improved from 35.2 ± 6.4 to 12.1 ± 4.5 in Group 1 and from 37.8 ± 5.7 to 13.6 ± 4.2 in Group 2. (The ROM and assessment scores expressed are the mean ± standard deviation.)

The 194 patients in whom Nitinol memory loops were implanted were analyzed based on the preoperative and 1-year postoperative ROM of each lumbar segment. The change of ROM in looped segments treated with PLIF was significantly reduced, but the change of ROM in looped segments without PLIF was not significant. The change of ROM at the segment adjacent to the loop was not significant, and the change of kyphosis reflected a slight recovery.

Conclusions

The Nitinol BioFlex dynamic stabilization system can achieve stabilization and simultaneously allow physiological movement, which can in turn decrease the degeneration of adjacent segments. When used with PLIF, the fusion rate can be expected to increase. The flexible Nitinol shape memory loop, a posterior dynamic stabilization device, is an adequate tension band that displays strength similar to the posterior ligamentous structures. In combination with PLIF at the main lesion, the BioFlex system or the Nitinol memory loop can provide posterior dynamic stabilization to the transitional upper or lower segments, enhance the fusion rate, reduce the adjacent-segment degeneration, and provide dynamic stabilization of the spine.

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Christopher E. Mandigo, Prakash Sampath and Michael G. Kaiser

✓Posterior dynamic stabilization in the lumbar spine is performed in an attempt to reduce loading across the intervertebral disc for the purpose of relieving pain and limiting degeneration while preserving motion. The AccuFlex rod system (Globus Medical, Inc.), a first-generation device, achieves this by changing the properties of the rod within the Protex pedicle screw–based rigid rod system. Helical cuts that have been created in the standard 6.5-mm rod allow for a limited range of motion while providing a posterior tension band that relieves a significant amount of disc loading. The AccuFlex rod system has been approved by the Food and Drug Administration for single-level fusion when used in conjunction with an interbody graft. In a study involving 170 patients who underwent fusion surgery for back pain, the 54 who received the AccuFlex construct had statistically similar fusion rates and outcomes (as assessed by visual analog scale and Short Form-16 scores) when compared with 116 patients treated with rigid rod fixation after 1 year of follow up. Future clinical studies will examine and provide information regarding the impact of AccuFlex on the incidence of adjacent-level disease. Information gained through the clinical experience with AccuFlex will serve as a foundation for the development of a stand-alone dynamic construct.

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Paul McAfee, Larry T. Khoo, Luiz Pimenta, Andy Capuccino, Domagoj Coric, Robert Hes, Bart Conix, Farbod Asgarzadie, Azmi Hamzaoglu, Yigal Mirofsky and Yoram Anekstein

patients requiring dorsal spinal procedures such as decompression was the posterior dynamic stabilization system known as Dynesys (Zimmer). The Dynesys was designed to preserve intersegmental kinematics and to alleviate loading at the facet joints. 26 , 28 The system uses standard closed-head conical pedicle screws, which are attached by a polyester cord that is enveloped in a polycarbonate cylindrical sheath. Results of biomechanical and cadaveric studies support the suggestion that implantation of the Dynesys system seems to restrict compression, extension, lateral

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Matthew McDonald, Robert Cooper and Michael Y. Wang

✓Facet disease is believed to play a major role in axial low-back pain and may prove in the future to be an important indication for posterior dynamic stabilization. However, the lack of good diagnostic tests and imaging methods for identifying this condition have made this entity obscure. Although single-photon emission computed tomography (SPECT) imaging is a highly sensitive and specific test, the images frequently lack adequate resolution, whereas computed tomography (CT) provides excellent resolution but lacks specificity.

Thirty-seven patients with back pain clinically attributable to facet disease underwent CT–SPECT fusion imaging of the lumbar spine. The SPECT images were obtained using a dual-head gamma camera equipped with VXGP high-resolution collimators using a 20% energy window centered at 140 keV and a 360° rotation totaling 128 projections at 16 seconds each. Transaxial CT images were transferred in the Digital Imaging and Communications in Medicine format to provide proper image overlay in the axial, sagittal, and coronal planes. Scanning for both modalities was performed using standard patient positioning. Patients with concordant images and symptoms then underwent joint injection and/or rhizotomy, which was performed by an independent physician.

Image fusion was successfully performed in all patients, and the image quality allowed definitive localization of the “hot” lesion in all cases, in contrast to conventional high-resolution SPECT scanning, which often led to problems differentiating L4/5 and L5/S1. In patients with solitary lesions, injection led to definitive pain resolution, even if temporary, in all cases with anesthetic blockade.

The CT–SPECT scanning modality combines the virtues of functional and anatomical imaging, aiding the clinician in making the diagnosis of painful facet arthropathy. This modality may prove useful for the selection of patients who are candidates for posterior dynamic stabilization.

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Jo Ann M. Eliason

Monthly Topic Editors, 2007 January issue: Posterior Dynamic Stabilization Topic Editors: Michael Y. Wang, M.D., and Michael W. Groff, M.D. February issue: Arachnoid Cysts Topic Editor: Mark D. Krieger, M.D. March issue: Brain Metastases Topic Editors: Jason P. Sheehan, M.D., and Jonas M. Sheehan, M.D. April issue: Hydrocephalus Topic Editor: Harold L. Rekate, M.D. May issue: Brain Edema Topic Editors: Jullian T. Hoff, M.D., and Richard Keep, Ph.D. June issue: Current Management of Nerve Tumors Topic Editors: Robert Tiel, M