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Fatih Ersay Deniz, Leonardo B. C. Brasiliense, Bruno C. R. Lazaro, Phillip M. Reyes, Anna G. U. Sawa, Volker K. H. Sonntag and Neil R. Crawford

greater extent of access will induce a greater amount of segmental instability. We therefore analyzed the biomechanics of the transpedicular discectomy in the thoracic spine and compared the effects on spinal stability after partial and complete removal of the facet joints. Methods Specimen Preparation Seven human cadaveric spine specimens (T6–11) with 3 cm of intact proximal ribs, intercostal ligaments, and costovertebral joints were used in this study ( Table 1 ). Specimens were obtained fresh frozen and thawed in a bath of normal saline at 30°C. Residual

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Richard P. Schlenk, Robert J. Kowalski and Edward C. Benzel

The correction of spinal deformity may be achieved by a variety of methods, each of which has advantages and disadvantages. The goals of spinal deformity surgery include reasonable correction of the curvature, prevention of further deformation, improvement of sagittal and coronal balance, optimization of cosmetic issues, and restoration/preservation of function. The failure to consider all these factors appropriately may result in a suboptimal outcome. Understanding fundamental biomechanical principles involved in the formation, progression, and treatment of spinal deformities is essential in the clinical decision-making process.

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Alejandro J. Lopez, Justin K. Scheer, Kayla E. Leibl, Zachary A. Smith, Brian J. Dlouhy and Nader S. Dahdaleh

T he craniovertebral junction (CVJ)—defined as the occiput, atlas, and axis—is a complex area that houses vital neural and vascular structures while achieving the most mobility of any segment within the spine. 63 It represents the transition between the brain and cervical spine. The majority of the spine’s rotation, flexion, and extension occur between the occiput, the atlas, and axis. 44 , 53 , 64 The biomechanics of motion and stability at the CVJ are unique at each vertebra and segment. An understanding of the complexities of the CVJ anatomy and

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Frank Kandziora, Robert Pflugmacher, Katrin Ludwig, Georg Duda, Thomas Mittlmeier and Norbert P. Haas

A nterior atlantoaxial plate fixation described by Schmelzle, et al., 26 represents a fixation method to be used after transoral odontoid resection, which involves an anterior transoral approach and exposure of the anterior aspect of the atlas and the odontoid process by making a midline incision in the posterior wall of the pharynx. Recently, the authors of biomechanical 10 and clinical 11, 15, 16 studies have shown that this procedure is a good alternative to established posterior atlantoaxial fixation procedures, but they have also indicated some

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Andrew E. Wakefield, Michael P. Steinmetz and Edward C. Benzel

The thoracic spine is a structurally unique region that renders it uniquely suceptible to thoracic disc herniation. Surgical management strategies are complicated, in part, by the regional anatomical and biomechanical nuances. Surgical approaches include posterior, posterolateral, and anterior routes. Each isassociated with specific indications and contraindications. The biomechanical principles and safe anatomical trajectories must be considered in the surgical decision-making process. These issues are discussed in the pages that follow.

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George J. Dohrmann, Manohar M. Panjabi and Dennis Banks

P recise quantitation of the trauma actually delivered to the spinal cord is necessary if useful conclusions are to be drawn from experiments on spinal cord contusion. Description of spinal cord trauma in terms of “gm-cm” units is no longer adequate. Monitoring of the trauma stimulus itself is needed. If a better understanding of the ensuing pathophysiological alterations within the traumatized spinal cord is to be had, it is important to detail the biomechanical parameters involved in the trauma. For a more complete comprehension of the dynamics of spinal

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Joseph F. Cusick, Narayan Yoganandan, Frank A. Pintar and John M. Reinartz

of the biomechanical effects of these iatrogenic changes gives rise to recommendations such as limiting surgical exposure, skeletal fixation, and/or fusion. This spectrum of recommendations demonstrates the lack of unanimity regarding the most efficacious operative procedures. The short- and long-term expectations for these commonly used procedures remain undefined for the surgeon. Previous studies of graded facetectomies in the vertebra-disc-vertebra model during compression-flexion loading monitored with continuous motion analysis showed significant differences

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Robert J. Kowalski, Lisa A. Ferrara and Edward C. Benzel

Bone fusion can be achieved by one or more of three methods: in situ, onlay, and interbody fusion. Interbody implants provide the spine with the ability to bear an axial load. They function optimally when placed along the neutral axis and produce little, if any, significant bending moment. Interbody implants may be comprised of bone, non-bone materials such as acrylic, or a combination of both such as in interbody cages. In this report the authors' goal is to provide some insight into the theoretical, as well as practical, biomechanical factors that influence bone fusion, focusing on interbody implants. They review the concept of stress shielding and its impact on fusion. With the attendant biomechanical nuances of the different regions of the spine, they discuss region-specific strategies involved in successful fusion. Finally, they review intraoperative techniques that will improve the chance of achieving a successful arthrodesis.

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Jonathan S. Hott, James J. Lynch, Robert H. Chamberlain, Volker K. H. Sonntag and Neil R. Crawford

described by Harms and Melcher. 8 This method, which creates what we call the LC1—PC2 construct (C-1 lateral mass screw to C-2 pars screw [ Fig. 1 ]), also allows concurrent placement of an interspinous cable/graft. Fig. 1. Diagram. A linkage connected from C-1 lateral mass screws to pars screws (“LC1—PC2” construct) is an alternative method for immobilizing C1–2. Interspinous cable and graft can be used simultaneously. Used with permission from Barrow Neurological Institute. In recent biomechanical studies investigators tested LC1—PC2 alone and in intact

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A. Giancarlo Vishteh, Neil R. Crawford, M. Stephen Melton, Robert F. Spetzler, Volker K. H. Sonntag and Curtis A. Dickman

. Based mainly on anecdotal experience, some authors have performed fusion across this joint by using bone grafts and internal fixation with a plate; 1 others have recommended internal fixation with a plate alone. 15 Still others have recommended that no graft or fixation device be used for patients undergoing less radical condylectomies. 17 No formal biomechanical studies have been performed of the occipitoatlantal (occiput [Oc]—C1) motion segment after such approaches. Likewise, the effects of unilateral condylectomy on the atlantoaxial (C1–2) motion segment have