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  • 1
    Electronic Resource
    Electronic Resource
    Simple equations in terms of displacements for finite axisymmetric deflections of shells of revolution
    Amsterdam : Elsevier
    International Journal of Non-Linear Mechanics 22 (1987), S. 1-13 
    Details
    ISSN: 0020-7462
    Source: Elsevier Journal Backfiles on ScienceDirect 1907 - 2002
    Topics: Mechanical Engineering, Materials Science, Production Engineering, Mining and Metallurgy, Traffic Engineering, Precision Mechanics , Physics
    Type of Medium: Electronic Resource
    URL: http://linkinghub.elsevier.com/retrieve/pii/0020-7462(87)90044-8
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  • 2
    Electronic Resource
    Electronic Resource
    Development of an opto-electronic positioning device for serial direct digital images of oral structures (2000)
    Copenhagen : Munksgaard International Publishers
    Journal of periodontal research 35 (2000), S. 0 
    Details
    ISSN: 1600-0765
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Medicine
    Notes: The aim of this study was to develop and test in vitro an opto-electronic positioning device for serial direct digital images of oral structures, i.e. to associate direct digital imaging with the principles of computer-aided surgery. This system registered positions of infrared light emitting diodes (LED) on carriers, establishing local coordinate systems. With LED markers attached on the sensor holder, the X-ray tube and a fix reference, the opto-electronic camera (Optotrak®) registered the geometric source/detector relation. A specially designed tracking and guidance software was developed which enabled the operator to reposition the X-ray source. A graphical user interface guided the operator in aligning 2 circles to the reference axis, one indicating the origin of the beam, the other its tip. In addition, depth control was provided. An in vitro calibration was performed. A sensor holder/bite block carried the Sens-A-Ray® sensor with a hair-cross. In front of the object a second hair-cross was fixed. A steel ball fixed to the center of the X-ray cone allowed to verify the alignment. The mean angulation error in the vertical plane was 0.06° and 0.04° in the horizontal one. Translation mean errors were small and ranged between −0.02 mm and 0.37 mm. The translation in the Z axis is negligible. This resulted in pairs of images suitable for digital subtraction. Although still in an experimental state, the results showed that opto-electronic navigation was useful to standardize projection geometry without any mechanical link and to achieve digital subtraction images based on direct digital imaging.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1034/j.1600-0765.2000.035004225.x
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  • 3
    Electronic Resource
    Electronic Resource
    On the derivation and comparative analysis of large rotation shell theories (1986)
    Springer
    Archive of applied mechanics 56 (1986), S. 145-160 
    Details
    ISSN: 1432-0681
    Source: Springer Online Journal Archives 1860-2000
    Topics: Mechanical Engineering, Materials Science, Production Engineering, Mining and Metallurgy, Traffic Engineering, Precision Mechanics
    Description / Table of Contents: Übersicht Im Rahmen der geometrisch-nichtlinearen ersten Schalenapproximation werden drei energiekonsistente Schalentheorien für große Rotationen hergeleitet. Die Schalengleichungen werden als Euler-Lagrange Gleichungen eines zugehörigen Variationsprinzips vom stationären Wert der potentiellen Gesamtenergie hergeleitet. Die numerische Anwendbarkeit wird anhand eines stark-nichtlinearen Schalenbeispiels nachgewiesen. Um die in dieser Arbeit hergeleiteten Theorien entsprechend einordnen zu können, wird für eine größere Zahl von in der Literatur angegebenen Schalengleichungen eine vergleichende Untersuchung mit numerischer Auswertung durchgeführt.
    Notes: Summary For the geometrically nonlinear first approximation theory of elastic shells three energy-consistent large rotation shell variants are constructed. The governing shell equations are derived as Euler-Lagrange equations of an associated variational principle of stationary total potential energy. The numerical applicability is considered for a highly nonlinear shell problem. To incorporate the presented theories into the frame of shell models published in the literature a comparative analysis is carried out for a large number of shell equations.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF00537243
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  • 4
    Electronic Resource
    Electronic Resource
    Results of lumbosacral distraction spondylodesis for the treatment of spondylolisthesis, failed-back syndrome, and lumbar instability (1992)
    Springer
    European spine journal 1 (1992), S. 117-124 
    Details
    ISSN: 1432-0932
    Keywords: Rachis lombaire ; Arthrodèse lombaire ; Spondylolisthésis ; Echec de la chirurgie lombaire ; Biomécanique ; Lumbar spine ; Spinal fusion ; Spondylolisthesis ; Failed-back syndrome ; Biomechanics
    Source: Springer Online Journal Archives 1860-2000
    Topics: Medicine
    Description / Table of Contents: Summary Various methods of lumbosacral fusions for the treatment of degenerative spinal diseases are used clinically. Results vary greatly depending on indication, type of fusion, implants, and method of evaluation. In a retrospective clinical and radiological examination after an average follow-up time of 3.9 years this study reports on the outcome of lumbosacral distraction spondylodesis (LSDS) in a consecutive series of 147 patients being fused for the treatment of spondylolisthesis, failed-back syndrome, or lumbar instability. LSDS consists of a posterolateral fusion together with an autologous corticocancellous H-graft wedged under distraction between the spinous processes of L4 and S1. With 81.0% good and excellent results this noninstrumented fusion technique showed the best outcome in patients with spondylolisthesis, while in cases with a failed-back syndrome or lumbar instability only 62.3% excellent to satisfying outcomes were noted. The rate of pseudarthrosis was 13.6% in the whole patient group; no major complications such as nerve root damage, postoperative neurological deficits, or spinal stenosis were found.
    Notes: Résumé Diverses méthodes de fusion lombo-sacrée sont utilisées dans le traitement des affections dégénératives rachidiennes. Les résultats sont très variables et dépendent de l'indicatio, du type de fusion, des implants et de la méthode d'évaluation. Par une analyse rétrospective clinique et radiologique après un délai postopératoire moyen de 3,9 mois, cette étude rend compte du résultat de la “spondylodèse lombo-sacrée en distraction” (LSDS) dans une série consécutive de 147 patients traités par arthrodèse pour spondylolisthésis, échec de la chirurgie lombaire ou instabilité lombaire. La LSDS consite en une greffe postéro-latérale associée à un greffon autologue cortico-spongieux en H enfoncé sous distraction entre les épineuses de L4 et de S1. Avec 81,0% de bons et excellents résultats, cette technique d'arthrodèse sans ostéosynthèse a montré ses meilleurs résultats dans le traitement des spondylolisthésis, tandis que dans les cas d'échec de la chirurgie lombaire ou d'instabilité lombaire il n'a donné que 62,3% des résultats excellents ou satisfaisants. Le taux de pseudarthroses était de 13,6% pour tout le groupe et l'on n'a pas trouvé de complications majeures comme une atteinte radiculaire, un déficit neurologique post-opératoire ou une sténose lombaire.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF00300938
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  • 5
    Electronic Resource
    Electronic Resource
    Viscoelasticity of the alar and transverse ligaments (1992)
    Springer
    European spine journal 1 (1992), S. 178-184 
    Details
    ISSN: 1432-0932
    Keywords: Rachis cervical supérieur ; Ligament alaire ; Ligament transverse ; Anatomie ; Biomécanique ; Upper cervical spine ; Alar ligament ; Transverse ligament ; Anatomy ; Biomechanics
    Source: Springer Online Journal Archives 1860-2000
    Topics: Medicine
    Description / Table of Contents: Summary The occipito-atlanto-axial joint is the most complex one of the human spine. Traumatic or inflammatory lesions in this region may lead to instability and neurological symptoms of clinical importance. This study reports the results of anatomical and biomechanical examination of 13 human upper cervical spine specimens and focuses on the viscoelastic behavior of the alar and transverse ligaments. Non-destructive tensile testing was performed on a uniaxial testing machine with 25 alar and 11 transverse ligaments at three different load rates of 0.1 mm/s, 1.0 mm/s, and 10.0 mm/s. The ligaments were further tested for relaxation over 300 s. Each ligament exhibited an initial neutral zone in which no tensile force could be measured during cyclic testing. This neutral zone was more significant in the alar ligaments than in the transverse ligaments with respect to the measured in situ length of the ligaments (11.2 vs 18.1 mm on average). Increasing axial deformation led to increased load in all ligaments. Hysteresis, i.e., the energy loss exhibited by viscoelastic material subjected to loading and unloading cycles, increased with higher displacement rates and higher tensile forces. In neutral position the alar ligaments were lax in all specimens. During axial rotation both alars tightened. Ligamentous resistance increased as the end of the range of motion (ROM) was approchaed during rotation. The neutral zone explains the laxity of the ligaments in midposition and allows mobility of the upper cervical spine with minimum expenditure of muscular energy. The ligaments become stiffer under higher loads and therefore contribute to a limitation of the ROM in the occipitio-atlanto-axial joint.
    Notes: Résumé L'articulation occipito-atlanto-axoïdienne est l'une des plus complexes du rachis humain. Les lésions traumatiques ou inflammatoires de cette région peuvent conduire à l'instabilité et à des troubles neurologiques importants. Cette étude rapporte les résultats d'une étude anatomique et biomécanique de 13 spécimens de rachis cervical supérieur et met l'accent sur le comportement visco-élastique des ligaments alaires et transverse. Des essais de mise en tension ont été réalisés sur 25 ligaments alaires et 11 ligaments transverses, en restant en deçà du point de rupture. Ils ont été effectués sur un appareil monoaxial, selon trois niveaux de charge différents: 0.1 mm/s, 1.0 mm/s et 10.0 mm/s. La détente ligamentaire a été en outre étudiée pendant 300s. Chaque ligament a montré une zone neutre initiale (NZ) dans laquelle aucune force de tension ne pouvait être mesurée au cours du cycle d'essai. Cette zone neutre était plus importante pour les ligaments alaires que pour les ligaments transverses, compte tenu de la longueur ligamentaire mesurée in situ (11.2 contre 18.1 mm en moyenne). L'augmentation de la déformation axiale a conduit à une augmentation des contraintes dans tous les ligaments. L'hystérésis, c'est à dire la perte d'énergie présentée par le matériel visco-élastique soumis à des cycles de mise en charge et décharge, augmentait avec l'amplitude du déplacement et l'intensité des forces de tension. En position neutre, les ligaments alaires étaient détendus sur tous les spécimens. Lors de la rotation axiale, les deux ligaments alaires se sont mis en tension. La résistance du ligament à la rotation s'est trouvée accrue en fin d'amplitude. La zone neutre explique la laxité des ligaments en position intermédiaire et permet la mobilité du rachis cervical supérieur avec un minimum de dépense d'énergie. Les ligaments se tendent sous des charges plus élevées et par conséquent contribuent à la limitation de ROM dans l'articulation occipito-atlanto-axoïdienne.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF00301310
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  • 6
    Electronic Resource
    Electronic Resource
    Pull-out strength of pedicle hooks with fixation screws: influence of screw length and angulation (1996)
    Springer
    European spine journal 5 (1996), S. 71-73 
    Details
    ISSN: 1432-0932
    Keywords: Thoracic spine ; Pedicle hook ; Fixation screw ; Biomechanics ; Pull-out test
    Source: Springer Online Journal Archives 1860-2000
    Topics: Medicine
    Notes: Abstract The pull-out force of thoracic spinal pedicle hooks secured by long fixation screws engaging the posterior portion of the vertebral endplate was measured. The perfomance of these hooks was compared with that of hooks using a shorter screw and different screw orientation such that the vertebral endplates were not perforated. The longer and differently angulated screws, engaging the endplate, significantly enhanced the fixation potential of the hooks.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF00307831
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  • 7
    Electronic Resource
    Electronic Resource
    A new approach to computer-aided spine surgery: fluoroscopy-based surgical navigation (2000)
    Springer
    European spine journal 9 (2000), S. S078 
    Details
    ISSN: 1432-0932
    Keywords: Key words Spine ; Implant ; Pedicle screws ; Navigation ; Fluoroscopy
    Source: Springer Online Journal Archives 1860-2000
    Topics: Medicine
    Notes: Abstract A new computer-based navigation system for spinal surgery has been designed. This was achieved by combining intraoperative fluoroscopy-based imaging using conventional C-arm technology with freehand surgical navigation principles. Modules were developed to automate digital X-ray image registration. This is in contrast to existing computed tomography- (CT) based spinal navigation systems, which require a vertebra-based registration procedure. Cross-referencing of the image intensifier with the surgical object allows the real-time image-interactive navigation of surgical tools based on one single registered X-ray image, with no further image updates. Furthermore, the system allows the acquisition and real-time use of multiple registered images, which provides an advanced multi-directional control (pseudo 3D) during surgical action. Stereotactic instruments and graphical user interfaces for image-interactive transpedicular screw insertion have been developed. A detailed validation of the system was performed in the laboratory setting and throughout an early clinical trial including eight patients in two spine centers. Based on the resulting data, the new technique promises improved accuracy and safety in open and percutaneous spinal surgery.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/PL00010026
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  • 8
    Electronic Resource
    Electronic Resource
    New means in spinal pedicle hook fixation (1995)
    Springer
    European spine journal 4 (1995), S. 114-122 
    Details
    ISSN: 1432-0932
    Keywords: Thoracic spine ; Biomechanics ; Pedicle screw ; Pedicle hook ; Pull-out test
    Source: Springer Online Journal Archives 1860-2000
    Topics: Medicine
    Notes: Summary Pedicle hooks which are used as an anchorage for posterior spinal instrumentation may be subjected to considerable three-dimensional forces. In order to achieve stronger attachment to the implantation site, hooks using screws for additional fixation have been developed. The failure loads and mechanisms of three such devices have been experimentally determined on human thoracic vertebrae: the Universal Spine System (USS) pedicle hook with one screw, a prototype pedicle hook with two screws and the Cotrel-Dubousset (CD) pedicle hook with screw. The USS hooks use 3.2-mm self-tapping fixation screws which pass into the pedicle, whereas the CD hook is stabilised with a 3-mm set screw pressing against the superior part of the facet joint. A clinically established 5-mm pedicle screw was tested for comparison. A matched pair experimental design was implemented to evauluate these implants in constrained (series I) and rotationally unconstrained (series II) posterior pull-out tests. In the constrained tests the pedicle screw was the strongest implant, with an average pull-out force of 1650 N (SD 623 N). The prototype hook was comparable, with an average failure load of 1530 N (SD 414 N). The average pull-out force of the USS hook with one screw was 910 N (SD 243 N), not significantly different to the CD hook's average failure load of 740 N (SD 189 N). The result of the unconstrained tests were similar, with the prototype hook being the strongest device (average 1617 N, SD 652 N). However, in this series the difference in failure load between the USS hook with one screw and the CD hook was significant. Average failure loads of 792 N (SD 184 N) for the USS hook and 464 N (SD 279 N) for the CD hook were measured. A pedicular fracture in the plane of the fixation screw was the most common failure mode for USS hooks. The hooks usually did not move from their site of implantation, suggesting that they may be well-suited for the socalled segmental spinal correction technique as used in scoliosis surgery. In contrast, the CD hook disengaged by translating caudally from its site of implantation in all cases, suggesting a mechanical instability. The differences in observed hook failure modes may be a function of the type and number of additional fixation screws used. These results suggest that additional screw fixation allows stable attachment of pedicle hooks to their implantation site. Hooks using additional fixation screws passing obliquely into the pedicle apparently provide the most rigid attachment. The second fixation screw of the prototype hook almost doubles the fixation strength. Thus, the prototype hook might be considered as an alternative to the pepdicle screw, especially in the upper thoracic region.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF00278923
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  • 9
    Electronic Resource
    Electronic Resource
    Compressive strength of interbody cages in the lumbar spine: the effect of cage shape, posterior instrumentation and bone density (1998)
    Springer
    European spine journal 7 (1998), S. 132-141 
    Details
    ISSN: 1432-0932
    Keywords: Key words Spine ; Interbody ; Fusion ; Compression ; Biomechanics ; Implant ; Posterior ; lumbar intervertebral fusion
    Source: Springer Online Journal Archives 1860-2000
    Topics: Medicine
    Notes: Abstract One goal of interbody fusion is to increase the height of the degenerated disc space. Interbody cages in particular have been promoted with the claim that they can maintain the disc space better than other methods. There are many factors that can affect the disc height maintenance, including graft or cage design, the quality of the surrounding bone and the presence of supplementary posterior fixation. The present study is an in vitro biomechanical investigation of the compressive behaviour of three different interbody cage designs in a human cadaveric model. The effect of bone density and posterior instrumentation were assessed. Thirty-six lumbar functional spinal units were instrumented with one of three interbody cages: (1) a porous titanium implant with endplate fit (Stratec), (2) a porous, rectangular carbon-fibre implant (Brantigan) and (3) a porous, cylindrical threaded implant (Ray). Posterior instrumentation (USS) was applied to half of the specimens. All specimens were subjected to axial compression displacement until failure. Correlations between both the failure load and the load at 3 mm displacement with the bone density measurements were observed. Neither the cage design nor the presence of posterior instrumentation had a significant effect on the failure load. The loads at 3 mm were slightly less for the Stratec cage, implying lower axial stiffness, but were not different with posterior instrumentation. The large range of observed failure loads overlaps the potential in vivo compressive loads, implying that failure of the bone-implant interface may occur clinically. Preoperative measurements of bone density may be an effective tool to predict settling around interbody cages.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/s005860050043
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  • 10
    Electronic Resource
    Electronic Resource
    The role of supplemental translaminar screws in anterior lumbar interbody fixation: a biomechanical study (1998)
    Springer
    European spine journal 7 (1998), S. 400-407 
    Details
    ISSN: 1432-0932
    Keywords: Key words Biomechanics ; Stability ; Implant ; Interbody ; fusion ; Translaminar screws
    Source: Springer Online Journal Archives 1860-2000
    Topics: Medicine
    Notes: Abstract The immediate stabilization provided by anterior interbody cage fixation is often questioned. Therefore, the role of supplementary posterior fixation, particularly minimally invasive techniques such as translaminar screws, is relevant. The purpose of this biomechanical study was to determine the immediate three-dimensional flexibility of the lumbar spine, using six human cadaveric functional spinal units, in four different conditions: (1) intact, (2) fixed with translaminar screws (TLS), (3) instrumented with anterior interbody cage insertion with the BAK system and (4) instrumented with BAK cage with additional TLS fixation. Flexibility was determined in each testing condition by measuring the vertebral motions under applied pure moments (i.e. flexion-extension, bilateral axial rotation, bilateral lateral bending) in an unconstrained manner. Anterior fixation with the BAK alone provided significant stability in flexion and lateral bending. Additional posterior TLS significantly reduced the motion in extension and axial rotation. TLS fixation alone resulted in smaller rotations than BAK fixation in all loading directions. Based on these results, it seems that interbody cage fixation with the BAK system stabilizes the spine in some, but not all, loading directions. The problematic loading directions of extension and axial rotation can be substantially stabilized by using translaminar screw fixation. However, one should emphasize that the degree of stability needed to achieve solid fusion is not known.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/s005860050097
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