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  • 1
    ISSN: 1432-0932
    Keywords: Biomécanique ; Implants ; Contraintes ; Rigidité ; Vis pédiculaires ; Biomechanics ; Implants ; Stress ; Stiffness ; Transpedicular fixation
    Source: Springer Online Journal Archives 1860-2000
    Topics: Medicine
    Description / Table of Contents: Summary A series of 216 biomechanical tests with 36 calf spines were performed to evaluate the rigidity of three newly developed prototypes of transpedicular fixation systems (Spine Fix, AO/ASIF prototype 1, AO/ASIF prototype 2) as compared to the already established Cotrel-Dubousset (CD) system. The Spine Fix system follows the same principle of spinal fixation as the CD system, while the two prototypes of the AO/ASIF group introduce a new concept of spinal reduction and fixation technique, using a three-dimensional adjustable fastening system of transpedicular screws to a longitudinal rod. This allows for correction and fixation of the instrumented vertebra segments in any position. During the tests the main point of interest was whether the newly gained degrees of freedom are associted with a loss of stiffness in the construct. Furthermore, the study evaluated whether transpedicular systems should be optimized from the technological point of view, or whether the stability and rigidity of these systems is determined mainly by the quality of pedicular anchorage. Load displacement was measured using a calf spine model with a precisely defined three-column lesion. Each implant was loaded up to 15 Nm in flexion, extension, lateral bending, and axial rotation. In all tests, the construct behaved in a highly linear fashion (r 2〉0.94). By continously measuring the forces and moments at the cranial end of the spine specimen high accuracy of the tests was achieved (standard deviation: x-axis, 1.74%; y-axis, 1.36%; z-axis, 1.21%). In general, the stifness was found to be highest in lateral bending, follwed by flexion/extension and axial rotation. Spine Fix was the stiffest implant in flexion/extension, AO/ASIF prototype 1 in lateral bending, and AO/ASIF prototype 2 in rotation. In comparison to the CD system (stiffness of CD=100%), differences in stiffness ranged from 77.3% prototype 1 to 140.8% Spine Fix in flexion, from 78.2% prototype 2b to 134.7% Spine Fix in extension, from 108.1% prototype 2b to 213.5% prototype 1 in lateral bending, and from 80.3% prototype 1 to 110.6% prototype 2 in axial rotation. The Spine Fix and prototype 2 systems showed equal or higher stiffness coefficients compared to the CD system. Prototype 1 is significantly more flexible, except in lateral bending, than the CD. From the technical point of view, the two AO/ASIF prototypes allow the correction and fixation of an instrumented vertebra in any position. Prototype 2, despite the additional joint between transpedicular screws and longitudinal rods, shows stiffness comparable to that of the CD system.
    Notes: Résumé Une série de 216 essais biomécaniques ont été effectués sur 36 rachis de veau, afin d'évaluer la rigidité de trois nouveaux prototypes de systèmes de fixation pédiculaires (Spine Fix, Prototype 1 de l'AO/ASIF, Prototype 2 de l'AO/ASIF), par rapport à celle du systéme CD maintenant bien établi. Le système Spine Fix suit la même philosophie de fixation rachidienne que le système CD, alors que les deux prototypes du groupe AO/ASIF introduisent un nouveau concept dans la technique de réduction et de fixation, utilisant un système de fixation tridimensionnel. Ceci permet la correction et la fixation des segments vertébraux instrumentés dans toutes les positions. Durant les essais, le principal point d'intérêt a été de déterminer si les degrés de liberté nouvellement acquis par les prototypes de l'AO/ASIF étaient associés à une perte de rigidité du montage. De plus, cette étude a été réalisée afin d'établir si ces systèmes de fixation pédiculaires doivent être améliorés du point de vue technologique, ou si leur stabilité et leur rigidité sont principalement déterminées par la qualité de l'ancrage pédiculaire. Le déplacement produit par la mise en charge a été mesuré en utilisant pour modèle un rachis de veau porteur d'une lésion bien définie des trois colonnes. Chaque implant a été soumis à une charge allant jusqu'à 15 Nm en flexion, extension, inclinaison latérale et rotation axiale. Dans tous les essais, le montage s'est comporté sur un mode hautement linéaire (r 2〉0.94). Par la mesure continue des forces et des moments à l'extrémité crâniale des spécimens rachidiens, l'évaluation des essais a pu bénéficier d'une haute précision (l'écart-type est de 1.74% sur l'axe des x, 1.35% sur l'axe des y, 1.21% sur celui des z). En général la rigidité a montré une valeur maximale lors de l'inclinaision latérale, cette valeur est un peu plus faible lors de la flexion/extension et de la rotation axiale. La rigidité maximale du Spine Fix est retrouvée lors de la flexion/extension, celle du Prototype 1 de l'AO/ASIF lors de l'inclinaison latérale, et celle du Prototype 2 lors de la rotation axiale. En comparant ces valeurs à celle du système CD (dont la rigidité serait arbitrairement estimée à 100%), les différences de rigidité lors de la flexion, s'étaient de 77.3% pour le Prototype 1, à 140.8% pour le Spine Fix. Lors de l'extension ces différences de rigidité s'étaient de 78.2% pour le Prototype 2b à 134.7% pour le Spine Fix; elles s'étaient de 108.1% pour le Prototype 2b, à 213.5% pour le Prototype 1 dans l'inclinaison latérale et de 80.3% pour le Prototype 1 à 110.6% pour le Prototype 2 en rotation axiale. Le Spine Fix tout comme le Prototype 2 ont montré des coefficient de rigidité égaux ou supérieurs à ceux du système CD. Le Prototype 1 est significativement plus flexible que le CD, sauf en inclinaison latérale. Du point de vue technique, les deux prototypes AO/ASIF permettent la correction et la fixation d'une vertèbre instrumentée dans n'importe quelle position. Le Prototype 2, malgré un dispositif de jonction supplémentaire entre les vis pédiculaires et les tiges, montre une rigidité comparable à celle du système CD.
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  • 2
    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.
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  • 3
    Electronic Resource
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    Springer
    Virchows Archiv 7 (1854), S. 158-162 
    ISSN: 1432-2307
    Source: Springer Online Journal Archives 1860-2000
    Topics: Medicine
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  • 4
    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.
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  • 5
    ISSN: 1420-9039
    Source: Springer Online Journal Archives 1860-2000
    Topics: Mathematics , Physics
    Notes: Summary A method is described for solving the boundary-value problem connected with the differential equation $$\frac{{d^2 x}}{{dt^2 }} = - f(x,t).$$ The problem is replaced by an integral equation with piecewise linear Kernel. The integral equation is solved by successive approximation and numerical integration. Numerical results are given working on an example of celestial mechanics.
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  • 6
    ISSN: 1432-5217
    Source: Springer Online Journal Archives 1860-2000
    Topics: Mathematics , Economics
    Description / Table of Contents: Summary The Swiss Federal Railways have decided to adopt the simultaneous formation with triangular schema for the composition of short distance goods trains wherever installations are suitable. Moreover the new conception will be used as a basis for planning building projects for new marshalling yards. This shunting procedure embodies considerable advantages as relatively few tracks and individual shunting manoeuvres are required. The necessary track capacity, however, and the number of runs required with a given number of waggons differs according to the sequencing of runs. The procedure described here produces a sequence which does not over-tax track capacity and at the same time keeps the number of runs to a minimum. This is an heuristic procedure which generally constitutes a sub-optimum solution but which has already proved its worth in practice.
    Notes: Zusammenfassung Die Schweizerischen Bundesbahnen haben sich entschieden, künftig für die Bildung der Nahgüterzüge in den dafür geeigneten Anlagen die Simultanformation mit dem Triangulärschema anzuwenden und die Bauprojekte für neue Rangierbahnhöfe auf dieser Basis zu planen. Dieses Rangierverfahren besitzt große Vorzüge, da man mit verhältnismäßig wenig Geleiseanlagen und Einzelmanövern auskommt. Allerdings ist der Bedarf an Geleisekapazität und die Zahl der Abläufe bei einem gegebenen Wagenaufkommen je nach Ablaufplan verschieden. Das hier beschriebene Verfahren erzeugt einen Ablaufplan, der die verfügbare Geleisekapazität nicht überbeansprucht und gleichzeitig die Anzahl der Abläufe möglichst klein hält. Es handelt sich um ein heuristisches Verfahren, das im allgemeinen nur eine suboptimale Lösung liefert, sich aber in der Praxis bereits gut bewährt hat.
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  • 7
    ISSN: 1435-2451
    Keywords: Pre-bending a plate ; Compression plate ; Flexural/torsional rigidity ; Plattenzugschraube ; Plattenüberbiegung ; Kompressionsplatte ; Biege-/Torsionsstabilität
    Source: Springer Online Journal Archives 1860-2000
    Topics: Medicine
    Description / Table of Contents: Zusammenfassung Die einer Knochenoberfläche exakt angepaßte Kompressionsplatte bewirkt bei der Stabilisierung einer Fraktur asymmetrische Kompression. Um ein plattenfernes Aufklaffen des Bruchspaltes zu verhindern, muß die Platte über dem Bruchspalt überbogen oder aber mit einer schrägen Plattenzugschraube am Knochen angeschraubt werden. Biege- und Torsionsstabilität wurden für verschiedene Techniken der Platten-Osteosynthese in vitro an Querosteotomien der Tibia untersucht. Die Ergebnisse zeigen, daß die schräge Plattenzugsehraube gleiche stabilitätsverbessernde Wirkung hat wie die für die klinische Anwendung sinnvolle Überbiegung der Platte. Die Kombination von schräger Zugschraube und Plattenüberbiegung bewirkt ein Höchstmaß an Biege- und Torsionsstabilität. Dieses Verfahren scheint daher für die Klinik empfehlenswert.
    Notes: Summary A compression plate, exactly contoured to the bone surface, produces asymmetric compression. To avoid opening of the fracture site opposite to the plate, the compression plate is to be used with additional means of pre-bending the plate or an inclined plate lag screw. Flexural and torsional stability provided by a plate lag screw, a pre-bent plate or the combination of both techniques have been studied in in vitro experiments in transverse osteotomies of the human tibia. The tests revealed that the use of an inclined plate lag screw increases the stability comparable to that produced by a pre-bent plate (for pre-bending in the range of a reasonable midspan deflection). The combination of both techniques seems to offer utmost stability.
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