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Objective: In navigated orthopedic surgery, accurate registration of bones is of major interest. Usually, this registration is performed using landmarks positioned directly on the bone surface. These landmarks must be exposed during surgery. Our goal is to avoid the exposure of bone surface for the sole purpose of registration by using an intraoperative ultrasound device that can localize the bone through tissue.Method: We propose an algorithm for the registration of CT and ultrasound datasets that takes into account the fact that ultrasound produces very noisy images (speckle) and shows only parts of the bone surface. This part is made from the CT dataset. Next, a surface volume registration is performed by searching for a position of the estimated surface that maximizes the average gray value of the voxels in the ultrasound dataset covered by the surface.Results: The algorithm was implemented and validated using an ex vivo preparation of a human lumbar spine with surrounding muscle tissue. On the basis of this data, the method has a large radius of convergence and a repeatability of 0.5 nun for displacement and 0.5 degrees for rotation.Conclusions: A robust algorithm for the registration of 3D CT and ultrasound datasets is presented. The computation time seems sufficiently short to permit intraoperative use.

We study the temperature dependence of the conductance and of the conductivity of a doped two-chain ladder system in the presence of a barrier or of a low impurity concentration, respectively, focusing on the effects of electronelectron interaction. Like in the purely 1-D case, the conductance vanishes at low temperature when the Luttinger-liquid exponent Kρ+ (for even-charge modes) is smaller than unity, despite the presence of dominant superconducting correlations. However, there is a region of repulsive interaction where perfect transmission across the barrier occurs. This is due to a renormalization of Kρ+ produced by the difference in Fermi velocities of the bonding and antibonding bands.

Ziel der Arbeit ist die exakte und genäherte Bestimmung der kritischen Last von elastischen Systemen mit nichtlinearem Vorbeulverhalten. Grundlage ist die Verschiebungsformulierung der Methode der finiten Elemente in inkrementell/iterativer Darstellung. Im ersten Hauptteil werden begleitende Stabilitätsuntersuchungen bei der exakten Auswertung der nichtlinearen Gleichungen behandelt. Hierzu gehören die Determinantenbeobachtung sowie Eigenwertuntersuchungen für den Lastparameter und die Eigenfrequenz. Im zweiten Hauptteil werden zwei verbesserte Anfangsstabilitätsanalysen zur genäherten Bestimmung der kritischen Last vorgestellt, die auf Reihenlösungen der Gleichgewichts- und Stabilitätsgleichungen basieren. In beiden Teilen bestätigen zahlreiche Beispiele die Brauchbarkeit der behandelten Verfahren.

The stability of a cylindrical shell subjected to wind load is analyzed using numerical solution methods. The multisegment direct integration as well as the finite element method are applied in linear analysis, and a nonlinear finite element algorithm is used to take into account the nonlinear prebuckling effects of the perfect and imperfect structure. The calculated results are compared with measurements, and good agreement is derived with respect to both stability limit and buckling mode.

Several letters to the editor are presented in response to articles in previous issues which include one that is related to profit being considered as a dirty word in the October 2010 issue, another about semi driving and one which relates the benefit of raw milk on having a good brain in the November 2010 issue.