K.-P. Schmitz

A comparison of the mechanical performance characteristics of seven drug-eluting stent systems†

Objectives: Mechanical properties of drug eluting stents (DES) will be measured to provide comparable numerical data to assess deliverability, and thus clinical performance. Background: DES are routinely used in coronary interventions to reduce the rates of restenosis and target vessel revascularizations. Current research is primarily concerned with issues related to late stent thrombosis. However, mechanical properties of DES are a critical determinant of deliverability, and consequently the ultimate arbiter of their clinical performance. Methods: Mechanical properties (pushability, trackability, crossability) were measured under standardized in‐vitro conditions. The vessel models were derived from typical vessel anatomy but adapted to the individual tests. Additionally, profile and bending forces of the stent segment of the delivery system were measured. Seven different commercially available balloon‐expandable coronary DES systems were included. All stents were 3.0 mm diameter with a stent length from 14 to 18 mm. Results: The pushability expressed as the ratio of distal force at a specific proximal push force (4N) ranged between 38.66 and 18.53%. The trackability as the mean track‐forces ranged from 0.551 N to 1.137 N. One stent system could not pass this test. The mean crossing forces at a 1.4 mm stenosis model ranged from 0.038 N up to 0.103 N. The mean crimped stent profiles ranged from 1.055 mm to 1.198 mm and the bending stiffness of the crimped stent was 17.22 to 47.20 Nmm2. Conclusion: Better understanding of mechanical properties of DES shall improve tactile skills of the interventionists during PCI and to improve criteria for DES selection in specific clinical settings.

Noninvasive assessment of stiffness and failure load of human vertebrae from CT-data.

A calculational method based on noninvasively derived data for the assessment of the mechanical quality of individual vertebrae is presented. Dimensional data obtained from serial, segmented CT scans were used as the geometric input for a newly developed finite element model designed to calculate stiffness and failure load of these complex bone structures. Mechanical, structural data for cancellous bone was obtained by measurements of the compressive strength and failure load of actual vertebral specimens obtained at autopsy. The stiffness and failure load calculated by the newly developed finite element model were compared with the data obtained from mechanical measurements of vertebral specimens. A high correlation between measured and calculated failure load was found (r = 0.89, p < 0.001, n = 16). The correlation between the failure load and bone mineral density (BMD) was significant (r = 0.82, p < 0.001, n = 16). A similar correlation between calculated and measured stiffness (r = 0.81, p < 0.001, n = 15) was also found using the finite element model described herein. Thus the newly developed calculation methodology has been verified and can be used to predict the failure load and stiffness of osteoporotic vertebrae using data obtained non-invasively from CT scans.

Noninvasive Assessment of Stiffness and Failure Load of Human Vertebrae from CT-Data. Nichtinvasive Abschätzung von Steifigkeit und Versagenslast von osteoporotischen Wirbelkörpern nach CT-Daten

A calculational method based on noninvasively derived data for the assessment of the mechanical quality of individual vertebrae is presented. Dimensional data obtained from serial, segmented CT scans were used as the geometric input for a newly developed finite element model designed to calculate stiffness and failure load of these complex bone structures. Mechanical, structural data for cancellous bone was obtained by measurements of the compressive strength and failure load of actual vertebral specimens obtained at autopsy. The stiffness and failure load calculated by the newly developed finite element model were compared with the data obtained from mechanical measurements of vertebral specimens. A high correlation between measured and calculated failure load was found (r = 0.89, p < 0.001, n = 16). The correlation between the failure load and bone mineral density (BMD) was significant (r = 0.82, p < 0.001, n = 16). A similar correlation between calculated and measured stiffness (r = 0.81, p < 0.001, n = 15) was also found using the finite element model described herein. Thus the newly developed calculation methodology has been verified and can be used to predict the failure load and stiffness of osteoporotic vertebrae using data obtained non-invasively from CT scans.

Biomechanical Aspects of Potential Stent Malapposition at Coronary Stent Implantation

Despite of the clearly improved restenosis rates with drug-eluting stents (DES), optimization of stent deployment is still important for favourable immediate and longterm results. This requirement is fed by the recently reported potential risk of late stent thrombosis which is among other factors assumed to be caused by stent malapposition. Stent malapposition appears frequently in clinical practice for DES as well as bare-metal stents (BMS).

Nichtinvasive Bestimmung von Steifigkeit und Versagenslast von osteoporotischen Wirbelkörpern

EINLEITUNG Mit zunehmender Verbesserung der biomechanischen Durchdringung medizinischer Problemstellungen gewinnt die Anwendung von Methoden der Strukturmechanik auf Knochenmechanik an Bedeutung. Die Erkenntnis, daß die Qualität von Knochen nicht allein vom Mineralgehalt, sondern auch von der Struktur abhängt, regte eine Untersuchung verschiedener Knochenarten mittels Finite-Elemente-Methode (FEM) an. Zahlreiche bisherige Arbeiten unterschiedlicher Autoren verwenden mit unterschiedlichem Aufwand und verschiedenen Genauigkeitsanforderungen die FEM zur Festigkeitsberechnung von Knochen, z.B. [1]. Am Ende der hier vorgestellten Arbeiten sollte ein nichtinvasives Diagnoseverfahren zur Untersuchung der biomechanischen Qualität von Wirbelkörpern stehen. Die Untersuchungen konzentrierten sich daher auf die Bestimmung von Geometrie und Materialeigenschaften von Wirbelkörpem aus computertomographischen (CT) Daten. Zur Bewertung der Validität des Verfahrens wurden die Ergebnisse mit Messungen an Leichenwirbelkörpem verglichen. METHODE Als Ausgangspunkt für die Entwicklung des Verfahrens dienten Transversalschnitte von Wirbelsäulenabschnitten (LWK 1-5) in Leichen. Die Untersuchungen wurden mit einem CT Siemens SOMATOM DRH durchgeführt. Als Schichtdicke und Schichtabstand wurden einheitlich 2 mm gewählt. Die Aufhahmeparameter entsprachen den üblichen für axiales Bonescanning (125 kV). Bei einer Bildgröße von 512x512 Pixeln ergibt sich eine Auflösung von ca. 0,5 mm. Ein Wirbelkörper wird durch ca. 12 Schnitte dargestellt. Die Geometrierekonstruktion und FE-Netzgenerierung erfolgte mittels eines selbstentwickelten Algorithmus, der aus den CT-Datensätzen automatisch ein FEMEingabefile generiert. Zur Segmentierung wurde eine Sequenz von Bildverarbeitungsoperatoren angewendet, die auf der Transformation des CT-Bilds in einen Merkmalsraum basiert. Ausgehend von der Überlegung, daß sich die zu segmentierenden Klassen des Bildes Kortikalis, Spongiosa und Umgebung nicht nur durch die CT-Zahl, sondern auch durch die Kompaktheit unterscheiden, wurde neben der CT-Zahl die lokale Varianz zur Segmentierung und Klassifikation der Bildinhalte verwendet. In diesem Falle beschreiben die Merkmale die Klassenzugehörigkeit wie folgt: • geringe lokale Varianz, hohe Hounsfieldzahl: Klasse Kortikalis • hohe lokale Varianz, hohe Hounsfieldzahl: Klasse Spongiosa • mäßige lokale Varianz, geringe Hounsfieldzahl: Klasse Umgebung Zur Segmentation wird ein Ausschnitt im Merkmalsraum definiert, dessen Punkte die Eigenschaften der Kortikalis haben (Abb. 1). Dieser Ausschnitt wird wie folgt festgelegt: _ Jl; (CT(x,y)>ctdef)fe(var(x,y)