Thomas Pressel

Mechanical properties of femoral trabecular bone in dogs

BackgroundStudying mechanical properties of canine trabecular bone is important for a better understanding of fracture mechanics or bone disorders and is also needed for numerical simulation of canine femora. No detailed data about elastic moduli and degrees of anisotropy of canine femoral trabecular bone has been published so far, hence the purpose of this study was to measure the elastic modulus of trabecular bone in canine femoral heads by ultrasound testing and to assess whether assuming isotropy of the cancellous bone in femoral heads in dogs is a valid simplification.MethodsFrom 8 euthanized dogs, both femora were obtained and cubic specimens were cut from the centre of the femoral head which were oriented along the main pressure and tension trajectories. The specimens were tested using a 100 MHz ultrasound transducer in all three orthogonal directions. The directional elastic moduli of trabecular bone tissue and degrees of anisotropy were calculated.ResultsThe elastic modulus along principal bone trajectories was found to be 11.2 GPa ± 0.4, 10.5 ± 2.1 GPa and 10.5 ± 1.8 GPa, respectively. The mean density of the specimens was 1.40 ± 0.09 g/cm3. The degrees of anisotropy revealed a significant inverse relationship with specimen densities. No significant differences were found between the elastic moduli in x, y and z directions, suggesting an effective isotropy of trabecular bone tissue in canine femoral heads.DiscussionThis study presents detailed data about elastic moduli of trabecular bone tissue obtained from canine femoral heads. Limitations of the study are the relatively small number of animals investigated and the measurement of whole specimen densities instead of trabecular bone densities which might lead to an underestimation of Youngs moduli. Publications on elastic moduli of trabecular bone tissue present results that are similar to our data.ConclusionThis study provides data about directional elastic moduli and degrees of anisotropy of canine femoral head trabecular bone and might be useful for biomechanical modeling of proximal canine femora.

A rapid prototyping model for biomechanical evaluation of pelvic osteotomies / Ein Modell zur biomechanischen Bewertung von Beckenosteotomien

Abstract The biomechanical consequences of Salter pelvic osteotomy are difficult to assess due to the complex three-dimensional anatomy of the pelvis. Therefore, models of the dysplastic pelvis are required to allow realistic biomechanical simulation of possible outcomes. A polyamide reversed-engineering model of the left hemipelvis and proximal femur was produced from a computed tomography dataset of an 8-year-old child with severe dysplasia of both hips using selective laser sintering. Hip joint forces before and after Salter osteotomy of the hip were measured using an experimental setup in which an industrial robot was exerting hip joint forces and moments representing one-legged stance. Hip extensor and abductor actuator forces were measured which counterbalanced the joint moments. The preoperative hip joint resultant force was 583 N (270% body weight), while after the operation a mean force of 266 N (120% body weight) was measured. The resulting bony model was geometrically accurate, while apparent joint incongruencies were due to the neglected cartilaginous structures in the model. The preoperative joint resultant force was within the limits reported in the literature. The results suggest that Salter innominate osteotomy not only increases joint contact area but also reduces the hip joint force. Zusammenfassung Die biomechanischen Auswirkungen der Salter-Beckenosteotomie sind wegen der komplizierten dreidimensionalen Anatomie des Beckens schwer abzuschätzen. Deswegen werden Modelle des dysplastischen Beckens benötigt, um die biomechanischen Folgen realistisch zu simulieren. Ein Polyamidmodell der linken Beckenhälfte und des proximalen Femurs wurde hergestellt mit dem selektiven Lasersinter-Verfahren auf Basis eines CT-Datensatzes eines achtjährigen Mädchens mit schwerer Hüftdysplasie. Hüftgelenkskräfte vor und nach der Salter-Osteotomie wurden mit einem Versuchsaufbau bestimmt, bei dem ein Industrieroboter Kräfte und Momente des Hüftgelenks im Einbeinstand erzeugte. Die Muskelkräfte der Abduktoren und Extensoren wurden bestimmt, mit denen die Momente im Gelenk ausgeglichen wurden. Die präoperative Hüftgelenksresultierende betrug 583 N (270% des Körpergewichts), nach der Operation wurde eine mittlere Kraft von 266 N gemessen (120% Körpergewicht). Das Modell des Knochens war geometrisch genau, allerdings fanden sich Inkongruenzen des Gelenks, da der Knorpel nicht berücksichtigt wurde. Die präoperative Hüftgelenkskraft liegt im Bereich der Werte, die in der Literatur berichtet werden. Die Ergebnisse weisen darauf hin, dass die Salter-Beckenosteotomie nicht nur die Kontaktfläche des Gelenks vergrößert, sondern auch die Hüftgelenkskraft reduziert.

FE-Simulation zur Lokalisierung hoch beanspruchter Bereiche in der Hüftpfanne von Endoprothesen / FE-analysis of surface stresses for the tribological system in total hip prostheses

Zusammenfassung Die Implantation einer Hüfttotalendoprothese ist ein operativer Eingriff, der in Deutschland häufig aufgrund eines fortgeschrittenen Hüftgelenkschadens sowohl beim Menschen als auch in der Veterinärmedizin beim Hund durchgeführt wird. Das Langzeitergebnis einer Hüfttotalendoprothese wird hauptsächlich durch die aseptische Lockerung der Prothese bestimmt; Abriebpartikel der Gleitpaarung sind für eine Lockerung mit verantwortlich. Zur Analyse der Oberflächenspannungen bei unterschiedlichen Gleitpaarungen wurde ein Finite-Element-Modell generiert, das auf den CAD-Daten einer handelsüblichen Hüfttotalendoprothese basiert. Nach Einleitung einer physiologischen Gelenkkraft wurden Spannungen in den Komponenten der drei Gleitpaarungen Keramik/Polyethylen, Keramik/Keramik und Metall/Polyethylen berechnet. Dabei zeigten sich bei der Gleitpaarung Keramik/Keramik deutlich höhere Spitzenspannungen als bei den beiden übrigen Gleitpaarungen. In weiteren Arbeiten sollen angepasste Prothesen entwickelt werden, die optimale Reib- und Dämpfungseigenschaften der Komponenten sicherstellen.

Correction: Mechanical properties of femoral trabecular bone in dogs

After the publication of this work [1], we became aware of the fact that the frequency of the ultrasound transmitter that we used for determining the elastic moduli of the trabecular bone specimens was not correctly specified. The oscillation frequency of the ultrasound transmitter was 2 MHz (and not 100 MHz as stated in our work) while we used a sampling rate of 100 MHz. In our publication, the oscillation frequency and sampling rate were confounded. Therefore also the statement in the discussion that we might have determined elastic moduli of trabecular bone tissue rather than the elastic properties of whole specimens because we used an ultrasound frequency > 2 MHz is wrong and has to be omitted.For measurement, the cubic bone specimens were not immersed in Ringers solution but only were kept moist all the time.Apart from these corrections concerning the methods and interpretation of the data, the results reported in our publication and the conclusions are absolutely correct.We apologize for the inconvenience that this inaccuracy may have caused.

The effect of functional splinting on mild dysplastic hips after walking onset

BackgroundFor treatment of Graf class IIb dysplastic hips at walking onset a treatment concept with abduction splints allowing patterns as walking and crawling under constant abduction control was investigated. However, as the splint still incapacitates child movements the research question remains whether the physiologically progressing maturation of hips can be significantly altered using such abduction splints for walking children.MethodsOf 106 children showing late hip dysplasia, 68 children treated with the Hoffman-Daimler (HD-splint) abduction splint were compared with 38 children with neglect of the abduction treatment in this retrospective study. Radiographic analyses were performed measuring the development of the age dependent acetabular angle.ResultsThe regression analysis for splint treatment showed a significant linear regression for both splint treatment and no splint treatment group (r2 = 0,31 respectively r2 = 0,33). No statistical difference between both treatment groups was apparent.ConclusionConsidering the characteristics of this study, there seems to be no strong rationale supporting the use of an abduction device in growing children. As no significant difference between treatment groups is apparent, a future controlled prospective study on splinting effects can be considered ethically allowed.