Andreas Schaller

Combined multi-body and finite element investigation of the effect of the seat height on acetabular implant stability during the activity of getting up

An important question in assessing the stability of a total hip arthroplasty is the effect of daily physical activities of patients. The aim of this study is to examine these effects when standing up from three different seat heights. A musculoskeletal body model has been modified to simulate the three different seat heights. The calculated muscle forces have been transferred to a finite element model of a pelvis. The pelvis model was created from a hemipelvis CT dataset. As an implant component, a metal socket with a polyethylene insert was used. A primary implantation situation was modelled. For the analysed patient activities the highest hip contact forces and the highest micromotions occur at the beginning of the motion. The results of this study show that standing up from a certain seat height can have a significant influence on the micromotions in the implant-bone interface.

Transient finite element analysis of a traumatic fracture of the zygomatic bone caused by a head collision

Midfacial fractures rank first concerning maxillofacial traumatology. Collisions of two heads or head to object are the main causes for these fractures. An investigation based on a transient simulation using the finite element method was performed. A biomechanical head model was created and tested. A transient collision of two heads was simulated. The results were compared to a typical real patient case. This comparison revealed an identical fracture pattern, which can be interpreted as a clinical match of the simulation. The results of this study show the validity of biomechanical investigations, which may serve as a method to better understand maxillofacial fracture patterns. These results will be used for the optimization of fracture therapy or trauma prevention in the future.

Biomechanical mechanisms of orbital wall fractures – A transient finite element analysis

As the biomechanical mechanisms of orbital wall fractures are still under research, three different fracture mechanisms were tested in a finite element based investigation. In contrast to earlier studies, a finer skeletal model and a transient dynamic simulation were used to test pure hydraulic, pure buckling and a mixed force transmission. Results showed that each set-up led to different orbital fracture patterns, which correlate well with clinical findings. Therefore the conclusion is that different mechanisms may act together explaining the variety of clinical fracture situations. Biomechanical testing has proven to be appropriate in answering questions regarding fracture mechanisms.

Biomechanical investigation of naso-orbitoethmoid trauma by finite element analysis

Naso-orbitoethmoid fractures account for 5% of all facial fractures. We used data derived from a white 34-year-old man to make a transient dynamic finite element model, which consisted of about 740 000 elements, to simulate fist-like impacts to this anatomically complex area. Finite element analysis showed a pattern of von Mises stresses beyond the yield criterion of bone that corresponded with fractures commonly seen clinically. Finite element models can be used to simulate injuries to the human skull, and provide information about the pathogenesis of different types of fracture.

Biomechanical investigation of the supraorbital arch - a transient FEA study on the impact of physical blows.

IntroductionAs fractures of the supraorbital region are far less common than midfacial or orbital fractures, a study was initiated to investigate whether fist blows could lead to fractures similar to those often seen in the midface.MethodsA detailed skull model and an impactor resembling a fist were created and a fist blow to the supraorbital region was simulated. A transient finite element analysis was carried out to calculate von Mises stresses, peak force, and impact time.ResultsWithin the contact zone of skull and impactor critical stress values could be seen which lay at the lower yield border for potential fractures. A second much lower stress zone was depicted in the anterior-medial orbital roof.ConclusionsIn this simulation a fist punch, which could generate distinct fractures in the midface and naso-ethmoid-orbital region, would only reach the limits of a small fracture in the supraorbital region. The reason is seen in the strong bony architecture. Much higher forces are needed to create severe trauma in the upper face which is supported by clinical findings. Finite element analysis is the method of choice to investigate the impact of trauma on the human skeleton.

Effects of various anchoring components and loading conditions on primary stability of acetabular revision implant.

Purpose In revision total hip arthroplasty, until today, orthopaedic surgeons are missing evidence-based guidelines on cementless acetabular cup fixation. Methods 5 finite element models were generated featuring the following anchorage strategies: 1 short peg, 1 long peg, 2 long screws, 3 short screws and zero anchoring components for reference. The micromotions at the implant-bone interface were analyzed for 3 different loadcases, “Seated leg-crossing” (joint force 940 N, impingement force 750 N), “Normal gait” (joint force 1820 N), and “Stumbling” (joint force 4520 N). Results Within the same loadcase, percentages of interface area below 28 µm are nearly identical in all anchorage strategies. The average percentage of interface area below 28 µm is 31% for “Seated leg-crossing”, 17% for “Normal gait”, and 11% for “Stumbling”. Maximal von Mises stresses in “Normal gait”, for example, reach 12 MPa in the short peg, 48 MPa in the long peg, 15 MPa in 1 of the 2 long screws, and 85 MPa in 1 of the 3 short screws. Conclusions Common orthopaedic practice, to use peg or screw fixation alternatively according to bone availability or other clinical aspects, can be confirmed. The short peg may be a good alternative to the long peg with regard to the preservation of bone stock. However, the current study implies that the extent of potential osseointegration depends less on the chosen anchorage strategy but strongly on postoperative loading conditions. Total hip patients should be instructed on adequate postoperative activities.

Maxillofacial fractures and craniocerebral injuries – stress propagation from face to neurocranium in a finite element analysis

BackgroundSevere facial trauma is often associated with intracerebral injuries. So it seemed to be of interest to study stress propagation from face to neurocranium after a fistlike impact on the facial skull in a finite element analysis.MethodsA finite element model of the human skull without mandible consisting of nearly 740,000 tetrahedrons was built. Fistlike impacts on the infraorbital rim, the nasoorbitoethmoid region, and the supraorbital arch were simulated and stress propagations were depicted in a time-dependent display.ResultsFinite element simulation revealed von Mises stresses beyond the yield criterion of facial bone at the site of impacts and propagation of stresses in considerable amount towards skull base in the scenario of the fistlike impact on the infraorbital rim and on the nasoorbitoethmoid region. When impact was given on the supraorbital arch stresses seemed to be absorbed.ConclusionsAs patients presenting with facial fractures have a risk for craniocerebral injuries attention should be paid to this and the indication for a CT-scan should be put widely. Efforts have to be made to generate more precise finite element models for a better comprehension of craniofacial and brain injury.AbstractHintergrundFrakturen des Gesichtsschädels gehen häufig mit intrakraniellen Verletzungen einher. Deshalb erschien es interessant, die Weiterleitung und Verteilung von Spannungen, wie sie bei einem Faustschlag auftreten, vom Gesichtsschädel zum Hirnschädel in einer Finite Elemente Analyse zu untersuchen.MethodenEin Finite Elemente Modell des menschlichen Schädels ohne Unterkiefer, welches aus zirka 740,000 tetraedrischen Volumeneinheiten bestand, wurde entwickelt. Die Einwirkung einer Kraft, die einem Faustschlag entsprach, auf den Infraorbitalrand, die Nasoorbitoethmoidregion und den supraorbitalen Bogen wurden simuliert. Die Weiterleitung der Spannungen wurde in einem zeitlichen Verlauf dargestellt.ErgebnisseDie Finite Elemente Simulation zeigte von Mises-Spannungen oberhalb des Wertes, ab dem Gesichtsschädelknochen frakturieren, im Bereich der Krafteinleitung und Fortleitung von Spannungen in Richtung auf die Schädelbasis in beachtlicher Höhe bei Auftreffen des Impaktors auf den Infraorbitalrand oder die Nasoorbitoethmoidregion. Bei Auftreffen der Kraft supraorbital scheinen die Spannungen absorbiert zu werden.SchlussfolgerungenDa Patienten mit Gesichtsschädelfrakturen ein Risiko für Schädel-Hirn-Traumata aufweisen, sollte eine entsprechende Überwachung erfolgen. Die Indikation für CT-Untersuchungen sollte großzügig gestellt werden. Genauere Finite Elemente-Modelle sind zum besseren Verständnis kraniofazialer Frakturen und Hirnverletzungen notwendig.