R. Leppek

Comparison of geometry-based and CT voxel-based finite element modelling and experimental validation.

The objectives of the paper presented here is the comparison of a geometry-based and voxel-based finite element (FE) method preprocessor of the human femur. The models were experimentally validated by strain gauge measurements (principal stress). The correlation coefficients (r) between the three methods (geometry-based FEM, voxel-based FEM, strain gauge measurements) were found to be in the range 0.91-0.94 (r2: 0.84-0.88). The relationships between the samples are highly significant (P = 0.001), where the strain gauge results are the independent variables. These results suggest that the validity with respect to the principal stress of a voxel-based modelling is similar to the validity of geometry-based modelling. In summary, therefore, we conclude that voxel-based meshing allows a straightforward interfacing with computerized tomography (CT) scans and might contribute to a clinically applicable FEM technology.

Validation data for periprosthetic bone remodelling theories

Periprosthetic adaptive bone remodelling after total hip arthroplasty (THA) has been frequently simulated in computer models, combining bone remodelling theory with finite element analysis. Unfortunately, there still subsist a lack of clinical data, which are necessary for validation of these simulation results. Therefore, the objective of the current project is to collect prospective volumetric bone density data with a clinical computerized tomography study in seven patients after THA. A retrospective study 12 years after implantation in 11 patients was added. A data set of about 100000 bone voxels for each femur was collected. In all prospective cases, the predominant change is seen during the first year. The average density reduction in the horizontal slices was between 50 and 150 Hounsfield units (HU) (approx. 10%; p<0.001) after 2 years. Loss of density is particularly strong distal of the minor trochanter and decreases from proximal to distal. For the 12 years retrospective study, the contralateral femur provided the control. Similar trends comparable to the prospective 2-year follow-up CT density values were seen in most cases with density reductions of up to 400 HU (30%). However, in one of these cases there was no difference between the operated and the control density. As far as we are aware, this is the first collection of fully prospective 3D validation data in vivo for periprosthetic adaptive bone remodelling theories. The data are also unique as they are suitable for direct patient-specific 3D finite element meshing and individual weight-related loading.

Die Anwendung voxelorientierter Femurmodelle zur Spannungsanalyse Generierung, Berechnung und Validierung CT-basierter FEM-Modelle - The Use of Voxel-Oriented Femur Models in Stress Analysis Preprocessing, calculation and validation of CT-based finite element models

A means of interfacing computed tomography with the finite element method for the analysis of stress distribution in human femurs is described. With the aid of data transfer and image processing programs, three-dimensional voxel models of four macerated and a fresh-frozen cadaveric femur were generated from CT data sets. The underlying basic principle is direct generation of model elements from one or more adjacent CT-voxels, which are characterised by their coordinates and local absorption coefficients. A new algorithm was used for smoothing the edges of the model. All the models were analysed with respect to their stress distribution under different load situations, and the results were compared with the data of a parallel study utilizing the strain-gauge technique. Using the method of linear regression, a high correlation (coefficient of determination r2: 0.80-0.91) was found between the calculated and experimentally measured principal stresses at the surface of the bones.

Magnetresonanztomographische Diagnostik der peripheren Durchblutung

Ziel:Eignung und kasuistische Anwendung der dynamischen kontrastverstärkten Magnetresonanztomographie (DCEMRT) zur Erfassung der peripheren Durchblutung im Unterschenkel.Patienten und Methodik:Bei einer Patientin mit peripherer arterieller Verschlusskrankheit (pAVK), einem Patienten mit koronarer Herzkrankheit (KHK) ohne klinische Zeichen einer pAVK, einer Normalperson mit ausreichender körperlicher Aktivität gemäß Freiburger Aktivitätsscore und zwei Leistungssportlern wird der krurale Muskelstatus mit einer isometrischen Maximalkraftmessung bestimmt. Nach Kalibrierung des speziell konstruierten Plantarflexionsergometers MRPEDALO® zur Durchführung auxotoner Muskelarbeit durch 1-minütige alternierende Fußextension und -flexion im MRTGerät wird die DCE-MRT vor und nach Belastung durchgeführt (T1w 2D-FLASH-GE-Sequenz mit TR/TE/α: 100 ms/6 ms/70°; Bildfeld: 400; Matrix: 81 × 256; Schichtdicke: 10 mm; Akquisitionen: 73 à 8,3 s; Messzeit: 9,24 min; Bolusapplikation von Magnevist®, Schering, 0,02 ml/kg KG, 20 ml NaCl Bolus, Flow 2 ml/s, kubitale 22G-Kanüle). Semiquantitative Kurvenauswertung mit DynaVision® (MeVis gGmbH).Ergebnisse:Die Messung der peripheren Durchblutung benötigt geeignete Belastungstests. Nach Muskelarbeit zwischen 52 Wattsekunden (Ws) und 244 Ws bzw. 0,65 W und 4,07 W, die in Relation zur trainingsbedingt stark unterschiedlichen isometrischen Maximalkraft für eine interindividuell annähernd vergleichbare Belastung sprechen, finden sich ausgeprägte Änderungen der Signalintensitäts-(SI-)Kurven für den Musculus peronaeus, in geringerem Ausmaß auch für den Musculus tibialis anterior, die mit der Time-to-Peak (TTP) und der Mean-Intensityto- Time-Ratio (MITR) sowie dem individuellen Muskelstatus korrelieren. Der Musculus gastrocnemius zeigt dagegen vergleichsweise geringe Kurvenänderungen. Die relative Verkürzung der TTP und Verlängerung der MITR nach Belastung, der eine verbesserte Mobilisierung der Durchblutungsreserve entspricht, ist bei Leistungssportlern besonders deutlich ausgeprägt. Die DCEMRT spricht darüber hinaus für individuell unterschiedliche Muskelkoordinationsstrategien trotz in der Größenordnung vergleichbarer Flexions- und Extensionsbelastung.Schlussfolgerung:Die nichtinvasive semiquantitative Messung der belastungsabhängigen Muskeldurchblutung der Unterschenkel ist mit der DCE-MRT möglich, bedarf aber für inter- und intraindividuelle Vergleiche einer weiteren Standardisierung. Die Methode besitzt diagnostisches Potential für das Therapiemonitoring sowie die Sport- und Rehabilitationsmedizin durch Visualisierung und Quantifizierung der peripheren Mikrozirkulation und additiver Information über die muskuläre Koordination.Purpose:This article describes the potential of dynamic contrast- enhanced magnetic resonance tomography (DCE-MRT) for the visualization and quantification of blood flow of lower leg muscles at rest and after individually adjusted muscular exercise.Patients and Methods:Five cases were chosen to exemplify the qualitative and semi-quantitative blood flow evaluation in the lower leg muscles. The crural muscle state was determined with an isometric maximal strength measurement from a female patient with peripheral arterial occlusive disease (pAVK), a male patient with coronary heart disease (KHK) without clinical signs of a pAVK, a volunteer with sufficient physical activity in accordance with the Freiburg Questionnaire of Physical Activity and two professional athletes. After calibration of the plantarflexion ergometer MR-PEDALO® (Figures 2a and 2b) for the execution of auxotonic muscle work a 1- minute alternating foot extension and flexion exercise on MRPEDALO® was performed in the MR machine. Instead of the lower leg splint shown in Figures 2a and 2b the MR coil fits exactly in MR-PEDALO® used for DCE-MRT. Mechanical work performed during the 1-minute exercise ranged from 52 watt seconds (Ws) to 244 Ws (0.65 W to 4.07 W), indicating similar interindividual work loads in relation to the individual maximum isometric strength. DCE-MRT was performed at rest and immediately after auxotonic exercise test (T1w 2DFLASH- GE sequence with TR/TE/α: 100 ms/6 ms/70°; field of view: 400; matrix: 81 × 256; slice thickness: 10 mm; acquisitions: 73 at 8.3 s each; total examination time: 9.24 min; bolus application of Magnevist®, Schering, 0.02 ml/kg kg, 20 ml bolus NaCl, flow 2 ml/s, 22G cannula in a cubital vein). Signal intensity (SI) curves were analyzed with DynaVision® (MeVis gGmbH, Bremen, Germany).Results:Measuring peripheral blood flow needs appropriate muscular stress tests. The SI-curves of the region of interest (ROI) representing the peroneus, tibialis anterior and gastrocnemius muscle run almost parallel at rest. Workloads between 52 Ws and 244 Ws (0.65 W and 4.07 W), similar in relation to the individual maximum isometric strength, induce distinctive changes of the upslope, wash-in, peak and washout of SI-curves preferably for the peroneus muscle and less predominant also for the tibialis anterior muscle and gastrocnemius muscle respectively. The first case, a 55-year-old female patient with peripheral arterial occlusive disease (pAVK) stage Fontaine IIb before (Figure 3a) and after (Figure 3b) percutaneous transluminal angioplasty (PTA) of a right femoral artery stenosis shows after interventional treatment a rapid post-exercise SI-increase in the peroneus muscle. The steeper SI-curve indicates a better contrast medium inflow due to an improved perfusion. The second case, a 65-year-old man suffering from coronary heart disease without clinical signs of pAVK (Figure 4) exercised with a workload of 92 Ws. After stress test the ROI for the peroneus muscle shows a clear intensity increase. After exercise the SI-curve for the tibialis anterior muscle shows a similar, but less predominant change while the shape of the SI-curve of the gastrocnemius muscle remains mainly identical. A 23-year-old male person with average physical activity (Figure 5) performed DCE-MRT of the left lower leg after stress test with 172 Ws demonstrating a rapid signal increase in the peroneus muscle while the synergistic tibialis anterior muscle and antagonistic gastrocnemius muscle show a comparatively slow contrast-medium wash-in. A 26-year-old male athlete (Figure 6) exercised with 196 Ws showing a rapid contrast medium inflow in the peroneus muscle and initially also in the synergistic tibialis anterior muscle. A contrast-medium wash-out appears in both muscles, while the shape of the gastrocnemius muscle SI-curve remains substantially unchanged. A 26-year-old female athlete (Figure 7) exercised with 244 Ws. Post exercise SI-curves show a distinctive and rapid increase of contrast medium wash-in with a sharp peak particularly in the peroneus muscle and similarly in the tibialis anterior and gastrocnemius muscle. After exercise all SI-curves show a wash-out phase.Conclusion:SI-curves show relative increase in correlation with Time-to-Peak (TTP) decrease and Mean-Intensity to Time Ratio (MITR) increase indicating blood flow reserve mobilization after exercise. Individual muscle state seems to be linked to muscle recruitment and muscle coordination reflected by post-exercise SI-curves. The gastrocnemius muscle shows comparatively low SI-curve changes after muscular load test. Further methodological standardization and optimization of the stress test is mandatory to assure intra- and interindividual comparisons. Due to direct visualization and quantitative evaluation of the peripheral microcirculation DCE-MRT has a diagnostic potential for monitoring therapeutic response in peripheral circulation disorders and sports medicine.

Suggestion for a Modular Topographic- Morphologic Classification of Proximal Humeral Fractures

AbstractBackground: The classification of proximal humeral fractures is a quite difficult problem. Both the Neer and the AO classification meet the requirements of a reliable classification system only in a limited form. In order to devise a more systematic, precise and reproducible classification system, the MTM classification = modular topographic and morphologic classification has been developed. It is based on an alphanumeric code. Standardized and good-quality anteroposterior and axillary radiographs are one of the main prerequisites for correct topographic and morphologic fracture assessment. In addition, fracture stability testing under fluoroscopic C-arm visualization and the use of CT scan may be necessary. Topographic Classification: The fractures are classified according to their determining topography into type A fractures = extraarticular, type B fractures = incomplete articular, and type C fractures = complete articular. The risk of avascular head necrosis increases from A to B and from B to C. The A, B, and C fractures are further classified topographically according to the fractures in the extraarticular segment, which are coded as G fractures = fractures of the greater tuberosity, L fractures = fractures of the lesser tuberosity, and M fractures = metaphyseal fractures. The fracture-dislocations are integrated in the MTM classification system and coded with the letter D. Morphologic Classification: The morphologic part of the classification is based on a catalog with four specifications, relevant for therapy and prognosis, arranged in the order of increasing severity: S1 = minimally displaced and stable; S2 = minimally displaced and unstable; S3 = displaced; S4 = displaced and comminuted. Defined criteria exist for the specifications and their practical application. In case of fractures with several main fragments, each single fracture is specified individually.

Thromboembolische Komplikationen bei mehrfachverletzten Patienten: ein unterschätztes Problem? Ergebnisse einer klinischen Beobachtungsstudie mit 50 Patienten

Zusammenfassung.Einleitung: Die Incidenz von thromboembolischen Komplikationen bei polytraumatisierten Patienten ist nur unzureichend bekannt. Die wenigen bisher vorliegenden Studien geben je nach Untersuchungsmethode Thromboseinincidenzen zwischen 1,4 und 63 % an. Eine systematische Screeninguntersuchung mit objektiven diagnostischen Maßnahmen hat sich als Routinemethode bisher nicht durchgesetzt. Methode: Von Januar 1996 bis Januar 1998 wurden 50 polytraumatisierte Patienten mit einem ISS-Score > 15 Punkten in eine klinische Beobachtungsstudie aufgenommen. Weitere Einschlusskriterien waren ein Mindestaufenthalt auf der Intensivstation von 72 Std. und eine minimale Beatmungsdauer von 72 Std. Bei allen Patienten wurde routinemäßig eine farbkodierte Duplexsonographie (FKDS) an beiden unteren Extremitäten vor Mobilisation bzw. vor Verlegung auf die periphere Station durchgeführt. Bei klinischem und/oder sonographischem Verdacht auf eine tiefe Beinvenenthrombose/Lungenembolie erfolgte eine Phlebographie/Pulmonalisangiographie. Ergebnisse: Das Durchschnittsalter der Patienten betrug 38 Jahre (Range 17–77 Jahre), die Schwere der Verletzung war mit einem durchschnittlichen ISS-Score von 40 Punkten (Range 16–70 Punkte) charakterisiert. Acht Patient verstarben während ihres Aufenthalts auf der Intensivstation im Rahmen einer Sepsis bzw. eines Multiorganversagens. Acht Patienten (19 %) wiesen thromboembolische Komplikationen auf. Schlussfolgerung: Die Thromboseincidenz bei polytraumatisierten Patienten ist als hoch einzuschätzen. Klinische Untersuchungsmethoden zum Erfassen von thromboembolischen Komplikationen stellen ein unsicheres Verfahren dar. Hier sollte die farbkodierte Duplexsonographie als routinemäßige objektive, nicht invasive Untersuchungsmethode zur Anwendung kommen und sich bei sogenannten Hoch-Risiko-Patienten hinsichtlich tiefer Beinvenenthrombosen etablieren.

Bone remodelling in humeral arthroplasty: follow-up using CT imaging and finite element modeling – an in vivo case study

Abstract Little material is available in the literature about remodelling of the human humerus after implantation of a shoulder hemiarthroplasty. A 73-year-old patient was examined by CT 4 years after implantation of a right shoulder hemiarthroplasty, and the bone density as represented by Hounsfield values was compared with the contralateral side. Additionally, a three-dimensional finite-element model was generated from the image data and analysed. Bone density was reduced around the prosthesis when compared with the contralateral side. The stresses were transmitted through the prosthesis, while low bone stresses were found surrounding the prosthesis. Distally from the prosthesis, high stresses were found. On the control side, a more homogeneous stress distribution was noted. The results could be explained by bone resorption around the prosthesis caused by stress shielding; this hypothesis has to be confirmed by future studies.

Die vollautomatische Erstellung implantatbestückter Voxelmodelle des Femurs zur FE-Analyse - Vergleich von direkter und nachträglicher Prothesenimplantation - Automatic Generation of Voxel Models of the Implant-bearing Femur for FE Analysis - Comparison of direct and subsequent prosthesis implantation

Two means of generating implant-bearing FE voxel models of the human femur are described and compared. The first method consists in the fully automatic production of a femur model from CT data and subsequent implantation of a prosthesis stem on the basis of a redefinition of existing voxel elements using geometrical data control. The second method processes the CT data of a femur already bearing a prosthesis. Here, the prosthesis elements are generated separately from the surrounding bony elements on the basis of the differing CT density thresholds. Although the two implantation algorithms produce comparable results, they have different fields of application. The method of numerical stress analysis involving automatic generation of voxel models can be applied to implant-bearing femurs. Clinical follow-up observations of patients with prostheses made of titanium can be supported by FE calculations.

Zahnimplantattestung am In-vivo-Finite-Elemente-Modell - Testing of Dental Implants Using an in vivo Finite Element Model

The finite element method (FEM) makes it possible to simulate biomechanical situations on a computer. In the present study the so-called voxel method [9, 14, 17, 18, 19] was used for the construction of the mandible model. For this, the relationship between the biological tissue (e.g. bone) and the corresponding attenuation coefficient of CT data (Hounsfield units = HU) were utilized. The CT data were obtained from an edentulous patient provided with a prosthesis borne on two titanium implants. In a parallel study, the bite forces of the same individual were measured. These were recorded digitally in three dimensions (cranio-caudal, anterior-posterior and left-right) The forces determined by a special program were then transferred to the FEM model implants. We were able to show that a bar joining the implants had a far greater effect on maximum equivalent stress than was expected from the measuring data alone. The highest stress at maximum occlusive force was lowered by 704 % on using the connecting bar. On chewing, a stress reduction of 59.9 % was observed. The reduction in stress achieved by the bar could, we believe, prolong the life of the implant.

Selektionseffekte -Ein Problem unfallchirurgischer Studien Stichprobenselektionseffekte und das Problem der Repräsentativität am Beispiel einer prospektiven randomisierten Studie zur HWS-Beschleunigungsverletzung

ZusammenfassungEinführung. Ein zentrales, aber wenig berücksichtigtes Problem für die Verwertbarkeit klinischer Studien sind systematische Fehler durch Selektionseffekte. Fragestellung. Da sich nur in wenigen unfallchirurgischen Publikationen Angaben zur Selektion von Studienpatienten finden, wurde anhand einer eigenen prospektiven, randomisierten Therapievergleichstudie zur frühfunktionellen Therapie von HWS-Beschleunigungsverletzungen Vorkommen und Bedeutung von Selektionseffekten auf den verschiedenen Stufen der Stichprobenselektion überprüft. Material und Methoden. Das Ausgangskollektiv und die Kollektive der studientauglichen Patienten, der initialen und der finalen Studienteilnehmer wurden hinsichtlich Alter, Geschlecht und weiterer soziodemographischer sowie unfallspezifischer Merkmale und klinischer Befunde verglichen. Ergebnisse. Im Beobachtungszeitraum vom 21.08.1997 bis 30.04.1999 wurden 732 Patienten wegen einer HWS-Beschleunigungsverletzung behandelt. 453 erfüllten die Ein- und Ausschlusskriterien. 107 im Recall-Verfahren ermittelten Studienteilnehmern standen 346 Escape-Patienten gegenüber. Das Studienkollektiv (n=119) reduzierte sich durch Drop-out-Fälle auf 80 Studienteilnehmer. Es wurden auf 2 Selektionsstufen signifikante Selektionseffekte festgestellt. Diese hatten einen unmittelbaren Einfluss auf die Auswahl geeigneter statistischer Testverfahren. Schlussfolgerungen. Unkontrollierte Selektionseffekte können die interne und externe Validität klinischer Studien gefährden. Nur ihre Analyse ermöglicht die korrekte Interpretation der Studiendaten und die Beurteilung der Übertragbarkeit der Ergebnisse auf Kollektive übergeordneter Selektionsstufen.AbstractIntroduction. The internal and external validity of studies is endangered by many factors, such as selection of subjects for inclusion. Selection bias itself is a major problem, but remains unmentioned and probably unexamined in the majority of published clinical trials in traumatology. Aim of the study. The aim of this investigation was to detect effects of subject selection which occurred during our own prospective intervention study. The clinical trial compared subjects with whiplash injury who were either treated by early mobilization or immobilization (soft collar). Material and Methods. Source population, eligible subjects, study participants and final study participants were compared for differences on various items like age, gender and further sociodemographic as well as crash related factors and clinical findings. Results. Between 21.08.1997 and 30.04.1999 a total of 732 patients was examined and treated after whiplash in our trauma department. The options for inclusion were met by 453 patients. While 346 escaped from the study, 107 agreed to participate. Of these another 39 patients dropped out of the study. Selection effects were detected on two different levels, leading to distinct statistical procedures from those proposed in the study protocol. Conclusions. Uncontrolled selection effects could undermine the interpretability of the results of clinical trials. Awareness of selection effects is mandatory regarding the applicability of these results to subjects, other than those in the group of the final study participants.