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M Thomsen

Publications and source records attributed to M Thomsen.

At least 37 records · Page 2Linked to original sources
PubMed2002

QSAR's based on partial order ranking.

The development of Quantitative Structure Activity Relationships (QSAR's) often relies heavily on the application of statistical methods such as multi-linear regression (MLR) or principal component analysis/partial least square (PCA/PLS). Partial order ranking (POR), which from a mathematical point of view is based on elementary methods of Discrete Mathematics, appears as an attractive and operationally simple and more general alternative since the method does not require specific functional relationships between the single descriptors or the end-points. The POR method allows ranking of a series of compounds, based on selected descriptors characterizing their structural and/or electronic nature (model diagram). The ranking of the compounds based on their end-points (experimental ranking) can then be compared to the model diagram. If the model diagram resembles the experimental ranking of the end-points under investigation, other compounds, not being experimentally investigated, can be assigned a rank in the model and hereby obtain an identity based on the known compounds. The present study elucidates the applicability of POR as a simple tool for QSAR modeling. Based on illustrative examples the POR approach to QSAR modeling will be presented with special focus on the precision and the uncertainties of the method, which will be discussed in terms of the number of descriptors and compounds involved. The advantageous interplay between POR and PCA, the latter being applied in order to reduce a possible large number of descriptors into a limited number of latent descriptors will be discussed.

Endpoint Determination↗
PubMed2002

Evaluation of empirical versus non-empirical descriptors.

In this case study, based on the notorious PCBs, the performance of selected empirical (EM) versus non-empirical and quantum chemical (NEM-QC) descriptors in multivariate QSARs have been evaluated. The informational content of the EM descriptors has been evaluated with respect to the physical understandable NEM-QC descriptors. Models for estimating key parameters for risk assessment have been developed, based on two-dimensional EM and three-dimensional NEM-QC descriptors, respectively. In spite of the simplicity of the two-dimensional descriptors, no evidence of lower predictive ability of the EM descriptors compared to the NEM-QC descriptors was observed. Homogeneity of variance within and between experimental methods, species and PCB congeners has been analysed, disclosing the importance for the need for handling uncertainty aspects of the results obtained by different laboratories and methods. The latter appears crucial for the model developments when using data from more than one source.

Animals↗
PubMed2002

Spatial micromovements of uncemented femoral components after torsional loads.

A novel method is presented which permits to assess implanted femoral components with regards to location of fixation and initial stability under cyclic torsional loads. The measurement apparatus tracks the spatial movement at several sites of stem and bone, allowing quantitative analysis of the micromotions and twisting of stem and bone, and the location of torque (force) transfer. Four types of prostheses were compared, which revealed striking differences in torque transfer. Our results for synthetic femurs are consistent with in vivo data on the osseointegration and radiolucensies observed for the stem types of this study. The method can be used to quantitatively compare various stem designs and implantation techniques.

Arthroplasty, Replacement, Hip↗
PubMed2002

Simultaneous orthotopic transplantation of carotid and aorta in the rat by the sleeve technique.

Graft vascular disease (GVD) remains the major limitation to long-term survival after solid organ transplantation. Aortic or carotid allografts in rats have been shown to be useful models because similar changes to those observed in man develop within weeks. Both immunological and non-immunological factors influence the process of GVD and a method that could permit rapid multiple arterial allotransplantation in the rat would be of great value. We performed simultaneous orthotopic aortic and carotid allotransplantations in 25 rats. The vessels were anastomosed using a sleeve technique. No immunosuppression was given. The animals were killed at 15, 30, or 60 days and histological analyses of the grafts were performed. The overall survival rate was 80% and the incidence of technical failure was very low. The histopathological aspect revealed typical progressive GVD. In conclusion, we have developed a new model of simultaneous aortic and carotid transplantation in rats. This model, which incorporates a modification of the sleeve anastomosis, is rapid and yields an easy tool to investigate immunological and non-immunological processes driving GVD.

Anastomosis, Surgical↗
PubMed2001

[Injury pattern in shoulder dislocation in the elderly patient].

PROBLEM: Recurrent shoulder dislocation is reported to be rare in older patients. While injuries of the glenoid labrum and the anterior capsule following primary dislocation in younger patients seem to determine the rate of instability, the pattern of damages in the elderly has rarely been studied. METHODS: 91 patients with an age older than 40 years with the diagnosis of primary (group A, n = 50) or recurrent (group B, n = 41) anterior shoulder dislocation were included in a prospective study. By analysis of the findings in x-rays, MRI, CT-scans and in 36 patients of diagnostic arthroscopy the pattern of intra- and periarticular pathology was evaluated. RESULTS: While the incidence of bony Bankart- and Hill-Sachs lesions was constantly spread over the age groups the pattern of soft tissue damages was different. Up to the age of 60 years lesions of the glenoid labrum were leading while in older patients the prevalence of rotator cuff tears became about 70 percent. CONCLUSIONS: The development of secondary osteoarthritis and persisting symptoms in older patients with rotator cuff tears after traumatic shoulder dislocation requires early diagnosis and appropriate treatment.

Adult↗
PubMed2001

[Developments in hip hemi-arthroplasty and theory of the link-chain dimeric hip prosthesis].

Aim of the study was to analyze the developments of hemiarthroplasties from the femur prosthesis to the bicentric bipolar prosthesis of today. Using the theories published 1990 by Nägerl about "dimeric link chains", nineteen bipolar-head systems were analyzed and biomechanically explained. Clinical findings, such as the problems of luxation and varus positioning of the monocentric bipolars, as well as the valgus, "self-centric" positioning of the bicentric prosthesis are discussed and explained correctly by physics. The hemiarthoplasty with the bicentric bipolar system of today has realized the principles of the dimeric link chains and is superior concept. Eleven of the sixteen today available systems work like this concept which was reached by try and error. The function of a bicentric system is solved now and explained by physics.

Equipment Failure Analysis↗
PubMed2001

[The importance to generate robot-assisted milled cavities for total hip replacement. A comparative experimental study: manual versus robotic preparation].

We present an experimental study to determine the effectiveness of computer assisted robotic bone preparation with regard to primary rotational stability in comparison to hand broaching. 5 different cementless stems were standardized implanted in synthetic femora and measured in a specially designed apparatus (displacement in 6 degrees of freedom). In addition the contact areas of the stems and the bone were visualized. The S-ROM- and ABG-stems were more stable in hand broached femora. The Osteolock-, G2- and Vision-2000-stems were more stable in the robot group. The mode of fixation of each implant could be characterized in relation to its position and its stiffness (primary rotational stability). The contact areas differed in some prosthesis in the way of preparation. The findings highlight the current difficulties in creating a perfect match of robotically milled cavity and stem geometry to achieve enhanced stability.

Arthroplasty, Replacement, Hip↗
PubMed2001

[Kinematics and biomechanics of the elbow joint].

An attempt has been made to describe the interaction of the different joint morphologies and the ligaments of the elbow in terms of the current literature. The physics of joint kinematics and biomechanics will be discussed and used to resolve current incomplete interpretations. The elbow is a very stable functional unit due to its bony tracking and "self-stabilising" ability especially in the humeroradial joint. In reconstruction and in total joint replacement, the different radii of curvature of the various surfaces have to be respected to redevelop full joint function.

Biomechanical Phenomena↗
PubMed2001

[Artefacts and ferromagnetism dependent on different metal alloys in magnetic resonance imaging. An experimental study].

The authors evaluated the significance of different metal alloys used in orthopaedic surgery in producing artefacts during magnetic resonance imaging. Several MRI sequences were tested and magnetic effects evaluated. Twelve discs made of different metal alloys from three manufacturers were examined. These discs were placed in a plastic box with a defined position in ultrasound gel. Then a sensitive, standard T1 weighted gradient echo sequence (TE: 4.1 ms; TR 9.4) was carried out in a coronal plane (Matrix 128/256). A Phillips Easy Vision workstation was used for image analysis. The largest area of artefact formation, including the surface size of the disc, was calculated using a special software program. In order to minimise the measurement error all discs were measured 10 times and the average value was determined. Then eight different sequences were run and measured in the same way. In a second series, all discs were placed separately on metric paper and subjected to the magnetic field of the MRI in order to detect possible motion secondary to the magnetic field applied. The different titanium alloys showed average distortion areas of from 245 mm2 (Ti6Al4V) to 349 mm2 (Ti5Al2.5Fe). Cobalt chrome alloys yielded differences of between 600 mm2 and 651 mm2 and iron alloys of between 902 mm2 (316L or Fe18Cr10NiMo) and 950 mm2 (Fe22Cr10Ni4Mn2MoNb) on average for the standard T1 weighted gradient echo. The artefact areas were dependent on the different sequences performed. For steel, (Fe18Cr10NiMo) areas of from 411 mm2 (T1TSE) to 2027 mm2 (EPI/3D/SPIR) were measured. All sequences studied produced different artefact pictures. None of the materials tested showed changes in position secondary to ferromagnetism. The size of signal distortion by MRI depends on the alloy making up the implanted material and the sequences used. The smallest artefacts occurred with the turbo-spin-echo sequences (TSE). The alloys tested in our study seem to carry no risk for patients of ferromagnetically induced secondary loosening caused by MRI scanning.

Alloys↗
PubMed2001

Treatment planning in severe scoliosis: the role of MRI.

The use of magnetic resonance imaging (MRI) in the preoperative investigation of children with idiopathic scoliosis is controversial. Syringomyelia and other intraspinal lesions may be risk factors for neurological injury during surgical correction. Our purpose was to investigate whether pathology of the neuraxis is associated with scoliosis and to detect lesions which may threaten neurological sequelae during distraction and instrumented correction. We obtained T1- and T2-weighted images of 40 children (28 girls, 12 boys), mean age 12.7 years with severe idiopathic scoliosis (Cobb angle 50-70 degrees) obtained in coronal, sagittal and axial planes from the posterior cranial fossa to the sacrum, and these were assessed by two neuroradiologists and an orthopaedic surgeon prior to further treatment planning. Abnormalities of the neuraxis were found in 24 patients (60%); five (12%) had two or more lesions. No abnormalities of the neuraxis were found in 16 patients (40%). There were 15 patients (38%) with intraspinal abnormalities who deteriorated clinically and nine (22%) who showed no clinical changes. We transferred 16 patients (40%) from the orthopaedic to the neurosurgical department for further assessment. Our results suggest that one should investigate the neuraxis with MRI before contemplating orthopaedic surgical correction of severe idiopathic scoliosis, because the findings may lead to a change of procedure.

Adolescent↗
PubMed2001

Partial order ranking-based QSAr's: estimation of solubilities and octanol-water partitioning.

Partial order ranking appears as an attractive alternative to conventional Quantitative Structure Activity Relationships (QSAR) methods, the latter typically relying on the application of statistical methods. The method seems attractive as a priori knowledge of specific functional relationships is not required. In the present study, it is demonstrated that QSAR models based on a partial order ranking approach can be used satisfactorily to predict solubilities and octanol-water partitioning for a selection of organic compounds exhibiting different structural and electronic characteristics. The uncertainty is validated using well-established LSER descriptors. Two requirements to the model with regard to precision prevail, i.e., the model must be able to rank the single compounds in the basis set correctly compared to the experimental data, and the model should be based on a basis set of compounds large enough to secure a satisfactorily fine-meshed net, taking the number of descriptors into account. In the present study, the model was able to rank 318 out of 319 comparisons correctly in the case of solubilities. The corresponding figures for the octanol-water partitioning were 407 out of 408. The precision and the uncertainties of the method which, were found closely related to the mutual interplay between the number of compounds and the number of descriptors is discussed in terms of the number of descriptors and compounds involved. The limitations of the method are discussed.

Forecasting↗
PubMed2001

Humic complexes of diethyl phthalate: molecular modelling of the sorption process.

The sorption mechanisms and complex formation between humic acid (HA) and a successively increasing number of diethyl phthalate (DEP) molecules have been studied theoretically using molecular mechanics, the number (n) of DEP molecules being varied from 1 to 30. The energy components of the single HA x nDEP complexes have been used as explanatory variables in a principal component analysis for exploring the presence of similarities/dissimilarities in the energetic properties of the individual xenobiotic complexes. The sorption can be explained in terms of a two-step mechanism. Absorption takes place as long as the host humic acid structure offers (a) enough internal docking space and (b) favorable interactions (energy release) with the guest molecule. This takes place for up to 7 DEP molecules. Further increase in the number to 30 DEP molecules will, due to the lack of free available internal voids, lead to surface controlled adsorption. The two-step sorption process apparently results in (a) a linear increase in energy gain by DEP bonds, and similarly (b) a constant incremental rise in molecular properties of the complexes such as volume and surface area. Three outstanding observations emerge: (1) Structural features at the atomic level (nanochemistry), such as partial atomic charges and high aromaticity of the humic acid, are observed to be dominating the intermolecular interactions in the complexes at the specific sorption sites. (2) Torsional relief and favorable changes in bonding energy also prevail for the growing complex. The latter indicates both the structural flexibility of the HA host and the stabilizing effect of DEP on the complex, by filling of the voids within the HA molecule. (3) The intermolecular forces are described mainly by hydrogen bonds (electrostatic energy) and interactions between dipole-dipole, such as carboxylic functions and uncharged moieties such as aromatic rings (van der Waals energy).

Absorption↗
PubMed2001

[Measurement of CSF flow in the spinal canal using MRI with an optimized MRI protocol: experimental and clinical studies].

PURPOSE: Measurement of the oscillating CSF flow in the spinal canal (SC) of healthy volunteers and in patients with post-traumatic syringomyelia (PTS) using an optimized MRI protocol as well as to determine whether stenosis induced velocity changes are detectable using MRI. METHODS: In 68 healthy volunteers quantitative studies of CSF flow in the cervical, thoracic, and lumbar regions were performed. First, an optimized sequence was developed and tested in 19 volunteers using four different flow-encoding velocities (4, 8, 12, 16 cm/s). Secondly, the optimized sequence was employed in 49 volunteers to measure the different CSF patterns in the cervical, thoracic, and lumbar spinal canals (CSC, TSC, LSC). Part three of the study, in which patients with PTS are being examined is still underway. We measured the maximum velocity (cm/s), the pixel area (mm2), and the stroke volume (ml/s). Using a flow model the velocities prior to and after compression with 5 different power levels were measured at the stenosis and at a distance of 70 cm. RESULTS: A total of 226 dynamic measurements have been performed--so far 76 in the first part (62 = 81.5% evaluable) and 150 in the second part--using the optimized sequence and optimal flow velocities. A flow-encoding sequence of 12 cm/s was found best in the CSC and one of 6 cm/s in the TSC and LSC. The maximum velocity in the CSC was 0.95 cm/s with the flow being directed caudal and 0.38 cm/s with the flow being directed cranial. In the TSC the values were 4.7 cm/s and 1.65 cm/s and in the LSC 0.96 cm/s and 0.59 cm/s. The highest velocities were found at the TSC, which has the smallest diameter compared to the CSC and LSC. In the 4 patients with PTS, the maximum velocities were between 0.09 cm/s and 0.97 cm/s with the flow being directed cranial and between 0.04 cm/s and 1.03 cm/s with the flow being directed caudal. The stroke volumina in the CSC were between 0.1 and 1.23 ml/s (mean: 0.48 ml/s) and 0.2 and 2.45 ml/s (mean: 0.66 ml/s) in the TSC and in the LSC 0.08 ml/s and 0.67 ml/s (mean: 0.29 ml/s). The results of the flow model studies showed an increase of velocity between 2.06 and 4.94 cm/s (mean: 3.31 cm/s) at the stenosis and 1.1 and 1.33 cm/s (mean: 1.23 cm/s) at a distance of 70 cm. CONCLUSION: Quantitative measurement of the oscillating CSF flow in the entire spinal canal (SC) is possible using an optimized MRI protocol as well as to detect stenosis induced velocity changes. Due to the high interindividual variability in the data of spinal CSF dynamics, further studies are necessary to collect normal data. The detection of movement of CSF in a post-traumatic spinal cord lesion may alter the therapeutic management.

Adult↗
PubMed2001

The use of Hasse diagrams as a potential approach for inverse QSAR.

Quantitative structure-activity relationships are often based on standard multidimensional statistical analyses and sophisticated local and global molecular descriptors. Here, the aim is to develop a tool helpful to define a molecule or a class of molecules which fulfills pre-described properties, i.e., an Inverse QSAR approach. If highly sophisticated descriptors are used in QSAR, the structure and then the synthesis recipe may be hard to derive. Thus, descriptors, from which the synthesis recipe can be easily derived, seem appropriate to be included within this study. However, if descriptors simple enough to be useful for defining syntheses recipes of chemicals were used, the accuracy of a numeric expression may fail. This paper suggests a method, based on very simple elements of the theory of partially ordered sets, to find a qualitative basis for the relationship between such fairly simple descriptors on the one side and a series of ecotoxicological properties, on the other side. The partial order ranking method assumes neither linearity nor certain statistical distribution properties. Therefore the method may be more general compared to many standard statistical techniques. A series of chlorinated aliphatic compounds has been used as an illustrative example and a comparison with more sophisticated descriptors derived from quantum chemistry and graph theory is given. Among the results, it was disclosed that only for algae lethal concentration, as one of the four ecotoxicological properties, the synthesis specific predictors seem to be good estimators. For all other ecotoxicological properties quantum chemical descriptors appear as the more suitable estimators.

Ecosystem↗
PubMed2001

Solubilities and surface activities of phthalates investigated by surface tension measurements.

Aqueous solutions of DEP (di-ethyl), DnBP (di-n-butyl), DnH(6)P (di-n-hexyl), and DEHP (di-[2-ethyl-hexyl]) phthalates have been investigated by use of surface tension measurements at temperatures between 10 and 35 degrees C. A tensiometric approach allows for the determination of unimeric solubilities and delta G degree, which is the standard Gibbs free energy change, for the dissolution of phthalates in water. The unimeric solubility of the phthalates increase with decreasing temperature. The delta G degree shows a linear increase with increasing phthalate alkyl chain length. The contribution of enthalpy (delta H degree) and entropy (delta S degree) to delta G degree were calculated from the temperature-dependent solubilities. The contributions of both delta H degree and delta S degree are negative and increase in magnitude with increasing alkyl chain length, suggesting hydrophobic interactions between phthalates and water. The ability of different phthalates to lower the surface tension decreases with increasing alkyl chain length, whereas the relative affinity for adsorption in the air-water interface increases drastically for long-chain phthalates. Despite the low surface activity of phthalates compared with that of common surfactants, they show significant affinity for adsorption in air-water interfaces of natural surface waters. This property, combined with their low solubilities, may affect the fate of these compounds within the natural environment, because they form emulsions above unimeric saturation in aqueous media.

Algorithms↗
PubMed2000

Chlamydia pneumoniae induces interleukin-10 production that down-regulates major histocompatibility complex class I expression.

Recently, it was demonstrated that CD8(+) T cells are important for the response against Chlamydia pneumoniae. By use of the human monocytic cell line U937 and human monocytes taken from peripheral blood, we investigated the effect of infection on various molecules critical for CD8(+) T cell function. A strong secretion of interleukin (IL)-10 by infected cells was observed, together with an inhibited expression of major histocompatibility complex (MHC) class I antigens, but without significant alteration of tumor growth factor-beta secretion or MHC class II expression. Recombinant IL-10 added to uninfected U937 cells decreased the expression of MHC class I, whereas blocking antibodies to IL-10 and its receptor abolished the C. pneumoniae-induced inhibition of MHC class I expression. Analysis of our data provides evidence that IL-10 secretion induced by C. pneumoniae infection of monocytic cells down-regulates the expression of MHC class I molecules and thereby might reduce the presentation of bacterial epitopes by MHC. This would decrease the ability of CD8(+) T cells to eliminate infected cells.

CD18 Antigens↗
PubMed2000

[Anchoring principles in hip endoprostheses. I: Prosthesis stem].

Total hip replacement is one of the most successful procedures in surgery. In Germany there is a great variety of cemented and uncemented implants with different principles of fixation. We reviewed the different principles, with particular reference to long-term clinical outcome, and discuss the most popular systems available in Germany. Despite many new developments which have been made in the field of cementless hip replacement, cemented total hip arthroplasty will remain the "gold standard" until prospective long-term data (> 15 years) of cementless systems become available.

Arthroplasty, Replacement, Hip↗