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At least 451 records · Page 25Linked to original sources
PubMed2002

Semiautomated registration using new markers for assessing the accuracy of a navigation system.

OBJECTIVES: New markers are described that can be used for an improved registration procedure and for the exact comparison of navigation systems. The advantages of the markers are demonstrated, together with an automated segmentation algorithm for locating the centroid of the markers in image space. Compared to manual registration, this method shows an improved registration accuracy. MATERIALS AND METHODS: The new markers are detected completely automatically within all scan images. This allows a semiautomatic registration, as a preregistration is performed via the algorithm. Furthermore, the exact coordinates within one scan slice are now determined with the calculation procedure. The calculated data from the preregistration were matched up with a manual preregistration and some reference data, so as to confirm the quality of this new algorithm. The accuracies of several manual and semiautomatic registrations were also compared. RESULTS: The average deviation between the coordinates of the algorithm and the reference data (coordinate measuring machine) was 0.3 mm. The standard deviation amounted to 0.131 mm. Comparing several manual registrations with the reference data showed that the middle fiducial registration error (FRE) was between 0.7 and 2 mm. In comparison, the FRE remained constant at around 0.7 mm for the semiautomatic registration procedure. CONCLUSIONS: The measured results show a significant improvement in the preregistration data using the new markers together with the algorithm. This improvement leads to a reproducible and more accurate registration. The combination of the new marker type with the automated segmentation algorithm minimizes the human error factor, and provides the opportunity to directly compare image-guided and robotic systems.

Algorithms↗
PubMed1976

The effects of pylon shape on bone-pylon interface performance in direct skeletal attachment.

The attachment of a prosthesis directly to the part of the skeleton remaining after an amputation offers many improvements over coupling schemes used in conventional prostheses. However, the stresses induced in the bone by the macrostructure of the attached prosthetic device must first be understood because they are a significant consideration in the design of a successful direct skeletal attachment (DSA) system. This investigation utilizes the finite element method of analysis to structurally model some possible DSA systems. Static stress-response models for an above-the-knee human femoral amputation are formulated to investigate the effect of variations in pylon macrostructure on system performance. The models approximate the initial stages of support following the insertion of a pylon into the medullary cavity of the bone and are also used to assess the bond strengths that must be developed between the bone and pylon for a "no-slip" condition to hold at their interface. The analyses indicate that either a shaped, marrow cavity-fit pylon or four 135 degree wedges with a complementary pylon are favorable geometries for a DSA system. The stress levels caused by these two geometries are on the order of 20% of the axial compressive strength of cortical bone. For each, local stress levels at the bone-biomaterial interface remain the critical parameters for investigation.

Artificial Limbs↗
PubMed2005

The influence of various occlusal materials on stresses transferred to implant-supported prostheses and supporting bone: a three-dimensional finite-element study.

The aim of this numerical analysis was to evaluate the amount and localization of stress that occurs with various materials used in implant-crown design under functional forces. Computer-aided design techniques and a finite-element stress analysis method were used for evaluation. A 4.1 x 10-mm implant placed in the mandibular second premolar area was simulated and analyzed. Simulation and analysis were performed with the use of COSMOS/M software and Pro/Engineer 2000i on a Dual Pentium III 1-GHz computer. Crown designs were as follows: porcelain fused to noble metal crown, porcelain fused to base metal crown, In-Ceram porcelain crown, and IPS Empress 2 porcelain crown. A 300-N vertical force was applied to the centric relation stop points of the crowns. The results of this study indicated that different types of restorative materials play an important role in the amount and distribution of the stresses in the superstructure and the implant. The highest stress values were observed in the IPS Empress 2 porcelain crown design (600 MPa). Porcelain fused to base metal and In-Ceram framework designs transferred less stress to abutment. Type of restorative materials used in implant crown designs are significant factors in the amount and distribution of the stresses on superstructure and implant under functional forces. Porcelain fused to base metal (149 MPa) and In-Ceram (173 MPa) crown designs induced higher von Mises stress values within the framework than porcelain fused to noble metal (108 MPa) and IPS Empress 2 (119 MPa) porcelain crown designs.

Biocompatible Materials↗
PubMed1996

A digital model of trabecular bone.

A 3D microCT dataset of bovine bone was used to create a digital 3D model simulating trabecular bone. The model serves a dual purpose: It allows for standard quantitative histomorphometric analysis and it approximates the reality e.g. of high resolution CT in vivo datasets of trabecular bone. Thus the model can potentially be used as a reference to develop 2D and 3D structural analysis algorithms applicable in vivo while it simultaneously allows verification of the results of these algorithms by standard histomorphometry. The model can be used as a standard to evaluate the impact of image processing techniques and of restrictions of imaging systems on the quantitative analysis of structural parameters describing a trabecular network. The model can be used for a comparison of 2D and 3D structural analysis methods and for an analysis of decreasing spatial resolution. The effects of segmentation and filtration can be studied separately and grayscale analysis is possible. As examples standard 2D histomorphometry and the analysis of topological parameters like node number and trabecular network length were applied to the model. The influence of spatial resolution was investigated by decreasing the spatial resolution of the digital model. The bone surface area determined by 3D surface triangulation was only 4% smaller than the surface area determined from the traditional 2D bone histomorphometric parameter bone surface/tissue volume (BS/TV) when 2D results were averaged over all slices of the 3D volume. However, BS/TV showed large (10%) variations among slices within the volume. Both histomorphometric and topological parameters were heavily influenced by spatial resolution and image segmentation. Our initial experience with the digital model indicates a need to investigate bone microstructure based on volume data or to average the 2D results of many slices.

Algorithms↗
PubMed2006

Oviduct contraction in Drosophila is modulated by a neural network that is both, octopaminergic and glutamatergic.

Fertility is a highly complex and regulated phenomenon essential for the survival of any species. To identify Drosophila fertility-specific neural networks, we used a GAL4/UAS enhancer trap genetic screen that selectively inactivates groups of neurons. We identified a GAL4 line (bwktqs) that has a female sterile phenotype only when it expresses the tetanus toxin light chain (TeTxLC). These flies lack oviduct contraction, lay almost no eggs, sperm accumulate in the oviducts, and fewer than normal are seen in the storage organs. In insects, two neuroactive substances are important for oviduct contraction: octopamine (OA), a monoamine that inhibits oviduct contraction, and glutamate (Glu), a neurotransmitter that induces contraction. It is known that octopaminergic neurons of the thoracic abdominal ganglion (TAG) modulate oviduct contraction, however, the glutamatergic neurons that innervate the oviduct have not been identified yet and the interaction between these two neuroactive substances is not well understood. Immunostaining experiments revealed that the bwktqs line trapped an octopaminergic neural network that innervates the genital tract. We show that wt like oviduct contraction in TeTxLC-inactivated flies can only be rescued by simultaneous application of Glu and OA suggesting that the abdominal bwktqs neurons are both octopaminergic and glutamatergic, the use of an agonist and an antagonist for Glu receptors as well as their direct visualization confirmed its participation in this phenomenon. Our work provides the first evidence that adult abdominal type II visceral innervations co-express Glu and OA and allows us to re-evaluate the previous model of neuronal network controlling insect oviduct contraction.

Animals↗
PubMed2001

Theory of the growth and evolution of feather shape.

We present the first explicit theory of the growth of feather shape, defined as the outline of a pennaceous feather vane. Based on a reanalysis of data from the literature, we propose that the absolute growth rate of the barbs and rachis ridges, not the vertical growth rate, is uniform throughout the follicle. The growth of feathers is simulated with a mathematical model based on six growth parameters: (1) absolute barb and rachis ridge growth rate, (2) angle of helical growth of barb ridges, (3) initial barb ridge number, (4) new barb ridge addition rate, (5) barb ridge diameter, and (6) the angle of barb ramus expansion following emergence from the sheath. The model simulates growth by cell division in the follicle collar and, except for the sixth parameter, does not account for growth by differentiation in cell size and shape during later keratinization. The model can simulate a diversity of feather shapes that correspond closely in shape to real feathers, including various contour feathers, asymmetrical feathers, and even emarginate primaries. Simulations of feather growth under different parameter values demonstrate that each parameter can have substantial, independent effects on feather shape. Many parameters also have complex and redundant effects on feather shape through their influence on the diameter of the follicle, the barb ridge fusion rate, and the internodal distance. Simulated isochrones-the loci, or sets, of feather cells of the same age-have the same oblique chevron-shaped position in the mature feather as fault bars, which are isochronic defects in the barbules created by a disruptions during development. Accurate simulation of fault bar shape and position confirms the uniform absolute growth rate hypothesis and the general realism of the model. The theory defines a six-parameter feather morphospace, and provides many predictions about the developmental determination of feather shape that can be tested with detailed observations and experiments on developing feathers. This theory also provides testable predictions about the changes in developmental mechanisms required to evolve different feather shapes to accomplish various functions.

Animals↗
PubMed1979

Comprehensive schemata on the histology of the liver with consequences in terminology.

New schemata of the liver are presented to discuss the combination of the three kinds of liver lobules known until today in a chalk-talk-manner. Terminology is also discussed. Further investigations are needed involving the construction and the vascular pattern of compound lobules of the three individial lobules of the liver in different species.

Liver↗
PubMed2001

The development of the skull in Acrochordus granulatus (Schneider) (Reptilia: Serpentes), with special consideration of the otico-occipital complex.

The skull of Acrochordus has been characterized by the absence of a crista circumfenestralis (a synapomorphy shared by all snakes), and by the absence of a recessus scalae tympani that in other squamates forms by subdivision of the embryonic metotic fissure. These traits have variably been identified as either plesiomorphic or paedomorphic. The study of the development of the osteocranium in a series of cleared and stained embryos of Acrochordus shows a close correspondence to the development of the skull in other snakes. The exception is the formation of the postorbital from two ossification centers. The significance of this observation, which might suggest the embryonic fusion of a postfrontal with a postorbital, remains enigmatic, as it is based on one side only of the skull of a single specimen. By contrast, a rudimentary and modified crista circumfenestralis can be identified in the skull of Acrochordus. Furthermore, absence of a recessus scalae tympani is not due to an undivided fissura metotica, but results from the obliteration of the anterior part of the metotic fissure instead. With respect to this character, Acrochordus is neither plesiomorphic, nor paedomorphic, but autapomorphic. This interpretation of the skull of Acrochordus is compatible with the sister-group relationships this genus shares with colubroid snakes.

Animals↗
PubMedDate not supplied

Inversion recovery image reconstruction with multiseed region-growing spin reversal.

A new algorithm is introduced for inversion recovery (IR) image reconstruction. The original complex image is modeled as a product of three factors: magnitude, polarity, and a smoothly changing phase factor. The simple binary polarity factor is first unified by a region-growing spin reversal (RGSR) operation, allowing the phase factor to be extracted. Multiplying the complex conjugate of the phase factor with the original complex data yields the desired IR contrast. The RGSR process is repeated with multiple seeds distributed in the field of view (FOV), and the results are added together, enabling disconnected tissues in the FOV to be handled. The extracted phase factor is filtered to reduce noise and artifacts, without losing useful information. The method is fully automatic and has been used practically in a large number of clinical examinations. The algorithm may also be useful for phase correction in simple proton spectroscopic imaging.

Algorithms↗
PubMed1988

Model analysis of factors influencing the prediction of muscle forces at the knee.

A three-dimensional stochastic mathematical muscle model of the knee joint has been developed and applied to a study in which the influence of both mechanical and physiological factors were examined in relation to the prediction of muscular forces about the joint. The model includes a representation of the proximal portion of the tibia and distal portion of the femur along with a mathematical expression of the patellar mechanism and 13 muscles crossing the knee joint. The model accounts for the rolling and gliding movement of the tibial-femoral articulation. The computational technique involves equilibrating three components of external moments at the knee joint to the internal moments generated by muscular forces and soft tissue. The variables contained in the moment equilibrium equation are randomly chosen based on the choice of the tibial-femoral contact point. The randomness of the variables, reflected in the final solution, defines a stochastic process in the context of the present model. Studies with the model indicated that a very important mechanical aspect of the model was the capability to simulate the moving contact point between the tibia and femur. The moving contact point increased the mechanical advantage of the quadriceps muscles by 50%, which corresponded to in vivo EMG measurements. Muscle force predictions during normal gait have shown the capability of the model to determine the presence of synergistic and antagonistic muscle action.

Biomechanical Phenomena↗
PubMed1996

Hamstrings and psoas lengths during normal and crouch gait: implications for muscle-tendon surgery.

Crouch gait, one of the most common movement abnormalities among children with cerebral palsy, is characterized by persistent flexion of the knee during the stance phase. Short hamstrings are thought to be the cause of crouch gait; thus, crouch gait is often treated by surgical lengthening of the hamstrings. In this study, a graphics-based model of the lower extremity was used in conjunction with three-dimensional kinematic data obtained from gait analysis to estimate the lengths of the hamstrings and psoas muscles during normal and crouch gaits. Only three of 14 subjects with crouch gait (four of 20 limbs with knee flexion of 20 degrees or more throughout stance) had hamstrings that were shorter than normal by more than 1 SD during walking. Most (80%) of the subjects with crouch gait had hamstrings of normal length or longer, despite persistent knee flexion during stance. This occurred because the excessive knee flexion was typically accompanied by excessive hip flexion throughout the gait cycle. All of the subjects with crouch gait had a psoas that was shorter than normal by more than 1 SD during walking. These results emphasize the need to consider the geometry and kinematics of multiple joints before performing surgical procedures aimed at correcting crouch gait.

Adolescent↗
PubMed1996

Correlation of bony ingrowth to the distribution of stress and strain parameters surrounding a porous-coated implant.

The ability of shear strains to inhibit bony ingrowth was investigated by use of a transcortical porous-coated cylindrical plug implant in a functionally isolated turkey ulna model in which the mechanical loading environment could be accurately controlled and rigorously defined. The distribution of ingrowth at the bone-implant interface was quantified following 8 weeks of in vivo loading consisting of 100 seconds per day of a 20 Hz sinusoidal stimulus sufficient to cause a local peak strain of approximately 100 microstrain in the cortex at the bone-implant interface in four turkeys. A nonuniform but repeatable pattern of bony ingrowth, from 33 +/- 6 to 72 +/- 6% (mean +/- SE), was observed. The mechanical environment in the vicinity of the bone-implant interface was calculated using a three-dimensional elastic orthotropic finite element model. The general stress-strain state of the bone as predicted by the finite element model was validated in two additional turkeys using four three-element rosette strain gauges, while high resolution moiré interferometry was used to determine the mechanical state of the region immediately adjacent to the implant itself. Shear strains and stresses were evaluated at the interface and correlated to the pattern of bony ingrowth circumscribing the implant interface. Linear regressions between ingrowth and both shear strain and shear stress were negative, with the values of R = -0.75 and R = -0.78 (p < 0.001), respectively, indicating significant inhibition of ingrowth where shear components were maximal. These results suggest that the minimization of shear stress and strain components is a major determinant in achieving successful ingrowth of bone into a prosthesis.

Animals↗
PubMed2004

Simulation of the regional manifestation of asthma.

Asthma presents serious medical problems of global proportions. Clinical data suggest that the disease occurs preferentially at regions designated by large (0 </= I </= 5), central (6 </= I </= 11), and small (12 </= I </= 16) airways, where I defines branching generations within lungs. Our straightforward hypothesis, therefore, was that the efficacies of pharmacologic drugs proposed for the treatment and prophylaxis of asthma would be enhanced via their targeted delivery to appropriate sites. Hence, we have developed a mathematical model describing the behavior and fate of inhaled aerosols. Original algorithms have been derived to detail the physical manifestation of asthma as distinct components of smooth muscle constriction and inflammation. We have conducted a systematic analysis of the relative effects of morphology, ventilation, and particle size on aerosol deposition. Different intensities of asthma were simulated by reducing airway diameters by prescribed amounts. To show the real clinical applications of modeling, we have also simulated the performance of a popular nebulizer. Regarding therapeutic implications, it is clear that disease-induced changes in airway morphologies have pronounced effects on the administration of inhaled drugs. Likewise, ventilation affects both the total aerosol mass deposited and its relative spatial distribution among airways. By formulating these effects, the computer code allows drugs (e.g., bronchodilators for constriction, steroids for inflammation) to be selectively deposited. We suggest, therefore, that the code can be used in a complementary manner with clinical studies and can be integrated into aerosol therapy regimens.

Aerosols↗
PubMed1983

A model for research on cochlear hypoxia.

The vessels emerging from one side of the basilar artery are supplied exclusively by the homolateral vertebral artery. Since blood flow is laminar through the vertebrobasilar system, mixing between two sides does not normally occur. Based on this fact, an experimental model for research on cochlear hypoxia is proposed and described. The animal's own blood flow in the vertebral artery is completely replaced by a stream of poorly oxygenated blood injected retrogradely through the ipsilateral axillary artery. In this way, the territory supplied by the vessels emerging from this side of the basilar artery, including the ear, is rendered hypoxic. The changes in the cochlear action potentials induced by the reduced oxygen supply are recorded by a chronically implanted electrode and analyzed.

Animals↗
PubMed2006

Anatomy and physiology of the basal ganglia: implications for deep brain stimulation for Parkinson's disease.

Central to surgical management of movement disorders is an understanding of the anatomy and physiology of the basal ganglia. The basal ganglia have been a target for neuromodulation surgery since Russell Meyers' pioneering works in the late 1930s. With the development of deep brain stimulation as the gold standard of surgical intervention for movement disorders, there has been a concomitant evolution in the understanding of the role the basal ganglia plays in the genesis of normal and abnormal motor behaviors. The fundamental concept of the cortico-striato-pallido-thalamocortical loop will be explored in the context of deep brain stimulation. The current targets for deep brain stimulation for Parkinson's disease, the subthalamic nucleus, the globus pallidus internus, and the ventral intermediate nucleus, will be discussed in the framework of the current physiological and anatomical models of Parkinson's disease (PD). Finally, the current understandings of the mechanisms underpinning the beneficial effects of deep brain stimulation for PD will be discussed.

Basal Ganglia↗
PubMed1995

Advanced non-animal microsurgical exercises.

For the novice microsurgeon, suturing simple lines on a static model ill prepares them for the technically difficult live model that is to follow. A practice card was therefore designed to make the transition easier. The basic building block is rubber glove sutured to form a simple tube. From this point, progressive exercises are carried out. These are (1) simple suturing, (2) tube anastomosis, (3) end-to-end anastomosis, (4) end-to-side anastomosis, (5) side-to-side anastomosis, (6) inequality of diameters, and (7) free graft placement. Since the student is faced with problems that are challenging and progressively complex, a noticeable improvement is seen in their skills. These models introduce the student to planning and instrumentation that they would deal with in the clinical situation. Once the student has mastered these exercises, he/she is ready to advance to the rat model.

Animals↗
PubMed1990

Effects of velocity profile of to-and-fro pulsatile flow on magnetic resonance signal intensity.

The effects of to-and-fro pulsatile flow, i.e., an oscillatory fluid motion with no net flow, on signal intensity in gated spin-echo magnetic resonance imaging are considered both theoretically and experimentally. On the basis of hydrodynamic principles, to-and-fro pulsatile flow at large Womersley numbers consists of uniform inner flow and boundary-layer-type flow adjacent to a tube wall. Therefore, the velocity profile is "trapezoidal" rather than parabolic at all times during the pulsation period. Contrary to the absence of phase dispersion and loss of signal within the inner flow where no velocity gradient exists, large velocity differences cause phase dispersion and, hence, loss of signal within the boundary layer, whose thickness is inversely proportional to the Womersley number. An understanding of these features of to-and-fro pulsatile flow provides the theoretical basis of cerebrospinal fluid flow phenomena in magnetic resonance imaging, since this type of flow exists in cerebrospinal fluid pathways.

Cerebrospinal Fluid↗