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At least 127 records · Page 7Linked to original sources
PubMed2001

Stress distribution around maxillary implants in anatomic photoelastic models of varying geometry. Part I.

STATEMENT OF PROBLEM: It is unclear which implant inclination and position are most favorable in relation to the supporting anatomy and loading direction in the maxilla. PURPOSE: This study was designed to examine stress distribution around implants in a 2-dimensional photoelastic anatomic model. MATERIAL AND METHODS: Two 2-dimensional photoelastic models were prepared with opposing 8-degree cylinder metal implant and molar teeth analogues. A frontal anatomic sectional plate model based on a CT section at the first molar was symmetrically loaded through its long axis. A midfacial rectangular model based on the same section was loaded in a different direction with varying supporting geometries. RESULTS: Stress distribution around the maxillary implant was highest in the buccal concavity at the apical buccal third and in the lingual concavity on intercuspal loading. No stress concentration occurred at the implant apex under the sinus for axial and nonaxial loading in both anatomic model geometries. On lateral loading, stress concentration was observed at the buccal concavity and at the implant neck. In the midfacial block model, principal stresses were concentrated at the maxillary implant neck on nonaxial loading and at the apex on axial loading. CONCLUSION: This 2-dimensional skull model showed different patterns of stress distribution among the maxillary implant, mandibular implant, and teeth. The highest principal stress concentration was seen at the buccal concavity of the maxillary implant; this may play a role in osseointegration with highly angled implants in the posterior maxilla. Differences in stress distribution between anatomic and nonanatomic models showed how the supporting geometry (for example, sinus/nasal anatomy), boundary conditions, and loading direction influence stress distribution.

Birefringence↗
PubMed2001

Stress distribution around maxillary implants in anatomic photoelastic models of varying geometry. Part II.

STATEMENT OF PROBLEM: Insufficient buccal bone volume can be a significant problem when loading dental implants in the maxilla. Increased potential for buccal fenestration and dehiscence can result in an exposed implant surface, mucosal irritation, decreased support, and potential implant failure. PURPOSE: The objective of this study was to model the stress distribution around maxillary implants by comparing simulated occlusal loading of maxillary implants in a 2-dimensional photoelastic anatomic model and a dry skull model. MATERIAL AND METHODS: Two model systems were used. First, a 2-dimensional photoelastic anatomic frontal skull sectional model was prepared in the first molar region. Left and right maxillary metal cylinder implant analogues inclined at 0 and 25 degrees to the sagittal plane were loaded in simulated intercuspation. Second, a dry skull lined with a photoelastic coating on the buccal aspect over an embedded cylinder implant was prepared in the first molar region. Principal stress concentration was photographed on axial and nonaxial implant loading. RESULTS: On simulated intercuspal loading, maximum stress concentration occurred at the buccal concavity in both the 2-dimensional anatomic photoelastic and skull models. There was no stress concentration at the apices of the maxillary implants in the 2-dimensional model. On lateral loading of the skull model, stress was distributed along the entire buccal aspect of bone adjacent to the implant, with a higher concentration at the buccal concavity. CONCLUSION: Preservation of buccal supporting bone volume is desirable to obtain a physiological modeling response and to enhance the facial plate. Insufficient bone volume may result in buccal fenestration or dehiscence, which can precipitate mucosal irritation, decreased support, and potential implant failure.

Birefringence↗
PubMed2000

Accuracy of a single equivalent moving dipole model in a realistic anatomic geometry torso model.

In this study, we investigated the accuracy of an algorithm to identify the spatial single equivalent moving dipole parameters in a realistic anatomic geometry torso model from potentials at the body surface. Specifically we investigated the effect of measurement noise, and dipole position and orientation in the accuracy of the algorithm. The boundary element method was used to calculate the forward potential distribution at 64 electrode positions on the body surface due to a point dipole. The mean and standard deviation of the distance of the true (obtained in the forward potential calculation) minus the estimated dipole location (obtained from the inverse algorithm) was estimated for each of the above three cases. Our results indicate that the dipole position has the most significant influence on the accuracy of our inverse algorithm.

Algorithms↗
PubMed2004

Anatomical statistical models and their role in feature extraction.

A detailed model of the shape of anatomical structures can significantly improve the ability to segment such structures from medical images. Statistical models representing the variation of shape and appearance can be constructed from suitably annotated training sets. Such models can be used to synthesize images of anatomy, and to search new images to accurately locate the structures of interest, even in the presence of noise and clutter. In this paper we summarize recent work on constructing and using such models, and demonstrate their application to several domains.

Brain↗
PubMed2005

Anatomically based modelling of the human skull and jaw.

We present here an anatomically based model of the human masticatory system that provides a framework for simulating the complex chewing process. The initial motivation for creating this model was the desire to have a computational model of the human jaw that can be used to simulate the action of simple bites, and to calculate the stresses and forces on the teeth that are involved. The model created also provides a platform that can be used to investigate other features of the masticatory system. To construct this global model, individual models of the bones of the skull and jaw were created from generic data sets. Geometric models of the muscles of mastication were also created and attached to the appropriate bones. To complete this initial model, representations of the crowns of the teeth were created and a basic model of the temporomandibular joint (TMJ) was included. The finite element method was used to solve for the stresses and strains created by the loading conditions during a clenching simulation involving the mandible bone. The model presented here is also discussed in relation to a model of the entire musculo-skeletal system being developed as part of the Physiome Project.

Computer Simulation↗
PubMed1995

Cells and circuits contributing to functional properties in area V1 of macaque monkey cerebral cortex: bases for neuroanatomically realistic models.

Anatomical and physiological data obtained from investigations of area V1 of the macaque monkey visual cerebral cortex have been used in 3 models outlining possible circuitry underlying functional properties of the region. The 3 models use, respectively, a fully implemented computer neural network, a mathematical formulation of interactions in a descriptive model of anatomical circuitry and a purely descriptive account of circuitry that could underlie particular functions. The 1st 2 models involve as part of their design an interpolation principle where afferents of opposite physiological property establish spatially offset but adjacent terminal fields and the postsynaptic neurons' dendrites have a continuum of different degrees of overlap into the 2 afferent pools and therefore different synaptic weights from the 2 afferents; this creates a functional and spatial gradient of response properties in the postsynaptic neurons between the properties of the different sets of afferents. The 3rd model examines lateral excitatory and inhibitory interactions in such gradients. Model 1 addresses the transformation of distinct thalamic axon properties to a gradient of response properties in postsynaptic spiny stellate neurons in layer 4C of V1. Model 2 proposes circuitry producing orientation specificity in V1 that begins by generating specificity of responses to orthogonal orientations; this is achieved by means of orthogonally oriented lateral axon projections made by the layer 4C spiny stellate neurons; this is followed by generation of a full cycle of orientation specificities by means of pyramidal neuron dendritic overlap across spatially separated fields of spiny stellate neuron axons responding preferentially to orthogonal orientations. Model 3 describes a circuitry to explain inhibitory and facilitatory interactions observed to occur in single unit responses when the classical receptive field is stimulated concurrently with the surround region. All the proposed models make predictions that can be tested by further anatomical and physiological experiments in the real visual cortex.

Animals↗
PubMed1985

Photoelastic effects of maxillary protraction on the craniofacial complex.

The conventional treatment of anterior crossbites has been the application of orthopedic force to the mandible to redirect its growth. However, in the patient with an underdeveloped maxilla, this treatment alone is not sufficient. Therefore, the purpose of this investigation was to study the orthopedic effects of maxillary protraction appliances in the treatment of anterior crossbites. A three-dimensional anatomic model of a human skull was fabricated with birefringent materials for photoelastic analysis. Three maxillary protraction appliances that utilized different anchorage units were used. The protraction forces placed on these appliances were parallel to the occlusal plane, a downward vector 20 degrees to the occlusal plane, and a combination of these two vectors. The resulting stress patterns were observed. The effects of the forces produced by the three appliances were transmitted to the maxilla and distant craniofacial structures. Both a parallel traction and a 20 degrees downward pull to the occlusal plane caused a constriction of the anterior portion of the maxilla. The parallel traction caused a counterclockwise (opening) rotation of the molar tooth and palatal plane. A 20 degrees downward force to the occlusal plane decreased this effect.

Bicuspid↗
PubMed2001

Calculation of induced current densities for humans by magnetic fields from electronic article surveillance devices.

This paper illustrates the use of the impedance method to calculate the electric fields and current densities induced in millimetre resolution anatomic models of the human body, namely an adult and 10- and 5-year-old children, for exposure to nonuniform magnetic fields typical of two assumed but representative electronic article surveillance (EAS) devices at 1 and 30 kHz, respectively. The devices assumed for the calculations are a solenoid type magnetic deactivator used at store checkouts and a pass-by panel-type EAS system consisting of two overlapping rectangular current-carrying coils used at entry and exit from a store. The impedance method code is modified to obtain induced current densities averaged over a cross section of 1 cm2 perpendicular to the direction of induced currents. This is done to compare the peak current densities with the limits or the basic restrictions given in the ICNIRP safety guidelines. Because of the stronger magnetic fields at lower heights for both the assumed devices, the peak 1 cm2 area-averaged current densities for the CNS tissues such as the brain and the spinal cord are increasingly larger for smaller models and are the highest for the model of the 5-year-old child. For both the EAS devices, the maximum 1 cm2 area-averaged current densities for the brain of the model of the adult are lower than the ICNIRP safety guideline, but may approach or exceed the ICNIRP basic restrictions for models of 10- and 5-year-old children if sufficiently strong magnetic fields are used.

Biophysical Phenomena↗
PubMed2002

SARs for pocket-mounted mobile telephones at 835 and 1900 MHz.

Increasingly, mobile telephones are becoming pocket-sized and are being left in the shirt pocket with a connection to the ear for hands-free operation. We have considered an anatomic model of the chest and a planar phantom recommended by US FCC to compare the peak 1 and 10 g SARs for four typical cellular telephones, two each at 835 and 1900 MHz. An agreement within +/- 10% is obtained between calculated and experimental 1 and 10 g SARs for various separations (2-8 mm) from the planar phantom used to represent different thicknesses of the clothing both for the antenna away from or turned back towards the body. Because of the closer placement of the antennas relative to the body, the peak 1 and 10 g SARs are considerably higher (by a factor of 2-7) for pocket-mounted telephones as compared to the SARs obtained using a 6 mm thick plastic ear head model--a procedure presently accepted both in the US and Europe. This implies that a telephone tested for SAR compliance against the model of the head may be severely out of compliance if it were placed in the shirt pocket.

Cell Phone↗
PubMed1997

A simple analog of visual field retinal projection.

Basic biophysical principles of the eye, such as the retinal projection of the visual field, bidimensional image inversion, blind point determination, visual field and retinal quadrants, perimetric principles, and determination of scotoma, are seen on a working anatomic model of the eye and its visual field that can easily be constructed at very low cost and from easily obtained materials: an opaque glass globe, a styrofoam hemisphere, a wood screen, bulbs, sockets, wire, and switches. This model has been used for many years in our undergraduate medical physiology courses and has replaced the classical model of candle image inversion by a converging lens.

Audiovisual Aids↗
PubMed2005

Functional hierarchy of coronary circulation: direct evidence of a structure-function relation.

The heart muscle is nourished by a complex system of blood vessels that make up the coronary circulation. Here we show that the design of the coronary circulation has a functional hierarchy. A full anatomic model of the coronary arterial tree, containing millions of blood vessels down to the capillary vessels, was simulated based on previously measured porcine morphometric data. A network analysis of blood flow through every vessel segment was carried out based on the laws of fluid mechanics and appropriate boundary conditions. Our results show an abrupt change in cross-sectional area that demarcates the transition from epicardial (EPCA) to intramyocardial (IMCA) coronary arteries. Furthermore, a similar pattern of blood flow was observed with a corresponding transition from EPCA to IMCA. These results suggest functional differences between the two types of vessels. An additional abrupt change occurs in the IMCA in relation to flow velocity. The velocity is fairly uniform proximal to these vessels but drops significantly distal to those vessels toward the capillary branches. This finding suggests functional differences between large and small IMCA. Collectively, these observations suggest a novel functional hierarchy of the coronary vascular tree and provide direct evidence of a structure-function relation.

Blood Flow Velocity↗
PubMed1988

Temperature profiles with respect to inhomogeneity and geometry of the human body.

Temperature profiles within the human body are highly dependent on the geometry and inhomogeneity of the body. Physical parameters such as density and heat conductivity of the various tissues and variables such as blood flow and metabolic heat production of different organs are spatially distributed and thereby influence the temperature profiles within the human body. Actual physiological knowledge allows one to take into account up to 54 different spatially distributed values for each parameter. An adequate representation of the anatomy of the body requires a spatial three-dimensional grid of at least 0.5-1.0 cm. This is achieved by photogrammetric treatment of three-dimensional anatomic models of the human body. As a first essential result, the simulation system has produced a realistic picture of the topography of temperatures under neutral conditions. Compatibility of reality and simulation was achieved solely on the basis of physical considerations and physiological data base. Therefore the simulation is suited to the extrapolation of temperature profiles that cannot be obtained experimentally.

Body Constitution↗
PubMed2006

Bone enhancement filtering: application to sinus bone segmentation and simulation of pituitary surgery.

The simulation of pituitary gland surgery requires a precise classification of soft tissues, vessels and bones. Bone structures tend to be thin and have diffuse edges in CT data, and thus the common method of thresholding can produce incomplete segmentations. In this paper, we present a novel multi-scale sheet enhancement measure and apply it to paranasal sinus bone segmentation. The measure uses local shape information obtained from an eigenvalue decomposition of the Hessian matrix. It attains a maximum in the middle of a sheet, and also provides local estimates of its width and orientation. These estimates are used to create a vector field orthogonal to bone boundaries, so that a flux maximizing flow algorithm can be applied to recover them. Hence, the sheetness measure has the essential properties to be incorporated into the computation of anatomical models for the simulation of pituitary surgery, enabling it to better account for the presence of sinus bones. We validate the approach quantitatively on synthetic examples, and provide comparisons with existing segmentation techniques on paranasal sinus CT data.

Algorithms↗
PubMed2003

Stereolithographic surgical templates for placement of dental implants in complex cases.

In recent years, dental implant rehabilitation has faced demands from prosthetic and esthetic arenas that call for increasingly ideal outcomes, which require precise surgical planning and placement. Anatomic limitations and bone quantity and quality can now be evaluated using more sophisticated radiographic techniques, although transferring this information to the surgical phase has been at best a difficult task. Recently, computer-aided design and manufacturing have made it possible to use data from computerized tomography to not only plan implant rehabilitation, but also to transfer this information to the surgery. One of these techniques uses stereolithography, a laser-driven polymerization process that fabricates an anatomic model and surgical templates. This novel approach is illustrated with two advanced cases, demonstrating that the technique not only allows for the precise translation of the treatment plan directly to the surgical field, but also offers many significant benefits over traditional procedures.

Bone Transplantation↗
PubMed2005

Anatomical hip model for the mechanical testing of hip protectors.

An anatomical hip model has been developed to simulate the impact load on the hip of a falling person wearing a hip protector. The hip consists of an artificial pelvis made of aluminium, linked by a ball-and-socket joint to an anatomically shaped steel femur (thigh bone). The femur is embedded in silicone material with a hip-shaped surface to allow realistic positioning of the protectors with accessory underwear. Additionally, the silicone simulates the damping and load-dispersal effect of soft tissue. A triaxial load sensor is integrated in the neck of the femur to measure the axial and cross-sectional force components in response to external impact forces on the hip. The performance of the hip model was investigated in drop tests and validated against biomechanical data. In a first series of measurements, the shock absorption of 10 different hip protectors, including both energy-absorbing and energy-shunting systems, was analysed. To determine the importance of hip protector placement, each protector was tested in the correct anatomical alignment over the hip and anteriorly displaced by 3 cm. Considerable differences were found between individual hip protectors in their effectiveness to reduce impact forces on the femur. Position of the hip protector also influenced the forces applied to the femur.

Accidental Falls↗
PubMed1990

An expert system for the labeling and 3D reconstruction of the coronary arteries from two projections.

In this paper we present a rule-based expert system for the automatic delineation and 3D reconstruction of the left coronary artery on standard RAO and LAO angiographic projections. The approach is based on the application of a general blood vessel model and on anatomical models which take into account the normal variations of the coronary artery structure. In a first step, the arteries are delineated by detecting the maximum intensity on the centerline of the vessels. Then, we label the blood vessel segments according to an anatomical model of the left coronary artery. In general, only 1-2 labels remain for each blood vessel segment. Finally, these results are used for an automatic 3D reconstruction of the left coronary artery from two projections. Results from clinical RAO and LAO angiograms will be presented.

Algorithms↗
PubMed2000

Current densities in a 2 mm resolution anatomically realistic model of the body induced by low frequency electric fields.

Current density distributions in a fine resolution (2 mm) anatomically realistic voxel model of the human body have been calculated for uniform, low frequency vertically aligned electric fields for a body grounded and isolated from 50 Hz to 10 MHz. The voxel phantom NORMAN is used which has a height of 1.76 m and a mass of 73 kg. There are 8.3 million voxels in the body differentiated into 37 tissue types. Both finite-difference potential and time-domain methods were used. Results are presented for the current density averaged over 1 cm2 in muscle, heart, brain and retina. Electric field values required to reach the NRPB and ICNIRP basic restrictions on current density are derived and compared with the external field guidelines from these standards.

Computer Simulation↗
PubMed1992

Numerical dosimetry at power-line frequencies using anatomically based models.

We have used the finite-difference time-domain (FDTD) method to calculate induced current densities in a 1.31-cm (nominal 1/2 in) resolution anatomically based model of the human body for exposure to purely electric, purely magnetic, and combined electric and magnetic fields at 60 Hz. This model based on anatomic sectional diagrams consists of 45,024 cubic cells of dimension 1.31 cm for which the volume-averaged tissue properties are prescribed. It is recognized that the conductivities of several tissues (skeletal muscle, bone, etc.) are highly anisotropic for power-line frequencies. This has, however, been neglected in the first instance and will be included in future calculations. Because of the quasi-static nature of coupling at the power-line frequencies, a higher quasi-static frequency f' may be used for irradiation of the model, and the internal fields E' thus calculated can be scaled back to the frequency of interest, e.g., 60 Hz. Since in the FDTD method one needs to calculate in the time domain until convergence is obtained (typically 3-4 time periods), this frequency scaling to 5-10 MHz for f' reduces the needed number of iterations by over 5 orders of magnitude. The data calculated for the induced current and its variation as a function of height are in excellent agreement with the data published in the literature. The average current densities calculated for the various sections of the body for the magnetic field component (H) are considerably smaller (by a factor of 20-50) than those due to the vertically polarized electric field component when the ratio E/H is 377 ohms. We have also used the previously described impedance method to calculate the induced current densities for the anatomically based model of the human body for the various orientations of the time-varying magnetic fields, namely from side to side, front to back, or from top to bottom of the model, respectively.

Electromagnetic Fields↗