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At least 1,207 records · Page 67Linked to original sources
PubMed1980

Production of thin CT sections and coronal and sagittal images by spatial filtering.

A method for the production of thin axial transverse sections, coronal views, and sagittal views from overlapped thick computed tomography (CT) scans has been developed. Thin slices are produced by spatially filtering the overlapped CT data using a filter that is a combination of square waves in the spatial domain. Resolution recovery and noise properties have been examined in phantomas and patients for a selection of filter parameters. Sample results of these studies are presented. The method has been applied to data from several hundred patient studies. Several of these cases are shown to illustrate the method.

Brain↗
PubMed1980

Computerized tomography and nuclear magnetic resonance.

In 1917 Radon gave a simple formula for reconstructing a function (f(x,y,z) from its integrals over all planes in three-dimensions. We give a simple algorithm for numerical quadrature of his formula and suggest its application to imaging the nuclear magnetic resonance spin density of an object.

Head↗
PubMed2001

Calcium concentration and pH of the periapical environment after applying calcium hydroxide into root canals in vitro.

The purpose of the present study was to determine both calcium concentration and pH in the periapical region after application of 1 of 4 different calcium hydroxide preparations into experimental root canals. Fifty root canal models were divided into five groups: group 1--calcium hydroxide was mixed with distilled water at a powder/water weight ratio of 38%; group 2--calcium hydroxide was mixed with distilled water at 44%; group 3--calcium hydroxide was mixed with distilled water at 50%; group 4--calcium hydroxide powder only was used; and group 5-the control group, in which nothing was applied to the canals. All samples were immersed in distilled water maintained at 37 degrees C. Calcium concentration and pH of the distilled water were measured after 3 days, 7 days, and then at weekly intervals up to 15 wk, during which time the storage medium was renewed after each measurement. Calcium concentration and the change in pH of the distilled water were statistically quicker and greater in groups 1 to 3 (mixture groups) than group 4 (powder only) (p < 0.05). The highest calcium concentration (peak Ca2+ release) was observed after 3 days for the mixture groups, whereas that for the powder only group was found at 7 days. Peak pH change was found after 14 days for the mixture groups, whereas that for the powder only group was found at 49 days. After peaking, all groups showed a decline of the pH over time. These results suggest that the time required for optimum intracanal activity when using calcium hydroxide mixtures is at least 2 wk.

Analysis of Variance↗
PubMed2001

The endodontic cube: a system designed for evaluation of root canal anatomy and canal preparation.

An extensively redesigned muffle system is presented incorporating improved design features, such as rigid external fixation and machined internal indexing to enable a more accurate, clinically relevant, and reproducible evaluation of root canal anatomy before and after preparation. Given the ability to directly observe and quantify changes in the root canal system, the information obtained using this design is comprehensive. In each tooth pretreatment evaluation provides the ideal control, reinforcing the suitability of this technique.

Dental Pulp Cavity↗
PubMed2001

Wear of nickel-titanium endodontic instruments evaluated by scanning electron microscopy: effect of ion implantation.

The present work analyzes possible increases in resistance to wear of nickel-titanium endodontic instruments that have undergone a process of ionic implantation. ProFile .04 taper #25 instruments were subjected to ionic implantation with bands of nitrogen ions of 250 KeV, currents in the order of 10 microA/cm2, and doses of 2 x 10(17) ions/ cm2. The instruments were used to make preparations in acrylic endodontic training blocks. Scanning electron microscopic investigations showed that after 60 s of work inside the endodontic training blocks nonionic implanted control instruments showed small modifications in their blades, and their tips showed the first signs of wear. After 240 s the control instruments showed consistent signs of wear and frequent changes to their surfaces. After 240 s of use the ionic-implanted instruments did not present any significant changes in the micromorphology of their surfaces. The implanted instruments did not manifest the typical signs of wear and did not show the surface changes that quickly affect the working life of untreated endodontic instruments manufactured from nickel-titanium.

Dental Instruments↗
PubMed1976

Three dimensional analysis of unmyelinated fibers in normal and pathologic autonomic nerves.

The technique of serial-section electron microscopy and diagrammatic three dimensional reconstructions has been used to assess normal and pathological unmyelinated nerve fibers from a peripheral autonomic nerve: the rat cervical sympathetic trunk. Within this predominantly unmyelinated nerve, there is a complex arrangement of axons into longitudinally oriented bundles brought together by chains of Schwann cells. Each bundle is subdivided into smaller components by the cytoplasmic processes of Schwann cells; such subdivisions, which are basal lamina-enclosed masses of Schwann cell cytoplasm, when viewed on cross-sectional electron micrographs, are termed Schwann cell units. The size and shape of each Schwann cell unit varies along the length of fibres, but the diameter of individual axons shows little variation over the segments studied. Axonal branching was not observed in normal unmyelinated nerves. Crush injury and x-irradiation produces different patterns of alteration in the axon-Schwann cell relationships of unmyelinated nerves. Following crush injury, Schwann cell processes increase in diameter and contain numerous small diameter axonal sprouts. Many of the regenerating axons remain thin while others reacquire a normal diameter. X-irradiation affects Schwann cells leading to retraction of their processes and the appearance of naked axonal segments.

Animals↗
PubMed1997

Corneal geometry reconstruction with the Keratron videokeratographer.

BACKGROUND: Color-coded corneal maps produced by computer-assisted videokeratographers (CAVKs) have become an indispensable tool for clinical understanding of corneal shape. However, Placido-based CAVKs are criticized as theoretically incapable of producing accurate corneal height information. PURPOSE: This paper describes how the Keratron (Optikon 2000, Rome, Italy) integrated design innovations to achieve accuracy and map use previously thought to be unobtainable. METHODS: The Keratron implemented a spherically unbiased surface reconstruction method that yields height, axial power, and instantaneous curvature without derivation of one quantity from another. Processing innovations resulted in sub-micron height accuracy. The Keratron includes axial power, instantaneous curvature, refractive maps, a height map (spherical offset), pupil edge detection, a "move axis" feature, process editing, indices, a contact lens program, and photorefractive keratectomy (PRK) simulation. CONCLUSIONS: The algorithms for surface reconstruction and the design solutions implemented in the Keratron resulted in accurate height, axial power, and instantaneous curvature measurement and valid, clinically useful maps, as well as additional user options and features.

Algorithms↗
PubMed1997

Modeling the cornea with the topographic modeling system videokeratoscope.

The algorithms and models used by the Topographic Modeling System (TMS) corneal topography systems are described. The systems are calibrated to report the power at a number of points along each of 256 meridians. The power reported is the power of an equivalent sphere. From the radius of the equivalent sphere and the optical properties of the videokeratoscope, shape and a more accurate estimate of power are calculated. The TMS differs from most other videokeratoscopic systems in that it estimates the power and does not use axial distance.

Cornea↗
PubMed2000

The accuracy and precision performance of four videokeratoscopes in measuring test surfaces.

In this study we evaluated the accuracy and precision of three placido-disk videokeratoscopes (the Keratron, Medmont and TMS) and one videokeratoscope that uses the raster-stereogrammetry technique (PAR-CTS) in elevation topography with six test surfaces. The test surfaces were a sphere, an asphere, a multicurve, and three bicurve surfaces. Each instrument performed well on certain test surfaces, but none of the instruments excelled on all of the surfaces. The results showed high accuracy of the Keratron and Medmont instruments in measuring the sphere, asphere, and multicurve surfaces, but not the bicurve surfaces. The precision of the Keratron and Medmont instruments were high. The TMS and PAR-CTS instruments showed poorer accuracy than the Keratron and Medmont instruments for the multicurve test surface but showed better performance for the bicurve surfaces. The PAR-CTS had the poorest performance in precision of the four instruments. The use of the Noryl spherical test surface instead of polymethyl methacrylate (PMMA) resulted in small differences in the accuracy performance of the placido-disk videokeratoscopes only.

Cornea↗
PubMed2003

Scatter and its implications for the measurement of optical image quality in human eyes.

PURPOSE: To investigate the effect of scatter on measurements of wavefront aberrations and point-spread functions in a model eye. METHODS: The wavefront aberrations of a model eye were measured using Hartmann-Shack wavefront sensing and crossed-cylinder aberroscope techniques and compared with its measured point-spread function in the presence of scattering media of different concentrations. RESULTS: The point-spread functions became broader as the concentration increased. Forward light scatter on both the light path into the eye and the light path out of the eye contributed to this broadening of the point-spread function. Neither the crossed-cylinder aberroscope nor wavefront sensing, which, respectively, measure the ocular wavefront aberrations for light entering the eye and leaving the eye, were affected by the scatter. CONCLUSION: We predict that by minimizing the contribution of the forward light scatter from one or other of these light paths by manipulating the size of the entrance and exit pupils, it should be possible to objectively assess narrow-angle forward light scatter in the eye by measuring and removing any confounding effect from wavefront aberration.

Diagnostic Techniques, Ophthalmological↗
PubMed1988

Surface-area increase in tissue expansion.

In this paper, the subject of surface-area gain in tissue expansion is approached from three sides. With the help of a mathematical approach, the theoretical increase in surface area was calculated in relation to the specifications of the expanders. In an in vitro model, these mathematical data were confirmed to be reliable. To determine the real surface-area gain that takes place in vivo, measurements on pigs were performed. It appears that only about 35 percent of the mathematically expected increase in surface area takes place. Moreover, a clear difference was observed between the surface-area gain for the different shapes of tissue expanders.

Animals↗
PubMed1989

Medical College of Alabama in Mobile, 1859-1920: a legacy of Dr. Josiah Clark Nott.

The first legislated orthodox medical school within Alabama was founded in Mobile in 1859, a legacy of Dr. Josiah Clark Nott. That it developed later than other Southern medical schools has been attributed to multiple factors, among them rural isolation, restricted communication, limited transportation, sparse population, cultural deprivation, and climatologic enervation. The rationale for a medical school within Alabama was also multifactorial: to supply physicians to rural Alabama, to reverse the economic and cultural drain among Alabamians that out-of-state education implied, and to educate medical students regarding the unique health care requirements of a predominantly rural Alabama populace. A medical school building was constructed east of the Mobile City Hospital, and was equipped with an elegant collection of anatomic models acquired by Nott during his travels in western Europe in 1859. After only two sessions, however, the War Between the States (1861 to 1865) forced the medical school to close, as faculty and students joined the Confederate forces. In 1868, with the continued involvement of Dr. W. H. Anderson as Dean, the institution was reopened. During the succeeding 52 years of its existence, the financially strapped medical school attempted to cope with evolving medical technologies and educational philosophies. Despite the commitment of the administrators and faculty, sociopolitical factors and insufficient economic support militated against the school's continued existence. Nott has been characterized as a physician, anatomist, anthropologist, and ethnologist. His opinions as revealed in his writings were controversial because they addressed sociopolitical and racial issues. Nevertheless, his commitment to the Medical College of Alabama in Mobile was unstinting, and he provided the major leadership role in its establishment.

Alabama↗
PubMed1988

An analytical investigation of the mechanics of spinal instrumentation.

Three-dimensional nonlinear finite element models of the intact L4-5 one motion segment/two-vertebrae and L3-5 two motion segments/three-vertebrae were developed using computed tomography (CT) films. The finite element mesh of the L4-5 motion segment model was modified to simulate bilateral decompression surgery. The mesh was further altered to achieve stabilization, using an interbody bone graft and a set of Steffee plates and screws. The model behavior of the intact specimen in all loading modes and of the stabilized model in compression, flexion, and extension modes were studied. The stresses in the cancellous bone region were found to decrease. The interbody bone graft, due to an overall decrease in stresses in the bone below the screw, transmits about 80% of the axial load as compared with 96% transmitted by an intact disc in an intact model. Thus, the use of a fixation device induces a stress shielding effect in the vertebral body. The results indicate that although the bone graft transmits lesser loads than the intact disc, it is active in transmitting loads. The presence of low stresses in the cancellous bone region and high localized stresses in the cortical pedicle region surrounding the screw, compared with the intact case, suggests that the screws are likely to become loose over time. The use of an interbody bone graft alone or in combination with any existing fixation device also induces higher stresses at the adjacent levels. This may be responsible for the adverse iatrogenic effects seen clinically.

Biomechanical Phenomena↗
PubMed1998

Influence of muscle morphometry and moment arms on the moment-generating capacity of human neck muscles.

STUDY DESIGN: The function of neck muscles was quantified by incorporating experimentally measured morphometric parameters into a three-dimensional biomechanical model. OBJECTIVE: To analyze how muscle morphometry and moment arms influence moment-generating capacity of human neck muscles in physiologic ranges of motion. SUMMARY OF BACKGROUND DATA: Previous biomechanical analyses of the head-neck system have used simplified representations of the musculoskeletal anatomy. The force- and moment-generating properties of individual neck muscles have not been reported. METHODS: A computer graphics model was developed that incorporates detailed neck muscle morphometric data into a model of cervical musculoskeletal anatomy and intervertebral kinematics. Moment arms and force-generating capacity of neck muscles were calculated for a range of head positions. RESULTS: With the head in the upright neutral position, the muscles with the largest moment arms and moment-generating capacities are sternocleidomastoid in flexion and lateral bending, semispinalis capitis and splenius capitis in extension, and trapezius in axial rotation. The moment arms of certain neck muscles (e.g., rectus capitis posterior major in axial rotation) change considerably in the physiologic range of motion. Most neck muscles maintain at least 80% of their peak force-generating capacity throughout the range of motion; however, the force-generating capacities of muscles with large moment arms and/or short fascicles (e.g., splenius capitis) vary substantially with head posture. CONCLUSION: These results quantify the contributions of individual neck muscles to moment-generating capacity and demonstrate that variations in force-generating capacity and moment arm throughout the range of motion can alter muscle moment-generating capacities.

Biomechanical Phenomena↗
PubMed1998

Prediction of load sharing among spinal components of a C5-C6 motion segment using the finite element approach.

STUDY DESIGN: A finite element model of the ligamentous cervical spinal segment was used to compute loads in various structures in response to clinically relevant loading modes. OBJECTIVE: To predict biomechanical parameters, including intradisc pressure, tension in ligaments, and forces across facets that are not practical to quantify with an experimental approach. SUMMARY OF BACKGROUND DATA: Finite element models of the cervical spine in their present form, because of inherent assumptions and simplifications, are not entirely satisfactory for studying the biomechanics of the intact, injured, and stabilized cervical spinal segment. METHODS: A three-dimensional finite element model of a C5-C6 motion segment was developed from serial computed tomographic scans of a ligamentous cervical spinal segment. This model included nonlinear ligament definition, fully composite intervertebral disc, fluid nucleus, and Luschka's joints. The model-based displacement predictions were in agreement with the experimental data. This model was used to predict load sharing and other related parameters in spinal elements in response to various loading modalities. RESULTS: In axial compression, 88% of the applied load passed through the disc. The interspinal ligament experienced the most strain (29.5%) in flexion, and the capsular ligaments were strained the most (15.5%) in axial rotation. The maximum intradisc pressure was 0.24 MPa in the flexion with axial compression mode (1.8 Nm + 73.6 N). The anterior and posterior disc bulges increased with the increase in axial compression (up to 800 N). CONCLUSIONS: The results provide new insight into the role of various elements in transmitting loads. The model represents significant and essential advancement in comparison with previous finite element models, making it possible for such models to be used in investigating a broad spectrum of clinically relevant issues.

Biomechanical Phenomena↗
PubMed1995

Angulation assessment of anterior single tooth root form implants: technical note.

A technique is presented to evaluate the angulation and position of an anterior single tooth root form implant by means of a surgical template, a pilot osteotomy, and an abutment guide. The space available for the restorative materials can be assessed and the angulation modified, if necessary, to improve the esthetic potential. If the angulation is satisfactory and sufficient room is available for the prosthesis, the surgeon can continue with implant placement at the initial site.

Cuspid↗
PubMed1998

Three-dimensional guidance system for implant insertion: Part I.

A technique is presented, with two sets of newly designed drills, that facilitates three-dimensional guided insertion of implants. A radiographic guide has been fabricated with titanium markers that are transferred into the CT scan reformatted images. The resulting cross-sectional and panoramic reformatted images provide the precise location for each implant. The buccolingual and mesiodistal inclinations of each implant are measured and transferred to a surgical guide that contains steel drill guide tubes. The starting point and three-dimensional orientation of each implant is then transferred to a pilot osteotomy. The widening of an osteotomy, with end-cutting drills of the usual design, can lead to the loss of orientation because a wider drill cannot be effectively guided by a smaller diameter osteotomy. A newly designed set of drills has been developed that maintains tracking of the original orientation as the incremental increase in width proceeds. The three-dimensional guidance system for implant insertion was first accomplished in vitro and then on patients.

Anatomy, Cross-Sectional↗