Reliability and accuracy of straightforward measurements for liver volume determination in ultrasound and computed tomography compared to real volumetry.

To evaluate the suitability of volume index measurement (VI) by either ultrasound (US) or computed tomography (CT) for the assessment of liver volume. Fifty-nine patients, 21 women, with a mean age of 66.8 ± 12.6 years underwent US of the liver followed immediately by abdominal CT. In US and CT imaging dorsoventral, mediolateral and craniocaudal liver diameters in their maximum extensions were assessed by two observers. VI was calculated by multiplication of the diameters divided by a constant (3.6). The liver volume determined by a manual segmentation in CT ("true liver volume") served as gold standard. True liver volume and calculated VI determined by US and CT were compared using Bland-Altman analysis. Mean differences of VI between observers were - 34.7% (- 90.1%; 20.7%) for the US-based and 1.1% (- 16.1%; 18.2%) for the CT-based technique, respectively. Liver volumes determined by semi-automated segmentation, US-based VI and CT-based VI, were as follows: 1.500 ± 347cm3; 863 ± 371cm3; 1.509 ± 432cm3. Results showed a great discrepancy between US-based VI and true liver volume with a mean bias of 58.3 ± 66.9%, and high agreement between CT-based VI and true liver volume with a low mean difference of 4.4 ± 28.3%. Volume index based on CT diameters is a reliable, fast and simple approach for estimating liver volume and can therefore be recommended for clinical practice. The usage of US-based volume index for assessment of liver volume should not be used due to its low accuracy of US in measurement of liver diameters.

[Reference values for echocardiographic parameters of trained and untrained Icelandic horses]. / Normwerterhebung echokardiogra- phischer Parameter bei trainierten und untrainierten Islandpferden.

The aim of this study was to establish echocardiographic reference values for healthy Icelandic horses. For this purpose cardiologic examinations were performed on 44 healthy trained and untrained Icelandic horses without known cardiologic or pulmonary disease. The atrial diameter of the trained horses were significantly greater than in the untrained horses and the left ventricular free wall diameter was also higher in the trained than in the untrained horses. These findings confirm that the changes of the heart caused by training which have previously been described by other authors in warmbloods, thoroughbreds, dogs and humans are present in Icelandic horses as well. However, in contrast to the findings in race horses, no enlargement of the left ventricle was found in trained Icelandic horses, which may indicate that training conditions of Icelandic horses are not comparable to those of race- or jumping-horses in high level training. The reference values established in this study will serve as basis for the current interpretation of the results of echocardiographic examinations in Icelandic horses. This should enable cardiologists to perform a more detailed and precise examination similar to cardiological examinations in warmbloods and race horses.

Laser cutting: influence on morphological and physicochemical properties of polyhydroxybutyrate.

Polyhydroxybutyrate (PHB) is a biocompatible and resorbable implant material. For these reasons, it has been used for the fabrication of temporary stents, bone plates, nails and screws (Peng et al. Biomaterials 1996;17:685). In some cases, the brittle mechanical properties of PHB homopolymer limit its application. A typical plasticizer, triethylcitrate (TEC), was used to overcome such limitations by making the material more pliable. In the past few years, CO2-laser cutting of PHB was used in the manufacturing of small medical devices such as stents. Embrittlement of plasticized PHB tubes has been observed, after laser machining. Consequently, the physicochemical and morphological properties of laser-processed surfaces and cut edges of plasticized polymer samples were examined to determine the extent of changes in polymer properties as a result of laser machining. These studies included determination of the depth of the laser-induced heat affected zone by polariscopy of thin polymer sections. Molecular weight changes and changes in the TEC content as a function of distance from the laser-cut edge were determined. In a preliminary test, the cellular response to the processed material was investigated by cell culture study of L929 mouse fibroblasts on laser-machined surfaces. The heat-affected zone was readily classified into four different regions with a total depth of about 60 to 100 microm (Klamp, Master Thesis, University of Rostock, 1998). These results correspond well with the chemical analysis and molecular weight measurements. Furthermore, it was found that cells grew preferentially on the laser-machined area. These findings have significant implications for the manufacture of medical implants from PHB by laser machining.

Pyrolytic carbon indentation crack morphology.

In studying fatigue and fracture behavior of brittle materials, Vickers diamond indentation cracks are often used. Many of the studies of indentation cracks use crack system models such as the radial-median crack or Palmqvist crack. These systems are also used to study small crack growth in brittle materials, and have been studied for pyrolytic carbon. However, the true morphology of these cracks in pyrolytic carbon coatings on graphite substrates have not been described. This study examined Vickers diamond and spherical ball indentation cracks in pyrolytic carbon coatings using several techniques, including serial metallographic cross sections, indentation fracture in bending, acoustic emission, and residual surface indentation scanning. The crack systems developed using these techniques were not typical of either radial median or Palmqvist systems. The morphology is unique to this material, possibly because of the coating thickness limitations. Given the difference in crack system, the application of standard indentation crack equations in studying fracture mechanics, especially for small cracks, must be questioned.

Effect of cavitation on pyrolytic carbon in vitro.

It has long been known that mechanical heart valves, when tested for durability using non-physiologic conditions common in accelerated testers, would cavitate. Until recently, cavitation was never observed in vivo. The discovery that a small number of Edwards-Duromedics heart valve explants indicated signs of cavitation erosion prompted a reassessment of the cavitation erosion potential of pyrolytic carbon (PyC). Analyses of the explanted valves indicated that cavitation may be accentuated by porous regions in the pyrolytic carbon coating from which most mechanical heart valves are constructed. Early studies have shown that for PyC (a) the resistance to cavitation erosion is comparable to that of aluminum, (b) the resistance to cavitation erosion is high initially, but with time the erosion rate accelerates, and (c) the cavitation erosion resistance is somewhat variable. In this study, similar experiments were performed utilizing polished pyrolytic carbon as well as microporous surfaces since microporous surfaces have been implicated as accelerating erosion. Within the accuracy of the measurement, we found no contributing acceleration due to the microporous nature of the pyrolytic carbon surfaces tested when compared to the polished surfaces. Examination of cross sections of samples exposed to cavitation conditions revealed the presence of extensive microcracking even without the presence of substantial surface erosion.

Finite element analysis of indentation tests on pyrolytic carbon.

The stresses which cause failure at contact areas between leaflets and orifices in pyrolytic carbon heart valves are evaluated. These contact stresses have previously been studied using Hertzian crack models that apply to monolithic material. Many heart valves are not monolithic pyrolytic carbon but a pyrolytic carbon deposited on graphite. Contact loads on these layered structures cause initial cracking in the pyrolytic carbon at the interface between pyrolytic carbon and graphite rather than Hertzian surface cracks. Increasing the load on layered structures will cause a secondary cracking (of Hertzian cracks) on the surface. The contact loading was simulated with a 5.1 mm diameter ball pressing against a flat sample of graphite coated with 0.26 mm of pyrolytic carbon on each surface. Finite element analysis of this model calculated the stresses associated with a range of loads causing no cracks through initial interface cracks and secondary surface cracks to complete failure. The calculated stresses are correlated with parallel laboratory experiments. A failure criterion for contact stresses is developed. The initial cracks at the graphite/pyrolytic carbon interface occur when the tensile stress in the pyrolytic carbon reaches 207 to 276 MPa and the compression stress in the graphite reaches 414 to 483 MPa. These initial cracks do not propagate immediately to the surface since they run into a high triaxial compression stress field. Circular surface cracks occur at the edge of the ball/pyrolytic carbon contact area at higher loads. These cracks require a shear stress of about 241 MPa and also require a tensile stress component. The results provide a criterion for designing contact regions in pyrolytic heart valves.

Pure pyrolytic carbon: preparation and properties of a new material, On-X carbon for mechanical heart valve prostheses.

BACKGROUND AND AIM OF THE STUDY: Historically, the pyrolytic carbon used in mechanical prosthetic heart valves contained small amounts of silicon, this being a necessary additive to achieve consistently the hardness required for wear resistance. New processing technology has allowed the deposition of pyrolytic carbon without silicon, while maintaining adequate hardness to ensure wear resistance. METHODS: A parametric study of coating parameters identified the conditions necessary to produce the optimal pure carbon material. RESULTS: In comparison with silicon-alloyed carbon, the pure carbon was found to be about 20% stronger, have a strain-to-failure about 25% higher and have a greater toughness. CONCLUSIONS: The enhanced strength, deformability and toughness of the new carbon permits designers to utilize component shapes and dimensions that could not be manufactured using the silicon-alloyed carbons. Such design features have hemodynamic benefits resulting in valve performance improvements.

Dynamic characterization of a new accelerated heart valve tester.

This paper presents a new accelerated prosthetic heart valve tester prototype that incorporates a camshaft and poppet valves. A three element Windkessel system is used to mimic the afterload of the human systemic circulation. The device is capable of testing eight valves simultaneously at a rate up to 1,250 cycles/min, while the flow rate, the pressure, and the valve loading can be monitored and adjusted individually. The tester was characterized and calibrated using a set of eight Carpentier-Edwards bioprostheses at a flow rate varying between 3 and 5 L/min. The experiment was carried out with the pressure difference across the closed heart valve maintained between 140 and 190 mmHg. Smooth and complete opening and closing of the valve leaflets was achieved at all cycling rates. This confirms that the velocity profiles approaching the test valves were uniform, an important factor that allows the test valves to open and close synchronously each time.

Closing behavior of a new bileaflet mechanical heart valve.

Recently, the in vivo cavitation potential has become a primary concern among manufacturers of new mechanical heart valves (MHV). An experimental/computational program was designed to investigate each of the flow parameters involved. It was established that the closing velocity of the leaflet holds the key to MHV cavitation. One of the novel concepts of the new bileaflet mechanical heart valve (1205-MHV) was its ability to operate with a relatively small angular excursion that led to a much smaller closure velocity at impact (as compared with control valves). This is believed to significantly reduce the cavitation potential. The 1205-MHV is characterized by a longer valve body, with the hinges protruding further upstream. The unique design allows the valve the freedom to open as much as 90 degrees. The closure velocities are reduced by a smaller leaflet excursion (50 degrees), combined with a floating hinge that allows absorption of part of the impact energy at closure. The impact velocities of the 1205-MHV leaflets at closure were measured by a laser sweeping technique that monitored the leaflet closing motion with a precision of 5 microseconds within the last 3 degrees before impact. The 27 mm 1205-MHV (the largest size) was tested in the program by mounting the valve in the mitral position of a physiologic mock circulatory loop. The valve was tested at 70, 90, and 120 bpm. The results were compared with those of a St. Jude Medical 29 mm MHV. The closure velocities measured with the 1205-MHV were significantly lower than those measured with the control valve.(ABSTRACT TRUNCATED AT 250 WORDS)

Assessment of explanted PTCA balloons.

The data presented here are part of a on-going study to define the surface characteristics and properties of explanted PTCA catheters in a further effort to address some of the ramifications of the re-use issue. PTCA balloon catheter were examined after angioplasty in one hundred and sixty-eight patients (n = 168). This series included six balloon types from three manufacturers. The fresh fixed and dehydrated balloons were examined at first with light microscopy and then in a scanning electron microscope. X-ray semiquantitative microanalysis and FT-IR-ATR analysis were also performed on the balloons. Because most blood proteins are water soluble, we examined unfixed balloons with a protein silver staining kit for detection of adhered proteins described by Heukeshoven. A further method for protein detection is the Lowry-analysis. With this method water insoluble proteins can be observed. Our study has shown convincingly that all deployed angioplasty catheters were coated with adherent protein layers. Plaque particles were found embedded in the surfaces of most of the balloons examined. Fissuring and micro tearing of balloon surfaces was noted. FT-IR-ATR analyses of the blood contacted balloon surfaces did not show any peaks indicative of proteins on the balloon surface. The silver staining method also did not show any evidence of protein adsorption on the balloons. On the other hand, the Lowry-analysis yielded clear evidence that water insoluble proteins were adherent to the balloon surfaces. The average measured protein concentration was 17 microg/ml.

On the durability of pyrolytic carbon in vivo.

The durability of pyrolytic carbon heart valve components was examined from the point of view of number implanted, documentation of wear on explanted components, and from the aspect of fatigue. Failures of pyrolytic carbon components were found to be few in number. A model describing the time course of events and successful usage of pyrolytic carbon components in heart valves was developed. The model is based on the yearly shipments of pyrolytic carbon components or valves, starting with 1969. The model indicates that about 2 million components have been successfully implanted, resulting in accumulative experience of over 10 million patient years. The wear of pyrolytic carbon, based on analysis of explanted heart valves, was found to be minimal. Additional wear data obtained on explanted components confirms the earlier observation that wear in vivo in less than that observed in vitro. The recently discovered fatigue behavior of pyrolytic carbon was found to have no demonstrated practical impact on the durability of pyrolytic carbon components used in existing mechanical heart valves.

The fatigue behavior of vapor-deposited carbon films.

Vapor-deposited carbon films (about 4000 to 5000 A thick) on stainless steel substrate were cyclically loaded to 10(6) cycles. The carbon films did not fail in fatigue at strain levels up to 13.12 x 10(-3). Rather, the failure in the carbon film occurred as a result of plastic deformation in the substrate; i.E., the failure was directly related to the endurance limit of the substrate material, which, when expressed as strain, was measured in this study to be about 8.0 - 10.88 x 10(-3). The endurance limit was also found to be very close to the elastic strain limit of the substrate. The implications of the findings for the use of carbon coated components in prosthetic devices are also discussed.

Gaseous flow through thin carbon films.

Thin carbon films, when used as coatings on prosthetic devices, must be a barrier to gases and physiological fluids. Using CO2 at room temperature, the gas permeability of carbon films ranging in thickness from about 200 to 500A was measured. The average permeability constant of 21 carbon films was determined to be 1.91 (+/- 1.02) x 10(-12) cm3-cm/cm2-sec-mmHg. This value is quite comparable to or smaller than that of nuclear graphites, which are considered to be impermeable to gases.

Developments in carbon prosthetics.

The majority of carbon-coated prosthetic devices in use today are coated with a unique form of carbon, low-temperature isotropic (LTI) carbon. The wide acceptance of this special form of carbon is a direct result of LTI carbon's demonstrated biocompatibility, its mechanical properties, and its inertness. The LTI carbon deposition process, however, places severe constraints on the size and type of substrate that can be coated. The substrates must be small so that they may be supported in a fluidized bed and further must be able to withstand temperatures in excess of 1200 degrees C. Recent technological advancements have removed the requirement that an object to be coated must be suspended in a fluidized bed and have also made possible the deposition of isotropic carbon at near room temperature. These developments expand the application of carbon-surfaced components into areas of prosthetics not previously possible. This paper describes some of the new applications and results.

The adhesion of thin carbon films to metallic substrates.

As part of the development of carbon-coated prosthetic devices, the adhesion of thin carbon films to metallic substrates has been studied. The bond strength of carbon films about 5000 A thick on Ti-6A1-4V and stainless steel was measured in a pull test and found to be greater than 4700 psi. Auger electron spectroscopy showed a reactive film/substrate interface. The ultimate bond strength was found to be dependent on the substrate and the deposition parameters.

Vapor-deposited carbon-coated tooth root implants: preliminary evaluation of a stylized tooth implant system in dogs.

The results of this preliminary evaluation of a tooth implant model combining a stylized tooth root design and a thin, highly biocompatible microporous carbon coating, although tentative, add support to the feasibility of developing a statisfactory system for the immediate replacement of selected teeth in healthy bone.

Preliminary histologic evaluation of a biocompatible mesh for tracheal reconstruction.

The purpose of this study was to evaluate a porous Dacron - urethane mesh tracheal prosthesis in large dogs with surgically created discontinuous defects of the cervical trachea. Some protheses were carbon coated in an attempt to improve biocompatibility. Histology was performed on 2 dogs. The mucosa regenerated entirely covering the prosthesis. Respiratory epithelium was formed over a large central portion of the tracheas.