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At least 1,135 records · Page 63Linked to original sources

PubMed2002

Evaluation of the effect of varying MPEG-2 compression ratios on digital coronary angiographic assessment of stenosis severity.

The purpose of this study is to evaluate the influence of MPEG-2 compression scheme on coronary angiography and to search the highest compression ratio at which no significant effect to accuracy of assessment of stenosis severity occurs. Forty-Four digital cine angiographies were used. Three cardiologists participated in a subjective study in which they read both uncompressed images and compressed images. Furthermore, an objective study was carried out to measure vessel stenosis ratio by using software. The influence of compression was evaluated by kappa statistics in case of subjective study and by both systematic error and random error in case of objective study. Kappa statistics between uncompressed image and compressed image at a ratio of 80:1 was significantly lower than that of other compression ratios such as 40:1. Similar results were obtained in objective evaluation. In this report, the authors provide the baseline for further studies on observer performance for motion images.

Coronary Angiography↗

PubMed2005

[Compression therapy in the treatment of acute deep vein thrombosis].

Especially in Europe patients with acute deep vein thrombosis (DVT) of the leg are treated with compression therapy. A look at the literature, however, reveals that the exact mechanism of compression therapy in the treatment of acute deep vein thrombosis is not completely understood. Effects on fibrinolysis, resolution of pain and swelling as well as a fixation of the thrombus towards the vessel wall have been discussed. The literature indicates that compression stockings were mostly used in the treatment of DVT although there is some evidence that inelastic compression bandages have a greater impact on resolution of swelling and pain. Large randomised multicenter trials are still not available. Furthermore, evidence-based data providing an advantage of thigh-length compression therapy in the treatment of acute DVT are lacking. Studies have, however, shown that compression therapy is a safe therapy and can reduce the development of postthrombotic syndrome after acute DVT.

Acute Disease↗

PubMed1992

Compression loading in vitro regulates proteoglycan synthesis by tendon fibrocartilage.

The regulation of proteoglycan synthesis in a fibrocartilaginous tissue by mechanical loading was assessed in vitro. Discs of bovine tendon fibrocartilage were loaded daily with unconfined, cyclic, uniaxial compression (5 s/min, 20 min/day) and the synthesis of large and small proteoglycans was measured by incorporation of [35S]sulfate. All discs synthesized predominantly large proteoglycan when first placed in culture. After 2 weeks in culture nonloaded discs synthesized predominantly small proteoglycans whereas loaded discs continued to produce predominantly large proteoglycan. The turnover of 35S-labeled proteoglycan was not significantly altered by the compression regime. Increased synthesis of large proteoglycans was induced by a 4-day compression regime following 21 days of culture without compression. Inclusion of cytochalasin B during compression mimicked this induction. Autoradiography demonstrated that cell proliferation was minimal and confined to the disc edges whereas 35S-labeled proteoglycan synthesis occurred throughout the discs. These experiments demonstrate that mechanical compression can regulate synthesis of distinct proteoglycan types in fibrocartilage.

Animals↗

PubMed1985

An in-vitro system for studying the effect of variable compressive forces on the mandibular condylar cartilage of the rat.

An in-vitro system enabled variable range of continuous and intermittent compressive forces to be applied to the cartilage of 4-day-old rats under serum-free culture conditions. A continuous compressive force below 3 g did not affect the growth of the cartilage; when the force exceeded 3 g, growth ceased. Under an intermittent compressive force up to 8 g, growth continued, but at a lower rate; over 8 g, growth ceased. When growth-restricting compression was removed or reduced, growth was reactivated until a new balance was achieved. The normal architecture of the different zones was altered in the non-compressed in-vitro controls, but not in cultured cartilage during the application of these compressive forces.

Animals↗

PubMed1993

Influence of the compression force on the intramedullary pressure development in reaming of the femoral medullary cavity.

The reaming process is considered the main damage factor in intramedullary nailing, as there are repeated increases in intramedullary pressure and cortical temperature, which can lead to aseptic cortex necroses and fat embolisms. In this study, the influence of the compression force on the increase in pressure is to be examined. The intramedullary pressure was measured in the middle of the diaphysis and in the metaphysis of human femora reamed under various compression forces. The compression force exerted by the surgeon was measured on polyurethane femora. It was observed that an increase in the compression force of 1.8 times led to an increase in the diaphyseal pressure of 4.7 times and of the metaphyseal pressure of 3.1 times. As surgeons exert high compression forces, peak pressures of 970 mmHg diaphyseally and 1150 mmHg metaphyseally must be expected in the hospital. If reaming is carried out with less compression force, the intramedullary pressure can easily be reduced by 79% in the area of the diaphysis and by 68% in the metaphysis.

Femoral Fractures↗

PubMedDate not supplied

The biphasic poroviscoelastic behavior of articular cartilage: role of the surface zone in governing the compressive behavior.

Surface fibrillation of articular cartilage is an early sign of degenerative changes in the development of osteoarthritis. To assess the influence of the surface zone on the viscoelastic properties of cartilage under compressive loading, we prepared osteochondral plugs from skeletally mature steers, with and without the surface zone of articular cartilage, for study in the confined compression creep experiment. The relative contributions of two viscoelastic mechanisms, i.e. a flow-independent mechanism [Hayes and Bodine, J. Biomechanics 11, 407-419 (1978)], and a flow-dependent mechanism [Mow et al. J. biomech. Engng 102, 73-84 (1980)], to the compressive creep response of these two types of specimens were determined using the biphasic poroviscoelastic theory proposed by Mak. [J. Biomechanics 20, 703-714 (1986)]. From the experimental results and the biphasic poroviscoelastic theory, we found that frictional drag associated with interstitial fluid flow and fluid pressurization are the dominant mechanisms of load support in the intact specimens, i.e. the flow-dependent mechanisms alone were sufficient to describe normal articular cartilage compressive creep behavior. For specimens with the surface removed, we found an increased creep rate which was derived from an increased tissue permeability, as well as significant changes in the flow-independent parameters of the viscoelastic solid matrix. permeability, as well as significant changes in the flow-independent parameters of the viscoelastic solid matrix. From these tissue properties and the biphasic poroviscoelastic theory, we determined that the flow-dependent mechanisms of load support, i.e. frictional drag and fluid pressurization, were greatly diminished in cartilage without the articular surface. Calculations based upon these material parameters show that for specimens with the surface zone removed, the cartilage solid matrix became more highly loaded during the early stages of creep. This suggests that an important function of the articular surface is to provide for a low fluid permeability, and thereby serve to restrict fluid exudation and increase interstitial fluid pressurization. Thus, it is likely that with increasing severity of damage to the articular surface, load support in cartilage under compression shifts from the flow-dependent modes of fluid drag and pressurization to increased solid matrix stress. This suggests that it is important to maintain the integrity of the articular surface in preserving normal compressive behavior of the tissue and normal load carriage in the joint.

Animals↗

PubMed1994

Differences between the tensile and compressive strengths of bovine tibial trabecular bone depend on modulus.

The conflicting conclusions regarding the relationship between the tensile and compressive strengths of trabecular bone remain unexplained. To help resolve this issue, we compared measurements of the tensile (n = 22) and compressive (n = 22) yield strengths, and yield strains, of trabecular bone specimens taken from 38 bovine proximal tibiae. We also studied how these failure properties depended on modulus and apparent density. To enhance accuracy, trabecular orientation was controlled, and each specimen had a reduced section where strains were measured with a miniature extensometer. We found that the mean yield strength was 30% lower for tensile loading. However, the difference between individual values of the tensile and compressive strengths increased linearly with increasing modulus and density, being negligible for low moduli, but substantial for high moduli. By contrast, both the tensile and compressive yield strains were independent of modulus and density, with the yield strain being 30% lower for tensile loading. Thus, the difference between the tensile and compressive strengths of bovine tibial trabecular bone depends on the modulus, but the difference between yield strains does not. This phenomenon may explain in part that conflicting conclusions reached previously on the tensile and compressive strengths of trabecular bone since the mean modulus has varied among different studies. Realizing that our data pertain only directly to bovine tibial trabecular bone for longitudinal loading, our results nevertheless suggest that failure parameters based on strains may provide more powerful and general comparisons of the failure properties for trabecular bone than measures based on stress.

Animals↗

PubMed1996

Long-term compressive property durability of carbon fibre-reinforced polyetheretherketone composite in physiological saline.

In total hip arthroplasty, concerns such as corrosion and stress shielding associated with stiff metallic femoral components have led to the development of low stiffness advanced fibre-reinforced polymer (FRP) composite femoral components. Carbon fibre-reinforced polyetheretherketone (CF/PEEK) composite material is now one of the primary material systems being considered for composite hip stem development. As a hip stem, a composite material must be able to support a complex state of stress in the in vivo environment without failure. Considering the loading conditions of a hip stem (superimposed compression and bending), and the fact that FRP composites typically possess lower compressive than tensile strength, the compressive behaviour of FRP composites becomes very important for femoral component design. This paper presents an investigation of the long-term durability of 0 degree and 90 degrees compressive strengths of CF/PEEK composite following physiological saline saturation. 0 degree and 90 degrees compressive moduli and Poisson ratio (v12) properties are also reported. Samples were tested following conditioning in physiological saline at 37, 65 and 95 degrees C for time periods from 0 to 5000 h. Dry samples were tested as controls. Results show no significant loss in compressive property values of the saline-saturated or the dry control samples as a function of conditioning time or temperature.

Benzophenones↗

PubMed1992

JPEG compression for PACS.

In the medical field, especially in diagnostic radiology, there still remains controversy over how and whether or not to compress X-ray images for storage and transmission. The joint Photographic Expert Group (JPEG) standard which has recently been agreed upon is the very attractive technique to archive and to transport images in medical fields. This technique is based on 'lossy' compression of images, which can handle not only X-ray images but also full-colored images, and is suitable for introduction into picture archiving and communicating systems (PACS). The images can be handled after compression as quite small clusters of data. For example, a single 2000 x 2000 x 12 bits chest X-ray image which is an 8 Mbyte image compressed at a 10:1 ratio could retain virtually all the visible quality of the original version, and would take 100 s to transmit at 64 kbits/s using the Integrated Services Digital Network (ISDN, 800 kbytes compressed file, 8 kbytes/s transmission theoretically). An important factor in the design of this technique is that this format relies on no specific hardware or software if using JFIF (JPEG File interchange Format). Soon this algorithm will be able to run on any workstation or on any PC in the world. Highly compressed images may be unsuitable for diagnostic purposes. However, they may be sufficient for reference images which will be needed in clinical fields.

Algorithms↗

PubMed1985

How to avoid compression: a model study of latex sphere grid sections.

The parameters which might play a role in the compression of plastic-embedded objects are studied. The compression is measured on spherical polystyrene latex particles, used as markers in the grid sectioning technique. By changing independently the hardness of the latex particles through a controlled electron irradiation and the hardness of the embedding medium, it is shown that compression is a local event depending only on the mechanical properties of the sectioned object and not on the properties of its surrounding materials. It is demonstrated on one hand that the intrinsic resin compression diminishes as resin hardness increases, and on the other hand that the latex compression can be completely eliminated after a preliminary irradiation by electrons (the electron-induced vulcanization is equivalent to a hardening). It is thus concluded that compression could be greatly reduced or eliminated if objects were sufficiently hardened during their preparation. Several preparation procedures for biological specimens are suggested. For comparison latex has also been irradiated 31/2 days near a 25 000-Ci 60Co gamma source and near the core of an 8-MW nuclear reactor. Neither of these irradiations was sufficient to produce a hardening equivalent to that of the electron irraidation.

Electrons↗

PubMed2005

Activation of chondrocytes calcium signalling by dynamic compression is independent of number of cycles.

Mechanical loading is necessary for the development and maintenance of healthy articular cartilage through the control of extracellular matrix synthesis and catabolism. However, the underlying process of chondrocyte mechanotransduction remains unclear. This study examined the influence of cyclic compression on intracellular calcium (Ca(2+)) signalling within isolated articular chondrocytes cultured in agarose constructs. A validated experimental system was developed for applying controlled cyclic cell deformation. Cell-agarose constructs were subjected to 1Hz cyclic compression between 0 and 10% gross strain for 1, 10, 100 or 300 cycles. The cells were subsequently visualised for 300s in the unstrained state using confocal microscopy and the Ca(2+) indicator, Fluo-4 AM. Within unloaded control constructs, a sub-population of approximately 50% of chondrocytes exhibited characteristic spontaneous Ca(2+) transients each lasting approximately 40-60s. Cyclic compression, for only 1 cycle, significantly up-regulated the percentage of cells exhibiting Ca(2+) transients in the subsequent 5min period (p<0.05). Increasing the number of cycles to 10 or 100 had no additional effect. The up-regulated Ca(2+) signalling was maintained for up to 5min before returning to basal levels. By contrast, 300 cycles were followed by Ca(2+) signalling that was not significantly different from that in unloaded controls. However, this response was shown to be due to the increased time following the start of compression. In conclusion, this study indicates that chondrocyte Ca(2+) signalling is stimulated by dynamic compression, probably mediated by cyclic cell deformation. The overall response appears to be independent of the number of cycles or duration of cyclic compression. The sustained up-regulation of Ca(2+) signalling after 1, 10 or 100 cycles suggests the involvement of an autocrine-paracrine signalling mechanism. Furthermore, the reduced response following 300 cycles indicates a possible receptor desensitisation mechanism. Therefore, Ca(2+) signalling may be part of a mechanotransduction pathway through which chondrocyte populations can modulate their metabolic activity in response to changing mechanical stimuli.

Animals↗

PubMed2005

Extradural compression of the sensorimotor cortex delays the acquisition but not the recalling of a lever-pressing task in Wistar rats.

The learning and recalling of a lever-press task (LPT) after brief unilateral extradural compression (EC) of the right sensorimotor cortex was studied in Wistar rats. All rats, regardless of the time-point for EC, were trained to lever press for food from D(day)1 to D6. On D8, the position of the active lever was changed to the right side of the operant box and performance was tested until D14. Total and active lever presses, as well as % errors were used to analyse the performance. Rats submitted to EC 24 h before initiating the LPT schedule (naïve-compressed group) showed delayed task acquisition and impaired performance until D10. No significant impairments were detected by D3 on a beam-walking test, excluding paresis as the cause to the delay. Rats submitted to EC after they learned the LPT (trained-compressed group) showed only mildly impaired post-compression performance with no effects on the recalling of the task. Using a progressive ratio LPT, the maximum number of presses to obtain a food-pellet (breaking point) was significantly reduced 24h after EC suggesting reduced motivation for the task early after brain injury. The delayed acquisition of the LPT in naïve-compressed rats was accompanied by consistent cortical, striatal and thalamic degeneration detected by Fluoro-Jade and anti-glial fibrillary acidic protein (GFAP) staining, whereas the improvement in the performance of this group was accompanied by a reduction of the cortical damage on D10. Recall of the LPT in trained-compressed rats was not altered by EC, suggesting the contribution of compensatory mechanisms.

Analysis of Variance↗

PubMed2005

Influence of the pressure on the properties of chromatographic columns. I. Measurement of the compressibility of methanol-water mixtures on a mesoporous silica adsorbent.

The compressibilities of aqueous solutions of methanol or acetonitrile containing 0, 20, 40, 60, 80 and 100% (v/v) organic solvent were measured with a dynamic chromatographic method. The elution volumes of thiourea samples (2 microL) in these solutions were measured at different average column pressures, adjusted by placing suitable capillary restrictors on-line, after the detector. The reproducibility of the measurements was better than 0.2%. In the range of average pressures studied (10-350 bar), the maximum change in elution volume of thiourea is 1.3% (in pure water) and 4.0% (in pure methanol). This difference is due to the different compressibilities of these pure solvents. For mixtures, the plots of the elution volume of thiourea versus the pressure are convex downward, which is inconsistent with the opposite curvature predicted by the classical Tait model of liquid compressibility. This difference is explained by the variation of the amount of thiourea adsorbed with the pressure. The deconvolution of the two effects, adsorption of thiourea and solvent compressibility, allows a fair and consistent determination of the compressibilities of the methanol-water mixtures. A column packed with non-porous silica particles was also used to determine the compressibility of methanol-water and acetonitrile-water mixtures. A negative deviation by respect to ideal behavior was observed.

Chromatography, Liquid↗

PubMed2006

Progress in characterization of Langmuir monolayers by consideration of compressibility.

Over decades, information about the rheological properties of the condensed monolayer phases has been obtained by introduction of a two-dimensional compressibility which is defined on the basis of the surface pressure-molecular area (Pi-A) features of the monolayer. Since the last decade, fundamental progress was attained in the experimental determination of the main characteristics of Langmuir monolayers in microscopic and molecular scale. Already smallest changes in the molecular structure of the amphiphile can result in changes in the molecular arrangement in the monolayer and thus, in changes of the main characteristics of the monolayer such as, the surface pressure-area per molecule (Pi-A) isotherms, the shape and texture of the condensed phase domains and the two-dimensional lattice structure. As the classical equations of state allowed only characterisation of the fluid (gaseous, liquid-expanded) state, thermodynamically based equations of state, which consider also the aggregation of the monolayer material to the condensed phase, have been developed. The present review focuses particularly to amphiphilic monolayers, the Pi-A isotherms of which indicate the existence of two condensed phases. For this case, the experimental results of the differences in the structure features and phase properties are discussed. The generalisation of the equation of state for Langmuir monolayers developed for the case that one, two or more phase transitions in the monolayer take place, is in agreement with the experimental results that the two-dimensional compressibility of the condensed phases undergoes a jump at the phase transition, whereas the compressibility is proportional to the surface pressure within one of the condensed phases. An example is presented which explains the procedure of the theoretical analysis of Pi-A isotherms indicating the existence of two condensed phases. An element of the procedure is the application of the general principle that the behaviour of any thermodynamic system is determined by the stability condition. An interesting anisotropy of the compressibility is revealed by GIXD studies of the S-phase of octadecanol monolayers. However, similar studies performed close to the LS-S-phase transition would result in a thermodynamically impossible negative compressibility. Close to this phase transition, the compressibility cannot be determined from the positions of the maxima because the monolayer is in a disordered state attributed to elastic distortions by fluctuations with the structure of the new phase in the surrounding matrix without destroying the quasi-long-range positional order.

Mechanics↗

PubMedDate not supplied

Ruptures of the medial head of the gastrocnemius ("tennis leg"): clinical outcome and compression effect.

UNLABELLED: The purpose of this study is to describe the clinical outcome of conservative treatment in patients with rupture of the medial head of the gastrocnemius ("tennis leg") occurring during exercise and training and the effect of the compression treatment. MATERIALS AND METHODS: Thirty patients (conservative treatment=15; compression treatment=15), aged 30-45 years, with clinically suspected rupture of the medial head of the gastrocnemius occurring during exercise and training were referred for ultrasonography (US) examination. All patients underwent US of the affected and contralateral asymptomatic limb. Follow-up clinical evaluation and US imaging of all patients each were performed at 1-week intervals during the month after injury and at 2-week intervals during the following 6 months. US findings were analyzed with respect to the integrity of the musculotendinous junctions of the gastrocnemius, the presence of a fluid collection, and the union time of the medial head of the gastrocnemius with the soleus muscle. In addition, we analyzed the clinical outcome of the compression treatment. RESULTS: Of the 30 patients who had an initial US examination after their injury, partial rupture of the medial head of the gastrocnemius muscle was identified in 11 patients (36.7%); the remaining 19 patients were diagnosed with complete rupture. Fluid collection between the medial head of the gastrocnemius and the soleus muscle was identified in 25 patients (83.3%). First union of the medial head of the gastrocnemius with the soleus muscle in the compressive group was significantly rapid than that of the conservative group (4.25 vs. 3.25 weeks; P=.02). Fluid collection between the two muscles after 1 month in the compressive group was significantly smaller than that in the conservative group (8.9 vs. 4.5 mm; P=.01). CONCLUSIONS: Early compressive treatment in patients with rupture of the medial head of the gastrocnemius decreases the hemorrhage amount and can be possible for early ambulation.

Adult↗

PubMed2005

The incidence of hepatic pseudolesions caused by focal rib compression as seen on multidetector row CT in patients of different hepatic function.

OBJECTIVE: To determine whether the incidence of pseudolesions of the liver caused by rib compression is correlated with liver function. MATERIALS AND METHODS: Multidetector row CT images of 150 consecutive cases were evaluated for the presence of pseudolesions caused by rib compression. Liver function was categorized into two groups mainly using the Child-Pugh classification: Group I-normal liver function and Grade A; Group II-Grades B and C. The number of focal deformities of the liver caused by rib compression was also counted. The incidence of pseudolesions based on number of patients or focal deformities of liver parenchyma by rib compression were compared between the two groups. RESULTS: There were 108 and 42 patients in Groups I and II, respectively. 169 and 41 focal deformities of the liver caused by rib compression were found in Groups I and II, respectively. Patient-based and deformity-based incidences of pseudolesions were 10.7% (16/150) and 7.6% (16/210), respectively. All pseudolesions were seen in patients of Group I. Both patient-based and deformity-based incidences of pseudolesions were significantly higher in Group I as compared with those in Group II. CONCLUSION: Pseudolesions of the liver caused by focal rib compression may be seen more frequently in patients with preserved liver function than in those with impaired function.

Adult↗

PubMed2005

Compression estimates using behavioral and otoacoustic emission measures.

Cochlear compression in normal-hearing listeners was estimated at octave frequencies from 250 to 4000 Hz using a forward-masking paradigm. Temporal masking curves (TMCs) for a 10-dB SL signal were obtained with two maskers -- one equal in frequency to the signal and another an octave below the signal. The ratio of the slope of the off-frequency function to that of the mid-level portion of the on-frequency function was computed as an estimate of the amount of compression at each frequency. Compression was less frequency selective at low frequencies, so an average of the off-frequency slopes at high frequencies (1000, 2000, and 4000 Hz) was used in computing the ratio for each signal frequency. Results indicated strong compression (approximately 0.15-0.30) at all frequencies using the averaged off-frequency slopes, indicating little difference in compression across frequencies. Distortion product otoacoustic emission (DPOAE) input-output (I-O) functions were obtained for each subject at 1000, 2000, and 4000 Hz. The slopes of the DPOAE I-O functions and the psychophysical growth rates were similar to one another, reinforcing the assumption that the forward-masking procedure is providing an estimate of cochlear compression, at least at frequencies from 1000 to 4000 Hz.

Acoustic Impedance Tests↗

PubMed2005

Compression forces generated by Mini bone screws--a comparative study done on bone model.

The compressive forces generated by the AO/ASIF 3.0 mm cannulated cancellous and 2.0 mm cortical screws, Mini-Acutrak and Herbert/Whipple small bone cannulated screws were measured in the laboratory with the use of simulated cancellous bone and a load cell washer as a means of quantifying their fixation capabilities. The Herbert/Whipple screw and the Mini-Acutrak screw were found to have nearly identical compression capabilities and provided more compression than the cortical screw. The AO/ASIF cannulated screw when used with a support screw demonstrated a compressive capacity twice that of the 2.0 mm cortical screw and higher than the headless Mini-Acutrak and Herbert/Whipple screws. The Mini-Acutrak screw produced about 70% of compression of the cancellous screw in spite of having a diameter almost half that of the cancellous screw. The Herbert/Whipple screw in spite of its larger size compared to the Mini-Acutrak produced almost the same amount of compression.

Biomechanical Phenomena↗