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At least 289 records · Page 16Linked to original sources

PubMed2000

Load-bearing and stress analysis of the human spine under a novel wrapping compression loading.

OBJECTIVE: To examine biomechanics of the human spine under a novel compression loading that follows the curvature of the spine.Design. The detailed response of the spine is predicted and compared under various types of compression loading at different postures. BACKGROUND: The posture and loading configuration could be so adjusted as to increase load-bearing capacity and stability of the spine in compression while minimizing the muscle activity and risk of tissue injury. METHODS: The nonlinear finite element formulation of wrapping elements sliding over solid body edges is developed and used to study the load-bearing capacity of simplified beam-rigid body thoracolumbar (T1-S1) and lumbosacral (L1-S1) spines under a wrapping compression force. The load-bearing and stress analysis of a detailed model of the lumbar spine, L1-S1, is also investigated under five wrapping loads resulting in differential compression forces at various levels. Follower load at L1, axially fixed compression at L1, and combined axially fixed compression and moments load are also considered for comparison. For the detailed model, the effect of changes in the position of wrapping elements and in the lumbar curvature on results are considered. RESULTS: The idealized wrapping loading stiffens the spine, allowing it to carry very large compression loads without hypermobility. It diminishes local segmental shear forces and moments as well as tissue stresses. CONCLUSIONS: In comparison to fixed axial compression, the compression loading by wrapping elements that follow the spinal curvatures increases the load-bearing capacity in compression and provides a greater margin of safety against both instability and tissue injury. Relevance These findings suggest a plausible mechanism in which postural changes and muscle activation patterns could be exploited to yield a loading configuration somewhat similar to that of the wrapping loading, i.e., the net reaction force at various levels passes through discs nearly normal to their mid-height plane. To alleviate hypermobility in compression, the wrapping loading could also allow for the application of meaningful compression loads in experimental as well as model studies of the multi-segmental spinal biomechanics.

Aged↗

PubMed2000

Aspiration in chest compression alone without mechanical ventilation in the head down position in dogs.

BACKGROUND: Previous work by the authors has shown that chest compressions alone without mechanical ventilation during cardiopulmonary resuscitation in the natural supine position was associated with pulmonary aspiration in dogs. The purpose of this investigation was to test the hypothesis that a head down position may prevent aspiration during chest compressions alone and whether oxygenation can be improved by simply insufflation of oral oxygen 10 min after cardiac arrest. METHODS: Cardiac arrest was induced in ten mongrel dogs which were anesthetized and paralysed. Eight underwent chest compressions alone in different head down positions using an automatic compressor at 9 kg compression force and 3 cm compression depth. The study was composed of two parts. Part 1 evaluated the effect of insufflation of 10 l/min O2, into the mouth of the dogs, 10 min after initiation of resuscitation, using chest compressions alone. Part 2 was designed to test our hypothesis that the head down position may protect the lungs from aspiration during chest compression alone. The mouths of the dogs were filled with mixed barium and the dogs underwent serial episodes of chest compressions, for 10 min each, in the 20 degree head down, 10 degree head down and the natural supine positions. Chest X-rays with antero-posterior and lateral views were taken to evaluate pulmonary aspiration. Two additional dogs underwent direct chest compression alone in the natural supine position and the time of chest compression was shortened to 5 min. RESULTS: All dogs in the natural position showed evidence of pulmonary aspiration of barium, five or six of the dogs showed tracheal aspiration in the 10 degree head down position, while no any barium was visualized in the tracheo-broncheal trees of the dogs in the 20 degree head down position. Supplemental oxygen in the mouth improved the mean PaO2 from 67 +/- 26 to 160 +/- 97 mmHg during chest compressions alone. CONCLUSION: Chest compression alone without mechanical ventilation in the supine position caused pulmonary aspiration in the unprotected airway in dogs. This complication could be prevented by adopting a 20 degree head down position. The 10 degree head down position seemed to reduce the severity of the pulmonary aspiration, but not enough to eliminate the danger altogether. Supplemental oxygen in the mouth can improve oxygenation in chest compressions alone.

Administration, Oral↗

PubMed2001

Emergency Department compression ultrasound to diagnose proximal deep vein thrombosis.

Emergency Department (ED) patients with suspected deep vein thrombosis (DVT) require an objective vascular study such as ultrasound (US) to confirm the diagnosis prior to treatment or disposition. A simple compression US test of the common femoral vein and popliteal vein reliably detects proximal DVT in symptomatic patients. Application of compression US in the ED by Emergency Physicians (EPs) has been tested in a single previous study. We evaluated the ability of ED compression US, performed by EPs, to diagnose proximal DVT as compared to duplex US performed in a vascular laboratory. A prospective, observational study was conducted on a convenience sample of patients presenting to an ED with lower extremity symptoms and signs suggestive of DVT. Patients with a history of DVT in the symptomatic extremity were excluded. Final diagnosis of DVT was made by color-flow duplex US performed in a vascular laboratory. ED compression US was performed by one of six EP sonographers. In compression US, DVT was diagnosed by the inability to compress the common femoral vein or popliteal vein. The examination was considered indeterminate if the veins could not be clearly identified or compressibility was equivocal. For statistical analysis, an indeterminate examination was considered positive. In those cases where ED compression US was discordant with duplex US, and not indeterminate, we retrospectively reviewed the US findings. There were 76 patients who completed the study, and 18 patients (24%) were diagnosed with DVT by duplex US, among whom ED compression US was positive in 14, negative in 2, and indeterminate in 2. Among 58 patients diagnosed without DVT by duplex US, there were 4 false-positive ED compression US examinations and 10 indeterminate examinations. In all, ED compression US was indeterminate in 12 patients (15.8%). Compared to duplex US, ED compression US had a sensitivity of 88.9% (95% C.I. 65.3-98.6%) and specificity of 75.9% (62.8-86.1). Negative predictive value was 95.7% (85.2-99.5). Among ED patients with the clinical diagnosis of possible DVT, negative ED compression US greatly reduces the likelihood of DVT, such that discharge and outpatient follow-up can be considered. Because of limited specificity, positive results require confirmation, but may justify immediate treatment pending follow-up testing. Indeterminate results can be expected in a significant number of patients and mandate further testing prior to disposition.

Emergencies↗

PubMed1996

Inelastic versus elastic leg compression in chronic venous insufficiency: a comparison of limb size and venous hemodynamics.

PURPOSE: Compression of the lower extremity is the mainstay of therapy in patients who have chronic venous insufficiency. We evaluated the ability of two forms of compression-elastic stockings and an inelastic compression garment-with air plethysmography to determine how well they corrected abnormal deep venous hemodynamics in patients who had class III chronic venous insufficiency and how well this correction was sustained over time. METHODS: Patients had measurements taken with no compression, with a 30 to 40 mm Hg below-knee stocking, and with the inelastic compression garment 2 hours and 6 hours after donning the garments. Therapies were compared with baseline and with themselves over time. RESULTS: Inelastic compression maintained limb size and reduced venous volume better than no compression or stockings over time (ankle circumference at 2 hr vs 6 hr: baseline, 24.7 +/- 7 cm vs 26.1 +/- 1.1 cm; stocking, 23.9 +/- 1.1 cm vs 26.2 +/- 1.2 cm; inelastic compression, 25.4 +/- 1.1 cm vs 25.4 +/- 0.9 cm; venous volume at 2 hr vs 6 hr: baseline, 97.5 +/- 14.1 ml vs 105.2 +/- 17.9 ml; stocking, 112.4 +/- 29.7 ml vs 77.5 +/- 13.2 ml; inelastic compression, 72.2 +/- 14.1 ml vs 56.1 +/- 10.2 ml). At 6 hours, the ejection fraction was increased and the venous filling index was significantly less with inelastic compression compared with the stocking and baseline (ejection fraction at 6 hr: baseline, 61.6% +/- 6.9%; stocking, 75.9% +/- 17.7%; inelastic compression, 78.8% +/- 12.2%). CONCLUSIONS: Inelastic compression has a significant effect on deep venous hemodynamics by decreasing venous reflux and improving calf muscle pump function when compared with compression stockings, which may exert their primary effect on the superficial venous system.

Adult↗

PubMed1997

Ultrasound-guided pseudoaneurysm compression: efficacy after coronary stenting and angioplasty.

PURPOSE: Ultrasound-guided compression of femoral pseudoaneurysms has often obviated the need for open operative repair. Increasing use of percutaneous interventional cardiac procedures has created pseudoaneurysms with a large-caliber arterial defect, often in patients who are placed immediately on anticoagulation therapy. This report describes the prospectively collected information from our vascular laboratory regarding ultrasound-guided compression of these pseudoaneurysms after cardiac procedures, both interventional and diagnostic. METHODS: Since March 1994 prospective data collection for patients who have undergone pseudoaneurysm compression in our vascular laboratory has recorded information including cardiac procedure, size of catheter or sheath, coagulation parameters, pseudoaneurysm size and location, and time to compression. Forty-one patients underwent attempted ultrasound-guided pseudoaneurysm compression after cardiac procedures: 19 after cardiac catheterization alone, seven after angioplasty, one after atherectomy, two after insertion and subsequent removal of an intraaortic balloon pump, and 12 after coronary stenting. RESULTS: Compression was successful overall in 88% of the patients (36 of 41). Successful compression of the pseudoaneurysm was seen in 95% after catheterization alone, 100% after angioplasty, 100% after atherectomy, and 100% after intraaortic balloon pumping, as compared with 67% after stenting (eight of 12 vs 28 of 29; p = 0.02). A sheath size of 9F or greater was a significant factor in predicting unsuccessful compression (three of eight vs two of 33; p = 0.04). Abnormal coagulation parameters were present in 20 of the 41 patients and was not significantly different in patients who were successfully or unsuccessfully treated (four of five vs 16 of 36; p = 0.40). CONCLUSIONS: Pseudoaneurysms after cardiac procedures and interventions can often be successfully compressed with an ultrasound-guided technique. The presence of abnormal coagulation parameters was not identified as a risk factor for compression failure and should not dissuade attempted compression. Stent placement was more likely to result in unsuccessful compression, and this appeared to be a result of the larger size of the arterial defect. Even in this setting, compression achieved obliteration of the pseudoaneurysm in more than half of the patients.

Aneurysm↗

PubMed1996

Impact of lossy image compression on accuracy of caries detection in digital images taken with a storage phosphor system.

Image compression may reduce storage needs whether in the lossless (reversible) or lossy (irreversible) form. The aims of the study were to evaluate (1) storage needs, (2) subjective image quality, and (3) accuracy of caries detection in digital radiographs compressed to various levels by a lossy compression method. The material consisted of 116 extracted human premolars and molars. The teeth were mounted three in a line and radiographed by the Digora system (Sorodex Medical Systems, Helsinki, Finland). The images were exported in tagged image file format and compressed with the Lempel-Ziv-Welch reversible and the Joint Photographic Experts Group irreversible compression algorithm on four levels. The total of 580 images were assessed by five observers on a 5-rank confidence scale for caries diagnosis. The observers subjectively judged image quality on an 11-point rank scale. With the reversible compression, images could be compressed to less than 50% of the original storage needs whereas the four irreversible compression factors compressed to 20%, 8%, 5%, and 3%, respectively. For occlusal surfaces, when receiver operating characteristic curve areas were increasingly smaller and the compression rate was higher. The difference between the original and the most compressed images was 14% (p = 0.1). The median quality score was above middle on the 11-point rank scale for all except the most compressed images (median score = 1). In conclusion, for caries diagnosis, compression ratio rates of 1:12 can be justified before accuracy and image quality is significantly affected.

Dental Caries↗

PubMed2001

Sources of age-related recognition difficulty for time-compressed speech.

Older people frequently show poorer recognition of rapid speech or time-compressed speech than younger listeners. The present investigation sought to determine if the age-related problem in recognition of time-compressed speech could be attributed primarily to a decline in the speed of information processing or to a decline in processing brief acoustic cues. The role of the availability of linguistic cues on recognition performance was examined also. Younger and older listeners with normal hearing and with hearing loss participated in the experiments. Stimuli were sentences, linguistic phrases, and strings of random words that were unmodified in duration or were time compressed with uniform time compression or with selective time compression of consonants, vowels, or pauses. Age effects were observed for recognition of unmodified random words, but not for sentences and linguistic phrases. Analysis of difference scores (unmodified speech versus time-compressed speech) showed age effects for time-compressed sentences and phrases. The forms of time compression that were notably difficult for older listeners were uniform time compression and selective time compression of consonants. Indeed, poor performance in recognizing uniformly time-compressed speech was attributed primarily to difficulty in recognizing speech that incorporated selective time compression of consonants. Hearing loss effects were observed also for most of the listening conditions, although these effects were independent of the aging effects. In general, the findings support the notion that the problems of older listeners in recognizing time-compressed speech are associated with difficulty in processing the brief, limited acoustic cues for consonants that are inherent in rapid speech.

Adult↗

PubMed2000

Duration, compression, and the aided loudness discomfort level.

OBJECTIVE: The purpose of this investigation is to determine how the unaided and aided loudness discomfort level (LDL) varies with the duration of the input signal and whether the electroacoustic characteristics of compression circuits affect this relationship in a manner that may alter the listener's dynamic range for short duration sounds. DESIGN: Ten hearing-impaired and 20 normal-hearing listeners participated. LDLs were determined for noise bursts of durations ranging in six steps from 32 to 1024 msec, using a two-alternative, forced-choice adaptive tracking procedure in which input level varied until LDL was achieved. LDLs were also obtained for continuous discourse, using a clinical procedure. Subjects were also given the opportunity to self adjust maximum output SPL to their LDL using either output limiting or volume controls in response to fixed 90 dB SPL noise bursts. Testing was conducted unaided and with hearing aids representing two analog (output compression limiting, wide dynamic range compression) and four digital compression circuits. Primary circuit contrasts included compression threshold, compression ratio, attack time and the presence or absence of unity gain at high levels. RESULTS: For the unaided condition, both normal-hearing and hearing-impaired subjects showed increasing LDLs with decreasing signal duration. Under aided conditions, circuits with compression thresholds of 45 to 50 dB SPL and compression ratios of 2:1 produced LDL functions that were similar in slope to the impaired listener's unaided functions. Slopes were steeper when the attack time was slow (128 msec) than when it was fast (2 msec). Circuits with compression ratios of 8:1 produced flat LDL duration functions (i.e., a loss of duration-dependent effects). Similar duration-dependent LDL effects were also observed when subjects adjusted their own hearing aid output characteristics in response to 90 dB noise bursts. CONCLUSION: For the unaided condition, results suggest that normal-hearing and hearing-impaired listeners can tolerate short duration sounds at higher levels than long duration sounds, a finding that has implications for hearing aid design. Circuits that preserve the relationship between duration and LDL should allow brief phonemes to be presented at higher levels without discomfort than circuits that do not, possibly resulting in greater audibility or speech recognition. Current results suggest that circuits with low compression thresholds, low compression ratios, and slow attack times might accomplish this objective better than circuits with high compression thresholds, high compression ratios and fast attack times.

Adult↗

PubMed2004

Intermittent venous compression, and the duration of hyperaemia in the common femoral artery.

External compression of limbs to below-diastolic pressure (venous compression) has been shown to produce a short-lived hyperaemia in supply arteries. Intermittent pneumatic compression is currently under investigation therefore as a treatment for peripheral arterial disease. The optimal timing of the compression will depend on the duration of hyperaemia produced by a particular duration of compression, and the purpose of this work was to test that link. Nineteen healthy volunteers underwent intermittent compression of one leg with two compression cycles - one compressing for 10 s each time, the other for 1 min. Blood flow velocities in the common femoral artery was shown to increase on release of the compression by 38% (inter-quartile range 27-56%) for the sequence with short duration compression, and by 57% (inter-quartile range 37-87%) for the longer sequence (difference, P = 0.005, Wilcoxon). The hyperaemia duration above the baseline level was 37 s (inter-quartile range 32-49 s) for the short sequence, and 54 s (inter-quartile range 37-76 s) for the longer sequence (difference, P = 0.001, Wilcoxon). The magnitude of the change in the compression duration was not equalled by the difference in hyperaemia duration, suggesting that the physiological mechanism behind the hyperaemia is unlikely to be due solely to simple accumulation of metabolites, and a myogenic mechanism remains possible. Therapies for peripheral arterial disease need not employ long duration compression, as a greater percentage of time will be spent in hyperaemia with short duration intermittent compression.

Adult↗

PubMed1995

Chest compression and ventilation rates during cardiopulmonary resuscitation: the effects of audible tone guidance.

OBJECTIVES: To determine: 1) whether chest compressions during CPR are being performed according to American Heart Association (AHA) guidelines during cardiac arrest; and 2) the effect of an audio prompt to guide chest compressions on compliance with AHA guidelines and hemodynamic parameters associated with successful resuscitation. METHODS: An observational clinical report and laboratory study was conducted. A research observer responded to a convenience sample of cardiac arrests within a 300-bed hospital and counted the rate of chest compressions and ventilations during CPR. To evaluate the effect of an audio prompt on CPR, health care providers performed chest compression without guidance using a porcine cardiac arrest model for 1 minute, followed by a second minute in which audio guidance was added. Chest compression rates, arterial and venous blood pressures, end-tidal CO2 (ETCO2) levels, and coronary perfusion pressures were measured and compared for the two periods. RESULTS: Twelve in-hospital cardiac arrests were observed in the clinical part of the study. Only two of 12 patients had chest compressions performed within AHA guidelines. No patient had respirations performed within AHA guidelines. In the laboratory, 41 volunteers were tested, with 66% performing chest compressions outside the AHA standards for compression rate without audible tone guidance. With guided chest compressions, the mean (+/- SD) chest compression rate increased from 74 +/- 22 to 100 +/- 3/min (p < 0.01). End-tidal CO2 levels increased from 15 +/- 7 to 17 +/- 7 torr (p < 0.01). Coronary perfusion pressure increased minimally with audible tone-guided chest compressions. CONCLUSIONS: The majority of Basic Cardiac Life Support--certified health care professionals did not perform CPR according to AHA-recommended guidelines. The use of audible tones to guide chest compression resulted in significantly higher chest compression rates and ETCO2 levels.

American Heart Association↗

PubMed2000

Unconfined compression of articular cartilage: nonlinear behavior and comparison with a fibril-reinforced biphasic model.

Mechanical behavior of articular cartilage was characterized in unconfined compression to delineate regimes of linear and nonlinear behavior, to investigate the ability of a fibril-reinforced biphasic model to describe measurements, and to test the prediction of biphasic and poroelastic models that tissue dimensions alter tissue stiffness through a specific scaling law for time and frequency. Disks of full-thickness adult articular cartilage from bovine humeral heads were subjected to successive applications of small-amplitude ramp compressions cumulating to a 10 percent compression offset where a series of sinusoidal and ramp compression and ramp release displacements were superposed. We found all equilibrium behavior (up to 10 percent axial compression offset) to be linear, while most nonequilibrium behavior was nonlinear, with the exception of small-amplitude ramp compressions applied from the same compression offset. Observed nonlinear behavior included compression-offset-dependent stiffening of the transient response to ramp compression, nonlinear maintenance of compressive stress during release from a prescribed offset, and a nonlinear reduction in dynamic stiffness with increasing amplitudes of sinusoidal compression. The fibril-reinforced biphasic model was able to describe stress relaxation response to ramp compression, including the high ratio of peak to equilibrium load. However, compression offset-dependent stiffening appeared to suggest strain-dependent parameters involving strain-dependent fibril network stiffness and strain-dependent hydraulic permeability. Finally, testing of disks of different diameters and rescaling of the frequency according to the rule prescribed by current biphasic and poroelastic models (rescaling with respect to the sample's radius squared) reasonably confirmed the validity of that scaling rule. The overall results of this study support several aspects of current theoretical models of articular cartilage mechanical behavior, motivate further experimental characterization, and suggest the inclusion of specific nonlinear behaviors to models.

Animals↗

PubMed1998

Pseudoaneurysm of femoral artery after catheterisation: treatment by a mechanical compression device guided by colour Doppler ultrasound.

BACKGROUND: Femoral artery pseudoaneurysm is a significant complication in patients undergoing diagnostic or therapeutic catheterisation. First choice treatment for pseudoaneurysm is freehand ultrasound guided compression repair, which is time consuming and uncomfortable for the patient and operator. AIM: To explore a mechanical compression device (FemoStop) as an alternative treatment for iatrogenic femoral artery pseudoaneurysm. METHODS: Fourteen patients with pseudoaneurysm were considered for treatment with FemoStop after a brief freehand ultrasound guided compression repair to confirm the compressibility of the lesion. The FemoStop compression was applied for 20 minutes. The result was controlled with colour Doppler ultrasound, and a second cycle of 20 minutes followed if necessary. RESULTS: FemoStop compression was successful in 13 of the 14 patients. The mean compression time was 33 minutes (range, 20-60). The mean number of compression periods was 1.6 (range 1-3). FemoStop compression was successful in all 11 patients not taking anticoagulants and in two of three patients receiving anticoagulants. The mean compression time in patients given oral or intravenous anticoagulants was longer (50 v 27 minutes). Colour Doppler ultrasound 12 hours after the procedure indicated no recurrence of pseudoaneurysm in the 13 patients with initial success. CONCLUSIONS: FemoStop compression for iatrogenic pseudoaneurysm is feasible, and as safe and effective as freehand ultrasound guided compression repair. It is more comfortable for the patient and operator, and probably more economical than freehand compression.

Adult↗

PubMed2002

Aberrant subclavian arteries: cross-sectional imaging findings in infants and children referred for evaluation of extrinsic airway compression.

OBJECTIVE: The purpose of our study was to describe patterns of airway compression identified on cross-sectional imaging in infants and children with either right aortic arch and aberrant left subclavian artery or left aortic arch with aberrant right subclavian artery. MATERIALS AND METHODS: Data from MR imaging and CT performed to evaluate pediatric patients for extrinsic airway compression were reviewed for cases that revealed an aberrant right or left subclavian artery. Clinical, endoscopic, and imaging findings in identified cases were reviewed. Recurrent patterns of extrinsic compression were reviewed among cases. RESULTS: Twelve patients with right aortic arch with aberrant left subclavian artery and nine patients with left aortic arch and aberrant right subclavian artery were identified. All 12 with right aortic arch with aberrant left subclavian artery had airway compression shown, with multiple sites or diffuse compression in six. Of these 12 patients, nine had compression at the level of the arch and aberrant subclavian artery (10 had Kommerell's diverticulum), and nine had compression of the distal airway in association with a midline descending aorta. Five of the nine patients with left aortic arch and aberrant right subclavian artery had airway compression shown, all at the level of the arch and aberrant subclavian artery. None of these compressions was associated with either Kommerell's diverticulum or midline descending aorta. CONCLUSION: Both right and left aberrant subclavian arteries can be associated with symptomatic airway compression, but the patterns of compression are different. The airway compression in right aortic arch with aberrant left subclavian artery is often associated with either Kommerell's diverticulum or midline descending aorta, whereas compression associated with left aortic arch and aberrant right subclavian artery is not.

Airway Obstruction↗

PubMed2005

Fracture patterns of the adolescent porcine spine: an experimental loading study in bending-compression.

STUDY DESIGN: To expose functional spinal units from adolescent porcine to mechanical flexion-compression and extension-compression to failure. The biomechanical, radiologic, magnetic resonance imaging, and histologic characteristics are described. OBJECTIVES: The aim of the present study was to investigate the fracture pattern of functional spinal units from adolescent porcine lumbar spines in in vitro compression loading and bending. SUMMARY OF BACKGROUND DATA: In several studies, it has been shown that the adolescent spine, especially the vertebral growth zones, is vulnerable to trauma. A high frequency of abnormalities affecting the spine has been found among athletes participating in sports with high demands on the back. The etiology of these abnormalities is still a controversial issue. METHODS: Sixteen functional spinal units obtained from eight adolescent male pigs were used. Eight functional spinal units were exposed to flexion-compression and eight functional spinal units to extension-compression loading to failure. They were examined with plain radiography and magnetic resonance imaging before and after the loading. The functional spinal units were finally examined macroscopically and histologically. RESULTS: Fractures/separations were seen in the growth zone anteriorly and more frequently, posteriorly in functional spinal units exposed to flexion-compression. In the extension-compression group, such injuries occurred only anteriorly. Only large fractures could be seen on plain radiographs and on magnetic resonance imaging. Macroscopically, a fracture/separation could be seen in 15 cases and histologically in all 16 cases. The median angle at failure for the flexion group was 17 degrees (range, 12-19) and for the extension group 17 degrees (range, 13-19 degrees). The median ultimate compression load in the flexion-compression group was 1894 N (range, 1607-3138 N) and in the extension-compression group 1801 N (range, 1158-2368 N). CONCLUSIONS: The weakest part of the growing porcine lumbar spine, when compressed into flexion- or extension-compression, was the growth zone. The injury was more extensive in extension loading than during flexion loading. Growth zone injuries of the adolescent spine may go undetected on plain radiographs and magnetic resonance imaging.

Age Factors↗

PubMed1993

Response of peripheral nerve to cyclic compression in a laboratory rat model.

Repetitive cyclic loading of a nerve has been proposed as a pathogenic factor in the development of occupational compression neuropathies. Little is known about the basic response of peripheral nerve to cyclic compression. We investigated the hypothesis that cyclic compression is more detrimental to nerve function than constant compression. We measured the amplitudes and velocities of distally evoked action potentials in the presence of constant or cyclic compression of the tibial nerve in rats. Seven groups were subjected to constant or cyclic compression for 6 h by a computer controlled, hydraulically activated compression chamber. Nerves were compressed with 0 (control group), 30, 60, or 90 mm Hg of constant pressure or 0-30, 20-50, or 30-60 mm Hg of cyclic compression for approximately 20,000 compression cycles. Action potentials were recorded every 15 min. The effects of cyclic compression on nerve conduction were equivalent to the effects of constant compression at the average applied pressure. Cyclic loading itself does not appear to be an important pathogenic factor in the development of nerve conduction block.

Action Potentials↗

PubMed2006

Irreversible JPEG 2000 compression of abdominal CT for primary interpretation: assessment of visually lossless threshold.

To estimate the visually lossless threshold for Joint Photographic Experts Group (JPEG) 2000 compression of contrast-enhanced abdominal computed tomography (CT) images, 100 images were compressed to four different levels: a reversible (as negative control) and irreversible 5:1, 10:1, and 15:1. By alternately displaying the original and the compressed image on the same monitor, six radiologists independently determined if the compressed image was distinguishable from the original image. For each reader, we compared the proportion of the compressed images being rated distinguishable from the original images between the reversible compression and each of the three irreversible compressions using the exact test for paired proportions. For each reader, the proportion was not significantly different between the reversible (0-1%, 0/100 to 1/100) and irreversible 5:1 compression (0-3%). However, the proportion significantly increased with the irreversible 10:1 (95-99%) and 15:1 compressions (100%) versus reversible compression in all readers (P < 0.001); 100 and 95% of the 5:1 compressed images were rated indistinguishable from the original images by at least five of the six readers and all readers, respectively. Irreversibly 5:1 compressed abdominal CT images are visually lossless and, therefore, potentially acceptable for primary interpretation.

Contrast Media↗

PubMed1985

Learning and interocular transfer of visual discriminations by goldfish with retinotectal compression.

Visual function mediated by a compressed retinotectal projection was examined by training goldfish with unilateral retinotectal compression to perform red/green and horizontal/vertical discriminations. Fish were trained monocularly via the compressed or the normal visual field using an aversive classical conditioning model. Interocular transfer was then examined to determine if the mechanisms mediating this transfer functioned normally after retinotectal compression and to compare interpretation of visual information via normal and compressed visual fields. Both visual discriminations were learned successfully using the normal or the compressed visual field. Learning deficits (relative to controls) were, however, observed in fish trained with the color discrimination using the compressed visual field, or the horizontal/vertical discrimination using the compressed or the normal visual field. Interocular transfer of the color discrimination from the compressed to the normal visual field or in the reverse direction was demonstrated to occur at approximately normal values. Interocular transfer of the horizontal/vertical discrimination was successful from the compressed to the normal visual field, but was reduced or absent in the opposite direction. The results indicate that analysis of the colors red and green is essentially normal after retinotectal compression, and that the pathways mediating interocular transfer of this color discrimination remain functional. There were, however, abnormalities in the mechanism mediating interocular transfer of pattern discriminations after retinotectal compression.

Animals↗

PubMedDate not supplied

Biomechanical changes after the augmentation of experimental osteoporotic vertebral compression fractures in the cadaveric thoracic spine.

BACKGROUND CONTEXT: Osteoporotic compression fractures are an important public health concern, leading to significant morbidity, mortality and economic burden. Cement augmentation procedures used to treat these fractures alter the biomechanics of the fractured segment, which could promote adjacent failure. However, if alignment is improved or restored, there will be less risk of adjacent failure. PURPOSE: To determine the effects of load (compression/flexion), adjacent vertebral location (superior/inferior) and augmentation on vertebral segment stiffness and adjacent vertebral strain in the upper and lower thoracic spine. STUDY DESIGN: Human cadaveric thoracic spine segments were tested under load control before and after the creation of experimentally augmented vertebral compression fractures. METHODS: Six T1-T5 and six T8-T12 segments were obtained from eight thoracic spines with known bone mineral density (BMD). Rosette strain gauges were applied to T2, T4, T9 and T11 to measure strain adjacent to the experimental fracture sites T3 and T10. Two compression fractures were created in succession, the first in flexion preceded by a weakening defect in T3 and T10 and the second created in an adjacent vertebra in compression without prior weakening. The first fracture was reduced with the inflatable bone tamp (IBT) and augmented with cement. Compression and flexion tests were performed before and after the first fracture while measuring vertebral cortical shear strain on T2, T4, T9 and T11 and stiffness of the entire segment. Strain and stiffness were compared by using a repeated measures analysis using adjacent vertebral location (superior/inferior), augmentation and load (compression/flexion) as factors. RESULTS: The mean BMD was 0.61+/-0.11 g/cm(2) (T1-T5) and 0.78+/-0.07 g/cm(2) (T8-T12). Stiffness in compression and flexion increased with load (p<.05, and p>.27, respectively). Augmentation reduced compressive and bending stiffness (p=.23, and p=.19, respectively), whereas the adjacent vertebral strain increased (p>.11). The adjacent strain in flexion was much greater than in compression (p<.03). Cement augmentation caused greater amounts of inferior than superior adjacent strain (p>.19). The applied moment at first fracture was 2.98+/-1.28 Nm (T1-T5) and 8.44+/-1.02 Nm (T8-T12). The compressive load at second fracture was 1122+/-993 N (T1-T5) and 2906+/-1008 N (T8-T12). Adjacent vertebral strain during the second compression and flexion tests exceeded that during the first compression and flexion tests (p=.11). Adjacent vertebral strain at second fracture exceeded that at first fracture (p=.007) and was greater on the superior adjacent vertebra than the inferior (p=.47). CONCLUSION: With axial compressive loads, the addition of flexion increases fracture risk. Cement augmentation of a fractured vertebral segment reduces stiffness while increasing both the superior and inferior adjacent cortical strain. This increment in strain that is greatest on the inferior adjacent vertebra effectively redistributes loads from the superior adjacent vertebra to the inferior adjacent vertebra, sparing the superior adjacent vertebra from failure.

Adult↗