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PubMed1991

Some factors affecting retention forces with the use of denture fixatives in vivo.

Using the UCL retentiometer, the effect of three proprietary denture fixatives has been clinically investigated in 14 dentate subjects wearing mucosal-borne acrylic palatal plates over periods of 6 hours. The results showed that retention forces with saliva for the well-fitting plates were significantly better when compared with the forces for the ill-fitting plates. A substantial improvement in retention on all occasions was seen for both types of plate after the application of fixatives. An interesting observation of this study was that the use of paste and liquid fixatives rendered the ill-fitting plates as retentive as the well-fitting plates with the same fixatives. The test fixatives, however, showed significant differences regarding the onset and duration of action, depending on their physical formulation. The paste and liquid fixatives were found to have more delayed action than the powder fixative, but maintained the improved retention for a post-insertion period of more than 6 hours for both well-fitting and ill-fitting plates. Investigation of the effect on retention of different quantities of fixatives showed that larger amounts were associated with greater retention. The findings of these studies predict that a single application of 0.6 ml of either the paste or liquid fixatives used in this study is a recommended quantity which would provide and maintain the desirable retention for more than 6 hours in most cases of well-fitting complete dentures and those with retention problems. The powder fixatives though very effective immediately, would, however, require repeated applications every 3 hours or so, depending to some extent upon the use that the product will be put to.

Carboxymethylcellulose Sodium↗
PubMed1998

[Fixation disparity with the Pola pointing test: not representative for eye position under natural viewing conditions].

BACKGROUND: According to certain findings obtained with the Zeiss Polatest, H.J. Haase defined a "Fixation Disparity Type One". In this diagnosis, the "Zeigertest" is particularly important. The Zeigertest consists of a central ring presented to both eyes for fixation, a vertical clock hand presented to the right eye and two markings at the six and twelve o'clock positions presented to the left eye. All parts are surrounded by a binocularly visible frame. Subjects with a "Fixation Disparity Type One" see a misalignment between the clock hand and the peripheral markings. We investigated (1) whether the perceived misalignment correlated with an objective deviation of the eyes from orthovergence and (2) whether subjects with a "Fixation Disparity Type One" had a deviation of the eyes from orthovergence when looking at a natural, i.e., fully fusionable object. SUBJECTS AND METHODS: Out of 303 medical students, 10 subjects with a "Fixation Disparity Type One" were selected and asked to indicate the perceived alignment or misalignment in the Zeigertest with a laser pointer. Two subjects without fixation disparity served as controls. The position of both eyes was recorded using the search coil technique. One of the 10 subjects with "Fixation Disparity Type One" had to be excluded due to excessive blinking. Experiment 1: In the beginning all parts of the Zeigertest were presented to both eyes (natural viewing condition). Then, the object for one of the eyes was switched off leaving the frame as the only fusional stimulus. The outcome variable was a refixation movement of the other eye. This experiment is similar to the unilateral cover test. Experiment 2: In the beginning all parts of the Zeigertest were presented to both eyes (natural viewing condition). Then, the original Zeigertest was switched on (clock hand presented only to the right eye, peripheral markings only to the left eye). The outcome variable was a change of vergence. RESULTS: Experiment 1: A significant refixation movement did not occur in any of the subjects. Experiment 2. In all 9 subjects with "Fixation Disparity Type One" the vergence changed significantly between 2.4 and 14.9 arcmin. The change of vergence correlated significantly with the angle of the perceived misalignment between clock hand and peripheral markings. CONCLUSION: A fixation disparity ascertained at the Zeigertest does not indicate a fixation disparity under natural viewing conditions.

Adult↗
PubMed1997

[Ophthalmoscopic evaluation of ocular fixation in children up to 2 years old].

BACKGROUND: The examination of fixation on the fundus oculi is one of the main points in the diagnosis of amblyopia. Therefore it is necessary to describe the fixation ability and to clarify at what age children can fixate foveally and at what age this examination can be carried out, considering not only physiological aspects, but also co-operation reasons. PATIENT AND METHODS: Fixation on the fundus oculi was examined in 111 children aged between 1 and 25 months with the ophthalmoscop-star. This examination was carried out in 64 cases by a second and in 15 cases by a third examiner on the same day. RESULTS: During the first 3 months of age children cannot fixate foveally, one can observe instead, a kind of scatter at the posterior pole in a diameter of about 5 degrees. In the course of the 4, and 5, month of age foveal fixation develops. Repeated examinations of the same child on the same day by a 2, and 3, examiner reveal a very high rate of accordance (78%) of their results. CONCLUSIONS: The examination of fixation on the fundus can be carried out at 4 to 5 months of age. At that age one can differentiate quite reliable between a foveal and eccentric fixation as well as scatter at the posterior pole. The rate of accordance 78% with this fixation examination by several examiners shows that same reliability of this examination carried out in school children and adults. Comparing this result with fixation examination carried out with the Haidinger brushes then one must calculate with a fault rate of about 20% when examining with the ophthalmoscope, considering that the examination with Haidinger brushes as a reference examination.

Adult↗
PubMed2001

Maintenance of soft tissue changes after rigid versus wire fixation for mandibular advancement, with and without genioplasty.

OBJECTIVE: This multisite prospective randomized clinical trial examined 2-year longitudinal soft tissue profile changes after bilateral sagittal split osteotomy for mandibular advancement by using rigid or wire fixation, with and without genioplasty. STUDY DESIGN: The study sample consisted of 127 subjects. The rigid-fixation group (n = 78) received 2-mm bicortical position screws, whereas the wire-fixation group (n = 49) received inferior border wires. In the rigid-fixation group, 35 subjects underwent genioplasty, whereas 24 subjects underwent genioplasty in the wire-fixation group. Soft tissue profile changes of labrale inferius, B-point, and pogonion were obtained from digitized cephalometric films taken immediately before surgery and up to 2 years after surgery. RESULTS: Regardless of fixation technique, subjects who had genioplasty in conjunction with the mandibular advancement had the largest surgical movement and the largest postsurgical change (P <.05). When all variables were constant, fixation technique was associated with maintenance of soft tissue change. Subjects who underwent rigid fixation maintained more soft tissue change than patients who underwent wire fixation. CONCLUSIONS: These findings suggest that subjects undergoing rigid fixation and genioplasty maintained the most soft tissue advancement.

Adult↗
PubMed1996

Fixation affects morphometric characters of Psoroptes cuniculi mites (Acari:Psoroptidae).

Thirteen morphologic characters were evaluated on Psoroptes cuniculi (Delafond) mites to determine if measurements for these variables were influenced by the use of different fixatives and varying periods of fixation. Morphologic structures composed primarily of chitin (outer opisthosomal setal lengths and right 3rd tarsal pedicle length) were not affected by either fixative type or duration of fixation. In contrast, alcohol and formalin fixation both resulted in significant reductions (P < 0.05) in mean body size measurements when compared with mites mounted in Hoyer's medium without prior fixation. The largest reduction in size was seen in mites fixed for the longest period, but there was no apparent association between size reduction and increasing concentration of alcohol or formalin. Surprisingly, when mites were remeasured 4 yr later, Hoyer's mounting medium was found to have had the opposite effect of fixation and caused an increase in body size. Mites mounted in Hoyer's medium without prior fixation had significantly increased body lengths, whereas mites mounted in Hoyer's following fixation in alcohol and formalin had significant increases in both body length and posterior body width. Our results indicate that fixation and mounting can significantly influence the morphometric analysis of mites and other arthropods. We recommend that morphometric studies be conducted using consistent methodology to reduce experimental bias, and these methodologies be reported in publications dealing with morphometric analyses.

Animals↗
PubMed2000

Hinged external fixation of the elbow: optimal axis alignment to minimize motion resistance.

OBJECTIVE: To establish an optimal single hinge axis position for application of hinged external fixation to the elbow joint. DESIGN: Cadaveric biomechanical investigation. SETTING: A customized motion transducer applied passive elbow motion to six cadaveric upper extremities. The instant rotation axis of the humero-ulnar articulation was determined from three-dimensional kinematic data acquired by an electromagnetic motion tracking system. For each specimen, an optimal fixator hinge position was calculated from these motion data. INTERVENTION: A prototype articulated external fixator was applied to the elbow, first with its hinge aligned along the computed optimal position. Then the fixator was mounted in sixteen distinct off-axis positions. MAIN OUTCOME MEASURE: Additional resistance to joint motion (in terms of energy) corresponding to deliberately introduced amounts of relative malalignment between the optimal elbow axis and the actual fixator hinge axis. RESULTS: Aligning the fixator hinge along the optimized axis position resulted in a minimal amount of energy (0.15 joules) needed to rotate the elbow through a prescribed range of motion. Malpositioning the hinge by ten millimeters caused up to ten times that amount of motion resistance. CONCLUSIONS: An optimal fixator hinge position can be determined to minimize the increase in motion resistance due to fixator application. The severely increased motion resistance associated with small amounts of malalignment between the fixator hinge and the anatomic elbow axis suggests the need for highly accurate fixator hinge application.

Cadaver↗
PubMed2002

Failure of fixation of tibial plateau fractures.

OBJECTIVE: To define the failure of fixation in tibial plateau fractures, detect its incidence, and determine the contributing factors. DESIGN: Mail survey and literature review were used to define the fixation failure; this definition was applied to a radiologic review of patients who were treated surgically for tibial plateau fracture at a Level 1 trauma unit for a three-year period from 1993 to 1995. PATIENTS AND SETTING: Forty-two consecutive patients treated surgically at our trauma unit for tibial plateau fractures were studied retrospectively, specifically for loss of fixation. Factors that might affect the fracture fixation were reviewed, including age, mechanism of injury, type of fracture, bone quality, severity of fragmentation, severity of displacement, time to surgery, operating time, fixation method, use of bone graft, postoperative bracing, and mobilization. MAIN OUTCOME MEASURES: The main outcome measure was failure of fixation, by using criteria defined by the result of a mail survey of experts and literature review. RESULTS: Using a strict definition of radiologic failure of fixation, we reported an overall 31 percent rate of failure of fixation: 79 percent in patients older than sixty years compared with 7 percent in younger patients. The statistically significant associations with loss of reduction were age more than sixty years, premature weight bearing, preoperative displacement, fracture fragmentation, and severe osteoporosis. CONCLUSION: It is logical to define failure of fixation using the same measures considered as indications for reduction and fixation. Using these strict criteria, the incidence of radiologic failure was much higher than previous published series. In the elderly this was unacceptably high, and treatment goals should be limited to restoring stability and alignment.

Adult↗
PubMed2004

The magnetic attraction of lower extremity external fixators in an MRI suite.

OBJECTIVES: To determine the magnetic attraction of 10 currently available lower extremity external fixators in the MRI suite and to test fixators with minimal or no magnetic attraction for the presence of temperature elevation during a 30-minute MRI scan. DESIGN: Consecutive testing of selected tibial external fixation models in an identical manner. SETTING: Level 1 trauma center. CONSTRUCTS: Ten commercially available external fixators used for lower extremity injuries applied to saw bone tibia. INTERVENTION: External fixators suspended in testing apparatus were placed at three locations in the MRI suite for magnetic testing. Fixators with minimal attraction were scanned for 30 minutes. MAIN OUTCOME MEASUREMENTS: Magnetic attraction was measured with a digital scale, and temperature measurements were obtained with a digital thermometer and thermocouple. RESULTS: The Ilizarov with carbon fiber rings, Richards Hex-Fix, and Large Synthes External Fixator had <1 kg of magnetic attraction at all three locations. No component of these fixators experienced >2 degrees of temperature elevation after a 30-minute MRI scan. The EBI Dynafix with Ankle Clamp, EBI Dynafix, and EBI Dynafix Hybrid along with the Hoffman II, Hoffman II Hybrid, Ilizarov with stainless steel rings, and Synthes Hybrid had >1 kg of magnetic attraction at all three locations. CONCLUSIONS: Many commercially available external fixators have components and spring clamps that have significant magnetic attraction to the MRI scanner. Fixators that have <1 kg of attraction do not experience significant heating during MRI.

Equipment Design↗
PubMed1996

Surgical neck fractures of the proximal humerus: a laboratory evaluation of ten fixation techniques.

OBJECTIVE: A biomechanical cadaver study was performed to compare the stability and ultimate strength of ten standard fixation techniques used for the treatment of surgical neck fractures of the proximal humerus. DESIGN: One hundred twenty (60 fresh frozen, 60 embalmed) proximal humerus specimens were selected and divided into two groups: fresh frozen specimens represented a nonosteopenic group and embalmed specimens an osteopenic group. Simulated fractures were created at the level of the surgical neck, reduced, and randomly assigned to one of ten methods of fixation (six fresh frozen and six embalmed specimens per fixation group). These constructs were then mechanically tested with the humeri oriented to create primarily shear loading of the fixation. RESULTS AND CONCLUSIONS: The T-plate and screws provided significantly stronger fixation (p < 0.005) in the fresh frozen specimens than all other methods. The Ender nails/tension band construct was the second strongest fixation technique, providing significantly stronger fixation (p < 0.01) than all the remaining techniques. Four Schanz pins with one pin placed through the greater tuberosity followed by the T-plate and screws provided the strongest fixation in embalmed specimens. Tension band fixation in both humeral groups was shown to provide the least effective fixation.

Biomechanical Phenomena↗
PubMed1995

Comparison of rigid plate versus wire fixation in the management of zygoma fractures: a long-term follow-up clinical study.

We evaluated retrospectively 101 patients to assess clinically the results of fixation methods on isolated unilateral zygoma fractures (mean follow-up 3.2 years). All patients were examined by an investigator who was blinded to the initial fixation method. Ninety-two patients were treated with wire or plate fixation alone. The major clinical parameters assessed were globe position abnormalities (enophthalmus and dystopia), malar projection, and cheek sensation. Rigid plate fixation achieved statistically better long-term malar symmetry (p = 0.002) and approached statistical significance in the achievement of more normal globe position (p = 0.06) compared with wire fixation alone. Cheek sensation showed a nonsignificant trend (p = 0.13) toward improvement with plate fixation. Other parameters that were evaluated were anatomic location of fixation points, number of fixation points, surgical approach and exposure, and breakdown of fixation dates by specialty. None of these factors showed any significant influence on outcome. The present study confirms what we have known to be true from our clinical experience--that rigid internal fixation is superior in minimizing or preventing long-term sequelae of facial fractures.

Bone Plates↗
PubMed1997

Pedicle screw fixation for nontraumatic lesions of the cervical spine.

STUDY DESIGN: This retrospective study was conducted to analyze the clinical results in 45 patients with nontraumatic lesions of the cervical spine treated by pedicle screw fixation. OBJECTIVES: To evaluate the effectiveness of pedicle screw fixation in reconstructive surgery for nontraumatic cervical spinal disorders. SUMMARY OF BACKGROUND DATA: Pedicle screw fixation for hangman's fracture of the axis and traumatic lesions of the middle and lower cervical spine has been reported; however, there have been no reports on pedicle screw fixation for nontraumatic lesions of the cervical spine. METHODS: Forty-five patients with nontraumatic lesions of the cervical spine underwent reconstructive surgery including pedicle screw fixation and fusion. Five patients underwent occipitocervical fixation for the lesion of the upper cervical spine, and one patient underwent separate occipitocervical fixation and cervicothoracic fixation. Cervical or cervicothoracic fixation was performed in 39 patients. Twenty-six of these patients underwent simultaneous laminectomy or laminoplasty. Supplemental anterior surgery was conducted for 15 patients. RESULTS: Solid fusion was obtained in all patients except eight with metastatic vertebral tumors who did not receive bone graft. Correction of kyphosis was adequate. There were no neurovascular complications, except one case of transient radiculopathy caused by screw threads. CONCLUSIONS: Pedicle screw fixation is a useful procedure for posterior reconstruction of the cervical spine. This procedure does not require the lamina for stabilization, and should be especially valuable for simultaneous posterior decompression and fusion. The risk to neurovascular structures, however, cannot be completely eliminated.

Adult↗
PubMed2004

Biomechanical analysis of transpedicular screw fixation in the subaxial cervical spine.

STUDY DESIGN: An in vitro biomechanical study to compare 2 different dorsal screw fixation techniques in the cervical spine with respect to primary stability and stability after cyclic loading. OBJECTIVES: To investigate if the biomechanical stability is better in pedicle screw or in lateral mass fixation. SUMMARY OF BACKGROUND DATA: In patients with poor bone quality who require multisegmental fixations, the current dorsal stabilization procedures in the subaxial cervical spine using lateral mass screws are often insufficient. Cervical pedicle screw fixation has been suggested as an alternative procedure, but there are still limited data available on the biomechanical differences between pedicle screw and lateral mass fixation. METHODS: A severe multilevel discoligamentous instability was created in 8 human cervical spine specimens (C2-C7). Dorsal stabilization was performed with the assistance of computer navigation (SurgiGate, Medivison, Switzerland) using either lateral mass or pedicle screw fixation. In the first part of the study, primary stability was measured by means of a multidirectional flexibility test. Then, specimens were divided into 2 groups, randomized for bone mineral density. Cyclic loading was applied with sinusoidal loads in flexion/extension (1000 cycles, +/-1.5 Nm, 0.1 Hz). Mechanical behavior of the specimens was determined by a flexibility test before and after the application of cyclic loads. Data analysis was performed by calculating the ranges of motion, and statistical differences were determined with the t test for group comparison. RESULTS: Pedicle screw fixation showed a significantly higher stability in lateral bending (pedicle screw range of motion 0.86 +/- 0.31 degrees; lateral mass range of motion 1.43 +/- 0.62 degrees; P = 0.037). No significant differences were seen in flexion/extension and axial rotation. After cyclic loading, the decrease in stability was less with pedicle screw fixation in all load directions. Differences in the decrease of stability were statistically significant in flexion/extension (pedicle screw 95.4 +/- 9.4%; lateral mass 70.5 +/- 9.8%; P = 0.010) and lateral bending (pedicle screw 105.3 +/- 5.0%; lateral mass 84.2 +/- 13.6%; P = 0.046), whereas there was no significant difference in axial rotation. CONCLUSIONS: The major finding of the current study was the higher stability of pedicle screws over lateral mass fixation with respect to primary stability and stability after cyclic loading. From a biomechanical point of view the use of pedicle screws in the subaxial cervical spine seems justified in patients with poor bone quality and need for multisegmental fixation.

Adult↗
PubMed2006

A biomechanical analysis of sublaminar and subtransverse process fixation using metal wires and polyethylene cables.

STUDY DESIGN: An in vitro biomechanical calf thoracic spine study. OBJECTIVE: To evaluate the biomechanical stability of sublaminar and subtransverse process fixation using stainless steel wires and ultra-high molecular weight polyethylene (UHMWPE) cables. SUMMARY OF BACKGROUND DATA: It is commonly held that transverse process fixation provides less stability than sublaminar fixation. To our knowledge, this is the first biomechanical study to compare the stability afforded by sublaminar fixation and subtransverse process fixation using metal wire and UHMWPE cable before and after cyclic loading. METHODS: There were 6 fresh-frozen calf thoracic spines (T4-T9) used to determine the sublaminar fixation stiffness and subtransverse process fixation stiffness in each group. Double strands of 18-gauge stainless steel wire, 3 and 5 mm-width UHMWPE cable (Nesplon; Alfresa, Inc., Osaka, Japan) were applied to each spine. Cyclic pure flexion-extension moment loading (2 Nm, 0.5 Hz, 5000 cycles) was applied after the initial stability was analyzed by measuring the range of motion. Statistical analyses were used to delineate differences between the various experimental groups. RESULTS: Subtransverse process wiring was more stable than sublaminar wiring after cyclic loading in flexion-extension (P < 0.05). There were no significant differences between each group in lateral bending and axial rotation after cyclic loading. Sublaminar stainless steel wiring was more stable than sublaminar 3 and 5-mm cable before and after cyclic loading in axial rotation (P < 0.01). Acute subtransverse process fixation using 3-mm cable was less stable after cyclic loading in axial rotation (P < 0.05). All other groups did not produce statistically significant differences. CONCLUSIONS: Subtransverse process fixation provides at least as much stability as sublaminar fixation. A 5-mm UHMWPE cable and stainless steel wire result in equivalent sublaminar and subtransverse process stability.

Animals↗
PubMed2003

Zinc-based fixative improves preservation of genomic DNA and proteins in histoprocessing of human tissues.

Advantageous preservation of histology and detailed cellular morphology has rendered neutral buffered formalin (NBF) the most widely used fixative in clinical pathology. Despite excellent morphology for routine diagnostics, a major drawback of NBF fixation is its detrimental effect on DNA and RNA quality. In addition to complicating analysis of genes and transcripts in complex tissues, NBF denatures proteins and thereby hampers immunohistochemical visualization of certain antigens. In the present study, we evaluated a zinc-based fixative (ZBF) regarding its effects on tissue morphology, quality of genomic DNA, and preservation of protein immunoreactivity in a broad spectrum of tissues. Four different modes of fixation were analyzed: ZBF-paraffin embedding, NBF-paraffin embedding, ZBF-fixation prior to snap-freezing, and immediate snap-freezing. Laser-assisted microdissection, allowing retrieval of a defined number of cells for PCR, was used to study DNA quality. Genomic DNA was analyzed using primers for beta2-microglobulin and the transferrin receptor. Immunohistochemistry was performed using nine antibodies. Tissue microarray blocks were used for analysis of morphology and immunoreactivity. Only slight impairment of morphologic qualities was found after ZBF-paraffin embedding, whereas ZBF prior to freezing resulted in a more crisp morphology compared with routine cryosections. A significantly higher DNA yield was observed in samples isolated from ZBF-paraffin-embedded tissues compared with NBF-paraffin-embedded tissues. Both yield and quality of DNA was comparable in frozen tissues irrespective to prior ZBF fixation. Immunoreactivity in paraffin-embedded tissue was superior in ZBF-fixated tissue compared with NBF-fixated for a majority of tested antibodies. Furthermore, for seven out of nine antibodies, antigen retrieval pretreatment proved unnecessary in ZBF-fixated tissue. Thus, despite a slight impairment of morphology, ZBF preserves protein structures well. We conclude that ZBF is superior to NBF for analysis of DNA and protein expression. Fixation of tissues in ZBF may also be an alternative strategy to freeze storage of tissue specimens, eg, in future bio-banks.

Carcinoma, Transitional Cell↗
PubMed2003

Engagement of visual fixation suppresses sensory responsiveness and multisensory integration in the primate superior colliculus.

Neurons in the intermediate and deep layers of the superior colliculus (SC) often exhibit sensory-related activity in addition to discharging for saccadic eye movements. These two patterns of activity can combine so that modifications of the sensory response can lead to changes in orienting behaviour. Can behavioural factors, however, influence sensory activity? In this study of rhesus monkeys, we isolate one behavioural factor, the state of visual fixation, and examine its influences on sensory processing and multisensory integration in the primate SC. Two interleaved fixation conditions were used: a FIX condition requiring exogenous fixation of a visible fixation point; and a FIX-BLINK condition, requiring endogenous fixation in the absence of a visible fixation point. Neurons of the SC were influenced by fixation state, exhibiting both lower levels of sensory activity and reduced multisensory interactions when fixation was exogenously engaged on a visible fixation point. These results are consistent with active visual fixation suppressing responses to extraneous stimuli, and thus demonstrate that sensory processing and multisensory responses in the SC are not dependent solely on the physical properties of the sensory environment, but are also dynamically influenced by the behavioural state of the animal.

Acoustic Stimulation↗
PubMed2007

Symbiotic nitrogen fixation in a tropical rainforest: 15N natural abundance measurements supported by experimental isotopic enrichment.

* Leguminous trees are very common in the tropical rainforests of Guyana. Here, species-specific differences in N(2) fixation capability among nodulating legumes growing on different soils and a possible limitation of N(2) fixation by a relatively high nitrogen (N) and low phosphorus (P) availability in the forest were investigated. * Leaves of 17 nodulating species and 17 non-nodulating reference trees were sampled and their delta(15)N values measured. Estimates of N(2) fixation rates were calculated using the (15)N natural abundance method. Pot experiments were conducted on the effect of N and P availability on N(2) fixation using the (15)N-enriched isotope dilution method. * Nine species showed estimates of > 33% leaf N derived from N(2) fixation, while the others had low or undetectable N(2) fixation rates. High N and low P availability reduced N(2) fixation substantially. * The results suggest that a high N and low P availability in the forest limit N(2) fixation. At the forest ecosystem level, N(2) fixation was estimated at c. 6% of total N uptake by the tree community. We conclude that symbiotic N(2) fixation plays an important role in maintaining high amounts of soil available N in undisturbed forest.

Ecosystem↗
PubMed1987

The effects of chemical fixation on the permeability of frog mesenteric capillaries.

1. We have investigated the effects on microvascular permeability of two chemical fixatives, glutaraldehyde/formaldehyde mixture and osmium tetroxide, both of which are used widely in electron microscopy. 2. The permeability of single perfused frog mesenteric capillaries was assessed using the technique of Michel (1980) to estimate the hydraulic conductance (Lp) of the vessel walls and the effective osmotic pressure (sigma delta pi) which could be exerted across them by the neutral macromolecule, Ficoll 70. 3. In each experiment, Lp and sigma delta pi were estimated in the same capillary before and after chemical fixation with either an aldehyde mixture (n = 6) or 1% osmium tetroxide (n = 6). In the absence of serum proteins in the perfusate, the values of Lp recorded prior to fixation were high. The effect of both fixatives was to reduce Lp. With aldehydes the mean value (+/- S.E.M.) of Lp fell from 22.1 (+/- 6.3) x 10(-3) to 5.7 (+/- 2.0) x 10(-3) microns s-1 cmH2O-1. With osmium tetroxide the mean Lp fell from 29.1 (+/- 8.5) x 10(-3) to 8.6 (+/- 3.6) x 10(-3) microns s-1 cmH2O-1. 4. The perfusates contained the neutral macromolecule, Ficoll 70, at a concentration of 60 mg ml-1 which exerted an osmotic pressure of 35 cmH2O in a membrane osmometer. In the absence of plasma proteins in the perfusate, sigma delta pi was only a fraction of this perfusate oncotic pressure prior to fixation. After aldehyde treatment there was a small but insignificant decrease in sigma delta pi. Osmium tetroxide, however, increased sigma delta pi from a mean (+/- S.E.M.) of 6.3 (+/- 1.6) cmH2O before fixation to 16.4 (+/- 2.8) cmH2O after fixation, P less than 0.01 using a paired t test. 5. In three experiments, the reflection coefficient of the capillary wall to NaCl sigma NaCl, was measured using the technique of Curry, Mason & Michel (1976) before and after osmium fixation. Fixation raised sigma NaCl from a mean value (+/- S.E.M.) of 0.009 (+/- 0.001) to one of 0.017 (+/- 0.004). Although these changes may be interpreted as an increase in the contribution to the total Lp of the capillary wall of channels which are available only to water, the changes are too small for such a mechanism to account for the increase in reflection coefficient to Ficoll 70. 6. The reduction in Lp following chemical fixation could represent an increase in the hydraulic resistance of either the capillary wall itself, or in the tissues surrounding the vessel.(ABSTRACT TRUNCATED AT 400 WORDS)

Animals↗
PubMed2006

Fixation stability using radial gratings in patients with age-related macular degeneration.

BACKGROUND: The fixation stability of patients with macular atrophy is generally worse than that of people without pathology. METHODS: The effects of 2 types of high-contrast fixation stimuli on fixation stability were compared between patients with longstanding age-related macular degeneration (AMD) and control subjects with normal vision. One stimulus was a 9-cycle square-wave radial grating measuring 5 degrees in diameter and the other a white 0.5 degrees disc. A video-based infrared eye tracker with remote optics was used to record eye position while participants fixated the stimuli in primary position of gaze for 6 to 7 s. Fixation stability was measured with a bivariate contour ellipse area (BCEA). RESULTS: For patients with AMD, fixation stability for the radial grating was largely independent of visual acuity, whereas fixation stability for the disc diminished with acuity. For the control observers, there were no differences in fixation stability for the 2 kinds of stimuli. INTERPRETATION: In clinical and research settings, radial gratings can be useful targets for fixation for patients with macular disease since they provide enough visual information to help maintain fixation stability. These findings have important implications for the design of clinical tests and procedures such as perimetry, multifocal electroretinography, and optical coherence tomography for patients with macular atrophies.

Adult↗