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Tissue fixation methods alter the immunohistochemical demonstrability of neurofilament proteins, synaptophysin, and glial fibrillary acidic protein in human cerebellum.

This study has examined the effect of postmortem autolysis, type, and duration of fixation on neurofilament, synaptophysin, and glial fibrillary acidic protein (GFAP) antigen decay as demonstrated by immunohistochemistry, using a streptavidin-biotin peroxidase method. The system used consisted of 5 normal cerebellar cortices. Time intervals, temperature, mode of fixation and storage, and staining technique were well controlled. Anti-neurofilament antibodies comprised SMI-31, MNF, and BF-10 against phosphorylated epitopes, and SMI-32 against a non-phosphorylated epitope. Bouin's and B5 fixative, and Sensofix gave best results, whereas formaldehyde and paraformaldehyde fixation gave much lower immunoreactivity. Phosphorylated neurofilament epitopes were less affected by aldehydes than unphosphorylated epitopes. GFAP staining was most consistent after Bouin fixation while the monoclonal antibody was much more sensitive to the fixative used than the polyclonal one. Aspecific background staining increased considerably after a postmortem interval of 24 hours. Synaptophysin immunoreactivity, as demonstrated by SY-38, proved very sensitive to prolonged fixation and was of poor quality following formaldehyde and paraformaldehyde fixation. Knowledge of antigen decay due to postmortem artifacts is essential for the correct evaluation of immunoperoxidase studies of autolyzed tissues that have been fixed and stored in different modes and for variable time interval.

Antibodies↗

Vying for dominance: dynamic interactions control visual fixation and saccadic initiation in the superior colliculus.

By the time you have reached this point, your daily count of alternating saccades and fixations will have increased considerably. So too will have your understanding of the dynamic interactions model. In the superior colliculi, visual fixation and saccadic initiation may be viewed as independent motor plans that compete for dominance across the intermediate layers. Extrinsic input modifies a point location on the retinotopic motor map that is shaped into a motor plan through the intrinsic circuitry of the superior colliculi. Independent motor plans compete for selection in a push-pull fashion and when a saccadic plan ultimately reaches threshold, it produces a strong burst of action potentials that shuts down the remaining regions of the intermediate layers. Modifying the activity of the intermediate layers changes these dynamic interactions in predictable ways. Enhancing the activity of one region facilitates nearby locations and inhibits distant locations. Diminishing the activity of one region inhibits nearby locations and facilitates distant locations. Such effects have been demonstrated in the neurophysiological activity of single cells (Munoz and Istvan, 1998; Olivier et al., 1999) and in behavior (Hikosaka and Wurtz, 1985; Munoz and Wurtz, 1993b). In addition to explaining visual fixation and saccadic initiation during basic saccadic tasks, the dynamic interactions model can explain changes in the timing of saccadic initiation that are observed when this task is modified. Namely, the gap effect, or decreased saccadic reaction times as a consequence of a gap period, occurs because removing fixation decreases the activity of fixation regions and, correspondingly, increases the excitability of saccadic regions. Express saccades, are a special instance of such dynamic interactions, in which decreased fixation activity and heightened motor preparation signals cause the target-related activity to be translated into a saccadic signal immediately. Finally, the slowing of saccadic initiation for antisaccades, can be interpreted as the consequence of multiple competing signals across the intermediate layers. It should be emphasized that the dynamic interactions that we have described in this chapter are not limited to the superior colliculi. On the contrary, similar interactions take place at many levels of the neuraxis (Moschovakis et al., 1996; Leigh and Zee, 1999; Schall and Thompson, 1999; Hikosaka et al., 2000; Munoz et al., 2000; Glimcher, 2001; Scudder et al., 2002). At this juncture, however, the dynamic interactions involved in producing visual fixation and saccadic initiation are better understood in the superior colliculi because of its well-organized motor map and its well-characterized neuronal elements. Although we are a long way from understanding how the brain controls visual fixation and saccadic initiation, we have made substantial progress in understanding these behaviors in the superior colliculi.

Fixation, Ocular↗

Fixational eye movements in normal and pathological vision.

Most of our visual experience is driven by the eye movements we produce while we fixate our gaze. In a sense, our visual system thus has a built-in contradiction: when we direct our gaze at an object of interest, our eyes are never still. Therefore the perception, physiology, and computational modeling of fixational eye movements is critical to our understanding of vision in general, and also to the understanding of the neural computations that work to overcome neural adaptation in normal subjects as well as in clinical patients. Moreover, because we are not aware of our fixational eye movements, they can also help us understand the underpinnings of visual awareness. Research in the field of fixational eye movements faded in importance for several decades during the late 20th century. However, new electrophysiological and psychophysical data have now rejuvenated the field. The last decade has brought significant advances to our understanding of the neuronal and perceptual effects of fixational eye movements, with crucial implications for neural coding, visual awareness, and perception in normal and pathological vision. This chapter will review the type of neural activity generated by fixational eye movements at different levels in the visual system, as well as the importance of fixational eye movements for visual perception in normal vision and in visual disease. Special attention will be given to microsaccades, the fastest and largest type of fixational eye movement.

Fixation, Ocular↗

Defining the temporal threshold for ocular fixation in free-viewing visuocognitive tasks.

Patterns of ocular fixation in free-viewing tasks reflect aspects of visual attention and cognition. To quantify these patterns, fixations must be identified in raw eye movement data by applying explicit spatial and temporal thresholds. A temporal threshold of 200 ms is commonly used in clinical visuocognitive research, despite having been originally derived from a study of eye movements in reading. We systematically explored temporal fixation thresholds below 200 ms, using biologically relevant (human face) and abstract (complex geometric) stimuli. Short fixations (<200 ms) significantly altered spatiotemporal patterns of fixation for both the face and geometric stimuli, by increasing the number of fixations and the scanpath length. A threshold of 100 ms was found to discriminate fixations from other oculomotor activity effectively, and was consistent with current physiological and visuocognitive models. Statistical examination of fixation duration data for each subject suggested the median was a less biased and more robust measure of central tendency than the mean.

Adult↗

Fixation stability in schizophrenia, bipolar, and control subjects.

A few investigators have suggested that visual fixation abnormalities may serve as an endophenotype of liability for schizophrenia. However, the data are equivocal. Conflicting reports regarding the specificity of fixation deficits to schizophrenia may be attributable to methodological differences. Thirty-four schizophrenia patients, 20 bipolar patients, and 30 non-patient controls were presented targets for central fixation. Fixation was scored in terms of frequency of saccades as well as qualitative ratings. Analysis of variance on the number of saccades produced during fixation revealed that the three groups did not differ. Similarly, we observed that the schizophrenia patients did not differ from either bipolar patients or controls in terms of ratings of fixation quality. It appears that schizophrenia patients are not characterized by poor visual fixation. The findings are discussed in terms of the viability of visual fixation as a marker of schizophrenic diathesis, as well as possible implications for the analysis of schizophrenia patients' visual search performance.

Adult↗

The role of fixation point and subjects' readiness in the occurrence of express saccades as revealed by the self-initiation paradigm.

The role of fixation and the subjects' response preparedness in producing express saccades were explored in seven human subjects. The occurrence frequencies of the express saccades were compared in the overlap (continuous presentation of fixation point), gap (fixation point offset 0-400 ms prior to target onset) and no-fixation tasks under the conventional and self-initiation paradigms. In the latter paradigm, the subjects, when ready, touched a sensor in order to ignite the target lamp with a delay time of 0-400 ms (target onset delay time). Therefore, the subjects' response preparedness might be expected to be higher than that in the normal paradigm and equated in each subject at the time when the subjects touched a sensor regardless of the paradigms. Although express saccades were produced neither in the normal overlap nor in the normal no-fixation tasks, they could be produced at the rate of 24 and 48% in the overlap and no-fixation tasks under the self-initiation paradigm, respectively. The highest occurrence frequency of express saccades was obtained when the gap paradigm was combined with the self-initiation paradigm with a delay time of 100 ms (62%). The value was higher by 20% than in the normal gap task. At a target onset delay time of 0 ms under the self-initiation paradigm, the occurrence frequency of express saccades was higher in the overlap task than in the gap task. These results suggest that the subjects' response preparedness has a potentiality to produce express saccades without fixation point offset and that fixation point offset at the same time of the target stimulus onset has an interference, rather than facilitatory, influence on the generation of express saccades.

Adult↗

Use of explicit criteria for total hip joint replacement fixation techniques.

We developed a tool to judge the appropriateness of indications and fixation mechanisms for total hip joint replacement (THJR) and applied it to a sample of patients. Criteria were developed using a modified Delphi panel judgment process, following the RAND methodology (RAM). We recruited, during 1 year, patients with a diagnosis of osteoarthritis undergoing THJR in five public hospitals. The appropriateness of the THJR intervention and the fixation mechanism was judged by the explicit criteria developed by a panel of experts. Of the 216 scenarios scored by the panel for the use of each of three fixation mechanisms, the cemented fixation was considered inappropriate in 69.5%, versus just 33.3% for the non cemented. Of those scenarios considered appropriate, the most appropriate mechanism of fixation was considered to be non cemented (74.4%), while cemented (17.8%) and hybrid (7.8%) scenarios were scored as appropriate less often. The previous explicit criteria were applied to 583 real patients. After evaluation of the interventions, 30% of the fixation mechanisms used were considered appropriate, while 21.8% inappropriate. Appropriate use of fixation mechanisms varied among hospitals. RAM can provide explicit criteria to help in clinical decision making and evaluating indications for a THJR intervention. Nevertheless, in the case of the appropriateness of fixation mechanisms, due to the lack of evidence, the panel criteria were biased towards the non cemented technique, which had important implications for the evaluation of some hospitals.

Arthroplasty, Replacement, Hip↗

Failure of posterior titanium atlantoaxial cable fixation.

BACKGROUND CONTEXT: The operative treatment of rotatory atlantoaxial instability remains controversial. The use of cable fixation has largely replaced the use of wire for interlaminar fixation. Although cable fixation offers biomechanical advantages over wire fixation, it is still at risk of fatigue failure. The authors were unable to locate any published reports of fatigue failure of titanium cables in the fixation of atlantoaxial instability. PURPOSE: The purpose of this case study is to describe an unusual case of fatigue failure of a titanium cable used to aid in atlantoaxial fusion for the treatment of rotatory atlantoaxial instability. STUDY DESIGN: Case study. METHODS: We reviewed the medical records and X-rays of a patient with rotatory atlantoaxial instability treated with posterior C1-C2 fusion and atlantoaxial fixation with a titanium multistranded cable, who developed fracture of the cable and migration of the cable into the spinal canal. RESULTS: The patient was revised with removal of the broken cable, repair of the pseudarthrosis and fixation with atlantoaxial screws. CONCLUSION: Interlaminar fixation with cables or wires is at risk for failure with potential migration of the wire or cable into the spinal canal. The authors found that failure of the cables or wire can be salvaged with application of transarticular screws.

Adult↗

The impact of fixation on the multifocal electroretinogram.

There are a number of variables which can influence the quality of multifocal ERG waveforms. In common with visual field measurements, fixation quality may be an important parameter on the integrity of the acquired data. A low cost, fixation-monitoring device was used to assess fixation quality on a group of normal volunteers. Data was successfully acquired while five subjects viewed a fixation target for a period of time equal to that of a single multifocal recording segment. The target was presented on a stationary grey background and as the central fixation mark on a 61-element multifocal flicker stimulus. The results show no significant difference with or without the flickering pattern. The percentage of samples falling within 1.2 degrees of the point of fixation was 51%. This suggests that fixation quality is adequate for scaled stimuli where the central element subtends 2.4 degrees. High resolution stimuli of less than 2.4 degrees may be more susceptible to fixation fluctuations during the recording process.

Adult↗

Look here but ignore what you see: effects of distractors at fixation.

Distractor interference effects were compared between distractors in the periphery and those placed at fixation. In 6 experiments, the authors show that fixation distractors produce larger interference effects than peripheral distractors. However, the fixation distractor effects are modulated by perceptual load to the same extent as are peripheral distractor effects (Experiments 1 and 2). Experiment 3 showed that fixation distractors are harder to filter out than peripheral distractors. The larger distractor effects at fixation are not due to the cortical magnification of foveal stimuli (Experiments 4 and 5), nor can they be attributed to cuing by the fixation point (Experiment 2), the lower predictability or greater location certainty of fixation distractors (Experiment 5), or their being in a central position (Experiment 6). The authors suggest that preferential access to attention renders fixation distractors harder to ignore than peripheral distractors.

Adolescent↗

Improved preservation of ultrastructural morphology in human spermatozoa using betaine in the primary fixative.

Improved preservation of the ultrastructural morphology of human spermatozoa was obtained by filtration onto a Millipore filter (0.45 micron pore size) followed by fixation for 1 h in a fixative containing 3% glutaraldehyde, 1% formaldehyde and 21 mM betaine in Hepes or cacodylate buffer. Betaine raised fixative osmolality only slightly but markedly improved the ultrastructural appearance of the spermatozoa; it is known to function as an osmoprotectant in other situations, and may protect the cells from osmotic damage during the initial stages of fixation. The method is suitable for oligozoospermic samples as they are concentrated when the seminal plasma passes through the filter. Utilizing fixation after swim-up, it is suitable also for highly viscous semen. The fixative gave equally good preservation of sperm heads and tails and can be stored prior to use. The Millipore filter did not dissolve until placed in acetone prior to embedding, simplifying the exchange of solutions during processing. Improved preservation of rat epididymal ultrastructure was obtained by addition of betaine to the fixative of Ito & Karnovsky (1968), using immersion fixation.

Animals↗

Microwave-enhanced fixation of rabbit articular cartilage.

Cartilage, being highly aqueous, is difficult to preserve for electron microscopy without artefacts. Microwave-enhanced fixation is suggested as a standard method for block samples of this material, with dimensions of up to 12 x 7 x 3 mm. Cartilage samples from the tibial plateau of adult rabbits were fixed by conventional, cryo- or microwave-enhanced fixation. Constant or cyclical microwave irradiation of samples, immersed in fixatives, was carried out to varying final solution temperatures. Microwave-enhanced fixation and staining is shown to be both rapid and reproducible, giving fine structural preservation. Below 323 K microwave fixation always gave excellent preservation of the fine structure within seconds. At higher temperatures thermal artefacts were introduced. In this study the microwave-enhanced fixation is equal in quality to the test conventional immersion fixation and is nearly as fast as cryo-preservation. It provides a standardized, reproducible fixation for morphological studies on cartilage with good process control.

Animals↗

Transcondylar screw fixation in anterior cruciate ligament reconstruction.

PURPOSE: The purpose of this study was to investigate the outcomes of 2 different methods of femoral fixation for arthroscopically assisted anterior cruciate ligament (ACL) reconstruction. TYPE OF STUDY: Double-blind prospective study. METHODS: Fifty-five patients were prospectively evaluated at 2-year follow-up. In group A, 24 patients underwent arthroscopic ACL reconstruction with bone-patellar tendon-bone and interference screw fixation. In group B, 31 patients underwent arthroscopic ACL reconstruction with bone-patellar tendon-bone and transcondylar screw fixation. Patients were evaluated by using the IKDC form, Lysholm-II form, Tegner scoring system, and KT-2000 arthrometer. RESULTS: In group A, final IKDC clinical evaluation achieved 0% grade A, 62.5% grade B, 25% grade C, and 12.5% grade D. In group B, 29.1% achieved grade A, 54.8% grade B, 12.9% grade C, and 3.2% grade D. The difference was statistically significant (P <.05). The injured versus normal side-to-side KT-2000 difference at maximum manual loading was 3.68 mm (SD, 1.71) in group A and 1.64 (SD, 2.05) in group B (P <.0001). CONCLUSIONS: This prospective study could not provide significant data suggesting that 1 method of fixation is superior to the other. Therefore, we consider both methods comparable in terms of stability and functional outcome at short-term follow-up. Transcondylar fixation offers the following advantages: fixation without intra-articular hardware, greater bone-to-bone fixation, and the possibility of fixation with breakage of the posterior femoral tunnel wall. Moreover, this method may be a useful alternative in revision ACL surgery.

Adolescent↗

Tendon healing in a bone tunnel. Part I: Biomechanical results after biodegradable interference fit fixation in a model of anterior cruciate ligament reconstruction in sheep.

PURPOSE: Interference fit fixation of soft-tissue grafts has recently raised strong interest because it allows for anatomic graft fixation that may increase knee stability and graft isometry. Although clinical data show promising results, no data exist on how tendon healing progresses using this fixation. The purpose of the present study was to investigate anterior cruciate ligament (ACL) reconstruction biomechanically using direct tendon-to-bone interference fit fixation with biodegradable interference screws in a sheep model. TYPE OF STUDY: Animal study. METHODS: Thirty-five mature sheep underwent ACL reconstruction with an autologous Achilles tendon split graft. Grafts were directly fixed with poly-(D,L-lactide) interference screws. Animals were euthanized after 6, 9, 12, 24, and 52 weeks and standard biomechanical evaluations were performed. RESULTS: All grafts at time zero failed by pullout from the bone tunnel, whereas grafts at 6 and 9 weeks failed intraligamentously at the screw insertion site. At 24 and 52 weeks, grafts failed by osteocartilaginous avulsion. At 24 weeks, interference screws were macroscopically degraded. At 6 and 9 weeks tensile stress was only 6.8% and 9.6%, respectively, of the graft tissue at time zero. At 52 weeks, tensile stress of the reconstruction equaled 63.8% and 47.3% of the Achilles tendon graft at time zero and the native ACL, respectively. A complete restitution of anterior-posterior drawer displacement was found at 52 weeks compared with the time-zero reconstruction. CONCLUSIONS: It was found that over the whole healing period the graft fixation proved not to be the weak link of the reconstruction and that direct interference fit fixation withstands loads without motion restriction in the present animal model. The weak link during the early healing stage was the graft at its tunnel entrance site, leading to a critical decrease in mechanical properties. This finding indicates that interference fit fixation of a soft-tissue graft may additionally alter the mechanical properties of the graft in the early remodeling stage because of a possible tissue compromise at the screw insertion site. Although mechanical properties of the graft tissue had not returned to normal at 1 year compared with those at time zero, knee stability had returned to normal at that time. There was no graft pullout after 24 weeks, indicating that screw degradation does not compromise graft fixation.

Absorbable Implants↗

A biomechanical comparison of screw and wire fixation with and without polymethylmethacrylate re-enforcement for acetabular osteotomy stabilization in dogs.

OBJECTIVE: Compare the biomechanical characteristics of screw and wire fixation with and without polymethylmethacrylate (PMMA) re-enforcement for acetabular osteotomy stabilization in dogs. ANIMALS: Pelves removed from 8 adult mixed breed dogs weighing between 25 and 30 kg. PROCEDURE: The pubic symphysis of each pelvis was split and a central transverse acetabular osteotomy was performed. One hemipelvis from each dog was stabilized with the composite fixation (interfragmentary Kirschner wire, two screws and a figure-of-eight orthopedic wire with PMMA). The contralateral hemipelves was stabilized with an interfragmentary Kirschner wire, two screws, and a figure-of-eight orthopedic wire without PMMA. All hemipelves were tested in bending by using a materials testing machine at a cross head speed of 5 mm/min. An extensometer was placed on the dorsomedial surface of the hemipelves centered over acetabular osteotomy to record distraction of the osteotomy during loading. A load/deformation curve and a load/distraction curve was produced for each hemipelvis. The slope for the initial linear portion of the load/deformation curve and the load/distraction curve, yield load and maximum load sustained were compared between repair groups using a paired t-test with P < .05 considered significant. RESULTS: The slope of the load/deformation curve was significantly greater (P = .001) for hemipelves stabilized with the composite fixation (mean +/- SD: 69 +/- 18 N/mm) compared with hemipelves stabilized without PMMA (mean +/- SD: 39 +/- 8 N/mm). There was no significant difference (P = .593) between repair groups in the slope of the load/distraction curves as measured on the extensometer. Yield load was significantly greater (P = .0002) for hemipelves stabilized with the composite fixation (mean +/- SD: 184 +/- 25 N) compared to hemipelves stabilized without PMMA (mean +/- SD: 74 +/- 12 N). Maximum load sustained was also significantly greater (P = .013) for hemipelves stabilized with the composite fixation (mean +/- SD: 396 +/- 71 N) compared to hemipelves stabilized without PMMA (mean +/- SD: 265 +/- 94 N). Failure of hemipelves stabilized with the composite fixation occurred primarily by ventrolateral bending of the cranial and caudal pelvic segments at the osteotomy site. Failure of hemipelves stabilized without PMMA occurred by ventrolateral bending of the cranial and caudal pelvic segments at the osteotomy site with pronounced concurrent ventrolateral rotation of the cranial pelvic segment. CONCLUSION: PMMA improves the mechanical characteristics of acetabular fracture fixation, at least in part by neutralization of rotational forces. The results of this study justify use of PMMA as a component of the composite fixation when repairing acetabular fractures.

Acetabulum↗

Assessment of the relationship between timing of fixation of the fracture and secondary brain injury in patients with multiple trauma.

BACKGROUND: It has been suggested that early fixation of a fracture is deleterious to eventual neurologic outcome. We undertook this study to determine whether the timing of fracture fixation is correlated to neurologic outcome. METHODS: We retrospectively reviewed patients with severe head and orthopedic injuries requiring fracture fixation. Patients were divided into two groups: early fracture fixation (< 24 hours after injury) and late fracture fixation (> 24 hours after injury). RESULTS: One hundred twenty-three patients met entry criteria. During fracture fixation, the early group had a significant 2-, 3-, and 2-fold increase in crystalloid, blood infusion, and blood loss, respectively. There was no difference in oxygen saturation and systolic blood pressure or episodes of cranial hypertension or hypoperfusion. There was no difference in outcomes as measured by in-hospital complications, stay in the intensive care unit or hospital, mortality rates, hospital discharge or follow-up Glasgow Coma Scores, or long-term orthopedic or neurologic results. CONCLUSIONS: Patients undergoing fracture fixation with severe head injury mandate monitoring of intracranial pressure and perfusion and tailored fluid resuscitation to meet specific organ end points. Integrating end organ perfusion and pressure with meticulous fluid status during the definitive repair phase may reduce the exposure to secondary brain injury in patients undergoing early fracture fixation.

Adult↗

Visual attention while driving: sequences of eye fixations made by experienced and novice drivers.

Eye fixations were recorded while novice and experienced drivers drove along three types of roads (rural, suburban and dual-carriageway). An analysis of the content of those fixations was performed in order to identify differences in the scanpaths that can be associated with skill acquisition and that can indicate a sensitivity to road type. This analysis itemized the part of the visual scene that was inspected with each fixation, and identified what the driver looked at as a function of what they had looked at previously. Single-fixation, two-fixation, and three-fixation patterns of eye-movements were identified. Differences in sequences of fixations were found between novice and experienced drivers on the three types of roads, with experienced drivers showing greater sensitivity overall, and with some stereotypical transitions in the visual attention of the novices. A number of individual sequences were identified, including a roadway preview pattern (alternating fixations between near and far views of the road ahead), and patterns involving mirror inspections that varied according to the road type.

Adult↗

How do readers know where to look next? Local information distributions influence eye fixations.

Skilled readers' eye movements were recorded as they inspected sentences in preparation for comprehension questions. The sentences were written around target words that had uneven distributions of information, in that the words were predictable, given either the first few letters or the last few letters. Some parts of these words can be described as being more important than others for successful word recognition, and the experiments asked whether inspection patterns reflect the uneven distributions. Four types of ten-letter word were used: words with highly redundant endings (e.g. engagement), with moderately redundant endings (e.g. repatriate), with moderately redundant beginnings (e.g. superstore), and words with informative beginnings and endings (e.g. amalgamate). Redundancy was defined operationally in terms of the number of possible completions of a word given the first few or last few letters. A highly redundant ending is therefore one that occurs frequently and cannot be used to identify the word. Words gaining just one fixation received this fixation nearer to the word's centre than in the case of the first fixation upon words gaining two fixations. The single fixation also had a longer duration, giving support to the notion of a convenient viewing location, which, when achieved, can lead to a net saving in inspection time. In the case of words with redundant endings gaining just one fixation, the fixation was nearer to the beginning of the word than in the case of words with informative endings, and this influence of word type upon the location of the first fixation was interpreted in favour of the parafoveal processing hypothesis of eye guidance during reading.

Adult↗