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Biomedical subjects

M F Land

Publications and source records attributed to M F Land.

At least 19 recordsLinked to original sources
PubMed2005

The effect of different adhesives on vinyl polysiloxane bond strength to two tray materials.

STATEMENT OF PROBLEM: Although stock trays often provide mechanical retention for elastomeric impression materials, manufacturers typically recommend the use of an adhesive, whether a stock or custom tray is used. Universal adhesives for vinyl polysiloxane (VPS) are used in practice, but comparative bond strengths have not been reported. PURPOSE: The aim of this study was to evaluate the bond of 3 VPS materials with a methylmethacrylate autopolymerizing and a light-polymerizing tray material, using the adhesive recommended by the manufacturer of the impression material, and 2 universal adhesives (paint-on and spray-on). MATERIAL AND METHODS: Ten specimens per group, for a total of 180 flat 15 x 15 x 20-mm specimens, were made from the 2 tray materials and finished to 320 grit with silicon carbide paper. Four paint-on adhesives (Coltene, Caulk, Kerr VPS, or universal VPS) and 1 spray-on adhesive (Sili Spray) were applied and dried following manufacturers' instructions or for 10 minutes. Three impression materials, Affinis, Aquasil, and Take I, were automixed and injected into a perforated cylinder positioned in a universal testing machine. Tray specimens were positioned against the open cylinder end in contact with the VPS material. Tensile tests (cross-head speed 5 mm/min) were conducted until adhesive separation failure. Mean values and standard errors of the adhesive strength were recorded in MPa for each material combination. Data were first analyzed with Welch analysis of variance (ANOVA) to determine overall significance, and then with a 3-way ANOVA and the Tukey HSD test to make pairwise comparisons, with alpha=.05 for all testing. RESULTS: For all impression materials tested, the universal spray-on adhesive consistently demonstrated significantly lower bond strengths than all other adhesives (P<.05). Equivalent or significantly (P<.05) higher bond strength values were found for the universal paint-on adhesive for the 3 impression materials tested. CONCLUSION: The use of GC paint-on universal adhesive provided significantly higher adhesive values than those obtained with the adhesives supplied by the manufacturers of the impression materials tested, with the exception of the Kerr impression and adhesive material combination where no significant differences were found.

Adhesives↗
PubMed2003

Rapid colour changes in multilayer reflecting stripes in the paradise whiptail, Pentapodus paradiseus.

The Paradise whiptail (Pentapodus paradiseus) has distinct reflective stripes on its head and body. The reflective stripes contain a dense layer of physiologically active iridophores, which act as multilayer reflectors. The wavelengths reflected by these stripes can change from blue to red in 0.25 s. Transmission electron microscopy revealed that the iridophore cells contain plates that are, on average, 51.4 nm thick. This thickness produces a stack, which acts as an ideal quarter-wavelength multilayer reflector (equal optical thickness of plates and spaces) in the blue, but not the red, region of the spectrum. When skin preparations were placed into hyposmotic physiological saline, the peak wavelength of the reflected light shifted towards the longer (red) end of the visible spectrum. Hyperosmotic saline reversed this effect and shifted the peak wavelength towards shorter (blue/UV) wavelengths. Norepinephrine (100 micromol l(-1)) shifted the peak wavelength towards the longer end of the spectrum, while adenosine (100 micromol l(-1)) reversed the effects of norepinephrine. The results from this study show that the wavelength changes are elicited by a change of approximately 70 nm in the distance between adjacent plates in the iridophore cells.

Animals↗
PubMed2001

Steering with the head. the visual strategy of a racing driver.

We studied the eye movements of a racing driver during high-speed practice to see whether he took in visual information in a different way from a normal driver on a winding road [1, 2]. We found that, when cornering, he spent most of the time looking close to, but not exactly at, the tangent points on the inside edges of the bends. Each bend was treated slightly differently, and there was a highly repeatable pattern to the way the track edge was viewed throughout each bend. We also found a very close relationship between the driver's head direction and the rate of rotation of the car 1 s later. We interpret these observations as indicating that the driver's gaze is not driven directly by tangent point location, as it is in ordinary driving. Instead, we propose that his head direction is driven by the same information that he uses to control steering and speed, namely his knowledge of the track and his racing line round it. If he directs his head at an angle proportional to his estimate of car rotation speed, this will automatically bring his head roughly into line with the tangent points of the bends. From this standardized position, he can use the expected movements of the tangent points in his field of view to verify, and if necessary modify, his racing line during the following second.

Automobile Driving↗
PubMed2001

In what ways do eye movements contribute to everyday activities?

Two recent studies have investigated the relations of eye and hand movements in extended food preparation tasks, and here the results are compared. The tasks could be divided into a series of actions performed on objects. The eyes usually reached the next object in the sequence before any sign of manipulative action, indicating that eye movements are planned into the motor pattern and lead each action. The eyes usually fixated the same object throughout the action upon it, although they often moved on to the next object in the sequence before completion of the preceding action. The specific roles of individual fixations could be identified as locating (establishing the locations of objects for future use), directing (establishing target direction prior to contact), guiding (supervising the relative movements of two or three objects) and checking (establishing whether some particular condition is met, prior to the termination of an action). It is argued that, at the beginning of each action, the oculomotor system is supplied with the identity of the required object, information about its location, and instructions about the nature of the monitoring required during the action. The eye movements during this kind of task are nearly all to task-relevant objects, and thus their control is seen as primarily 'top-down', and influenced very little by the 'intrinsic salience' of objects.

Eye Movements↗
PubMed2000

On the functions of double eyes in midwater animals.

Midwater predators often have double eyes consisting of a large upward-pointing part with a narrow field of view and high resolution, and a small downward-pointing part with a wide field of view and low resolution. In crustaceans with compound eyes the different eye parts are of basically similar construction, but in fishes the downward-pointing regions may employ unusual optical systems with unknown image-forming capabilities. It has been suggested that the upward-directed parts are used to detect silhouettes of animals against the residual daylight, whereas the lower parts look out for luminescent organisms. Here I calculate the sizes that apposition compound eyes would need to attain in order to fulfil these tasks, and the way that size should vary with depth. It is concluded that silhouette detection is much the more demanding task, and becomes increasingly difficult as light levels decrease. For this reason the upward-pointing parts must increase rapidly with depth. This is not the case with luminescence detectors, where the task is most difficult near the surface because of upwelling background light, and becomes easier with depth. On the whole these predictions fit well with the sizes and shapes of real midwater eyes, especially in the case of the hyperiid amphipods.

Animals↗
PubMed2000

From eye movements to actions: how batsmen hit the ball.

In cricket, a batsman watches a fast bowler's ball come toward him at a high and unpredictable speed, bouncing off ground of uncertain hardness. Although he views the trajectory for little more than half a second, he can accurately judge where and when the ball will reach him. Batsmen's eye movements monitor the moment when the ball is released, make a predictive saccade to the place where they expect it to hit the ground, wait for it to bounce, and follow its trajectory for 100-200 ms after the bounce. We show how information provided by these fixations may allow precise prediction of the ball's timing and placement. Comparing players with different skill levels, we found that a short latency for the first saccade distinguished good from poor batsmen, and that a cricket player's eye movement strategy contributes to his skill in the game.

Adult↗
PubMed1999

The effects of skill on the eye-hand span during musical sight-reading.

The eye-hand span (EHS) is the separation between eye position and hand position when sight-reading music. It can be measured in two ways: in notes (the number of notes between hand and eye; the 'note index'), or in time (the length of time between fixation and performance; the 'time index'). The EHSs of amateur and professional pianists were compared while they sight-read music. The professionals showed significantly larger note indexes than the amateurs (approximately four notes, compared to two notes), and all subjects showed similar variability in the note index. Surprisingly, the different groups of pianists showed almost identical mean time indexes (ca. 1 s), with no significant differences between any of the skill levels. However, professionals did show significantly less variation than the amateurs. The time index was significantly affected by the performance tempo: when fast tempos were imposed on performance, all subjects showed a reduction in the time index (to ca. 0.7 s), and slow tempos increased the time index (to ca. 1.3 s). This means that the length of time that information is stored in the buffer is related to performance tempo rather than ability, but that professionals can fit more information into their buffers.

Adult↗
PubMed1999

Visual optics: The sandlance eye breaks all the rules.

The eyes of the sandlance differ from those of other fish, both optically and in the kinds of movements they make. The predatory behaviour of these tiny fish not only makes their lifestyle similar to that of a chameleon, but has led to several extraordinary examples of convergence in the visual system.

Animals↗
PubMed1999

Motion and vision: why animals move their eyes.

Nearly all animals with good vision have a repertoire of eye movements. The majority show a pattern of stable fixations with fast saccades that shift the direction of gaze. These movements may be made by the eyes themselves, or the head, or in some insects the whole body. The main reason for keeping gaze still during fixations is the need to avoid the blur that results from the long response time of the photoreceptors. Blur begins to degrade the image at a retinal velocity of about 1 receptor acceptance angle per response time. Some insects (e.g. hoverflies) stabilise their gaze much more rigidly than this rule implies, and it is suggested that the need to see the motion of small objects against a background imposes even more stringent conditions on image motion. A third reason for preventing rotational image motion is to prevent contamination of the translational flow-field, by which a moving animal can judge its heading and the distances of objects. Some animals do let their eyes rotate smoothly, and these include some heteropod molluscs, mantis shrimps and jumping spiders, all of which have narrow linear retinae which scan across the surroundings. Hymenopteran insects also rotate during orientation flights at speeds of 100-200 degrees s-1. This is just consistent with a blur-free image, as are the scanning speeds of the animals with linear retinae.

Animals↗
PubMed1999

Investment strength as a function of time and temperature.

OBJECTIVES: The purpose of this investigation was to evaluate and compare the compressive strength characteristics of selected investments and to determine if these change as a function of time and temperature after mixing. METHODS: Two phosphate-bonded investments and one gypsum-bonded investment were selected and 288 cylindrical specimens were fabricated (n = 9). Baseline compressive strength values were determined for each material at room temperature and compressive strength measurements were made for all materials at 700 degrees C, and at 872 degrees C only for phosphate-bonded investments. Each material was tested at 2, 6, 12, and 24 h after mixing. Specimens were heated at 15 degrees C/min and heat soaked for 5 min in a burnout oven at the applicable testing temperature. They were transferred to a preheated chamber on an Instron testing machine and subjected to a compressive load at a crosshead speed of 2 mm/min until failure. The resulting strength data are presented in MN/m2. RESULTS: The gypsum-bonded investment did not exhibit marked differences in strength as a function of either time or temperature. The phosphate-bonded investments, on the other hand, demonstrated considerable changes in compressive strength and differed in compressive strength characteristics at 700 degrees C and 872 degrees C respectively. CONCLUSIONS: At elevated temperatures, all materials approximated peak strength 2 h after initial mixing. At room temperature, the phosphate-bonded investments in this study were not significantly stronger than the gypsum-bonded investment material. However, they exhibited increased compressive strength as a function of time and temperature that was considerably higher than that exhibited by the gypsum-bonded investments.

Analysis of Variance↗
PubMed1998

Dental luting agents: A review of the current literature.

STATEMENT OF PROBLEM: The practice of fixed prosthodontic has changed dramatically with the introduction of innovative techniques and materials. Adhesive resin systems are examples of these changes that have led to the popularity of bonded ceramics and resin-retained fixed partial dentures. Today's dentist has the choice of a water-based luting agent (zinc phosphate, zinc polycarboxylate, glass ionomer, or reinforced zinc oxide-eugenol) or a resin system with or without an adhesive. Recent formulations of glass ionomer luting agents include resin components (resin-modified glass ionomers), which are increasingly popular in clinical practice. PURPOSE: This review summarizes the research on these systems with the goal of providing information that will help the reader choose the most suitable material. MATERIAL: The scientific studies have been evaluated in relation to the following categories: (1) biocompatibility, (2) caries or plaque inhibition, (3) microleakage, (4) strength and other mechanical properties, (5) solubility, (6) water sorption, (7) adhesion, (8) setting stresses, (9) wear resistance, (10) color stability, (11) radiopacity, (12) film thickness or viscosity, and (13) working and setting times. In addition, guidelines on luting-agent manipulation are related to available literature and include: (1) temporary cement removal, (2) smear layer removal, (3) powder/liquid ratio, (4) mixing temperature and speed, (5) seating force and vibration, and (6) moisture control. Tables of available products and their properties are also presented together with current recommendations by the authors with a rationale.

Dental Cements↗
PubMed1997

The knowledge base of the oculomotor system.

In everyday life, eye movements enable the eyes to gather the information required for motor actions. They are thus proactive, anticipating actions rather than just responding to stimuli. This means that the oculomotor system needs to know where to look and what to look for. Using examples from table tennis, driving and music reading we show that the information the eye movement system requires is very varied in origin and highly task specific, and it is suggested that the control program or schema for a particular action must include directions for the oculomotor and visual processing systems. In many activities (reading text and music, typing, steering) processed information is held in a memory buffer for a period of about a second. This permits a match between the discontinuous input from the eyes and continuous motor output, and in particular allows the eyes to be involved in more than one task.

Cognition↗
PubMed1997

Custom-cast post fabrication with a thermoplastic material.

Problems associated with fabrication of custom-cast posts with a plastic rod and autopolymerizing resin include voids in the pattern, locking of the pattern into undercuts, and resin fracture during removal of the pattern. This article describes a system for fabricating custom-cast posts with a thermoplastic resin to address these problems. The system consists of a plastic support rod and a thermoplastic resin. A custom post pattern can be quickly and easily produced by trimming the plastic rod, heating the resin either over a direct flame or in a low-temperature glue gun and seating it into a lubricated prepared canal. The resin flows well and thereby reduces voids. The resin is elastic enough to be removed from small undercuts without locking into the canal or fracturing.

Dental Casting Technique↗
PubMed1997

Visual acuity in insects.

The acuity of compound eyes is determined by interommatidial angles, optical quality, and rhabdom dimensions. It is also affected by light levels and speed of movement. In insects, interommatidial angles vary from tens of degrees in Apterygota, to as little as 0.24 degrees in dragonflies. Resolution better than this is not attainable in compound eyes of realistic size. The smaller the interommatidial angle the greater the distance at which objects--prey, predators, or foliage--can be resolved. Insects with different lifestyles have contrasting patterns of interommatidial angle distribution, related to forward flight, capture on the wing, and predation on horizontal surfaces.

Journal Article↗
PubMed1996

Smear layer instability caused by hemostatic agents.

The effect of hemostatic agents, other than a 15.5% Fe2(SO4)3 solution, on prepared tooth structure is unknown. The purpose of this study was to (1) compare the effect of six commonly used hemostatic solutions and two nondental astringents on the dentinal smear layer and (2) determine whether different responses caused by product and/or time could be established. Standardized dentinal smear layers were exposed to eight astringent solutions for 30, 120, and 300 seconds (n = 6). A total of 144 SEM photographs at x2400 magnification were ranked according to predetermined criteria for five categories of smear layer removal and etching of underlying tooth structure. There were significant differences (p < 0.001) caused by the solution, exposure time, and their interaction. Greatest smear layer removal was observed with 21.3% AlCl3-6 hydrate, 8% racemic epinephrine HCl, and 15.5% Fe2(SO4)3 solutions at longer exposures. These caused significantly more removal than did almost pH neutral tetrahydrozoline or oxymetazoline (p < 0.05).

Acid Etching, Dental↗
PubMed1994

Where we look when we steer.

Steering a car requires visual information from the changing pattern of the road ahead. There are many theories about what features a driver might use, and recent attempts to engineer self-steering vehicles have sharpened interest in the mechanisms involved. However, there is little direct information linking steering performance to the driver's direction of gaze. We have made simultaneous recordings of steering-wheel angle and drivers' gaze direction during a series of drives along a tortuous road. We found that drivers rely particularly on the 'tangent point' on the inside of each curve, seeking this point 1-2 s before each bend and returning to it throughout the bend. The direction of this point relative to the car's heading predicts the curvature of the road ahead, and we examine the way this information is used.

Automobile Driving↗