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PubMed2006

Potential effect on the retinoscopic reflex of scleral expansion surgery for presbyopia.

PURPOSE: In this initial study, we model the impact of crystalline lens movement and tilt, which we postulate are a potential consequence of scleral expansion surgery (SES). We demonstrate the possibility that these lead to an improvement in near vision with no concurrent restoration of lens accommodative function. With the implantation of scleral expansion bands (SEB), Schachar predicts an increase in the amplitude of accommodation not observed objectively. We postulate that the SEB implants cause the crystalline lens to be shifted forward and become misaligned. Using mathematical models of the optics of the human eye, we assess the effects of our postulate on the power of the eye, the depth of field, optical aberrations, and the appearance of the horizontal retinoscopic reflex. METHODS: Using anatomically based models of the human eye, the postulated effects of SES are theoretically modeled and optically analyzed in Code V for unintentional lens anterior movement, tilts, and decentrations of up to 1 mm, +/- 3 degrees , and +/- 0.3mm, respectively. The transverse aberrations are calculated before and after SES. Because it has been reported that the appearance of the retinoscopic reflex is consistent with the presence of excess aberrations, we also predict the appearance of the one-dimensional retinoscopic reflex for our models. The change in refractive error is also determined. RESULTS.: Unintentional lens shift, tilt, and decentration, as might occur as a result of SES surgery, would result in an increase in the total power of the eye along with an increase in asymmetric aberrations and little change in symmetric aberrations. The calculated appearances of the retinoscopic reflexes pre- and postsurgery are consistent with observations in the literature. CONCLUSIONS: Scleral expansion surgery could produce an improvement in near vision through an unintentional anterior displacement of the crystalline lens in combination with excess tilts and decentrations rather than as a result of a restoration of lens accommodation. There is a need for measurements and further analysis of the optical and visual properties of SES patients postsurgery.

Accommodation, Ocular↗
PubMed1991

Control of end-point forces of a multijoint limb by functional neuromuscular stimulation.

A multivariable feedback controller was designed and tested for regulating the magnitude and orientation of the force vector at the end point of a multijoint limb in contact with an isometric load. The force vector was produced by electrical stimulation of muscles. To achieve arbitrary control of end-point force magnitude and orientation, two coupling issues must be dealt with by the control system. First, there is a geometric coupling between the end-point force vector and joint torques. The amplitude and orientation of the force vector depend on the limb geometry. Second, torques at two joints may be coupled due to activation of muscles that cross them (biarticular coupling). To eliminate the geometric coupling, a transformation of controller error from the Cartesian space to the joint space was employed. A multivariable proportional-plus-integral (PI) control law was used to calculate muscle activation based on the transformed controller error. Centralized and decentralized controls were investigated for decoupling the effects of biarticular muscles. The results obtained from cat experiments showed that the magnitude and orientation of the end-point forces of the cat hindlimb could be regulated by this controller. In the presence of strong biarticular coupling, centralized control yielded better performance than decentralized control during transient responses. Both control strategies could decouple the biarticular muscle at steady state. When no biarticular coupling was present, centralized control sometimes performed worse than decentralized control. This is the first step in the simultaneous control of multiple joints by functional neuromuscular stimulation (FNS). The controller has broad potential applications in FNS neural prostheses.

Animals↗
PubMedDate not supplied

Developmental trends in children's pretend play.

The developmental trends in pretend play were investigated in children 2-6 years of age (18 in each of five age groups) by examining changes in pretend action and speech separately. Play behaviour was assessed by using a selected set of Duplo Lego toys. Interest focused on occurrence of decentration, decontextualization and integration at different age levels. The proportions of decentred and decontextualized acts, action integrations and play themes, increased linearly with age. Changes in substitutive and inventive actions were, however, more minor than expected. Single-scheme combinations did not reveal any essential aspect of the development of children's symbolic competence. In this sense, multischeme combinations were more important in revealing the children's way of organizing toy material. Linear age trends were not found for language measures. The use of decentred utterances, language integrations and linguistically expressed themes were individual-specific rather than age-related. Issues for studying pretend play in language-impaired groups are also suggested.

Age Factors↗
PubMed1982

Local protein synthesizing activity in axonal fields regenerating in vitro.

The goldfish retinal explant system of Landreth and Agranoff was used to study endogenous protein synthesizing activity of retinal ganglion cell axons regenerating in culture. Light and electron microscopic examination of axonal fields showed that axons were free of nonneural cell investment. Decentralized axons were incubated with a mixture of tritiated amino acids, and direct quantitative microanalysis of protein and tritium radioactivity was carried out on individual axonal fields. Our findings showed that radioactive amino acids were incorporated into axonal protein in a manner that inhibited significantly by cycloheximide, but not by chloramphenicol. Decentralized axons appeared to maintain their viability for at least 3-4 h. Axonal fields maintaining their central connections to the explant incorporated 3H-amino acids at an apparent rate that was similar to decentralized axonal fields. Labeled material transported into axonal fields from ganglion cell bodies appeared in significant amounts after a delay of 2-3 h. Fluorographic patterns of axonal proteins after labeling with either 3H-amino acids or [35S]methionine and separated by microelectrophoresis indicated that primarily tubulin and, to a lesser extent, actin were labeled. Our findings indicate that goldfish retinal ganglion cell axons regenerating in vitro exhibit measureable endogenous protein-synthesizing activity.

Amino Acids↗
PubMed1980

Effects of clonidine on canine cardiac neuroeffector structures controlling heart rate.

1 In intact dogs anaesthetized with pentobarbitone, clonidine (10 mug/kg, i.v.) produced a sustained decrease in heart rate. This effect was significantly smaller in vagotomized dogs in which the sympathetic drive to the heart was either left intact or experimentally created by continuous electrical stimulation of the decentralized cardioaccelerator nerve. In the latter preparation, the negative chronotropic action of clonidine was reversed by an intravenous injection of phentolamine, whereas in the former experimental situation it was antagonized only by an intravenous plus an intravertebral artery injection of phentolamine.2 In dogs with denervated hearts the tachycardia produced by electrical stimulation of the cardioaccelerator nerve was accompanied by a rise in noradrenaline overflowing into the coronary sinus plasma. Clonidine inhibited both these effects and phentolamine restored them to pre-clonidine levels.3 Clonidine decreased heart rate in dogs with an intact parasympathetically innervated heart and decentralized stellate ganglia. When the low basal heart rate of this preparation was elevated by electrical stimulation of the cardioaccelerator nerve, clonidine had a negative chronotropic effect, the degree of which was similar to that observed in intact dogs.4 Clonidine neither modified baseline heart rates of dogs with denervated hearts nor the levels of heart rate which in this preparation were reduced by a sustained electrical stimulation of the right vagus or increased by intravenous infusions of either isoprenaline or noradrenaline.5 These findings indicate that in the intact dog, bradycardia induced by clonidine resulted both from a reduction of sympathetic drive and from a concomitant increase in parasympathetic tone. The latter action did not occur at the level of cardiac neuroeffector structures since it was observed only in the presence of centrally connected vagal pathways. The inhibition of cardiac sympathetic tone was of both peripheral and central origin. Clonidine, in fact, diminished the quantity of noradrenaline overflowing into the coronary sinus plasma in cardiac denervated dogs with a tachycardia elicited by electrical stimulation of the decentralized cardioaccelerator nerve. This peripheral effect was probably due to an activation of alpha-adrenoceptors located on sympathetic nerve terminals since it was antagonized by phentolamine. However, in vagotomized dogs (intact sympathetic pathways) intravenous phentolamine failed to antagonize the heart rate effects of clonidine which were abolished by a subsequent injection of phentolamine into the vertebral artery. Thus, the clonidine-induced inhibition of both the peripheral and central sympathetic drive to the heart would appear to be mediated via alpha-adrenoceptors.

Animals↗
PubMed1992

The mediation of increased duodenal alkaline secretion in response to 10 mM HCl in the anaesthetized rat. Support for the involvement of capsaicin-sensitive nerve elements.

Experiments were performed on chloralose-anaesthetized rats. Bicarbonate secretion by the duodenal mucosa was continuously recorded by use of an in situ-titration technique. Exposing the duodenal segment to 10 mM HCl over 5 min increased the bicarbonate transport by about 65%. This acid-induced secretory response was resistant to neural decentralization by means of bilateral cervical vagotomy and/or splanchnicotomy. Furthermore, in the decentralized state, serosal application of the local anaesthetic lidocaine or tetrodotoxin (TTX) lowered basal duodenal bicarbonate secretion. Despite the presence of lidocaine or TTX, exposure of the duodenal mucosa to 10 mM HCl induced a net increase in secretion with a magnitude similar to that observed in the control situation with intact nerves. A 5-min exposure period of the decentralized duodenal segment to capsaicin (1.2 mg ml-1) raised bicarbonate secretion, a response which also occurred in the presence of lidocaine (serosa). Tachyphylaxis to capsaicin blocked the secretory response to 10 mM HCl and inhibited the response to luminal prostaglandin E2 (1.5 10(-5) M) by 80%. The present results indicate that luminal exposure to 10 mM HCl activates capsaicin-sensitive primary afferents which, locally within the submucosa or epithelium, activate the bicarbonate secretion.

Animals↗
PubMed1986

Activity of in situ stellate ganglion neurons of dogs recorded extracellularly.

Activity was recorded from 145 neurons in the in situ stellate ganglia of 36 dogs. The activity of 28 of these neurons, most of them located in the ganglia's cranial medial region, was related to the cardiac cycle primarily during systole. The activity of 16 of these cardiovascular-related neurons was modified by gentle mechanical distortion of the superior vena cava (1), heart (4), or thoracic aorta (11). Forty-one of the neurons were modified by respiration, with 17 being phase-locked to the respiratory cycle. Other neurons were activated by gentle mechanical distortion of localized regions of the thoracic wall (21% of all neurons), neck (18%), skin of the left foreleg (10%), or the mediastinum adjacent to the stellate ganglion (3%). Acutely decentralizing the stellate ganglion abolished the spontaneous activity of some, but not all, of these neurons including the respiratory or cardiovascular-related neurons. In the intact or acutely decentralized stellate ganglion, few neurons were activated by single short duration (1-4 ms) stimuli delivered to nerves attached directly or indirectly to the ganglion; however, most were activated by brief high frequency stimuli delivered in trains of 20-200 ms, or by single stimuli lasting 20-200 ms. As most cardiovascular, respiratory, or neck-related neurons in the stellate ganglion were not activated by single brief stimuli delivered to the cardiopulmonary nerves or vagosympathetic trunks, presumably they did not project their axons into the neck or thoracic organs. Thus, they were considered to be interneurons. It is postulated that interneurons in stellate ganglia can be modified by afferent receptors located in tissues of the neck, lungs, heart, or great thoracic vessels, whether the ganglion is intact or acutely decentralized. In addition, neurons in the stellate ganglion can be modified by mechanoreceptors located in the thoracic wall, abdominal wall, foreleg, or adjacent mediastinum. The majority of these neurons are activated by trains of impulses rather than single short duration impulses.

Action Potentials↗
PubMed1988

Cardiac effects produced by long-term stimulation of thoracic autonomic ganglia or nerves: implications for interneuronal interactions within the thoracic autonomic nervous system.

Electrical stimulation of an acutely decentralized stellate or middle cervical ganglion or cardiopulmonary nerve augments cardiac chronotropism or inotropism; as the stimulation continues there is a gradual reduction of this augmentation following the peak response, i.e., an inhibition of augmentation. The amount of this inhibition was found to be dependent upon the region of the heart investigated and the neural structure stimulated. The cardiac parameters which were augmented the most displayed the greatest inhibition. Maximum augmentation or inhibition occurred, in most instances, when 5-20 Hz stimuli were used. Inhibition of augmentation was overcome when the stimulation frequency was subsequently increased or following the administration of nicotine or tyramine, indicating that the inhibition was not primarily due to the lack of availability of noradrenaline in the nerve terminals of the efferent postganglionic sympathetic neurons. Furthermore, as infusions of isoproterenol or noradrenaline during the period of inhibition could still augment cardiac responses, whereas during the early peak responses they did not, the inhibition of augmentation does not appear to be due primarily to down regulation of cardiac myocyte beta-adrenergic receptors. The inhibition was modified by hexamethonium but not by phentolamine or atropine. Inhibition occurred when all ipsilateral cardiopulmonary nerves connected with acutely decentralized middle cervical and stellate ganglia were stimulated, whereas significant inhibition did not occur when these nerves were stimulated after they had been disconnected from the ipsilateral decentralized ganglia. Taken together these data indicate that the inhibition of cardiac augmentation which occurs during relatively long-term stimulation of intrathoracic sympathetic neural elements is due in large part to nicotinic cholinergic synaptic mechanisms that lie primarily in the major thoracic autonomic ganglia. They also indicate that long-term stimulation in intrathoracic sympathetic neural elements with frequencies as low as 2 Hz may augment the heart as much as higher stimulation frequencies, depending upon the structure stimulated and the cardiovascular parameter monitored.

Adrenergic beta-Antagonists↗
PubMed1985

Metabolic mapping of a cardiac reflex mediated by sympathetic ganglia in dogs.

The hypothesis tested was that some cardiac reflexes can be mediated by the neural mechanisms in thoracic sympathetic ganglia in addition to those existing in the spinal cord or brain stem. In mongrel dogs anesthetized with pentobarbital sodium the left stellate (SG) and middle cervical (MCG) ganglion were decentralized, and the left thoracic vagosympathetic trunk was cut cranial to the MCG and caudal to the MCG but above the heart. The left thoracic vagosympathetic trunk below the MCG was then stimulated afferently. A single bolus of 2-[14C]deoxyglucose was injected intravenously and the stimuli repeated periodically toward the decentralized ganglia for 45 min. The heart and ganglia were then removed, frozen, and sectioned for autoradiography. Significant increases in glucose utilization were found in both the SG and MCG compared with ganglia from nonstimulated control animals. Significant increases in the glucose utilization of the endocardial third of the left ventricle were also observed in the reflexly stimulated hearts. Neural mechanisms in the acutely decentralized SG and MCG could modify cardiac glucose utilization within specific areas of the heart. In addition, such ganglionic mechanisms may be concentrated within the caudal half of the SG. These data suggest that many neurons in the thoracic autonomic ganglia may be involved in local cardiac reflexes.

Afferent Pathways↗
PubMed1991

Dispersion of refractoriness in canine ventricular myocardium. Effects of sympathetic stimulation.

In 18 dogs on total cardiopulmonary bypass, the average interval between local activations during artificially induced ventricular fibrillation (VF interval) was measured from extracellular electrograms, simultaneously recorded from up to 32 ventricular sites. VF intervals were used as an index of local refractoriness, based on the assumption that during ventricular fibrillation, cells are reexcited as soon as they have recovered their excitability. In support of this, microelectrode recordings in two hearts during ventricular fibrillation did not show a diastolic interval between successive action potentials. Refractory periods determined at a basic cycle length of 300 msec with the extrastimulus method correlated well with VF intervals measured at the same sites. Thus, this technique allows assessment of spatial dispersion of refractoriness during brief interventions such as sympathetic stimulation. The responses to left, right, and combined stellate ganglion stimulation varied substantially among individual hearts. This was observed both in dogs with an intact (n = 12) and decentralized (n = 6) autonomic nervous system. Individual ventricular sites could show effects of both left and right stellate ganglion stimulation (42% of tested sites) or show effects of left-sided stimulation only (31%) or right-sided stimulation only (14%). In 13% of sites, no effects of stellate stimulation were observed. Apart from these regional effects, the responses could be qualitatively different; that is, within the same heart, the VF interval prolonged at one site but shortened at another in response to the same intervention, although shortening was the general effect and prolongation the exception. Whenever sites responded to stellate ganglion stimulation with a shortening of VF interval, this shortening was approximately 10% for left, right, or combined stimulation, whether the autonomic nervous system was intact or decentralized. In six of 12 hearts in the intact group, there was a distinct regional effect of left stellate ganglion stimulation; in the other six hearts, the effects were distributed homogeneously over the ventricles. In three hearts, the effect of left stellate ganglion stimulation was strongest in the posterior wall, and in the other three hearts, in the anterior wall. The effects of right stellate ganglion stimulation were restricted to the anterior or lateral part of the left ventricle. Dispersion of VF intervals increased after left and combined stellate ganglion stimulation in the intact group and after right stellate ganglion stimulation in the decentralized group, but not significantly in every heart. This points to a marked individual variation with regard to the effects of sympathetic stimulation on electrophysiological properties of the heart.

Action Potentials↗
PubMed2001

Influence of Stiles-Crawford effect apodization on spatial visual performance with decentered pupils.

Using theoretical estimates of the optical-transfer function and line-spread function as image-quality criteria, we predicted the influence of the Stiles-Crawford effect (SCE) on both optical performance of the eye and subjective measurements of transverse aberrations when pupils are decentered. The SCE was modeled as a pupil apodization. The SCE appears to improve image quality by providing compensation for aberrations induced by pupil decentration, but this improvement is usually small. When a criterion of the placement of the image is used as the centroid of the line-spread function, an average SCE reduces the influence of pupil decentration on subjective transverse chromatic aberrations (TCA's) for 5-mm-diameter pupils by 30%. This reduction is much less than that obtained by previous experimental studies of TCA, and possible reasons for this discrepancy are discussed. Decentering the SCE produces an appreciable shift in subjective TCA for 5-mm-diameter pupils of 1.4 arc min per 1-mm decentration (at wavelengths 433 and 622 nm).

Color Perception↗
PubMed1999

Point-of-care testing: the views of the working group of the Dutch Association of Clinical Chemistry.

There is clearly a demand for near-patient laboratory testing. Modern technological developments support its implementation, but these are not the only preconditions that need to be satisfied in order to successfully introduce decentralized testing. In part 2 of this paper we give a definition of decentralized testing. Part 3 presents the advantages and disadvantages. This list is a summary taken from the suggested literature. Part 4 examines the formal position of the clinical chemist. Of major importance to the situation in the Netherlands are the Care Institutions Quality Act and the Individual Health Care Professions Act. Part 5 discusses professional standards. Part 6 outlines practical considerations which might support the implementation of decentralized testing. The clinical chemist should play an active role in drawing up a discussion paper for his own hospital.

Chemistry, Clinical↗
PubMedDate not supplied

The Memphis diabetes continuing care program.

Since 1963 a network of nurse-operated, physician-backed decentralized clinics has provided continuing care for more than 5000 diabetic patients referred from the medical center clinics. Protocols that provide therapy goals and management details are used by the nurses and nutritionists in this network. To reduce fragmentation of care, intercurrent illnesses as well as other chronic diseases are treated using protocols in these clinics or in the home. This study examines certain outcome data in a subset of 556 diabetic patients under continuing care over a 7-yr period in this network, with comparisons being made to care before referral. Blood glucose is maintained at comparable levels in both decentralized and hospital clinics. Blood pressure levels in hypertensive patients are maintained in a satisfactory range. Total hospitalization rates are reduced by 47%. For ketoacidosis and amputation, hospitalization is decreased by 69% compared with the experience before referral. The maintenance care costs are decreased substantially compared with costs before referral due to the less expensive ambulatory services and the reduced need for hospitalization. The data support the concept that decentralization is an effective means of providing continuing care to patients with diabetes mellitus.

Ambulatory Care↗
PubMed1995

The development of radiotherapy in Denmark during 100 years. From radiology to oncology.

The evolution of radiotherapy in Denmark is traced from its early inception in 1896 to the first three radium centres in 1913-1914, the establishment of which caused a roar of protests among the surgeons of that time. Private initiative pioneered the Radium Foundation which raised money for radium and financed erection of new buildings for the three centres in the 1930's. Radiotherapy became a separate speciality in 1950. The early 1960s saw the introduction of megavoltage therapy and the first promising results from chemotherapeutic management of solid tumors. The consequent referral of patients to centres for non-surgical therapy created a need for two new centres (Aalborg and Herlev) and called for a gradual closing down of decentralized low-voltage treatment at county level. However, the decentralization of health care in 1970 partly reversed this trend and some patients were therefore referred for decentralized treatment at major county hospitals. Such treatment mainly consisted of adjuvant or palliative chemotherapy, though in a few countries palliative therapy was supported by low-voltage therapy. In 1987 the medical speciality of radiotherapy was officially renamed oncology.

Denmark↗
PubMedDate not supplied

Posterior chamber phakic intraocular lenses to correct high myopia: a comparative study between Staar and Adatomed models.

PURPOSE: To determine the feasibility of using posterior chamber phakic intraocular lenses (PIOLs) to treat high myopia, comparing two different models, Staar and Adatomed. METHODS: Twenty-four eyes from 12 patients were studied prospectively. A phakic Staar IOL was implanted in one eye of each patient, and the other eye received a phakic Adatomed IOL. Patients with uveitis or ocular trauma prior to ocular surgery, diabetic retinopathy, or capsular pseudoexfoliation were excluded. The mean preoperative spherical equivalent refraction was -16.00 +/- 5.05 D for the Staar group and -15.39 +/- 2.83 D for the Adatomed group. Average follow-up was 32.4 months (range, 19 to 46 mo) for the Adatomed group and 18.3 months (range, 11 to 21 mo) for the Staar group and included evaluation of intraocular pressure, intraocular lens pigment deposits, lens decentration, anterior subcapsular cataract, and visual acuity. RESULTS: Spectacle-corrected and uncorrected visual acuity improved in all eyes in both groups. No statistically significant differences in visual acuity gain were observed with the two materials (Student t-test, P = .08 for the Staar group and P = .6 for the Adatomed group), although the gain in visual acuity was somewhat greater with the Staar PIOLs. The difference in mean intraocular pressure before surgery and at last follow-up was 1.5 mmHg for the Staar group and 2.3 mmHg for the Adatomed group (P = .36). The incidence of lens pigment deposits was the same in both groups (41.66%), with deposits in 5 of the 12 eyes in both groups. The incidence of lens decentration was higher in the Adatomed group (5/12; 41.66%) than in the Staar group (2/12; 16.7%). Anterior subcapsular cataract was higher in the Adatomed group (4/12; 33.3%) than in the Staar group (3/12; 25%). CONCLUSIONS: There was a higher incidence of lens decentration and anterior subcapsular cataract in the Adatomed group than in the Staar group.

Adult↗
PubMedDate not supplied

Aberration-sensing and wavefront-guided laser in situ keratomileusis: management of decentered ablation.

PURPOSE: To clarify the feasibility of aberration-sensing and wavefront-guided laser in situ keratomileusis (LASIK) to manage grossly decentered ablation and to discuss the limitations of the technology. METHODS: Three patients with previous decentrations of the ablation zone between 1.5 to 2.0 mm were scheduled for wavefront-guided LASIK. All patients reported monocular diplopia and halos. Wavefront aberrations were measured with a Tscherning-type aberrometer. Laser ablation was done with a WaveLight Allegretto in a one-step procedure with ablation profiles calculated only from the individual wavefront map. Decentrations were determined from corneal topography. RESULTS: Three months after surgery, patient WM and patient SU had gained uncorrected and best spectacle-corrected visual acuity. The root mean square-wavefront error decreased up to 61% and 33%, respectively, for total and higher order aberrations (Zernike modes of 3rd order and higher). There was significant enlargement of the optical zone determined by corneal topography, and both patients no longer reported diplopia and halos at 3 months postoperatively. The optical aberration of the third patient (RE), after a 5.00-D overcorrection with a 2-mm decentration, was too high for aberration-sensing; retinal images obtained from the wavefront device were too smeared and not of sufficient contrast. In addition, this patient had a residual corneal thickness of 416 microm and thus wavefront-guided LASIK was not done. CONCLUSIONS: Wavefront-guided LASIK offers a new way of managing grossly decentered laser ablations. Unfortunately, there are still patients who have aberrations too large for wavefront sensing or with other clinical limitations such as a residual corneal thickness too thin for further treatment.

Adult↗
PubMedDate not supplied

Ablation centration in laser in situ keratomileusis for hyperopia: comparison of VISX S3 ActiveTrak and VISX S2.

PURPOSE: To compare ablation centration and outcome measurements in laser in situ keratomileusis (LASIK) for hyperopia using the pupil-tracking VISX S3 ActiveTrak or the nontracking VISX S2 excimer laser. METHODS: In a retrospective study, 49 consecutively treated hyperopic eyes (32 patients) that had LASIK by the VISX StarS3 ActiveTrak were compared to 49 control-matched eyes treated with the VISX StarS2 without pupil-tracking. Primary outcome variables including ablation centration, uncorrected visual acuity, best spectacle-corrected visual acuity, manifest refraction, complications, and induced cylinder analyzed by vector analysis were evaluated 3 months postoperatively. RESULTS: Hyperopic sphere ranged between plano and +5.50 D and cylinder between 0 and +2.75 D. Ninety-five of 98 eyes (96.9%) were available for analysis at 3 months. Of these 95, 52 eyes could be used for analysis of ablation centration. Mean decentration of the ablation zone from the entrance pupil was 0.30 +/- 0.20 mm in tracked eyes (n=31) and 0.41 +/- 0.39 mm in nontracked eyes (n=21), P=.17. Two eyes (6.5%) in the tracked group were significantly decentered between 0.5 and 1 mm from the pupil center. In the nontracked group, decentration was between 0.5 and 1 mm in one eye (4.8%) and greater than 1 mm in two eyes (9.5%). CONCLUSIONS: Comparable ablation centration in LASIK for hyperopia was achieved between actively-tracked and nontracked eyes. Decentrations (greater than 1 mm) were not seen with a tracking system in this study. Visual and refractive results were similar between the VISX StarS3 ActiveTrak and VISX StarS2 laser systems.

Cornea↗
PubMedDate not supplied

Limitations of correcting spherical aberration with aspheric intraocular lenses.

PURPOSE: Aspheric intraocular lenses (IOLs) are deigned to correct spherical aberration in pseudophakic eyes. We predict the benefit from correcting spherical aberration based on simulations and aberrometry of pseudophakic eyes implanted with spherical IOLs. METHODS: Ray tracing was performed through a model eye with an equi-biconvex spherical IOL and with a spherical aberration-correcting aspheric IOL. The IOLs were increasingly tilted and/or displaced, and the resulting transverse aberrations of 169 rays were transformed into Zernike coefficients for different pupil sizes. The benefit from correcting spherical aberration at individual esopic pupils was investigated by canceling C4(0) in the sets of Zernike coefficients for 41 eyes implanted with spherical IOL. RESULTS: Both the model eye and the real eye data predict that age-related miosis reduces spherical aberration in the eye implanted with a spherical IOL to approximately 1/3 of the spherical aberration at a 6-mm pupil. A reduction of similar magnitude occurs when spherical aberration-induced non-paraxial defocus is corrected by a spectacle lens. For natural mesopic pupils, canceling the Zernike C4(0) coefficient improved the objective image quality at a rate similar to changing defocus by 0.05 diopters. Average decentration and tilt levels diminish the lead of aspheric IOLs over spherical IOLs, depending on the direction of decentration. CONCLUSIONS: The benefit from correcting spherical aberration in a pseudophakic eye is limited for some or all of the following reasons: wearing glasses, age-related miosis, tilt and decentration of IOL, small contribution of spherical aberration to all aberrations, and intersubject variability.

Aged↗