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Biomedical subjects

Sverker Norrby

Publications and source records attributed to Sverker Norrby.

17 recordsLinked to original sources
PubMed2006

Development of an accommodating intra-ocular lens--in vitro prevention of re-growth of pig and rabbit lens capsule epithelial cells.

Cataract surgery is routinely performed to replace the clouded lens by a rigid polymeric intra-ocular lens unable to accommodate. By implanting a silicone gel into an intact capsular bag the accommodating properties of the natural lens can be maintained or enhanced. The implantation success of accommodating lenses is hampered by the occurrence of capsular opacification (PCO) due to lens epithelial cell (LEC) growth. In order to prevent LEC proliferation, a treatment regime using actinomycin D, cycloheximide and water was developed. The effectiveness of treatment was analyzed using an in vitro, MTT-based cell culture system and an ex vivo pig eye model in which the implanted lens-in-the-bag is cultured as a whole. LEC were exposed to treatment solutions for 5 min, then the cells were allowed to recover and to re-colonize the substratum. MTT conversion by cells was transiently inhibited by cycloheximide dissolved in water and by water alone. Exposure to actinomycin D resulted in a lasting inhibition of MTT conversion and consequently cell proliferation. These in vitro data could not be fully reproduced in the ex vivo pig eye model due to essential differences between both models. Treatment with actinomycin D containing solutions, however, resulted in a nearly complete absence of cells on the capsular wall. The pig eye model is a promising approach to further evaluate the effects of peri-surgical treatment during the accommodating intra-ocular lens implantation.

Animals↗
PubMed2006

Artificial crystalline lens.

If presbyopia is caused by hardening of the crystalline lens, replacing it with a material with mechanical properties similar to the young crystalline lens should restore accommodative ability. Such a silicone material has been developed. Refilling the capsular bag with this material results in 3 to 5 D of accommodation in primates in response to pilocarpine.

Animals↗
PubMed2006

Accommodative lens refilling in rhesus monkeys.

PURPOSE: Accommodation can be restored to presbyopic human eyes by refilling the capsular bag with a soft polymer. This study was conducted to test whether accommodation, measurable as changes in optical refraction, can be restored with a newly developed refilling polymer in a rhesus monkey model. A specific intra- and postoperative treatment protocol was used to minimize postoperative inflammation and to delay capsular opacification. METHODS: Nine adolescent rhesus monkeys underwent refilling of the lens capsular bag with a polymer. In the first four monkeys (group A) the surgical procedure was followed by two weekly subconjunctival injections of corticosteroids. In a second group of five monkeys (group B) a treatment intended to delay the development of capsular opacification was applied during the surgery, and, in the postoperative period, eye drops and two subconjunctival injections of corticosteroids were applied. Accommodation was stimulated with carbachol iontophoresis or pilocarpine and was measured with a Hartinger refractometer at regular times during a follow-up period of 37 weeks in five monkeys. In one monkey, lens thickness changes were measured with A-scan ultrasound. RESULTS: In group A, refraction measurement was possible in one monkey. In the three other animals in group A, postoperative inflammation and capsular opacification prevented refraction measurements. In group B, the maximum accommodative amplitude of the surgically treated eyes was 6.3 D. In three monkeys the accommodative amplitude decreased to almost 0 D after 37 weeks. In the two other monkeys, the accommodative amplitude remained stable at +/-4 D during the follow-up period. In group B, capsular opacification developed in the postoperative period, but refraction measurements could still be performed during the whole follow-up period of 37 weeks. CONCLUSIONS: A certain level of accommodation can be restored after lens refilling in adolescent rhesus monkeys. During the follow-up period refraction measurements were possible in all five monkeys that underwent the treatment designed to prevent inflammation and capsular opacification.

Accommodation, Ocular↗
PubMed2005

Clinical application of the lens haptic plane concept with transformed axial lengths.

PURPOSE: To clinically evaluate the lens haptic plane (LHP) concept in combination with thick-lens ray tracing for intraocular lens (IOL) power calculation. SETTING: St. Erik's Eye Hospital, Stockholm, Sweden. METHODS: Prospective study of normal cataract cases implanted with Pharmacia CeeOn 809C IOL. Axial length was measured by A-scan. The measured value was first transformed by addition of a constant value to correct for systematic error. Using the transformed axial length and corneal radius measured by keratometry, the LHP position was determined. Knowing the IOL design and the power implanted, expected refractive outcome was calculated and compared to manifest refraction at 6 weeks in terms of mean absolute error (MAE). Thick-lens ray tracing in the paraxial limit was used for the optical calculation. RESULTS: The mean transformed axial length was 23.87 mm. An LHP position algorithm in linear terms of transformed axial length and corneal radius gave an MAE of 0.38 D. There was no trend with axial length. On the present data, the Holladay 1, Hoffer Q, and SRK/T formulas produced MAEs of 0.39 D, 0.39 D, and 0.41 D, respectively, with optimized formula constants. The differences were not statistically significant (P > .05). CONCLUSIONS: The LHP concept in combination with thick-lens ray tracing achieved MAE comparable to that with currently used formulas. The lack of trend with axial length is important for patients with short and long eyes.

Cornea↗
PubMed2005

The Dubbelman eye model analysed by ray tracing through aspheric surfaces.

Dubbelman and co-workers have determined intraocular spacings and surface shapes in living eyes by means of corrected Scheimpflug images in a large number of subjects of different age at several levels of accommodation. They give relationships for key anterior segment parameters as a function of age and level of accommodation. These are used in this paper to build a schematic eye incorporating aspheric surfaces. This eye model is analysed by means of ray tracing with a technique developed for use with a common spreadsheet computer program. The Dubbelman eye model appears to be well corrected for spherical aberration. Compared with measurements on real eyes it agrees well in general, but spherical aberration is negative, while in real eyes it tends to be positive.

Accommodation, Ocular↗
PubMed2004

Using the lens haptic plane concept and thick-lens ray tracing to calculate intraocular lens power.

PURPOSE: To develop a methodology for intraocular lens (IOL) power calculation in which the task of predicting the postoperative position of the IOL is separated from the calculation itself. SETTING: Pharmacia, Groningen, The Netherlands. METHODS: The minimum biometry input needed for IOL power calculation is the mean anterior corneal radius and axial length of the eye. The lens haptic plane (LHP) is the plane where the IOL haptics make contact with eye tissue. It is an anatomical site (eg, the equator of the capsular bag) and is independent of the IOL model. The position of the IOL optic in relation to the LHP is determined from the exact design of the IOL. Gullstrand's eye model is adopted to obtain the posterior corneal radius, thickness of the cornea, and refractive indices of the eye media. Thick-lens ray tracing in the paraxial limit is used for the optical calculation. RESULTS: A spreadsheet is given for the calculation. CONCLUSIONS: The methodology developed allows for IOL power calculation from first principles (ie, using true physical distances, radii, and refractive indices as input for the optical calculation).

Algorithms↗
PubMed2004

Adaptive optics simulation of intraocular lenses with modified spherical aberration.

PURPOSE: Adaptive optics systems can be used to investigate the potential visual benefit associated with correcting ocular wave-front aberration. In this study, adaptive optics techniques were used to evaluate the potential advantages and disadvantages associated with intraocular lenses (IOLs) with modified spherical aberration profiles. METHODS: An adaptive optics vision simulator was constructed that allows psychophysical tests to be performed while viewing targets through any desired ocular wave-front profile. With this simulator, the subjective visual performance of four subjects was assessed by letter acuity and contrast sensitivity (at 3, 6, and 15 cyc/deg) for two different values of induced spherical aberration. The values of spherical aberration were chosen to reproduce two conditions: the average amount measured in pseudophakic patients with implanted IOLs having spherical surfaces and the complete correction of the individual's spherical aberration. Visual performance was assessed in both white and green light, at best focus and for defocus of +/-0.5 and +/-1.0 D. RESULTS: There was an average improvement in visual acuity associated with the correction of spherical aberration of 10% and 38% measured in white and green light, respectively. Similarly, average contrast sensitivity measurements improved 32% and 57% in white and green light. When spherical aberration was corrected, visual performance was as good as or better than for the normal spherical aberration case for defocus as large as +/-1 D. CONCLUSIONS: Correcting ocular spherical aberration improves spatial vision in the best-focus position without compromising the subjective tolerance to defocus.

Adult↗
PubMed2003

Effect of intraocular lens implantation on visual acuity, contrast sensitivity, and depth of focus.

PURPOSE: To determine the role of spherical and irregular aberrations in the optics of the natural eye and after intraocular lens (IOL) implantation in terms of visual acuity, contrast sensitivity, and depth of focus. SETTING: Laboratory of Experimental Ophthalmology, University of Groningen, Groningen, The Netherlands. METHODS: Visual acuity and defocus-specific contrast sensitivity in 11 pseudophakic patients (IOL group) and 27 age-matched phakic subjects were compared. The results were obtained psychophysically. Spherical and irregular aberrations were subsequently estimated by comparing the measured myopic shift (optimum focus of contrast sensitivity at 4 cycles per degree [cpd] compared to that at 16 cpd) and depth of focus with those of theoretical eye models with varying amounts of irregular and spherical aberrations. RESULTS: The best corrected visual acuity and best corrected contrast sensitivity in the IOL group did not significantly differ from that in the phakic group. The depth of focus was larger in the IOL group at a pupil diameter of 6.0 mm (P<.05). Comparison with theoretical eye models suggested a higher amount of spherical aberration in the IOL group; irregular aberration was almost the same in both groups. CONCLUSIONS: There was a higher amount of spherical aberration in the IOL group, related to a larger depth of focus, without loss of contrast sensitivity at optimum focus or loss of visual acuity. This might contribute to better quality of vision in pseudophakic subjects than in presbyopic phakic subjects.

Contrast Sensitivity↗
PubMed2003

Effect of methods of myopia correction on visual acuity, contrast sensitivity, and depth of focus.

PURPOSE: To psychophysically measure spherical and irregular aberrations in patients with various types of myopia correction. SETTING: Laboratory of Experimental Ophthalmology, University of Groningen, Groningen, The Netherlands. METHODS: Three groups of patients with low myopia correction (spectacles, soft contact lens, and Intacs) and 4 groups with high myopia correction (spectacles, rigid contact lens, Artisan claw lens, and laser in situ keratomileusis [LASIK]) had through-focus contrast sensitivity measurements to establish the myopic shift and depth of focus. From these 2 parameters, spherical and irregular aberrations were determined using theoretical eye models and geometric optics. Visual acuity, stray light, and predictability were also studied. RESULTS: There were no differences in best corrected visual acuity (BCVA) or best corrected contrast sensitivity between the low myopia groups. The Intacs group had a significantly larger depth of focus (P<.05). The results in the soft contact lens group were comparable to those in a human eye model with an average amount of spherical and irregular aberrations. The LASIK group had worse uncorrected visual acuity (UCVA) and best corrected contrast sensitivity than the spectacles, rigid contact lens, and Artisan claw lens groups (P<.05) due to the amount of spherical and irregular aberrations present after LASIK. The low and high myopia spectacles groups had average amounts of spherical and irregular aberrations. CONCLUSIONS: Neither surgical techniques nor contact lenses resulted in BCVA or best corrected contrast sensitivity that surpassed the values measured in the best corrected spectacles groups. The Artisan claw lens performed better than LASIK in UCVA, predictability, and best corrected contrast sensitivity.

Adult↗
PubMed2003

Comparison of 2 A-scans.

PURPOSE: To compare 2 A-scan instruments with regard to differences in measured results for the same patient sample. SETTING: St. Erik's Eye Hospital, Stockholm, Sweden. METHODS: In a study to evaluate the lens-haptic plane concept of intraocular lens (IOL) power calculation, 148 patients eligible for cataract surgery were measured with 2 different A-scan instruments (BVI Axis and Sonomed 1500). The axial length (AL), anterior chamber depth (ACD), and lens thickness (LT) results were analyzed for systematic differences and random errors. RESULTS: The Sonomed 1500 measured systematically longer than the BVI Axis for AL (0.41 mm) and ACD (0.28 mm), although the correlation was good (r = 0.99 and r = 0.87, respectively). The LT correlated poorly (r = 0.18) and showed no systematic trend. The relative random errors (standard deviations) in ACD (7.2%) and LT (18.6%) were larger than that of the AL (0.8%). The systematic difference in the AL corresponds to a 1.0 diopter difference in the A-constant. CONCLUSIONS: The large random errors in the ACD and LT reduce their value as predictors of postoperative IOL position in formulas that use them. Systematic differences in AL can be large enough to require separate formula constants for different pieces of equipment. If this is the situation in 1 setting, there is a risk of mistakes. This confusion could be avoided if there were an agreed standard and a universal calibration procedure for instruments intended for AL measurement.

Anterior Chamber↗
PubMed2003

Reduction of trend errors in power calculation by linear transformation of measured axial lengths.

PURPOSE: To find a method to improve the refractive outcome in short eyes and long eyes without sacrificing the outcome in normal eyes. SETTING: St. Erik's Eye Hospital, Stockholm, Sweden. METHODS: In a prospective study, 148 patients eligible for cataract surgery were measured with 2 different A-scans (BVI Axis, B.V. International; Sonomed 1500, Sonomed Inc.). Refraction was determined 6 weeks postoperatively. The postoperative refraction was compared with the refraction predicted by the Holladay 1, Hoffer Q, and SRK/T formulas; formula constants were optimized to give a zero mean error. The mean absolute error (MAE) was used as an outcome measure. RESULTS: The BVI Axis measured consistently shorter than Sonomed 1500. The mean axial lengths (ALs) were 23.033 mm and 23.435 mm, respectively. With the BVI Axis, an MAE of 0.44 diopter (D), 0.44 D, and 0.47 D was obtained, with the Holladay 1, Hoffer Q, and SRK/T formulas, respectively, with a trend toward undercorrecting short eyes and overcorrecting long eyes. The MAE with the Sonomed 1500 was 0.38 D, 0.39 D, and 0.40 D, respectively. By adding 0.402 mm to each measured value in the BVI Axis data set, the mean AL was transformed to 23.435 mm. With the transformed data, the MAE improved to 0.42 D, 0.43 D, and 0.44 D, respectively, with a reduced trend toward undercorrection and overcorrection. The 0.04 D difference between the instruments, although not statistically significant, may depend on measurement precision. Extending the concept of transformation, a minimum MAE of 0.41 D was obtained with the Holladay 1 at a mean AL of 24.0 mm, 0.43 D with Hoffer Q at 23.9 mm, and 0.40 D with SRK/T at 24.4 mm. The trend toward undercorrection and overcorrection was eliminated at the optimum for each formula. CONCLUSIONS: There were systematic differences in measured AL depending on equipment. Thus, the calculated powers differed and caused error in the degree of compliance between the labeled formula constant of an intraocular lens and the equipment used. Although personalization of formula constants reduces the mean error, in general a trend toward undercorrection of short eyes and overcorrection of long eyes will persist. Transforming the AL scale can eliminate the trend error and improve the overall refractive outcome. Transformation to a population mean AL of about 24.0 mm was close to optimum for the 3 formulas.

Cataract↗
PubMed2002

Corneal optical aberrations and retinal image quality in patients in whom monofocal intraocular lenses were implanted.

OBJECTIVES: To compare retinal image quality and optical corneal aberrations in patients in whom monofocal polymethyl methacrylate intraocular lenses (IOLs) were implanted with those in healthy subjects of a similar older age (60-70 years old) and to use the results to suggest improved optical designs of IOLs to maximize retinal image quality. METHODS: A double-pass apparatus was used to measure retinal image quality for 3-, 4-, and 6-mm pupil diameters. Corneal aberrations for a 4-mm pupil were calculated by a ray-tracing technique from the elevations provided by corneal topography. Two groups of 20 subjects of a similar older age were studied: in one group, polymethyl methacrylate monofocal IOLs were implanted; and in a second group, healthy subjects were used as a reference. RESULTS: The average retinal image quality was similar in older healthy patients and in patients in whom IOLs were implanted, with both groups having a significantly worse image quality than healthy younger subjects (aged 20-30 years). Both groups were more tolerant to defocus than younger subjects. CONCLUSIONS: The average retinal image quality of patients in whom IOLs were implanted was worse than that of healthy younger subjects despite the good optical quality of isolated IOLs. This apparent paradox can be understood by the nature of the aberration coupling in the eyes that undergo implantation. The ideal substitute for the natural lens is not an IOL with the best-isolated optical performance, but rather one designed to compensate for the aberrations of the cornea-a design somehow inspired by the crystalline lens of younger subjects.

Aged↗
PubMed2002

Anterior chamber depth measurement: a-scan versus optical methods.

PURPOSE: To evaluate methods of measuring anterior chamber depth (ACD) before and after cataract surgery. SETTING: St. Erik's Eye Hospital, Stockholm, Sweden. METHODS: A-scan (BVI Axis) and Scheimpflug imaging (Nidek EAS-1000) were used to measure ACD preoperatively and 6 and 18 weeks after phacoemulsification with implantation of a poly(methyl methacrylate) (PMMA) intraocular lens (IOL) in 23 patients. Because of a large systematic difference between the 2 methods, measurement with Orbscan (Orbtek) and optical pachymetry (Haag-Streit) were included when measurements were repeated 36 weeks postoperatively. A t test for paired observations was used for statistical analysis. RESULTS: Preoperatively, the mean A-scan measurements were significantly shorter than the Scheimpflug values: 3.05 mm +/- 0.36 (SD) and 3.37 +/- 0.35 mm, respectively (P < .001). At 6 weeks, the difference was more pronounced: 3.73 +/- 0.26 mm and 4.65 +/- 0.33 mm, respectively (P < .001). At 36 weeks, the A-scan and Scheimpflug values remained unchanged. The results of the Scheimpflug measurements were confirmed with optical pachymetry and Orbscan analysis. CONCLUSIONS: There was good agreement between results obtained with 3 methods based on optical principles. Considering the basic and simple measurement principle of these instruments, they appear to provide the correct result. The A-scan equipment used in this study is unsuitable for determination of ACD in eyes with PMMA IOLs.

Anterior Chamber↗
PubMedDate not supplied

Hydrogels for an accommodating intraocular lens. An explorative study.

In this study it was investigated whether hydrogels could be used for an accommodating lens. The requirements of such a hydrogels are a low modulus, high refractive index, transparency, and strength. Since conventional hydrogels do not possess this combination of properties, a novel preparation method and new polymers are introduced. As starting materials poly(1-hydroxy-1,3-propanediyl), poly(ethylene-co-vinyl alcohol), poly(vinyl alcohol), and poly(allyl alcohol) were used. The first three were cross-linked with a number of diisocyanate compounds. Network formation was performed at low concentrations in a good solvent. Mixing of the polymer solution and cross-linker appeared to be crucial for transparency. Poly(1-hydroxy-1,3-propanediyl), cross-linked with a slow reacting diisocyanate block, shows the most promising properties with respect to refractive index, transparency, tensile strength, and modulus. Poly(allyl alcohol) hydrogel was made by compression molding. The hydrogel was transparent and had a high refractive index and low modulus. It was concluded that hydrogels could be used as accommodating lens material.

Accommodation, Ocular↗