Biomedical subjects

Cynthia J Roberts

Publications and source records attributed to Cynthia J Roberts.

7 recordsLinked to original sources

PubMed2007

Determining corneal power using Orbscan II videokeratography for intraocular lens calculation after excimer laser surgery for myopia.

PURPOSE: To assess the accuracy of Orbscan II slit-scanning videokeratography for intraocular lens (IOL) calculation in eyes with previous photorefractive surgery for myopia. SETTING: Private practice, St. Louis, Missouri, USA. METHODS: Corneal power (K) was measured by manual keratometry, Placido-based videokeratography (Atlas), slit-scanning videokeratography (Orbscan II), and contact lens overrefraction in 21 post-photoablation eyes having cataract surgery. Postoperative data collected after phacoemulsification were used to back-calculate corneal power (BCK). The BCK values were statistically compared at 3.0 to 6.0 mm central Orbscan II curvature and power measurements, including total axial power, total tangential power, total mean power, and total optical power. Similar comparisons were made to Atlas curvature at the 0.0 to 10.0 mm zones. RESULTS: The mean corneal power after refractive surgery based on BCK values using the Holladay 2 formula (BCK H2) was 39.35 diopters (D) +/- 2.58 (SD). The mean manual value (40.52 +/- 1.95 D) and Atlas-based values were statistically higher than BCK H2 values (P<.001). The mean corneal power calculated from historical data was 39.33 +/- 2.70 D (P = .83 to BCK H2; n = 19) and from contact lens overrefraction, 41.38 +/- 3.11 D (P = .19; n = 5). Orbscan II parameters (n = 21) of the total mean power (3.0 mm, 39.10 +/- 2.63 D), total tangential power (3.0 mm, 39.11 +/- 2.60), total axial power (5.0 mm, 39.19 +/- 2.55 D), and total optical power (3.0 mm, 39.08 +/- 2.78 D; 4.0 mm, 39.39 +/- 2.76 D) were statistically similar to both the historical and BCK H2 values (P>.11). If used prospectively, 80.9% of eyes would have been within +/-0.50 D of the targeted refraction using a 4.0 mm total optical power, 76.2% using a 5.0 mm total axial power, and 42.1% using the historical method. CONCLUSION: The Orbscan II 5.0 mm total axial power and 4.0 mm total optical power can be used to more accurately predict true corneal power than the history-based method and may be particularly useful when pre-LASIK data are unavailable.

Cataract↗

PubMed2006

Changes in corneal biomechanics and intraocular pressure following LASIK using static, dynamic, and noncontact tonometry.

PURPOSE: To compare the preoperative and postoperative measurement of corneal biomechanical properties and intraocular pressure (IOP) using Goldmann applanation tonometry (GAT), the ocular response analyzer (ORA), and the Pascal dynamic contour tonometer (PDCT) in eyes undergoing myopic laser in situ keratomileusis (LASIK). DESIGN: Prospective, nonrandomized clinical trial. METHODS: IOP was measured in 66 myopic eyes before and after LASIK by GAT, ORA, and PDCT in a randomized sequence. Metrics of corneal biomechanical properties (corneal hysteresis [CH], corneal resistance factor [CRF], and ocular pulse amplitude [OPA]) were recorded. RESULTS: After LASIK, there was a reduction in mean corneal pachymetry of 90.2 mum and in IOP measurements with GAT (Delta = -1.8 +/- 2.8 mm Hg; P < .01), ORA-Goldmann (Delta = -4.6 +/- 2.8 mm Hg, P < .01), and ORA-corneal compensated (Delta - 2.1 +/- 2.6 mm Hg; P < .05). However, there was no statistically significant difference between preoperative and postoperative IOP measurements taken by PDCT (Delta = -0.5 +/- 2.6 mm Hg). Postoperatively, CRF decreased by 28.6% (P < .01), CH by 16.2% (P < .01), and OPA by 1.8% (P = .32). CONCLUSIONS: Measurement of IOP with PDCT appears to be relatively immune to changes in corneal biomechanics and pachymetry after LASIK, in comparison to GAT and ORA measures of IOP. PDCT and ORA both showed statistically lower variation in measurement than GAT. LASIK produced a marked decline in CH and CRF, which may reflect respective changes in the viscous and elastic qualities of the post-LASIK cornea. In contrast, there was no statistical change in OPA.

Adult↗

PubMed2006

Ultrasonic model and system for measurement of corneal biomechanical properties and validation on phantoms.

Non-invasive measurement of biomechanical properties of corneas may provide important information for ocular disease management and therapeutic procedures. An ultrasonic non-destructive evaluation method with a wave propagation model was developed to determine corneal biomechanical properties in vivo. In this study, we tested the feasibility of the approach in differentiating the mechanical properties of soft contact lenses as corneal phantoms. Three material types of soft contact lenses (six samples in each group) were measured using a broadband ultrasound transducer. The ultrasonic reflections from the contact lenses were recorded by a 500MHz/8-bit digitizer, and displayed and processed by a PC. A reference signal was recorded to compute the normalized power spectra using Fast Fourier Transformation. An inverse algorithm based on least-squares minimization was used to reconstruct three parameters of the contact lenses: density, thickness, and elastic constants lambda+2micro. The thickness of each sample was verified using an electronic thickness gauge, and the averaged density for each type of lenses was verified using Archimedes' principle and manufacturer's report. Our results demonstrated that the ultrasonic system was able to differentiate the elastic properties of the three types of the soft contact lenses with statistical significance (P-value<0.001). The reconstructed thicknesses and densities agreed well with the independent measurements. Our studies on corneal phantoms indicated that the ultrasonic system was sensitive and accurate in measuring the material properties of cornea-like structures. It is important to optimize the system for in vivo measurements.

Biomechanical Phenomena↗

PubMed2005

Influence of corneal biomechanical properties on intraocular pressure measurement: quantitative analysis.

PURPOSE: To understand and quantify intraocular pressure (IOP) measurement errors introduced by corneal variables during applanation tonometry using a cornea biomechanical model. SETTING: Department of Ophthalmology, Biomedical Engineering Center, The Ohio State University, Columbus, Ohio, USA. METHODS: The model assumed an overall resultant pressure that was based on the summation of the applanation pressure, the true IOP, and the surface tension caused by the tear film to determine the final deformation of the corneal apex during IOP measurement. Corneal resistance was varied according to the cornea's biomechanical properties, thickness, and curvature, and the effect of each variable on the accuracy of IOP tonometry readings was examined quantitatively. RESULTS: The model demonstrated that tonometry readings do not always reflect true IOP values. They deviate when corneal thickness, curvature, or biomechanical properties vary from normal values. Based on the model, predicted IOP readings have a 2.87 mm Hg range resulting from the variation in the corneal thickness in the normal population and a 1.76 mm Hg range from the variation in the corneal radius of curvature. Considering that Young's modulus of the corneal varies from 0.1 to 0.9 MPa in the normal population, the model predicts tonometry IOP readings will have a range of 17.26 mm Hg because of the variation in this corneal biomechanical parameter alone. CONCLUSIONS: The simulation based on the model demonstrated quantitatively that variations in each corneal variable cause errors in tonometry IOP readings. The simulation results indicate that differences in corneal biomechanics across individuals may have greater impact on IOP measurement errors than corneal thickness or curvature.

Biomechanical Phenomena↗

PubMed2005

Topographic and biomechanical differences between hyperopic and myopic laser in situ keratomileusis.

PURPOSE: To evaluate the size, shape, and uniformity of the videokeratographic functional optical zone (FOZ) after laser in situ keratomileusis (LASIK) in 2 cohorts of patients with equivalent amounts of preoperative myopic or hyperopic astigmatism. SETTING: Pepose Vision Institute, St. Louis, Missouri, USA. METHODS: Eyes with myopic or hyperopic astigmatism (n=27 in each group) that had LASIK with the Visx Star S3 laser were retrospectively selected to match for level of preoperative refractive error. Slit-scanning videokeratography was performed preoperatively and 6 months postoperatively and analyzed using custom software. The FOZ was calculated by analyzing refractive power maps using a region-growing algorithm. Difference maps were generated from slit images and compared for interval change in corneal elevation, tangential curvature, and refractive power. The difference maps were also averaged (mean difference maps) for each target population. A Zernike decomposition of corneal first-surface elevation was performed to compare postoperative values with baseline parameters. RESULTS: The mean postoperative refractive sphere at 6 months was -0.17 diopter (D) +/- 0.66 (SD) and +0.25 +/- 0.85 D in the myopia group and hyperopia group, respectively, and the mean postoperative astigmatism, -0.49 +/- 0.32 D and -0.65 +/- 0.52 D, respectively (P=.11). Based on the refractive power maps, the mean preoperative and postoperative myopic FOZ was 33.09 +/- 7.30 mm(2) and 30.94 +/- 5.43 mm(2), respectively, and the mean hyperopic FOZ, 33.19 +/- 7.96 mm(2) and 37.99 +/- 6.88 mm(2), respectively. After LASIK, there was an increase in magnitude of negative anterior corneal surface spherical-like Zernike values in the myopia group (P<.0001) and an increase in magnitude of positive spherical-like Zernike values in the hyperopia group. Postoperatively, significant induction of corneal surface horizontal coma was noted in hyperopic eyes (P<.0001). Hyperopic eyes, on average, had larger topographic FOZs after LASIK, but with less uniformity of curvature and power change than myopic eyes. CONCLUSIONS: Hyperopic LASIK, which involves more transition points along the ablation diameter, produced a less uniform topographic FOZ than typical myopic treatments. Less predictable biomechanical changes from the circumferential release of tension on collagen bundles after midperipheral hyperopic ablation and greater variation in beam centration and the angle of incidence may contribute to the greater variability in corneal curvature and power in hyperopic LASIK than in myopic LASIK.

Astigmatism↗

PubMed2005

Model for nonectatic increase in posterior corneal elevation after ablative procedures.

PURPOSE: To evaluate the response of the posterior cornea after uneventful laser in situ keratomileusis (LASIK) with scanning-slit videokeratography using the change in elevation as a measure of biomechanical remodeling. SETTING: Hong Kong Sanatorium and Hospital, Hong Kong, China. METHODS: Three schematic models of possible posterior corneal response mechanisms to laser refractive surgery were proposed: model A, isolated central bulging; model B, forward shift over a large region of the posterior surface; model C, backward peripheral movement into the anterior chamber. A retrospective study was performed to determine which model was consistent with posterior corneal behavior in 2380 eyes of 1255 patients after myopic LASIK performed with the Technolas 217 laser (Bausch & Lomb). Patients received preoperative and 6-month postoperative Orbscan I (version 2.0, Bausch & Lomb) examinations. Preoperative posterior elevation maps were subtracted from postoperative maps of 2380 eyes using 3 fitting protocols. Difference maps were averaged and the results compared to the proposed schematic models. All topography data processing was done using the Ohio State University Corneal Topography Tool. RESULTS: Significant central increases in posterior elevation corresponded with significant peripheral decreases in posterior elevation. This pattern is consistent with schematic model C, backward peripheral movement of the posterior cornea, possibly due to a differential pattern of swelling. CONCLUSIONS: Increases in central posterior corneal elevation after LASIK appear to be dominated by backward peripheral corneal swelling into the anterior chamber rather than forward "bulging" of the central posterior cornea. This is consistent with stable remodeling of the corneal shape due to a surgically induced change in structure and fluid balance, rather than an ectatic event.

Astigmatism↗

PubMed2004

Functional magnetic resonance imaging studies of pain: an investigation of signal decay during and across sessions.

BACKGROUND: Several investigations into brain activation caused by pain have suggested that the multiple painful stimulations used in typical block designs may cause attenuation over time of the signal within activated areas. The effect this may have on pain investigations using multiple tasks has not been investigated. The signal decay across a task of four repeating pain stimulations and between two serial pain tasks separated by a 4-min interval was examined to determine whether signal attenuation may significantly confound pain investigations. METHODS: The characteristics of the brain activation of six subjects were determined using whole brain blood oxygenation level-dependent functional magnetic resonance imaging on a 1.5-T scanner. Tasks included both tingling and pain induced by transcutaneous electrical stimulation of the median nerve. The average group maps were analyzed by general linear modeling with corrected cluster P values of less than 0.05. The time courses of individual voxels were further investigated by analysis of variance with P values of less than 0.05. RESULTS: Significant differences between pain and tingling were found in the ipsilateral cerebellum, contralateral thalamus, secondary somatosensory cortex, primary somatosensory cortex, and anterior cingulate cortex. Highly significant signal decay was found to exist across each single pain task, but the signal was found to be restored after a 4-min rest period. CONCLUSIONS: This work shows that serial pain tasks can be used for functional magnetic resonance imaging studies using electrical nerve stimulation as a stimulus, as long as sufficient time is allowed between the two tasks.

Adult↗