Clinical performance of an innovative back surface multifocal contact lens in correcting presbyopia

PURPOSE

This prospective study was designed to subjectively and objectively evaluate the performance of an aspheric multifocal back surface rigid gas permeable (RGP) contact lens. The multifocal element of this lens design consisted of an aspheric optical zone that varied according to the patient's ametropia, corneal topography, and required reading addition.

METHODS

We fit 28 presbyopic subjects with an aspheric multifocal back surface RGP contact lens (age range: 45 to 68 years). Reading additions ranged from +0.75 D to +2.50 D. Subjects were assessed initially and at 2, 6, and 12 weeks for ocular changes, visual performance, and subjective responses.

RESULTS

We required 116 RGP lenses to achieve an acceptable fit and visual acuity in 28 subjects (55 eyes). At the final visit, the distance logMAR acuity with the multifocal contact lens (+0.12 +/- 0.10) was not statistically different (t = -0.623, P = -0.5388) from spectacle acuity at the initial visit (+0.10 +/- 0.12). The near logMAR acuity with the multifocal contact lens at the final visit (0.36 +/- 0.12) was not statistically different from that for near acuity with spectacles at the initial visit (0.33 +/- 0.13). No slit lamp signs worsened during the study. A reduction in myopia of 0.67 D was noted by the final visit. Spectacle blur was noted if the acuity at the initial refraction was compared to the acuity with the same refraction at the final visit (t = -3.287, P = 0.0028) but not when the refractive changes were incorporated (t = 1.058, P = 0.3127). All subjects rated the performance of the lenses very highly: comfort, 86%; distance acuity, 83%; near acuity, 73%; and stability of vision, 74%. Twenty-four subjects (86%) chose the multifocal contact lens as their preference.

CONCLUSION

We demonstrated that a multifocal design is able to provide acceptable distance and near correction for presbyopic patients. The aspheric geometry required can be optimized for a given patient by considering his/her degree of ametropia, as well as the corneal topography.

Evaluation of the EyeSys model II computerized videokeratoscope. Part II: The repeatability and accuracy in measuring convex aspheric surfaces

PURPOSE

To evaluate the precision and repeatability of a Placido disc-based computerized videokeratoscope, using convex surfaces of varying eccentricities and apical radii designed to simulate the range of topographical variations of the human cornea, rather than the purely spherical surfaces used in most previous studies.

METHODS

Form Talysurf analysis was used to verify the exact form of 12 Perspex convex surfaces. The EyeSys model II videokeratoscope was used to measure the sagittal radii of curvature twice at known points on each surface. The raw data tables were analyzed to assess the repeatability and accuracy for both central and peripheral points on each surface. The relationship between these factors and the eccentricity was investigated.

RESULTS

For central radii the instrument showed high correlation (r = 0.996) between actual and measured values. There was a small instrumental bias of +0.042 mm and the 95% limits of agreement were narrow (+0.121 to -0.037 mm), indicating clinically acceptable accuracy. The accuracy decreased slightly as the p-value decreased (greater peripheral flattening). For peripheral radii, the overall accuracy compared well to central radii, with an average bias of +0.022 mm and maximum error in 95% of cases 0.083 mm (bias +1.96 x SD). However, for surfaces where p = 0.50, the bias was +0.049 and maximum error in 95% of cases 0.110 mm. Repeatability for the aspheric surfaces was shown to be high (SD +/- 0.01 mm in all quadrants).

CONCLUSIONS

The accuracy of the EyeSys corneal analysis system (CAS) in measuring central and peripheral radius of curvature was shown to be dependent on the shape of the surface to be measured. For more rapidly flattening surfaces, a decrease in accuracy was found for both central and peripheral radius of curvature, which in clinical terms is thought acceptable.

Evaluation of the EyeSys model II computerized videokeratoscope. Part I: Clinical assessment

PURPOSE

To compare the agreement and repeatability of keratometry measurements with simulated keratometry measurements from the EyeSys (model II) corneal analysis system (CAS). Furthermore, to establish any relationship between the repeatability of peripheral corneal measurements and the spatial location at the cornea.

METHODS

Measurements using a Bausch & Lomb keratometer in 19 eyes (19 subjects) were compared with the simulated keratometry values using the EyeSys CAS. Repeatability of the keratometer (14 eyes, 14 subjects) and the EyeSys simulated keratometry values (10 eyes, 10 subjects) was assessed by comparing the difference of two measurements. Peripheral repeatability of the EyeSys CAS was compared in a similar manner in a sample of 10 eyes (10 subjects).

RESULTS

The 95% confidence limits (-0.529 to +0.149 D) showed a lack of agreement between both instruments. With respect to the repeatability, both the keratometer (SD = +/- 0.103 D) and the EyeSys (SD = +/- 0.072 D) were found to exhibit approximately similar degrees of repeatability. The repeatability of peripheral corneal measurements using the EyeSys CAS showed changes in repeatability dependent on the corneal meridian and the distance from the point of alignment. Measurements along the superior and nasal meridians showed poorest repeatability. Repeatability was also found to deteriorate away from the point of alignment.

CONCLUSIONS

EyeSys simulated keratometry values were not interchangeable with the keratometer. This could be due to differences in the method of alignment or measurements from different areas on the corneal surface. Repeatability of both the Bausch & Lomb keratometer and the EyeSys CAS was found to be similar. The spatial dependency of peripheral corneal radius repeatability measurements could be attributed to interference of the ocular adnexa in those areas of measurement.

Current trends in modern orthokeratology

The present article describes the basic concepts and principles of modern orthokeratology. Early investigators postulated a variety of theories. However, controlled clinical studies have shown these methods to be both unpredictable and also modest in their ability to correct myopia. These traditional techniques involved fitting lenses according to a 'rule-of-thumb' and clinicians had no means of accurately evaluating corneal topography. More recently, with the significant advances in corneal topography systems and the application of reverse geometry lenses (lenses where the secondary curve steepens) certain investigators have concluded that the technique can rapidly reduce greater levels of myopia with greater predictability. The procedure involves a more scientific sagitta based fitting philosophy and predictability is defined according to corneal asphericity.

Membrane dipole potentials, hydration forces, and the ordering of water at membrane surfaces

We have compared hydration forces, electrical dipole potentials, and structural parameters of dispersions of dipalmitoylphosphatidylcholine (DPPC) and dihexadecylphosphatidylcholine (DHPC) to evaluate the influence of fatty acid carbonyl groups on phospholipid bilayers. NMR and x-ray investigations performed over a wide range of water concentrations in the samples show, that in the liquid crystalline lamellar phase, the presence of carbonyl groups is not essential for lipid structure and hydration. Within experimental error, the two lipids have identical repulsive hydration forces between their bilayers. The higher transport rate of the negatively charged tetraphenylboron over the positively charged tetraphenylarsonium indicates that the dipole potential is positive inside the membranes of both lipids. However, the lack of fatty acid carbonyl groups in the ether lipid DHPC decreased the potential by (118 +/- 15) mV. By considering the sign of the potential and the orientation of carbonyl groups and headgroups, we conclude that the first layer of water molecules at the lipid water interface makes a major contribution to the dipole potential.