Aberration changes of the corneal anterior surface following discontinued use of rigid gas permeable contact lenses

AIM: To record aberrations with a corneal topographic device on the anterior surface of the cornea at different time-points prior to wearing and following discontinued use of rigid gas permeable (RGP) contact lenses. The effect of wearing RGP on the anterior surface of the cornea was discussed to provide guidance for clinical refractive error correction.METHODS: The study objects were 24 eyes from 24 patients. All patients underwent identical examination procedures prior to lens use, as well as afterwards, including slit-lamp examination, non-contact tonometer measurement, computer optometry and corneal curvature measurement, subjective refraction test, and corneal topography analysis. The patients wore contact lenses everyday for 1 month and then discontinued. Corneal topographies were recorded at certain time points of 30 minutes, 1 day, 3, 7 and 14 days following use.RESULTS: Total corneal aberration at each time point following discontinued use of RGP contact lenses was less than the time point prior to use. Detailed results were as follows: root mean square (RMS) (pre)=(1.438± 0.328)μm, RMS (30 minutes) =(1.076 ±0.355)μm, RMS (1 day) =(1.362 ±0.402)μm, RMS (3 days) =(1.373 ±0.398)μm, RMS (7 days) =(1.387 ±0.415)μm, and RMS (14 days) = (1.430±0.423)μm. Results showed that at 30 minutes after discontinued use of RGP contact lenses, almost all 2 ndand 3 rd-order aberrations change. Quadrafoil Z10 and spherical Z12 of the 4 th-order were also changed. Alterations to Z5, Z6, and Z12 at 1 day after discontinued use were significant differences compared with the time period prior to RGP use: Z5 and Z6 decreased, and Z12 increased slightly. Z5 and Z6 remained decreased at 3 days after discontinued use, but Z9 and Z10 continued to increase and Z12 returned to levels prior to RGP use. At 14 days after discontinued use, all aberrations were notsignificantly different from the values prior to use.CONCLUSION: The use RGP contact lenses greatly reduced total aberration of the anterior surface of the cornea. Changes to 2 ndand 3 rd-order aberrations (including Z3, Z4, Z5, Z6, Z7, and Z8) were more significant. Following discontinued use of RGP contact lenses, the majority of lower order aberrations returned to original levels in a short period of time. During this process, a transient higher order aberration appeared, but all changes disappeared within 14 days after discontinued use of RGP contact lenses.

Effects of rigid gas permeable contact lens on morphological parameters and vision-related quality of life in keratoconus patients

AIM: To evaluate the effect of rigid gas permeable contact lens(RGP-CL) on corneal morphological parameters and vision-related quality of life in keratoconus(KC) patients.METHODS: Totally 57 eyes of 30 KC patients who were followed-up for more than two years, including 17 RGP wearers(32 eyes) and 13 non-wearers(25 eyes) were retrospectively analyzed. Initial medical history and corneal topography were collected at baseline. Corneal topography, corneal aberration, optical coherence tomography, and vision-related quality of life questionnaires were performed at the last follow-up.RESULTS: According to corneal topography, increase of the flattest keratometric values was higher in RGP wearers than in non-wearers(P=0.038). The morphological parameters, including symmetry index of front corneal curvature(P=0.004) and Baiocchi-Calossi-Versaci index front(P=0.047), were lower in RGP wearers than in nonwearers. Vertical coma was smaller in RGP wearers than non-wearers in 3.0, 5.0, 6.0, and 7.0 mm pupil diameters, respectively(P<0.05). The environmental triggering domain of ocular surface disease index was worse in RGP wearers as compared to non-wearers(P=0.003). At the last followup, there were no significant differences in constituent ratios of KC progression, corneal thickness topography, epithelial thickness topography, morphological parameters of corneal topography, and other questionnaire scores between the two groups(all P>0.05).CONCLUSION: Long-term use of RGP does not worsen KC but may cause corneal epithelial remodeling to increase symmetry of corneal anterior surface, reduce corneal vertical coma and improve visual quality. However, RGP wearing causes a slight decrease in vision-related quality of life. The occurrence of ocular surface symptoms is mainly associated with environmental triggering factors.

Axisymmetric smooth contact for an elastic isotropic infinite hollow cylinder compressed by an outer rigid ring with circular profile

A contact problem for an infinitely long hollow cylinder is considered. The cylinder is compressed by an outer rigid ring with a circular profile. The material of the cylinder is linearly elastic and isotropic. The extent of the contact region and the pressure distribution are sought. Governing equations of the elasticity theory for the axisymmetric problem in cylindrical coordinates are solved by Fourier transforms and general expressions for the displacements are obtained. Using the boundary conditions, the formulation is reduced to a singular integral equation. This equation is solved by using the Gaussian quadrature. Then the pressure distribution on the contact region is determined. Numerical results for the contact pressure and the distance characterizing the contact area are given in graphical form.

Evaluation of a Modified Scleral Contact Lens as a Riboflavin Delivery Device for Corneal Collagen Crosslinking

Introduction: Keratoconus is a complex corneal disease that reduces visual acuity by progressively modifying the corneal shape and thickness, usually producing myopia and irregular astigmatism. Corneal collagen crosslinking with riboflavin + ultraviolet-A radiation (CXL) has become a widely accepted treatment for progressive keratoconus. During CXL, riboflavin administration is performed by repeated manual instillation of solution drops on the cornea for 30 minutes, a procedure that is often uncomfortable for many patients and that consumes surgical facilities and staff resources. In this study, especially modified scleral contact lenses (MSCL) were employed for delivering riboflavin to the cornea during CXL. Objective: The study aimed at evaluating the safety and efficacy of MSCL as a drug delivery system, verifying if anterior chamber flare confirms riboflavin penetration and describes the impact on patient comfort and optimization of surgical staff and facility resources. Material and Method: This study included 8 eyes of 6 patients aged 16 - 25 years old with history of progressive keratoconus. After mechanical removal of corneal epithelium, the concave surface of the modified scleral contact lens was filled with riboflavin solution and the lens was placed on the patient’s eye during 30 minutes. The lens design allows the formation of a riboflavin layer between the lens and the exposed corneal stroma to facilitate riboflavin penetration. Patients with lens were allowed to stand up and wait for the second UVA phase outside the surgical room. Riboflavin diffusion was confirmed by biomicroscopic examination of the corneal stroma and anterior chamber with the lens in place. Patients returned and the lens was removed before UV-A irradiation at 3 mW/cm2 for 30 minutes. Statistical analysis was performed by comparing the following parameters of each patient pre- and post-CXL: Spherical equivalent (Sph.Eq.), Mean simulated keratometry (SimK-m) and corrected distance visual acuity (CDVA) using the Wilcoxon method for non-parametric data (p Results: The MSCL allowed patients to be transferred from the surgical room to wait for corneal impregnation with riboflavin. The MSCL was effective in delivering riboflavin to the cornea as confirmed by biomicroscopic examination of the cornea and anterior chamber. No intraoperative or postoperative complications were observed. MSCL use improved the patient comfort and reduced the burden on surgical staff and facilities. All analyzed parameters showed statistically significant differences pre- and post-CXL: Sph.Eq. p = 0. 018 (Median: -2.50;Average: -2.52);SimK-m p = 0. 006 (Median: 47.92;Average: 45.56). CDVA p = 0. 012 (Median: -0.45;Average: 0.42). Conclusion: The MSCL is a safe and efficacious device for riboflavin delivery during CXL. The present study permits slit lamp observation of anterior chamber flare to confirm riboflavin penetration, and provides added safety and comfort for the patient and convenience to healthcare providers by optimizing the use surgical facilities and staff. Keratometric, visual and refractive results were similar to those reported in the literature for CXL with manual riboflavin instillation. Additional studies with larger numbers of patients are needed to confirm the study findings.

Useful zero friction simulations for assessing MBS codes Pascal’s formula giving wheelsets frequency for zero wheel-rail friction

Present study provides a simple analytical formula,the“Klingel-like formula”or“Pascal’s Formula”that can be used as a reference to test some results of existing railway codes and specifically those using rigid contact.It develops properly the 3D Newton-Euler equations governing the 6 degrees of freedom(DoF)of unsuspended loaded wheelsets in case of zero wheel-rail friction and constant conicity.Thus,by solving numerically these equations,we got pendulum like harmonic oscillations of which the calculated angular frequency is used for assessing the accuracy of the proposed formula so that it can in turn be used as a fast practical target for testing multi-body system(MBS)railway codes.Due to the harmonic property of these pendulum-like oscillations,the squareω2 of their angular frequency can be made in the form of a ratio K/M where K depends on the wheelset geometry and load and M on its inertia.Information on K and M are useful to understand wheelsets behavior.The analytical formula is derived from the first order writing of full trigonometric Newton-Euler equations by setting zero elastic wheel-rail penetration and by assuming small displacements.Full trigonometric equations are numerically solved to assess that the formula providesω2 inside a 1%accuracy for usual wheelsets dimensions.By decreasing the conicity down to 1×10^(−4) rad,the relative formula accuracy is under 3×10^(−5).In order to test the formula reliability for rigid contact formulations,the stiffness of elastic contacts can be increased up to practical rigidity(Hertz stiffness×1000).

Nonstationary plane contact problem in theory of elasticity for conformal cylindrical surfaces

A numerical–analytical approach is described to investigate the process of impact interaction between a long smooth rigid body and the surface of a circular cylindrical cavity in elastic space. A non-stationary mixed initial boundary value problem is formulated with a priori unknown boundaries moving with variable velocity. The problem is solved using the methods of the theory of integral transforms, expansion of desired variables into a Fourier series, and the quadrature method to reduce the problem to solving a system of linear algebraic equations at each time step. Some concrete numerical computations are presented.The cylindrical body mass and radius impact on the proile of the transient process of contact interaction has been analysed.