Deformations and Ruptures in Human Lenses With Cortical Cataract Subjected to Ex Vivo Simulated Accommodation.

Purpose: Human cortical opacities are most commonly accompanied by changes in lens fiber structure in the equatorial region at the lens nucleus-cortex interface. Cortex and nucleus have different elastic properties, which change with age. We therefore subjected ex vivo lenses to simulated accommodation and studied the internal deformations to better understand the mechanism of cortical cataract formation. Methods: Nine human donor lenses (33-88 years old) were tested using a bespoke radial stretching device for anterior eye segments. Seven of the lenses exhibited cortical cataracts. The other two lenses, without cataract, were used as controls. Frontal and cross-sectional images of the lens obtained during stretching facilitated measurements on equatorial lens diameter and central lens thickness in the stretched and unstretched states. Results: Stretching caused the lens equatorial diameter to increase in all cases. Conversely, the lens central thickness showed no systematic variation during stretching. For four of the lenses with cortical cataract, ruptures were observed during stretching at the nucleus-cortex boundary adjacent to the cortical cataracts. Ruptures were not observed in the control lenses or in the three other lenses with cortical cataract. Conclusions: Internal ruptures can occur in aged ex vivo lenses subjected to simulated disaccommodation. These ruptures occur at the nucleus-cortex interface; at this location, a significant stiffness discontinuity is expected to develop with age. It is hypothesized that ruptures occur in in vivo lenses during accommodation-or attempted accommodation.

Anterior lens capsule strains during simulated accommodation in porcine eyes.

Experimental protocols have been developed to measure the spatial variation of the mechanical strains induced in the lens capsule during ex vivo lens stretching. The paper describes the application of these protocols to porcine lenses. The deformations and mechanical strains developed in the anterior capsule during each experiment were determined using full field digital image correlation techniques, by means of a speckle pattern applied to the lens surface. Several speckling techniques and illumination methods were assessed before a suitable combination was found. Additional data on the cross section shape of the anterior lens surface were obtained by Scheimpflug photography, to provide a means of correcting for lens curvature effects in the determination of the strains developed in the plane of the capsule. The capsule strains in porcine lenses exhibit non-linear behaviour, and hysteresis during loading and unloading. Peripheral regions experience higher magnitude strains than regions near the lens pole. The paper demonstrates the successful application of a procedure to make direct measurements of capsule strains simultaneously with ex vivo radial lens stretching. This experimental technique is applicable to future investigations on the mechanical characteristics of human lenses.

Validation of the Nuclear Cataract Grading System BCN 10.

PURPOSE: To evaluate a new nuclear cataract grading system which is intended as a surgical guidance system to predict lens hardness before cataract surgery. METHODS: The new BCN 10 grading system consists of frontal and cross-sectional slit-lamp images of human eye lenses, ranging from a completely transparent lens nucleus to a totally black nuclear cataract. Validation was done with 9 observers for 110 cases. Two modalities were applied, and observers were asked to use only whole digits and then half digits for grading. RESULTS: Repeatability with regard to test-retest differences showed a mean limit of agreement of 1.70 for whole digits and 1.32 for half digits. The absolute test-retest difference was close to zero for low as well high degrees of cataracts. Reliability for the entire group of 9 observers yielded an intraclass correlation coefficient which was within the same confidence interval, i.e., 0.991-0.995, for whole digits and half digits. CONCLUSIONS: BCN 10 grading repeatability was not affected by the severity of the cataract. It showed very good repeatability. Repeatability was significantly higher when the observers used half digits compared to whole digits. Reliability was found to be very good as well, independently of the use of whole or half digits.

Experimental protocols for ex vivo lens stretching tests to investigate the biomechanics of the human accommodation apparatus.

PURPOSE: To explore alternative experimental protocols to investigate the biomechanical behavior of the crystalline lens and zonules using ex vivo stretching. METHODS: Radial stretching tests were conducted on the anterior segment (consisting of lens, zonules, ciliary body, and sclera) of four pairs of presbyopic human donor eyes. A simple mechanical model is used to describe the behavior of the anterior segment when tested in this way. Each pair of samples was initially stretched with the ciliary body intact. One sample was retested after cutting the ciliary body radially, and the other sample was retested after removing the lens. RESULTS: The external forces needed to stretch the sample with the ciliary body intact were significantly greater than for the tests in which the ciliary body had been cut. The forces measured with the ciliary body intact and lens in situ were comparable to the sum of the forces measured in the tests in which the ciliary body had been cut (lens in situ) and the forces measured in the tests on the intact ciliary body with the lens removed. CONCLUSIONS: When stretching tests are conducted on the anterior segment, significant circumferential tensions develop in the ciliary body. This means that the forces applied to the lens and zonules cannot be related directly to the forces applied by the external loading system. If radial cuts are introduced in the ciliary body prior to testing, however, then this difficulty does not arise.

Resolving refractive error after cataract surgery: IOL exchange, piggyback lens, or LASIK.

PURPOSE: To evaluate the efficacy, predictability, and safety of three different procedures (intraocular lens [IOL] exchange, piggyback lens implantation, and LASIK) to correct residual refractive error following cataract surgery. METHODS: A retrospective multicenter study comprised 65 eyes of 54 patients that underwent phacoemulsification, resulting in a unacceptable final refractive error. Eyes were divided into three groups: eyes that had an IOL lens exchange (17 eyes), eyes that had a piggyback lens implanted (20 eyes), and eyes that had LASIK (28 eyes). RESULTS: No differences between the IOL exchange and piggyback lens groups in the spherical equivalent, sphere, or cylinder were found (P = .072, .436, and .081, respectively). The LASIK group showed a statistically significant reduction in spherical equivalent and refractive cylinder when compared with the IOL exchange group (P < .001 and P = .001, respectively). The LASIK group showed statistically significant reduced refractive cylinder in comparison with the piggyback lens group (P = .002). The median efficacy index was 0.58 (range: 0.28 to 0.93), 0.75 (range: 0.65 to 0.92), and 0.91 (range: 0.85 to 1.14) in the IOL exchange, piggyback lens, and LASIK groups, respectively. Statistically significant differences were found between the IOL exchange and LASIK groups (P = .004) and the piggyback lens and LASIK groups (P = .003). No statistically significant differences were detected in the safety index among groups (P = .094). The predictability (±1 diopters of final spherical equivalent) was 62.5% of eyes in the IOL exchange group, 85% of eyes in the piggyback lens group, and 100% of eyes in the LASIK group. CONCLUSIONS: The three procedures were effective. The LASIK group showed the best outcomes in efficacy and predictability.

Perioperative complications and clinical outcomes of intraocular lens exchange in patients with opacified lenses.

BACKGROUND: To evaluate the perioperative complications and the outcomes of intraocular lens (IOL) exchange in patients with opacified lenses. METHODS: Retrospective multicentrical consecutive series of cases that comprised 22 eyes from 21 patients who had previous phacoemulsification with implantation of an IOL in the capsular bag and developed severe late opacification of the IOL. All patients had loss of vision and reported light disturbances. The IOLs were explanted and replaced with new IOLs. The perioperative complications were evaluated. The best spectacle-corrected visual acuity (BSCVA) before and after the surgery was compared. RESULTS: The mean time lapsed between the original cataract surgery and the IOL exchange surgery was 89.1 ± 33.6 [48-216] months. The IOL exchange was uneventful in 14 eyes (63.6 %). Anterior vitrectomy was needed in seven cases (31.8 %). Other complications included zonular dehiscence in one case (4.5 %). In most of the cases, 14 eyes (63.6 %), the IOL was implanted in the sulcus. The most explanted IOL was the Hydroview H60M (Bausch & Lomb). The mean BSCVA (LogMAR) before and after the surgery were 0.57 ± 0.69 (0.10-3) and 0.18 ± 0.22 (0.0-1.10) respectively (t paired test, p < 0.001). After the operation, 20 eyes (90.9 %) achieved a BSCVA ≤ 0.3. No eye lost 1 or more lines of corrected vision after the surgery. CONCLUSIONS: IOL exchange surgery, although associated with a high incidence of complications, restores and significantly improves the visual acuity of patients with opacified IOLs.

Elastic properties of human lens zonules as a function of age in presbyopes.

PURPOSE: To investigate the elastic properties of human lens zonules as a function of age in presbyopes. METHODS: We studied 16 presbyopic human donor eyes (ages 47-97). Anterior eye sections with crystalline lens, zonules, ciliary body, and sclera were stretched radially. The stretching device consisted of a chamber filled with balanced salt solution and eight radial hooks to hold the anterior eye section. Radial stretching was created with a stepper motor connected to a digital outside micrometer for linear displacement and digital balance for force measurement. Three eye globes were used to test our methodology. For 13 eye globes, the spring constant, elastic modulus of the zonular system, and Young's modulus of the zonules were calculated. RESULTS: We found linear dependence for force-elongation and force-strain relationships at all ages. In young presbyopic eyes (ages 47-60), the Young's modulus of the zonules was 340 mN/mm(2), whereas in older eyes (ages 83-97) it was significantly lower at 270 mN/mm(2). However, the correlation coefficient between Young's modulus and age (47-97 years) was not significant with P = 0.063. CONCLUSIONS: The zonular system in presbyopic eyes was linear elastic, and the Young's modulus of the zonules decreased 20% from presbyopic age to late presbyopic age. However, there was no significant correlation between Young's modulus and age in presbyopes.