Binkhorst Lecture (Part 1). Experimental cataract surgery

Refractive prediction by various intraocular lens formulas using optical biometry and effect of ocular parameters on their accuracy

Purpose:

To assess the prediction accuracy of intraocular lens (IOL) formulas and study the effect of axial length (AL), central corneal thickness (CCT), anterior chamber depth (ACD), and lens thickness (LT) on the accuracy of formulas using optic biometry.

Methods:

This study was performed on 164 eyes of 164 patients who underwent uneventful cataract surgery. Ocular biometry values were measured using Lenstar-900, and intraocular lens (IOL) power was calculated using the SRK/T, SRK II, Hoffer Q, Holladay 2, and Barrett Universal II formulas. We evaluated the extent of bias within each formula for different ocular biometric measurements and explored the relationship between the prediction error and the ocular parameters by using various IOL formulas.

Results:

The summarization of refractive prediction error and absolute prediction error for each IOL formulation was performed after adjusting the mean refractive error to zero. The deviation in the error values was minimum for SRK/T (0.265) followed by Holladay 2 (0.327) and Barret (0.382). Further, SRK/T had the lowest median (0.15) and mean (0.198) absolute error as compared to other formulations. For the above formulations, 100% of the eyes were in the diopter range of ±1.0. It was observed that the overall distribution of error was closer to zero for SRK/T, followed by Holladay 2 and then Barrett.

Conclusion:

In summary, we found that accuracy was better in SRK/T formula. We achieved a better understanding of how each variable in the formulas is relatively weighed and the influencing factors in the refraction prediction.

Evaluation of the biocompatibility and fixation of a new silicone intraocular lens in the feline model

The biocompatibility and fixation of a new silicone intraocular lens was evaluated in the cat eye. Following extracapsular lens extraction, 14 cats were implanted with a silicone lens (SLM 2/UV type) with polypropylene modified J loops in one eye and a poly(methyl methacrylate) (PMMA) lens (Perspex CQ) of a similar design in the fellow eye. Half the lenses were placed in the ciliary sulcus and half in the capsular bag. The eyes were evaluated for up to one year. Neither lens material showed any signs of toxicity clinically or histopathologically. Both lenses achieved stable fixation in the capsular bag; however, some inflammatory reaction and lens dislocation were noted with sulcus placement of both lens types. The amount of proteinaceous and cellular debris on the explanted silicone lenses was significantly less than that on the PMMA lenses as assessed with methylene blue staining. All retrieved lenses were optically clear and no significant change in the lens surface morphology, clarity and/or optical properties was observed.

Corneal endothelial cell loss, cystoid macular edema, and iris-supported intraocular lenses

Endothelial cell loss, persistent cystoid macular edema and a chronic low-grade uveitis may complicate intracapsular cataract extraction combined with iris supported intraocular lenses. Clinical examination of 19 eyes of 17 patients, at 1.1 to 5 years after cataract surgery, and correlation with wide field specular microscopy and fluorescein angiography, documents this problem. The mean central corneal endothelial cell count was 497 cells/mm2 (standard deviation, 119). In ten cases, focal edema was localized to the corneal periphery. Cystoid macular edema was present in all but one case. Intermittent cornea/implant touch or low-grade intraocular inflammation, possibly from iris/implant contact, may explain the natural history of the endothelial cell loss and cystoid macular edema. Monitoring endothelial cell counts and macular function in patients with iris-supported implants may afford the early recognition of this problem. In such cases, early implant removal may alter the natural history and preserve corneal and macular function.

Short- and long-term endothelial cell loss associated with cataract extraction and intraocular lens implantation

A group of 249 patients undergoing cataract extraction with intraocular lens implantation underwent specular microscopic endothelial cell photographs before surgery and eight weeks, one year, and two years after surgery. The intraocular lenses used were the Shearing posterior chamber lens and the transiridectomy clip lens after extracapsular cataract extraction and the Medallion iris suture lens after intracapsular cataract extraction. Thirty-seven patients who underwent cataract extraction without implantation served as controls. Eight weeks after surgery, there was no statistical difference between the groups with intraocular lenses and the groups without them. One year and two years after surgery, endothelial cell loss had occurred in all groups except the group who underwent extracapsular cataract extraction without intraocular lens implantation--a group in which the patients were younger. The long-term endothelial cell loss was greater after intracapsular cataract extraction and was greater in the presence of an implant. There was statistically more endothelial cell loss with the transiridectomy clip lens than with the posterior chamber lens eight weeks and one year after surgery; the patients with posterior chamber lenses had continued to have more endothelial cell loss during the second postoperative year.

Serial corneal endothelial cell loss with lathe-cut and injection-molded posterior chamber intraocular lenses

We compared endothelial cell loss of patients implanted with lathe-cut posterior chamber lenses and those implanted with injection-molded lenses over a three-year postoperative period. Results were based on more than 2,500 measurements of corneal endothelial density. Although the technique of cataract extraction (anterior chamber phacoemulsification, posterior chamber phacoemulsification, or planned extracapsular extraction) significantly affected cell loss (P less than .01), the type of implant (lathe-cut or injection-molded) did not. Significant continuing endothelial cell loss did not occur during the first three postoperative years with injection-molded lenses. There was, however, a statistically significant 7% to 15% additional cell loss after surgery over the first two to three postoperative years with lathe-cut implants. There have been no cases of corneal endothelial decompensation developing after implantation of injection-molded or lathe-cut lenses. Because a standard field clinical specular microscope was used in this study, cell counting errors cannot be ruled out as a cause of these findings.

Quality control, and changing indications for lens implantation. The Seventh Binkhorst Medal Lecture-1982

A total of 776 specimens yielded significant data on quality control and manufacturing defects in lens implants and on the biodegradation of lens implant and suture materials. Improved quality and an understanding of durability of materials in vivo has permitted wider safe use of intraocular lenses.

Rate of endothelial cell loss in the early postoperative period after cataract surgery

Central corneal endothelial cell density was estimated in 33 consecutive patients preoperatively and at monthly intervals for 6 months after cataract surgery. The patients were divided into 3 groups by a random procedure to have either intracapsular extraction or intracapsular extraction plus Federov 4-loop iris clip lens, or extracapsular extraction with Binkhorst 2-loop iridocapsular lens. In all groups the greatest cell loss had occurred by one month and thereafter no accelerated loss was apparent except in individual patients. The limitations of such a method of assessment are discussed.

Monitoring for continuing endothelial cell loss with cataract extraction and intraocular lens implantation

For a period of three years, more than 2,700 endothelial cell density measurements were performed in over 1,000 patients undergoing cataract extraction with and without intraocular lens implantation. We concluded the following from the data: (1) Significant cell loss occurred with all surgical procedures, averaging between 11.6% and 25.6%, depending on type of procedure and implant used; anterior chamber phacoemulsification without intraocular lens implantation caused the most cell loss. (2) Patients with Medallion suture implants produced by one of two manufacturers studied demonstrated statistically significant cell loss during the first (15.6% additional cells lost) and second (22.9% additional cells lost) postoperative years following the initial cell loss due to surgery. This evidence for continuing cell loss with the Medallion suture lens correlated with the clinical observation of an increasing incidence of late bullous keratopathy in these patients. (3) No other surgical procedure or implant type, including Choyce and Shearing implants and Medallion style lenses from another manufacturer, demonstrated any evidence of continuing endothelial cell loss following the initial loss due to surgery.

Endothelial physiology and intraocular lens implantation

The endothelium is the cellular monolayer which lines the posterior surface of the cornea. This layer is important in clinical ophthalmology because it is vital to maintenance of the transparency of the cornea and vision through its pump and barrier functions which limit the ingress of fluid into the cornea from the aqueous. When the function of the corneal endothelium becomes compromised, the corneal stroma swells as it hydrates. Subsequently, epithelial bullae form with painful recurring epithelial erosions, and finally corneal scarring and blindness result. The relatively vulnerable position of the corneal endothelium renders it susceptible to iatrogenic injury during intraocular procedures, especially IOL implantation: the poor regenerative (mitotic) capacity of the human corneal endothelium limits its ability to recover normal function once it is injured.