When not to correct Astigmatism

Should refractive surgery be employed to correct 0.75 D or less of cylinder? The question poses a dilemma for surgeons since even standard ablations in photorefractive keratectomy and LASIK to correct myopia can induce a mean astigmatism of approximately 0.50 D. While toric intraocular lenses (IOLs) are an option for pseudophakic patients with astigmatism, the possible astigmatism induced by the corneal incision and the rotational and tilt errors during IOL positioning can limit the efficacy of correcting small amounts of astigmatism. Spanish researchers conducted a study to evaluate how small amounts of astigmatism affect visual acuity and the minimum astigmatism values that should be corrected to achieve maximum visual performance. They used wavefront sensing to measure astigmatism and higher-order aberrations in 54 normal young eyes with astigmatism ranging from 0.0 to 0.5 D. They corrected astigmatism using a purpose-designed cross-cylinder device. They calculated optical image-quality metrics for three conditions: natural astigmatism, corrected astigmatism and astigmatism only (with all HOAs removed). There was no significant correlation between the amount of astigmatism and visual acuity. The correction of astigmatism improved visual acuity for only high-contrast letters from 0.3 D, but with a high variability between patients. Lowcontrast visual acuity changed randomly as astigmatism was corrected. The correction of astigmatism increased the mean imagequality values but there was no significant correlation with visual performance. The deterioration in image quality given by astigmatism higher than 0.3 D was limited by higher order aberrations. The researchers conclude that under clinical conditions, the visual benefit of precise correction of astigmatism less than 0.5 D would be limited.

EA Villegas et al., JCRS, “Minimum amount of astigmatism that should be corrected”, Volume 40, Number 1, 13-19.

 

Femtocat learning curve

Femtocataract surgery has purported advantages of improved accuracy and safety. However, it is not without complications. In order to assess the incidence of complications investigators conducted a retrospective analysis of hundreds of femtosecond laser-assisted cataract surgeries. Overall, the study revealed miosis in 32 per cent of cases, conjunctival redness or haemorrhage in 34 per cent, capsule tags and bridges in 20 per cent, anterior tears in four per cent, endothelial damage due to cut within endothelial layer in three per cent and suction break in two per cent. No complications required vitrectomy. However, all of these complications occurred within the first 100 cases. The researchers conclude that with cautious surgical technique, the complications seen in this study can be avoided.

Z Nagy et al. JCRS, “Complications of femtosecond laser-assisted cataract surgery”, Volume 40, Number 1, 20-28.

PCO and different IOL types

In spite of ongoing improvements in IOL design and surgical technique, posterior capsule opacification (PCO) remains the most prevalent complication after cataract surgery. A randomised controlled clinical study conducted over a 12-year period compared PCO incidence seen in three types of IOL: a round-edged heparin-surfacemodified PMMA IOL, a round-edged silicone IOL or a sharp-edged hydrophobic acrylic IOL. After 12 years, there was no significant difference in the fraction or severity of PCO between the silicone IOL and acrylic IOL. The HSM PMMA IOL had a significantly higher PCO fraction than the silicone IOL, but not more than the acrylic IOL. There was no difference in PCO severity between the HSM PMMA IOL and the other two IOLs. The silicone IOL had higher median capsulotomy-free survival (>150 months) than the acrylic IOL (108 months) and the HSM PMMA IOL (53 months). Overall survival without Nd:YAG capsulotomy did not differ between the acrylic and silicone IOLs or between the silicone and HSM PMMA IOLs. However, overall survival was significantly better with the acrylic IOL than with the HSM PMMA IOL.

M Rønbeck et al., JCRS, “Posterior capsule opacification with 3 intraocular lenses: 12-year prospective study”, Volume 40, Number 1, 70-76.