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Catriona Hamer, Nabil Habib, Hetal Buckhurst, Christine Purslow, Phillip J Buckhurst, ; Do toric IOL calculators accurately predict the shift in orientation of the steepest corneal meridian following cataract surgery?. Invest. Ophthalmol. Vis. Sci. 2015;56(7 ):1914.
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© ARVO (1962-2015); The Authors (2016-present)
To provide accurate astigmatic visual correction after cataract surgery, a toric intraocular lens (IOL) needs to be accurately aligned with the steepest post-operative corneal meridian. Toric IOL calculators, based on the oblique cross cylinder formulae, are used to predict the orientation of the post-operative steepest meridian. These calculators assume that both corneal astigmatism and surgical incisions’ astigmatic effect act like two thin toric lenses in contact. The study aims to verify the accuracy of pre-surgical toric IOL calculator algorithms designed to predict the change in the corneal meridian orientations following cataract surgery.
Prospective interventional study where 145 subjects (74.8±9.6 years) had small incision sutureless cataract surgery with a clear corneal incision placed obliquely to the steepest corneal meridian. Scheimpflug tomography was conducted pre-operatively and at 3-6 weeks post-operatively. The true location of the corneal incision was assessed postoperatively through slit lamp examination. Measurements were used to determine the predicted postoperative steepest meridian shift and the actual axis change that occurred following the surgery.
The median pre-operative corneal astigmatism was 0.74D (IQR 0.45, 1.10D). The mean location of the superior-temporal corneal incision was 7.6±56.9° from the steepest corneal meridian. The predicted median shift in steepest axis was 11.75° (IQR 6.1, 24.8°) towards the incision according to the toric calculators, but the median actual change was only 4.8° (IQR -5.0, 20.2°). The toric calculators significantly overestimated the overall change in axis for this cohort (p<0.001), and a poor correlation was found between the predicted and actual corneal axis change (Ƭ=0.12, p= 0.28).
The oblique cross cylinder formulae used in toric IOL calculators overestimate the shift in orientation of the steepest corneal meridian following cataract surgery. These findings would suggest that positioning a toric IOL according to the predictions of the calculator can result in greater misalignment than simply positioning the lens according to the original corneal steepest meridian.
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