Abstract
Purpose To determine the significance of any association between intersessional changes in ocular residual astigmatism (RA) and astigmatism at corneal front (FSA) and back (BSA) surfaces following uneventful routine phacoemulsification. Methods Astigmatism was evaluated by autorefractometry and subjective refraction and at both the corneal surfaces with Orbscan II™ (Bausch & Lomb) over central 3 mm and 5 mm optical zones at 1, 2, and 3 months after routine phacoemulsification in 103 patients implanted with monofocal nontoric intraocular lenses (IOLs, one eye/patient). Data were subjected to vector analysis to determine the actual change (Δ) in astigmatism (power and axis) for the refractive and Orbscan II findings. Results The number of cases that attended where ΔRA was ≥0.50 DC between 1 and 2 months was 52 by autorefractometry and 36 by subjective refraction and between 2 and 3 months was 24 by autorefractometry and 19 by subjective refraction. Vector analysis revealed significant correlations between ΔFSA and ΔRA for data obtained by autorefractometry but not by subjective refraction. At all times, ΔBSA was greater than ΔFSA (p < 0.01). Key findings for ΔBSA values over the central 3 mm zone were between (A) the sine of the axis of ΔRA (y) and sine of the axis of ΔBSA (x) for the data obtained by autorefractometry (between 1 and 2 months, y = 0.749 − 0.303x, r = 0.299, n = 52, p=0.031) and subjective refraction (between 2 and 3 months, y = 0.6614 − 0.4755x, r = 0.474, n = 19, p=0.040) and (B) ΔRA (y) and ΔBSA (x) powers between 2 and 3 months postoperatively for the data obtained by autorefractometry (ΔRA = 0.118 ΔBSA + 0.681 r = 0.467, n = 24, p=0.021) and subjective refraction (ΔRA = 0.072 ΔBSA + 0.545 r = 0.510, n = 19, p=0.026). Conclusion Changes in the ocular residual refractive astigmatic error after implanting a monofocal nontoric IOL are associated with changes in astigmatism at the back surface of the cornea within the central optical zone.
Highlights
To determine the significance of any association between intersessional changes in ocular residual astigmatism (RA) and astigmatism at corneal front (FSA) and back (BSA) surfaces following uneventful routine phacoemulsification
Could unexpected modulation of corneal back surface astigmatism contribute to changes in ocular residual astigmatism between one time point and the next? An understanding of the corneal back surface astigmatism is important in relation to toric intraocular lens (IOL) power selection [1,2,3]
If corneal back surface astigmatism plays an important role in toric IOL power selection, could changes in astigmatism at this surface have impact on changes in ocular residual astigmatism in cases implanted with nontoric IOLs? A brief review of the literature shows that the mean radius of curvature at this surface ranges from 5.8 mm to 6.8 mm [1, 13,14,15,16]
Summary
Larysa Tutchenko ,1,2 Sudi Patel, Oleksiy Voytsekhivskyy, Mykhailo Skovron, and Olha Horak. To determine the significance of any association between intersessional changes in ocular residual astigmatism (RA) and astigmatism at corneal front (FSA) and back (BSA) surfaces following uneventful routine phacoemulsification. Changes in IOL position, tilt, corneal curvature, and the cumulative errors associated with IOL power estimation are likely sources of unexpected refractive surprises. An unpredicted change in astigmatism at the front surface of the cornea will affect ocular residual astigmatism following phacoemulsification. Could unexpected modulation of corneal back surface astigmatism contribute to changes in ocular residual astigmatism between one time point and the next? Ignoring corneal back surface astigmatism can lead to errors in estimating the surgically induced astigmatism and IOL power selection [1,2,3,4,5,6,7,8,9,10].
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