The Impact of Changes in Corneal Back Surface Astigmatism on the Residual Astigmatic Refractive Error following Routine Uncomplicated Phacoemulsification

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.

Evaluation of Refractive Accuracy of ORA and the Factors Impacting Residual Astigmatism in Patients Implanted with Trifocal IOLs During Cataract Surgery: A Retrospective Observational Study

“…25 Steep meridian incision on the anterior corneal surface can induce off-steep meridian changes, leading to a torsional effect on TCA. 26 Changes in the posterior corneal surface have been shown to correlate with residual astigmatism as the posterior corneal surface usually possesses an ATR astigmatism with a mean of 0.37 D. [27][28][29] Furthermore, relying solely on ACA may overestimate true postoperative refractive astigmatism in WTR and underestimate it in ATR, leading to cylindrical inaccuracy after cataract surgery. 8,11 In the present study, residual astigmatism positively correlated with postoperative TCA and not with postoperative ACA.…”

Evaluation of statistical correction strategies for corneal back surface astigmatism with toric lenses: a vector analysis

“…Since the Baylor nomogram, which recommends different toricity ranges for corneal astigmatism with or against the rule, was published by Koch et al, many articles have been published by different researchers pointing out the contribution of the corneal back surface to the (total) corneal astigmatism. [6][7][8][9][10]14,16,17,[21][22][23] Currently, there are several strategies for considering the effect of the corneal back surface in tIOL power calculation: If a tomographer is available, both surfaces of the cornea can directly be measured, and the total corneal power can be derived by calculating the spherocylindrical equivalent power (generalized Gullstrand formula) or front vertex power of the cornea based on a thick lens model. 8,13 Most of the tomographers provide data on total/real/refractive power of the cornea where both surfaces are considered, but we have no insight as to the calculation concept of how these data are derived from the measured front and back surface curvature (because this information is mostly undisclosed by the manufacturers).…”

“…Meanwhile, several articles have been published focusing on the contribution of the corneal back surface to the astigmatism of the entire eye. [6][7][8][9][10][11][12][13][14][15][16][17] Currently, in addition to the Baylor nomogram which describes the indication of lens toricity values for different ranges of with or against the rule keratometric astigmatism, there are the Abulafia-Koch formula, the Goggin nomogram, and the La Hood nomogram. 3,6,7,9,10,[18][19][20] In addition to using statistical models to predict the contribution of the corneal back surface to the total corneal astigmatism, there are currently several strategies for measuring the curvature of both corneal surfaces.…”

Prediction of total corneal power from measured anterior corneal power on the IOLMaster 700 using a feedforward shallow neural network

“…To overcome a subdivision into with-therule, against-the-rule, or oblique astigmatism, and to eliminate potential annealing effects (mostly in the vector component 45°/135°) when mixing left and right eyes, we decided to set up a multi-output neural network which predicts the three output parameters (CPEQ, CPAST 0°a nd CPAST 45°) in one step from the input parameters KEQ, KAST 0°K AST 45°, AL, CCT, ACD, LT, W2W and the patient's age. Overall, this correction is an estimate from a statistical evaluation of a set of clinical data, and in the individual case, it could be appropriate or result in an overor under-correction of the real corneal back surface astigmatism (Reitblat et al 2016;Savini et al 2017;Olsen & Jeppesen 2018;Mohammedi et al 2019;Tutchenko et al 2020;Nakano et al 2021).…”