The influence of routine uncomplicated phacoemulsification on the orthogonality of the cornea

Abstract

Purpose: The aim of this study was to determine the effect of routine uncomplicated phacoemulsification on the orthogonal distribution of mass within the central optical zone of the cornea. Methods: Astigmatism at both corneal surfaces was evaluated using Orbscan II (Bausch &and Lomb) before and up to 3 months after routine phacoemulsification (one eye/patient). The data were subjected to vector analysis to estimate the pre-and postoperative total astigmatism of the cornea (TCA). Results: Reporting the chief findings in minus cylinder (diopters, DC) over the central 3 mm (A) and 5 mm (B) optical zones. Mean TCA powers (±sd) at pre- and 3-months postop were A) –4.45DC (±2.00) and –5.69DC (±2.69), B) –2.91DC (±2.22) and –2.71DC (±1.60). Change in mean power was significant over 3 mm ( P < 0.01, n = 49) but not over 5 mm. Inter-zonal differences were significant ( P < 0.01). There was a significant linear relationship between the change in TCA power ( y = preoperative-postoperative) and TCA at preoperative stage (x) where, A) y = 0.45x + 3.12 ( r = 0.336, n = 49, P = 0.018), B) y = x + 2.65 ( r = 0.753, n = 49, P = <0.01). Over the central 3 mm zone only, change (preoperative-postoperative) in axis (°) of TCA (y 1 ) was significantly associated with TCA axis at preoperative stage (x 1 ) where y 1 = 1.391x 1 -0.008x 1 2 -0.701 ( r = 0.635, n = 49, P < 0.01). Conclusion: Changes in TCA power and axis at 3 months postop, determined using Orbscan II, are indicative of orthogonal alterations in the distribution of corneal tissue. Over the central 3 mm zone, the association between y 1 and x 1 shows that a change in TCA axis is more profound when preoperative axis is near 90° i.e., against-the-rule.

Spotlight on the Corneal Back Surface Astigmatism: A Review

“…Total corneal astigmatism is modified by routine phacoemulsification and this alteration appears to be driven by the change in corneal back surface astigmatism on an individual case-by-case basis. 36 But, to what extent does corneal back surface astigmatism vary within a typical sample drawn from a normal population? Table 1 lists the mean corneal back surface astigmatic power values reported in some of the key publications on this topic.…”