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 (y₁) was significantly associated with TCA axis at preoperative stage (x₁) where y₁ = 1.391x₁-0.008x₁²-0.701 (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 y₁ and x₁ shows that a change in TCA axis is more profound when preoperative axis is near 90° i.e., against-the-rule.

Keywords: Astigmatism; corneal; surface; total.

Figure 1

Example of Orbscan plots before (a) and after (b) phacoemulsification. The screenshots show topography of both corneal surfaces, distribution of corneal thicknesses and astigmatic powers and axes over the central 3 and 5mm central zones for the anterior surface. In a similar manner, the functionality of the instrument also yields astigmatic values for the posterior surface. Differences in both surface astigmatism and thickness are observed by glancing over the two images

Figure 2

The double-angle plot showing the actual changes in total corneal astigmatism (ie the changes induced by surgery itself and commonly referred to as the surgically induced astigmatism or SIA) at 1 month postop over central 3mm optical zone. Each circle represents SIA power of 2DC. Most data points occupy the left side indicating that the SIA, over this central zone, was more inclined towards 90° (ie along the vertical) less so towards 180° (ie the horizontal)

Figure 3

Change in total corneal astigmatism (y) at two months postop and preop total corneal astigmatism (x) over central 3mm optical zone. The association between the 2 parameters is represented by y = 0.47x+3.49 (r = 0.354, n= 83, p=<0.01)

Figure 4

Change in anterior corneal astigmatism (y) at two months postop and preop anterior corneal astigmatism (x) over central 3mm optical zone. The association between the 2 parameters is represented by y = 0.78x+0.79 (r = 0.847, n=83, p<0.01).

Figure 5

Change in posterior corneal astigmatism (y) at two months postop and preop posterior corneal astigmatism (x) over central 3mm optical zone. The association between the 2 parameters is represented by y = 0.46x+3.21 (r = 0.375, n=83, p=<0.01).

Figure 6

Change in axis of total corneal astigmatism(y) at two months postop and axis of preop total corneal astigmatism(x) over central 3mm optical zone. The association between the 2 parameters is represented by y = 1.129x- 0.006x²-1.813 (r= 0.755, n=83, p<0.01).