Comparing Surgical-Induced Astigmatism through Change of Incision Site in Manual Small Incision Cataract Surgery (SICS)

Various strategies currently available to the cataract surgeon can safely and effectively reduce corneal astigmatism during lensbased surgery. These techniques require careful preoperative surgical planning particularly as regards incision design and site. Employing advanced enhancement techniques (such as corneal relaxing incisions) and/or toric intraocular lens (IOL) technology may also help surgeons to achieve postoperative emmetropia [2].


Introduction
Cataract surgery has become one of the most common and successful procedures in ophthalmology. In addition to improving visual acuity (VA), one of the goals of modern cataract surgery is to reduce pre-existing astigmatism (PEA), a factor that may reduce VA and affect the quality of vision [1].
Various strategies currently available to the cataract surgeon can safely and effectively reduce corneal astigmatism during lensbased surgery. These techniques require careful preoperative surgical planning particularly as regards incision design and site. Employing advanced enhancement techniques (such as corneal relaxing incisions) and/or toric intraocular lens (IOL) technology may also help surgeons to achieve postoperative emmetropia [2].
As regards the choice of incision location, previous studies reported that superior incision induces greater corneal astigmatic change (against-the-rule astigmatism) (ATR), than temporal incision (which induces with-the-rule astigmatism) (WTR) [3][4][5]. In addition, with-therule astigmatism is preferred as it frequently allows better uncorrected visual acuity [6]. Others recommend choosing the location of corneal incision based on preexisting astigmatism by placing the incision on the steeper corneal meridian (on-axis incision) to reduce significant preoperative astigmatism [7].
While phacoemulsification remains the more advanced and technically superior method of cataract surgery, it is not always appropriate either from a cost perspective or the density of the cataracts involved [8]. Small incision manual extracapsular techniques (SICS) -the first choice alternative to phacoemulsification-retains most of the advantages of "phaco" giving visual results equivalent to phacoemulsification at lower cost. However, the larger incision used induces greater astigmatism than phacoemulsification [9].
In view of the previous findings, the study aimed to compare between the surgically induced astigmatism (SIA) in manual SICS by superior (on-axis) incision and temporal incision; to determine the possibility of reduction of preoperative "with the rule astigmatism" (WTR) in patients undergoing SICS.

The exclusion criteria were
• Complicated cataract, and pseudo exfoliation.
• Pre-existing pathology other than cataract causing diminution of vision such as corneal opacities, macular disorders and optic nerve diseases. • Any patient who had undergone previous ocular surgery (trabeculectomy, refractive or retinal detachment surgery).
The patients were scheduled for Manual SICS at the department of Ophthalmology Ain-Shams University Hospitals. All surgeries were done under peribulbar anesthesia. The incision architecture was similar in the 2 groups. A 6 mm scleral straight incision, 1.5 mm from the limbus was made with a number 15 Bard Parker blade. A funnel shaped sclerocorneal pocket incision was created with a crescent knife. One side-port was made 90 degrees apart of the scleral tunnel with a 15 degree angulation knife. With a 3.2 keratome, the anterior chamber was entered 1.5 mm into the clear cornea and the internal incision was enlarged sideways to 8 mm. A single piece PMMA IOL of 6mm optic size and 12.5 mm total size was implanted into the capsular bag. No sutures were given.
Postoperatively, topical prednisolone acetate 1% eye drops were administered six times a day in the first week and gradually tapered every week over six weeks. Topical ofloxacin eye drops 0.3% were administered four times a day for the first ten days and then discontinued.
Patients were thoroughly examined postoperatively on days 1, 7, 15 and 45 after surgery. The preoperative and postoperative keratometric readings (at day 45 of follow-up visit), for each eye were used for statistical analysis. Amplitude of astigmatism was calculated from the difference in the keratometric value in the steeper and flatter meridian, using the plus cylinder notation.
The SIA was then calculated for each eye using vector analysis method [10]: astigmatism vectors are drawn using a protractor and a ruler; A horizontal baseline is drawn, on which a center point is marked. First, the preoperative vector (K1) is depicted, by drawing a line from the center outward, at an angle double the preoperative astigmatism angle, and equal in length to the magnitude of the preoperative astigmatism. For the magnitude, a convenient scale can be chosen (in the present study, the scale used was one dioptre = one centimeter).
Next, the postoperative vector (K3) is drawn, again beginning at the center and going outwards. These two lines are joined. This third line is the resultant vector and represents SIA (K2). The junction of this line and the postoperative vector is marked as the 'head' , and its other end as tail. To determine the exact SIA, this line is measured and the reading is converted into dioptric value using the earlier scale. This is the magnitude of SIA.

Results
This study included 35 eyes of 24 patients. There were no significant differences in the age, sex, or preoperative astigmatism between the two groups ( Table 1).
Applying vector analysis method, the SIA was calculated, (scale used was, one dioptre = one centimeter); In group A (superior incision), the mean preoperative astigmatism was 1.63 D @ 93; the mean postoperative astigmatism was -0.42 D @ 11. The mean SIA was calculated as -2.1 D @ 5 (Figure 3a).   In group B (temporal incision), the mean preoperative astigmatism was 1.38 D @ 84; the mean postoperative astigmatism was 1.84 D@ 92. The mean SIA was calculated as 0.7 D @ 112 (Figure 3b).
On comparing the mean postoperative astigmatism between group A and B, a statistically significant difference was found (p<0.01). The mean SIA was significantly higher in group A (-2.1 D+0.9) of ATR astigmatism than in group B (0.7 D+0.52) of WTR astigmatism (P<0.01) ( Table 2).

Discussion
The term "refractive cataract surgery" has come to represent a reality for our cataract patients. In order to achieve excellent visual results, the effect of astigmatism on postoperative vision must be minimized [1].
Today's cataract incisions provide better control of surgically induced astigmatism, either by using temporal approach to produce "astigmatically neutral" surgery or by using on-axis incision to induce astigmatism at the steep axis to counteract preexisting astigmatism [11].
In the present study, the astigmatism induced by a superior (onaxis) incision versus temporal incision in manual SICS in eyes with preoperative "with the rule" corneal astigmatism was compared. It was measured using vector analysis method, which is a simple, powerful, and accurate method. However, it is tedious requiring patience [10] The SIA was found to be significantly lower in the temporal group compared to that in the superior group. This is in agreement with previous studies [3][4][5].
The superior incision induced 2.1 D of "against the rule astigmatism". This was greater than SIA by superior incision in phacoemulsification (1.5 D) as reported by Tejedor and Murube [12]. which is explained by the larger incision size in SICS. Also it was higher than that mentioned by Haldipurkar et al. [13] who reported 1.2 D in SICS. The high SIA in this study may help to neutralise a pre-existing WTR astigmatism of 2 D, and to reduce significant WTR astigmatism of > 2 D using superior incision.
On the other hand, temporal incision induced an average SIA of 0.7 D of WTR astigmatism which is a low value. Hence, it doesn't jeopardize low levels of WTR astigmatism.
In conclusion, the high SIA induced by superior incision may prove useful when aimed at reducing high levels of preoperative corneal WTR astigmatism (around 2 D). On the other hand, temporal incision is recommended in patients with low levels of preoperative WTR astigmatism. The exact cut-off value is to be studied.

Group B (Temporal incision) P value
No. of patients 13 11 No