Phacoemulsification has become a routine procedure for cataract extraction in most parts of the world. In order to obtain the advantages of a self-sealing sutureless incision at a low cost, ophthalmologists in the developing world are performing manual small incision cataract surgery (SICS) as an alternative. High astigmatism is an important cause of poor uncorrected visual acuity after cataract surgery.12 The aim of the study is to compare the astigmatism induced by a superior, supero-temporal and temporal incision in manual SICS.
Materials and Methods
Forty-five eyes of 42 patients (24 Male / 18 Female) with a mean age of 55.4 years (range 29 - 72 years) were included in the study. The inclusion criteria were patients with keratometric astigmatism of 1.5 D or less, good fixation and cataract upto grade 4 nuclear sclerosis. The higher grades of nuclear sclerosis cases were excluded to keep the uniformity in size and architecture of the incision. They were subdivided randomly into three groups of 15 eyes each. Group A received superior incision, group B received superotemporal incision and group C eyes received a temporal incision. Preoperatively, a full ophthalmic examination including keratometry and 'A' scan biometry was done by the surgeon (NG). Keratometry was performed using a manual keratometer (Bausch and Lomb). All surgeries were done by one surgeon (NG) under peribulbar anaesthesia. Manual SICS with viscoexpression technique was used. The incision architecture was similar in all three groups. A 6 mm scleral frown incision, 1.5 mm from the limbus was made with a 15 number Bard Parker blade. A funnel shaped sclerocorneal pocket incision was created with a diamond crescent knife. Two side ports were made 180 degrees apart on either side of the scleral tunnel with a diamond knife. With a diamond 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 intraocular lens of 6mm optic size and 12.5 mm total size was implanted into the capsular bag. Patients were examined on days 1, 4, 10, 40 and 90. Betamethasone 0.1% eye drops were administered six times a day in the first postoperative week and gradually tapered every week over six weeks. Ofloxacin eye drops 0.3% were administered four times a day for the first ten days and then discontinued. Uncorrected and best-corrected visual acuity and keratometry were recorded on each visit. All postoperative work including refraction was done by the surgeon (NG). There were no intraoperative or postoperative complications. All patients were followed up without any dropout.
Preoperative and postoperative (day 90) keratometric readings and refraction (day 90) were used for analysis. All calculations were performed using the surgically induced astigmatism (SIA) Calculator Version 1.0 a free software programme. Amplitude of preoperative and postoperative astigmatism was calculated from the difference in the keratometric value in the steeper and flatter meridian, using the plus cylinder notation. Astigmatism was considered a vector with a magnitude equal to this value, directed towards the steeper meridian. For example, keratometry values of 43.5 x 900 and 44 x 1800 would imply astigmatism of 0.5 D 1800. The amplitude of SIA was also calculated for each eye from the preoperative and postoperative amplitudes using the SIA software programme. Mean and standard deviation was calculated for all the individual values obtained in each group Table 1 - Amplitude of astigmatism).
The data were also analysed using Cartesian coordinates based analysis using Holladay's system,3 whereby each of the astigmatic vectors was converted into x and y values. These x and y values were independently averaged and reconverted into the astigmatic vector form, producing mean pre- and postoperative astigmatic values for the group. This mean astigmatism, called the centroid, represents the true mean of the data set, which takes into consideration all elements forming the data. From the pre- and postoperative x and y values, SIA x and y values were also calculated for each eye. These were averaged and converted into a vector form which gives the SIA centroid (Table 2). These data were also displayed and analysed using double angle plots (DAP) as depicted in (Figures 1 - 3). A highly localised clustering of points on the DAP indicated a homogeneous group with true representative of its centroid with a high predictive value.
The amplitude of preoperative astigmatism was similar and around 0.5 D in the three groups (Table 1). The amplitude of postoperative astigmatism Table 1 was higher in group A (1.45+0.94) than in group B (0.43+0.27) and in group C (0.67+0.65). The amplitude of surgically induced astigmatism Table 1 was also higher in group A (1.36+1.03) than in group B (0.51+0.49) and in group C (0.40+0.40).
Analysis of the SIA centroids Table 2 shows that 1.28 D of horizontal steepening (or vertical flattening) was induced by making a superior incision. Surgically induced astigmatism in the supero-temporal group was 0.20 D at 23.7° whereas the temporal incision induced an average horizontal flattening (or vertical steepening) of about 0.37 D.
The SIA vectors in the superior group on the DAP (Figure 1) show clustering, implying a high predictive value of the centroid obtained,that is, 1.28 D x 2.9. This indicates that making a superior incision consistently induced an average of 1.28 D of horizontal steepening. The SIA vectors in the superotemporal group were well spread out on the DAP (Figure 2) implying a low predictive value. The SIA vectors of the temporal group were also well clustered (Figure 3) implying a high predictive value and a temporal incision consistently induced an average horizontal flattening of about 0.37 D.
Manual SICS is an alternative for phacoemulsification but the astigmatism is higher due to the larger size of incision. Burgansky et al4 have shown an increase in astigmatism with an increase in incision size. In their study by vector analysis, the mean induced astigmatism was 0.6 ± 0.3 D for 6 mm incision, 0.75 ± 0.67 D for a 6.5 mm incision and 1.36 ± 0.77 D for a 7 mm incision. Kimura et al5 have shown by vector analysis that surgically induced astigmatism is less with an oblique incision (1.02 ± 0.66 D) than with a superior incision (1.41 ± 0.72 D).
The temporal location is farthest from the visual axis and any flattening due to the wound is less likely to affect the corneal curvature at the visual axis. When the incision is located superiorly, both gravity and eyelid blink tend to create a drag on the incision.
These forces are neutralised better with temporally placed incisions because the incision is parallel to the vector of the forces. With the rule astigmatism induced by a temporal incision is advantageous because most elderly cataract patients have preoperative against the rule astigmatism. The superotemporal incision also, is probably free from effect of gravity and eyelid pressure and tends to induce less astigmatism.
Astigmatism induced in manual SICS done with a superior, superotemporal and temporal scleral tunnel incision have been compared. Modern methods of astigmatic data handling require that both amount and axis of astigmatism be given equal importance, using a mathematically validated method.6 Accordingly, the method described by Holladay et al was employed.3 Holladay's method is more difficult to comprehend than vector analysis, but it has been chosen over the latter as it gives more accurate results.7 The study found that induced astigmatism was lower in the temporal and superotemporal groups compared to that in the superior group. Astigmatism in the superotemporal and temporal groups was comparable.
The superior incision is likely to induce about 1.28 D of astigmatism as suggested by the centroid (1.28 D x 2.9 degrees). This is a definite clinical gain, as one can aim to neutralise a pre-existing astigmatism of, say, 1.25 D x 90 degrees. In conclusion, a shift in the incision site to the superotemporal or temporal sclera is recommended except in patients with a pre-existing 'with the rule' astigmatism of about 1D. Small sample size is a weakness of this study. Our conclusions need to be validated with a larger study.
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