AN EVALUATION OF UNEXPECTED REFRACTIVE OUTCOMES FOLLOWING TORIC IOL IMPLANTATION FOR ASTIGMATISM: A SECTOR SUBTRACTION GRAPHICAL METHOD FOR CALCULATING THE EFFECTIVE ASTIGMATIC CORRECTION
Aim. To present and test a simple graphical procedure for determining the actual effective astigmatic correction applied after implanting toric intra-ocular lenses using the planned and residual astigmatic corrections.
Methods. Nineteen cases where a range of toric intra-ocular lenses were implanted were refracted by objective means one year post-op. Of the 19, 14 cases presented with residual astigmatism ≥0.25DC. Using the presented geometric expression, the actual effective astigmatic correction was calculated for each of the 14 cases.
Results. The pre-op planned astigmatic corrections ranged from –1.50DC to –7.50DC. The residual astigmatic values at one year did not exceed –1.75DC. Residual astigmatism resulted from a combination of IOL rotational misalignment (either clockwise or anti-clockwise) coupled with under or overcorrection. The mean (±s.d) rotational misalignment was 1.6° (±7.5°) ranging from 0 to 44°. A signifi cant association between the actual effective cylinder power (y1), planned cylinder power (x1) and sine of planned cylinder axis (x2) was found as follows , y1 = 0.807x1 + 0.199x2 – 0.372 (F= 25.43, r= 0.907, p<0.001, n=14, rpower= 0.899, ppower<0.001, raxis= –0.011, paxis= 0.97).
Conclusion. The simple graphical procedure identifi ed the root cause of unexpected residual refractive errors after toric IOL implantion. In general there was a tendency towards underdelivery of IOL power and axis misalignment. The procedure allows the cataract surgeon to quickly identify the root cause of unexpected post-op refractive error and assist in providing better management options to enhance patient satisfaction.
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