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Abstract Preoperative IOL power calculation is a must to avoid undesirable postoperative refractive errors. With the newer generation IOL power calculation formulas, there is general agreement that the pseudophakic refraction in eyes of average axial length can be predicted with reasonable accuracy. However prediction errors still occur specially in unusually long and short eyes, and it is in such eyes that the calculated IOL power may differ significantly from one formula to another. The aim of this work is to evaluate the accuracy ofBU formula in comparison to other commonly used second and third generation theoretical and regression derived formulas, namely; Hoffer, Binkhorstll, SRKII, Holladay and SRK/T formulas. In this study 35 eyes of 30 patients (9 males and 21 females) were subjected to planned extracapsular cataract extraction with primary PCIOL implantation. All patients had full ophthalmic examination and biometry (keratometric reading and ultrasonic axial length measurement). Errors in axial length measuements were avoided as much as possible by using Ascan probe mounted on the slit-lamp, avoiding any compression on the cornea, proper allignment and taking only the readings fulfilling the criteria of good A-scan. Postoperatively, the spherical equivalent (power of the sphere added algebrically to t he power of the cylinder) was measured 2 months after sugery. The errors in power prediction within 0.5D, ID, 2D and greater than 2D were calculated for each eye by each of the formulas ·67·, ------- - used in the study. Statistical evaluation included the standard error of estimate in diopters for each of the studied formulas. Six eyes (2 average, I long and 3 short eyes) had k-readings beyond the limits ofBU formula (Hamed et al., 1996). Thus the values of M-factor (the short and long eye correction factor related to keratometric reading) were not precisely specified and we assumed them in the same way as (Hamed et al., 1996). Using retrograde study and regression analysis, the M-factor was readjusted for those 6 eyes and the error in power prediction as well as SEE were again calculated. In eyes of average axial length the BU formula after its readjustment showed the best results (100% of eyes were predicted accurately within 20) compared to SRKII formula (94.44%), Holladay and SRK/T formulas (88.89%). In moderately long eyes it performs well and its prediction accuracy (100% of eyes were predicted accurately within 20) is similar to that of SRKII and SRKIT formulas (100%) and better than that of Holladay formula (85.71 %). However, the number of eyes was limited in this group so it needs further evaluation on wide range of eyes of long axial length. In unusually short eyes, the prediction accuracy ofBU formula after its readjustment (70% of eyes were predicted accurately within 20) is somewhat less than that of SRKII, SRK/T and Holladay formulas (90%), but still is better than that of Binkhorstll (60%) and Hoffer formulas (40%). - 68- The BlJ formula is a pronusrng formula tor PC-lOL power calculation. As in other regression derived formulas, application of the formula on large number of eyes with wide range of axial length together with retrograde study and regression analysis as well as revision of its correction factors further improves its accuracy and makes it superior to other available formulas namely; SRKlI, SRK/T and Holladay formulas. Errors in taking preoperative measurements, specially the axial length, have been shown to be the most significant source of error when using any of the modem formulas for calculating appropriate implant power. The variability of the A-scan readings is inversely proportional to the accuracy of the implant power calculated. Finally, we suggested another way of writing the BU formula, inorder to make it easier in calculating the IOL power, as presented in (table, 18). |